- Email: [email protected]
Antigen processing and presentation in transplantation
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Antigen processing and presentation in transplantation Hugh Auchincloss Jr* and Heena Sultan For many years the direct stimulation of T cells in response to donor MHC antigens expressed on donor antigen-presenting cells has been the focus of transplantation immunology. Indirect recognition in response to peptides of donor antigens presented by self MHC molecules on recipient antigen-presenting cells has not generally been considered an important feature of graft rejection. Recent evidence suggests that indirect responses may be more important than previously considered and the new emphasis on indirect pathways in allograft rejection has raised new issues, many of which are unresolved.
Addresses Massachusetts General Hospital, Department of Surgery and Transplantation, White 51 OB, Fruit Street, Boston, MA 02114, USA *e-maih [email protected] Current Opinion in Immunology 1996, 8:681-687
normal mechanism for T cell stimulation [4]. Although this pathway of stimulation was considered by La Rosa and Talmage [5] as a possible explanation for graft rejection in cases involving minor histocompatibility mismatches and although stimulation of an 'indirect' response during graft rejection was demonstrated several years ago by the phenomenon of 'cross-priming' (the presentation of minor antigens from the donor by recipient MHC class I molecules) [6], the indirect pathway of T cell stimulation has not generally been considered to be as important as direct stimulation. During the past several years, however, increasing evidence has accumulated emphasizing the role, and perhaps even the dominance, of the indirect pathway in allograft rejection [7]. This paper reviews some of the key information supporting that assertion and considers some of the interesting issues which have emerged in light of the contribution of indirect T cell stimulation to graft rejection.
© Current Biology Ltd ISSN 0952-7915 Abbreviations APC antigen-presentingcell TCR T cell receptor
Introduction
T h e direct T cell response to foreign MHC antigens has generally been thought to be the distinguishing feature of cell-mediated allogeneic immunity [1]. Defined as the stimulation of T cells by the allogeneic MHC antigens on allogeneic APCs, direct T cell stimulation occurs because of the similarity of allogeneic and self MHC molecules. Because of this similarity, allogeneic MHC molecules are nearly unique among foreign proteins in that they do not require processing and presentation as peptides in order to stimulate a T cell response [2]. Furthermore, when measured in vitro, the frequency of precursor T cells for a direct response is about one hundred-fold greater than for a response to peptides of ordinary environmental antigens. In addition to the extraordinary strength of the direct pathway when measured in vitro, evidence from in vivo experiments has supported the idea that direct recognition is critical in graft rejection, including the particular importance of MHC antigen matching in graft survival and the survival of some types of grafts after removing donor APCs [3]. In contrast to direct T cell stimulation, the natural physiological mechanism of T cell stimulation is for TCRs to recognize peptides of foreign proteins that have been processed by self APCs and presented by self MHC molecules. In transplantation immunology this physiologic response has been referred to as indirect presentation even though the term fails to convey the sense that this is the
Testing the importance pathway
of the indirect
In the past, the effort to demonstrate an important role for the indirect pathway has been hampered by the concurrent potential for direct stimulation that is so strong by in vitro assays. Therefore, some of the initial studies suggesting that indirect stimulation might play a role in vivo came from studies of xenogeneic graft rejection using species combinations in which direct T cell stimulation in vitro was weak or absent. For example, Moses et al. [8] showed that the primary in vitro response of mouse T cells to discordant stimulating cells was weak. On the other hand, the strong secondary response in vitro after xenograft rejection occurred almost entirely through the indirect pathway since the response depended on the presence of recipient APCs. Pierson et al. [9] and other investigators [10] found that xenograft rejection in vivo was especially dependent on CD4 + T cells and suggested from the in vitro data that an indirect response might be important in xenograft rejection. Studies of the role of the indirect pathway in allograft rejection required more subtle manipulations. For example, Fabre and co-workers [11",12] showed that immunization of recipients with peptides of allogeneic MHC antigens (which could only stimulate an indirect response) led to accelerated graft rejection. Although the difference in time to rejection was very small in some of these experiments, the results did suggest that an indirect response was one way that a recipient could respond to allogeneic grafts. Suciu-Foca and co-workers [13] showed evidence that the rejection episodes in human cardiac allograft recipients was associated with stimulation of an indirect T cell response.
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More compelling experiments showing that an indirect response could initiate allograft rejection became possible once mice which lacked MHC class II antigens were generated by targeted gene disruption [14]. Since grafts from these mice could not stimulate a direct CD4 ÷ T cell response, rapid rejection of their tissues in a CD4+-dependent manner suggested that indirect T cell stimulation could lead to graft rejection [15]. Subsequent in vitro studies, using T cells from mice which had rejected MHC class II deficient skin, supported this conclusion [16]. Although for many years evidence has indicated that physiological indirect responses occur during graft rejection, and although the evidence of recent years has suggested that such a response can provide a possible mechanism for graft rejection, the most interesting data of the past several years has suggested that indirect T cell stimulation may even be the dominant pathway involved in allograft rejection. For example, studies by Sayegh et al. [17] have indicated that the downregulation of T cell responses by thymic administration of peptides from allogeneic MHC antigens can lead to prolonged survival of subsequent aliografts. MHC peptides could not themselves have affected direct T cell responses, suggesting that indirect T cell stimulation was critical in the rejection process. In addition, Steele et al. [18"°] have shown that T cell help for B cell IgG alloantibody production after skin transplantation in mice depends on the ability to generate an indirect T cell response. These results have helped stimulate a new emphasis on the indirect pathway in allograft rejection but they have also raised many important questions in transplantation immunology which have not yet been resolved.
Which peptides stimulate an indirect response? Major histocompatibility antigens have the special feature that they can be recognized directly by TCRs and that they are especially important in causing rapid graft rejection. As foreign proteins, however, their peptides can also be processed and presented by the MHC molecules on recipient APCs, thereby generating an indirect response. Minor histocompatibility antigens, on the other hand, are peptides of donor proteins which can be processed, and presented by self MHC molecules and recognized as foreign by recipient T cells [1,2]. Individual minor histocompatibility antigens generally cause much slower graft rejection than do MHC antigens. These considerations raise a perplexing issue: if both MHC peptides and minor antigen peptides can be recognized indirectly, and if the indirect response is really the dominant pathway in graft rejection, then why should graft rejection be stronger when there are MHC antigenic differences? The early demonstration of cross-priming indicated that minor histocompatibility antigens can be recognized indi-
rectly [6]. Nonetheless, many of the recent studies of the indirect pathway have focused on recognition of peptides of MHC molecules, with the largely implicit assumption that these peptides are for some reason more important than minor peptides [19-21,22",23,24,25°-28",29-31]. No published studies to date, however, have actually compared the strength of indirect stimulation generated by peptides of minor compared to major histocompatibility antigens. Since matching for MHC antigens makes a difference in graft survival and if the indirect pathway is truly dominant, this assumption may be valid. It is, however, an assumption that requires further testing. Analysis of the endogenous peptides that have been eluted from MHC molecules have shown a surprisingly high frequency of peptides derived from MHC molecules. One likely explanation for this finding is simply that when self MHC molecules fail to fold or load peptides effectively (and are therefore degraded in the cytoplasm of self APCs) their peptides are the ones in the best location for subsequent presentation by newly forming self MHC molecules. This explanation for the frequent endogenous presentation of self MHC peptides, however, does not explain why peptides from exogenous MHC molecules should have a special advantage for presentation by the indirect pathway. Two other possibilities remain: first, allogeneic MHC molecules may be available in greater quantity than minor histocompatibility proteins (this could occur because MHC molecules are more frequently expressed, or because their location on the surface membrane makes them more easily available for uptake by self APCs, or because B cells are more likely to be stimulated by MHC than minor antigens and thus to serve as effective APCs for the presentation of MHC peptides); second, the T cell repertoire could be biased toward recognition of allogeneic MHC peptides presented by self MHC molecules (this might occur if the frequent endogenous presentation of self MHC peptides by self MHC molecules leads to positive selection in the thymus, especially for TCRs that recognize self MHC molecules presenting peptides of slightly different MHC proteins). Such explanations are speculative until clear evidence is obtained showing the dominance of MHC compared to minor peptides in presentation by the indirect pathway. Nonetheless, the evidence of the importance of the indirect pathway in graft rejection and the evidence that matching for MHC antigens matters in graft survival suggests a new concept: that MHC matching may matter primarily because of the importance of MHC peptides in stimulating indirect T cell responses rather than because of the importance of the direct recognition of intact MHC molecules.
Which APCs are important in the direct and indirect pathways? A significant difference between the direct and indirect pathways of sensitization is that the types of APCs
Antigen processingand presentationAuchinclossand Sultan
available in each case are likely to be different, a difference that may be important in determining the degree and character of T cell activation. Direct T cell activation is likely to involve two kinds of APCs that will vary with the type of graft. First, many tissues have their own specialized APCs (such as Langerhans' cells in the skin or Kupffer cells in the liver). Second, endothelial cells of vascularized grafts appear to have APC function, especially if activated [32,33]. T h e density of these donor APCs is different in different tissues [34] and also varies over time, as bone marrow derived donor cells are replaced by cells of recipient origin. Indirect T cell activation, on the other hand, depends on the APCs in the recipient's draining lymph nodes and spleen, including monocytes, lymphoid cells and dendritic cells. T h e availability of these APCs for indirect responses is probably less variable. Recent studies have indicated that dendritic cells [35"] and monocytes [32] have the capacity to stimulate indirect responses but that recipient B cells are not essential as APCs when recipient T ceils are sensitized to minor antigens [36°1. Activated B cells, however, do have APC function and may be especially important in stimulating some T cell responses [37]. Although so far few transplantation experiments have been performed to determine the different types of T-cell responses that might occur during graft rejection depending on the types of APCs available, many nontransplant experiments suggest that cytokine profiles of T cells do depend on the character of the APC. Furthermore, a reasonable hypothesis is that the APCs in draining lymph nodes are the ones best suited to stimulate immune responses, while the APCs in nonlymphoid organs are more likely to carry antigens away from that site and to downregulate local immune responses. Thus, an important issue in studying direct versus indirect responses is to determine the nature and the capacities of the different APCs involved.
Can both class I as well as class II molecules present donor antigens indirectly? A basic tenant of classical immunology has been that peptides of exogenous antigens are presented by M H C class II molecules whereas peptides of endogenous antigens are presented by M H C class I molecules. Applied to transplantation immunology, this principle would suggest that indirect responses to exogenous donor antigens ought to be limited to CD4+ cells responding to modified self M H C class II molecules. Despite this prediction, transplantation studies involving minor antigen disparate grafts from M H C homozygous donors to M H C heterozygous recipients have shown that CD8 ÷ T cells can be sensitized to the minor antigens presented in association with recipient M H C class I molecules [6]. This 'cross-priming' phenomenon suggests that exogenous antigens can be presented by M H C class I molecules. Subsequently, nontransplantation experiments have also
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indicated that M H C class I molecules can present exogenous antigens under a variety of circumstances. Bevan has recently reviewed three major mechanisms by which this may occur [38°]: first, phagocytes can shunt protein from phagosomes into the cytosol for loading onto the surface M H C class I molecules; second, phagocytes can digest the ingested lysosomal content and then load peptides onto surface M H C class I molecules; and third, heat shock proteins which are released by dying cells and serve as peptide carriers for professional APCs. These heat shock proteins normally play a role in the transfer of peptides from proteasomes to M H C class I molecules and can gain access to the cytosol of professional APCs. While it is no longer in doubt that indirect presentation of donor antigens by recipient M H C class I molecules can occur, there is uncertainty over whether this form of indirect presentation is important in graft rejection. Except in cases where donor and recipient M H C class I antigens are matched, CD8 ÷ cells sensitized by peptides presented in the context of recipient M H C class I molecules, will not find such a determinant expressed by donor cells. Therefore, CD8 + cells sensitized indirectly can only participate in graft destruction by the production of cytokines, not by their direct interaction with or lysis of donor cells.
Are there limitations on the ability of indirect helpers to co-operate with direct effectors? Another central principle of immunology is that regulatory interactions between lymphocytes generally require physical proximity. In some cases this may be accomplished by simultaneous expression of helper and cytotoxic determinants for CD4 + and CD8 ÷ T cells respectively, on the same APC. This was described as a 'three cell' model by Mitchison and O'Mailey for the generation of cytotoxic T cells [39]. In other cases, physical linkage may be accomplished by a cognate interaction between the helper and effector cell (such as CD4 ÷ recognition of the modified M H C class II antigens of B ceils in the generation of antibody responses). In transplantation immunology, studies by Keene and Forman [40] have similarly suggested that helper and cytotoxic cells involved in graft rejection are best sensitized by APCs expressing both determinants, while studies by Rosenberg et al. [41] indicated that the two determinants had (at least) to be generated by the same donor graft. To the extent that the effector CD8 + T cells mediating graft rejection must recognize determinants expressed on the cells of the donor graft, the use of the indirect pathway for the sensitization of helper cells appears to violate the principal of physical linkage. If helper cells are sensitized by recipient APCs in the draining lymph node, but effector cells are sensitized by donor cells in the tissue graft, there are no obvious mechanisms that would tend to bring these populations together in a tight physical association.
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Lee et al. [16] addressed this issue using MHC class II deficient skin grafts which forced sensitization to occur through the indirect pathway. These investigators concluded that under some circumstances, sensitization of CD8 ÷ cells by antigens expressed on donor APCs could be shown to depend on help from a CD4+ indirect response. While these results (and the general effectiveness of the indirect pathway in producing graft rejection) suggest that a three cell linkage is not absolutely required for helper/effector interactions in graft rejection, it remains unlikely that helper and effector cells can be sensitized entirely independently. A degree of physical association might occur if direct and indirect sensitization take place within the same draining lymph node. It remains to be determined, however, exactly what constraints are involved when help is generated indirectly for effector cells sensitized by the direct pathway.
Is there an indirect effector mechanism? One way to avoid the physical linkage problem in the case of graft rejection would be to drop the assumption that the effector cells responsible for graft destruction must recognize determinants expressed on donor cells. If cells stimulated by the indirect pathway can themselves destroy grafts, then there would be no need for them to cooperate with effector cells that have been sensitized directly. T h e idea that there might be an indirect effector mechanism would seem contrary to the results of studies by Rosenberg and Singer [42] using grafts from semiallogeneic tetraparental donors which are composed of cells of both donor and recipient origin. The apparent selectivity of the tissue destruction in these experiments suggested that effector mechanisms involve cells with donor specificity. On the other hand, studies by Winn and collegues [43] using skin grafts in which only the APCs were foreign to the recipient, indicated that rejection of an entire graft could occur when only some of the cells within it expressed foreign determinants. Wecker et al. [44"'] addressed this issue by placing MHC class II deficient skin grafts onto severe combined immunodeficiency mice that had been reconstituted only with CD4+ T cells. T h e y found that the MHC class II deficient skin was rejected very slowly in the absence of effector cells which could recognize the remaining MHC class I alloantigens. Similarly, Markmann et al. [45°,46,47] found that MHC deficient cardiac and skin allografts were not rejected by normal recipients, suggesting that an indirect response was not sufficient to cause destruction of these tissues. Overall, the evidence at this point suggests that an indirect effector mechanism can participate in graft destruction, albeit inefficiently in many circumstances. Indirect effector mechanisms are likely to be most effective as donor APCs are replaced by recipient APCs over time, when there are a large number of antigenic disparities and when certain types of tissues are involved, such as islets. Therefore,
indirect effector mechanisms may be especially important in chronic rejection and in xenogeneic transplantation.
Indirect responses and tolerance induction The ability to prolong graft survival in some cases by depleting donor APCs [48] and the subsequent demonstration that cells stimulated by non-APCs become anergic [49], suggested that graft survival and tolerance induction might be achieved by manipulating donor APCs to render them incompetent. Many strategies to accomplish this (for example, by blockade ofcostimulatory molecules) have been used successfully in rodent models [50",51]. The current emphasis on the importance of the indirect pathway, however, suggests that the mechanisms underlying these strategies probably involve more than simply disrupting donor APC function. In the first place, simple depletion of donor APCs would not generally be expected to be effective if indirect presentation of donor antigens can effectively sensitize T cells. Therefore, if an indirect response is important, strategies to prevent APC function must be aimed equally at recipient APCs, with the limitation that recipient APCs must be allowed to regain their normal function at some point in order to restore immunocompetence. It would appear, therefore, that strategies to induce tolerance by manipulation of APC function must generate a capacity to maintain tolerance even after normal APC function returns. The currently popular idea that tolerance can be induced by immune deviation of cytokine responses is especially attractive precisely because it includes a self-reinforcing component. Once deviated toward Th2 production, the Th2 cytokines both inhibit subsequent T h l responses and promote Th2 production. Nonetheless, the perpetration of a capacity to generate an indirect response after graft rejection suggests that tolerance induced by the temporary manipulation of APC function is likely to become unstable, to some degree, over time.
A second implication stemming from the emphasis on the indirect pathway is that it is not necessarily as important to introduce donor cells or intact donor antigens when manipulating the immune system toward tolerance as it is to introduce the peptides of donor antigens. Given the possibility that MHC peptides are particularly important in indirect sensitization (discussed earlier), several investigators have suggested that MHC peptides could be used [131 or are effective [17,27",28"] in promoting tolerance induction.
Conclusion: why did the direct response appear so important for so long if indirect recognition is so important? The strongest evidence for the special importance of the direct response to allogeneic MHC antigens in allograft rejection came from the powerful in vitro data. The existence of a primary in vitro response and the dramatically higher precursor frequency for T cells responding by direct stimulation was hard to ignore. On the other hand, the
Antigen processing and presentation Auchincloss and Sultan
survival time for skin grafts in mice is not very different for grafts mismatched only for MHC antigens compared to those mismatched only for multiple minor antigens, despite the vigorous primary mixed lymphocyte reaction in the first case and its complete absence in the other. Thus, evidence that in vitro assays of alloreactivity poorly predict the strength of graft rejection in vivo has been available for many years. T h e evidence that indirect responses may be more important than direct ones in vivo simply reemphasizes the limitations of our in vitro assays.
References and recommended reading
T h e greater strength of MHC antigen disparities compared with isolated minor antigen disparities in causing graft rejection also suggested the importance of direct recognition. If, however, MHC peptides are better at stimulating indirect responses than peptides of minor antigens, then the importance of MHC antigen matching is equally consistent with the suggestion that the indirect pathway predominates. T h e ability in some cases to prolong graft survival by depleting donor APCs probably represents the most puzzling challenge to the model emphasising indirect responses. There are at least three possibilities to explain this discrepancy. First, donor APCs may be important, not as the stimulating cells in graft rejection, but rather as the vehicles required to bring donor proteins to draining lymph nodes in order to stimulate an indirect response. This is a well recognized function of APCs in peripheral tissues [52]. Second, APCs may represent one of the critical targets for the effector mechanism of graft destruction as suggested by the studies of Winn and collegues [43]. Thus donor APC depletion may remove, at least temporarily, one of the important components in graft destruction. Third, even if the stimulation of helper responses occurs primarily through the indirect pathway, the stimulation of direct effector cells may still require the presence of donor APCs. This last consideration highlights a critical aspect of any discussion of direct versus indirect pathways in graft rejection. As we emphasize the newly recognized importance of indirect T cell stimulation, it is currently far too early to discard completely the long-standing idea that direct recognition of donor antigens plays a critical role in graft rejection. More likely, we will come to realize that both indirect and direct pathways contribute to graft rejection and that our effort should be to understand the interaction between these two responses. It is, in fact, the availability of two different sets of APCs which represents the fundamental difference between aJlograft rejection and all other immune responses.
Acknowledgements Some of the work reported by the authors was supported in part by National Institutes of Health grants HL36372, AI-38397 and HL18646. Heena Suhan was a Medical Research Fellow supported by the Howard Hughes Medical Institute.
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Papers of particular interest, published within the annual period of review, have been highlighted as: • *-
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11. •
Benham AM, Sawyer GJ, Fabre JW: Indirect T cell allorecognition of donor antigens contributes to the rejection of vascularized kidney allografts. Transplantation 1995, 59:1028-1032. These authors demonstrate the rapid rejection of vascularized allografts in animals that have been sensitized by the indirect pathway. 12.
FangmannJ, Dalchau R, Fabre JW: Rejection of skin allografts by indirect allorecognition of donor class I major histocompatibility complex peptides. J Exp Mad 1992, 175:1521-1529.
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Reed EF, Hong B, Ho E, Harris PE, Weinberger J, Suciu-Foca N: Monitoring of soluble HLA alloantigens and enti-HLA antibodies identifies heart allograft recipients at risk of transplant-associated coronary artery disease. Transplantation 1996, 61:566-5?2.
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Lee RS, Grusby MJ, Glimcher LH, Winn HJ, Auchincloss H Jr: Indirect recognition by helper cells can induce donor-specific cytotoxic T lymphocytes in vivo. J Exp Med 1994, 179:865-872.
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Sayegh MH, Perico N, Gallon L, Imberti O, Hancock WW, Remuzzi G, Carpenter CB: Mechanisms of acquired thymic unresponsiveness to renal allografts. Transplantation 1994, 58:125-132.
18. •*
Steele DJR, Laufer TM, Smiley ST, Ando Y, Grusby MJ, Glimcher LH, Auchincloss H Jr: Two levels of help for B cell alloantibody production. J Exp Med 1996, 183:699-703. This article demonstrates that the indirect response is required for alloantibody class switching. The results emphasize the importance of the indirect response in alloreactivity.
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Benichou G, Takizawa AP, Ho TP, Killion CC, McMillan M, Sercarz EE: Immunogenicity and tolerogenicity of selfmajor histecompetibility complex peptides. J Exp Mad 1990, 172:1341-1346.
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Benichou G, Takizawa PA, Olson CA, McMillan M, Sercarz EE: Donor major histocompatibility complex (MHC) peptides are presented by recipient MHC molecules during graft rejection. J Exp Med 1992, 175:918-924.
21.
Watschinger B, Gallon L, Carpenter CB, Sayegh MH: Mechanisms of allorecegnition: in vivo primed T cells recognize major histocempatibility complex polymorphisms presented as peptides by responder antigen-presenting cells. Transplantation 1994, 57:572-576.
22. •
Gallon L, Watschinger B, Murphy B, Akalin E, Sayegh MH, Carpenter CB: The indirect pathway of allorecognition: the occurrence of self-restrictad T cell recognition of a l I o - M H C peptides early in acute renal ellograft rejection and its inhibition by conventional immunosuppression. Transplantation 1995, 59:612-616. This paper illustrates several points regarding the indirect pathway: first, the accelerated rejection of rat renal allografts subsequent to sensitization with alIo-MHC peptides; second, the susceptibility of the indirect pathway to conventional immunosuppressive therapy; and third, the role of recipient genotype in allorecognition. 23.
Benham AM, Fabre JW: Elucidation of key peptide determinants involved in an indirect T-cell allorecegnition pathway of rat kidney allogreft rejection. Transplant Proc 1995, 27:547-548.
24.
Benham A, Fabre JW: Fine specificity of peptide determinants for indirect T cell recognition of class I MHC alloantigens. Transplantation 1994, 58:1236-1240.
33.
Page C, Thompson C, Yacoub M, Rose M: Human endothelial stimulation of allogeneic T cells via a CTLA-4 independent pathway. Transpl Immunol 1994, 2:342-347
34.
Auchincloss H Jr, Mayer T, Ghobrial R, Winn HJ: T cell subsets, bm mutants, and the mechanism of allogeneic skin graft rejection. Immuno/Res 1989, 8:149-164.
35. •
Kalhs P, White JS, Gervassi A, Storb R, Bean MA: In vitro recall of proliferative and cytolytic responses to minor histocompatibility antigens by dendritic cell enriched canine peripheral blood mononuclear cells. Transplantation 1995, 59:112-118. This study highlights the importance of the type APC used in antigen presentation and demonstrates the direct in vitro presentation of minor antigens by dendritic cells after priming in a canine model. 36. •
Epstein MM, Di Rosa F, Jankovic D, Sher A, Matzinger P: Successful T cell priming in B cell-deficient mice. J Exp Med 1995, 182:915-922. Using a genetically B cell deficient murine model the authors show that B cells are not essential for either CD4 + or CD8 + T cell priming in various systems. 37.
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38. he
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Shirwan H, Learner M, Wang HK, Makowka L, Cramer DV: Peptides derived from (x-helices of allogeneic class I major histocompatibility complex antigens are potent inducers of CD4+ and CD8 + T cell and B cell responses after cardiac allogreft rejection. Transplantation 1995, 59:401-410. These authors demonstrate the indirect presentation of MHC class I peptides during the rejection of cardiac allografts in the rat.
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25. •
Terness P, Dufter C, Otto G, Opelz G: Induction of acute rejection by indirect recognition of donor MHC antigens. Transplant Proc 1995, 27:457-458. This group, like [25"], demonstrates the role of indirect recognition of donor MHC antigens in induction of acute cardiac allograft rejection in an otherwise weakly incompatible donor-recipient rat combination. 27. •
Chowdhury NC, Jin M, Hardy MA, Oluwcle SF: Donor-specific unresponsiveness to murine cardiac allografts induced by intrathymic-soluble alloantigens is dependent on alternate pathway of antigen presentation. J Surg Res 1995, 59:91-96. This article, along with [17], is another example in which donor-specific unresponsiveness is achieved by the induction of intrathymic soluble alloantigens via the indirect pathway. 28. •
Saltovitch D, Morris PJ, Wood KJ: Donor-type single MHC locus products combined with anti-CD4 mAb can induce tolerance to cardiac allogrefts which is dependent on active suppression. Transplant Proc 1995, 27:119-120. This article demonstrates the induction of tolerance via an active suppression mechanism utilizing donor-type single MHC locus products. The model uses donor specific transfusions and anti-CD4 + treatment. The results suggest again that tolerance induced by indirect responses may be suftficient to prevent graft rejection. 29.
Suciu-Foca N, Liu Z, Harris PE, Reed El:, Cohen DJ, Benstein JA, Benvenisty AI, Mancini D, Michler RE, Rose EA eta/.: Indirect recognition of native HLA alloantigens and B-cell help. Transplant Proc 1995, 27:455-456.
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44. •.
Wecker H, Grusby MJ, Auchincloss H Jr: Effector cells must recognize antigens expressed in the graft to cause efficient skin graft rejection in SCID mice. Transplantation 1995, 59:1223-1227. In this article, the authors place MHC class II deficient skin grafts on severe combined immunodeficiency mice that are reconstituted only with CD4 + ceils. The results demonstrate that skin graft rejection only occurs very slowly when effector cells cannot recognize antigens expressed in the graft itself. 45.
Campos L, Naji A, Deli BC, Kern JH, Kim JI, Barker CF, Marl
46.
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47.
Markmann JF, Campos L, Bhandoola A, Kim JI, Desai NM, Bassiri H, Claytor BR, Barker CF: Genetically engineered grafts to study xenoimmunity: a role for indirect antigen presentation in the destruction of major histocompatibility complex antigen deficient xenografts. Surgery 1994, 116:242-249.
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Jenkins MK, Schwartz RH: Antigen presentation by chemically modified splenocytes induces antigen*specific T cell unresponsiveness in vitro and in vivo. J Exp Med 1987, 165:302-319.
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50. o-
Sayegh MH, Akalin E, Hancock WW, Russell ME, Carpenter CB, Linsley PS, Turka I_A: CD28-B7 blockade after alloanUgenic challenge in vivo inhibits Thl cytokines but spares Th2. J Exp Med 1995, 181:1869-1874. This article illustrates that blockade of the CD28-B7 pathway after alloantigenic challenge induces donor-specific acceptance of vascularized organ alIografts. Also, it shows that blockade inhibits Thl but spares Th2 responses in vivo.
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Baliga P, Chavin KD, Qin L, Woodward J, Lin J, Linsley PS, Bromberg JS: CTLA41g prolongs allograft survival while suppressing cell-mediated immunity. Transplantation 1994, 58:1082-1090.
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Steinman RM, Witmer-Pack M, Inaba K: Dendritic cells: antigen presentation, accessory function and clinical relevance. Adv Exp Med Biol 1993, 329:1-9.
Antigen processing and presentation in transplantation Hugh Auchincloss Jr* and Heena Sultan For many years the direct stimulation of T cells in response to donor MHC antigens expressed on donor antigen-presenting cells has been the focus of transplantation immunology. Indirect recognition in response to peptides of donor antigens presented by self MHC molecules on recipient antigen-presenting cells has not generally been considered an important feature of graft rejection. Recent evidence suggests that indirect responses may be more important than previously considered and the new emphasis on indirect pathways in allograft rejection has raised new issues, many of which are unresolved.
Addresses Massachusetts General Hospital, Department of Surgery and Transplantation, White 51 OB, Fruit Street, Boston, MA 02114, USA *e-maih [email protected] Current Opinion in Immunology 1996, 8:681-687
normal mechanism for T cell stimulation [4]. Although this pathway of stimulation was considered by La Rosa and Talmage [5] as a possible explanation for graft rejection in cases involving minor histocompatibility mismatches and although stimulation of an 'indirect' response during graft rejection was demonstrated several years ago by the phenomenon of 'cross-priming' (the presentation of minor antigens from the donor by recipient MHC class I molecules) [6], the indirect pathway of T cell stimulation has not generally been considered to be as important as direct stimulation. During the past several years, however, increasing evidence has accumulated emphasizing the role, and perhaps even the dominance, of the indirect pathway in allograft rejection [7]. This paper reviews some of the key information supporting that assertion and considers some of the interesting issues which have emerged in light of the contribution of indirect T cell stimulation to graft rejection.
© Current Biology Ltd ISSN 0952-7915 Abbreviations APC antigen-presentingcell TCR T cell receptor
Introduction
T h e direct T cell response to foreign MHC antigens has generally been thought to be the distinguishing feature of cell-mediated allogeneic immunity [1]. Defined as the stimulation of T cells by the allogeneic MHC antigens on allogeneic APCs, direct T cell stimulation occurs because of the similarity of allogeneic and self MHC molecules. Because of this similarity, allogeneic MHC molecules are nearly unique among foreign proteins in that they do not require processing and presentation as peptides in order to stimulate a T cell response [2]. Furthermore, when measured in vitro, the frequency of precursor T cells for a direct response is about one hundred-fold greater than for a response to peptides of ordinary environmental antigens. In addition to the extraordinary strength of the direct pathway when measured in vitro, evidence from in vivo experiments has supported the idea that direct recognition is critical in graft rejection, including the particular importance of MHC antigen matching in graft survival and the survival of some types of grafts after removing donor APCs [3]. In contrast to direct T cell stimulation, the natural physiological mechanism of T cell stimulation is for TCRs to recognize peptides of foreign proteins that have been processed by self APCs and presented by self MHC molecules. In transplantation immunology this physiologic response has been referred to as indirect presentation even though the term fails to convey the sense that this is the
Testing the importance pathway
of the indirect
In the past, the effort to demonstrate an important role for the indirect pathway has been hampered by the concurrent potential for direct stimulation that is so strong by in vitro assays. Therefore, some of the initial studies suggesting that indirect stimulation might play a role in vivo came from studies of xenogeneic graft rejection using species combinations in which direct T cell stimulation in vitro was weak or absent. For example, Moses et al. [8] showed that the primary in vitro response of mouse T cells to discordant stimulating cells was weak. On the other hand, the strong secondary response in vitro after xenograft rejection occurred almost entirely through the indirect pathway since the response depended on the presence of recipient APCs. Pierson et al. [9] and other investigators [10] found that xenograft rejection in vivo was especially dependent on CD4 + T cells and suggested from the in vitro data that an indirect response might be important in xenograft rejection. Studies of the role of the indirect pathway in allograft rejection required more subtle manipulations. For example, Fabre and co-workers [11",12] showed that immunization of recipients with peptides of allogeneic MHC antigens (which could only stimulate an indirect response) led to accelerated graft rejection. Although the difference in time to rejection was very small in some of these experiments, the results did suggest that an indirect response was one way that a recipient could respond to allogeneic grafts. Suciu-Foca and co-workers [13] showed evidence that the rejection episodes in human cardiac allograft recipients was associated with stimulation of an indirect T cell response.
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More compelling experiments showing that an indirect response could initiate allograft rejection became possible once mice which lacked MHC class II antigens were generated by targeted gene disruption [14]. Since grafts from these mice could not stimulate a direct CD4 ÷ T cell response, rapid rejection of their tissues in a CD4+-dependent manner suggested that indirect T cell stimulation could lead to graft rejection [15]. Subsequent in vitro studies, using T cells from mice which had rejected MHC class II deficient skin, supported this conclusion [16]. Although for many years evidence has indicated that physiological indirect responses occur during graft rejection, and although the evidence of recent years has suggested that such a response can provide a possible mechanism for graft rejection, the most interesting data of the past several years has suggested that indirect T cell stimulation may even be the dominant pathway involved in allograft rejection. For example, studies by Sayegh et al. [17] have indicated that the downregulation of T cell responses by thymic administration of peptides from allogeneic MHC antigens can lead to prolonged survival of subsequent aliografts. MHC peptides could not themselves have affected direct T cell responses, suggesting that indirect T cell stimulation was critical in the rejection process. In addition, Steele et al. [18"°] have shown that T cell help for B cell IgG alloantibody production after skin transplantation in mice depends on the ability to generate an indirect T cell response. These results have helped stimulate a new emphasis on the indirect pathway in allograft rejection but they have also raised many important questions in transplantation immunology which have not yet been resolved.
Which peptides stimulate an indirect response? Major histocompatibility antigens have the special feature that they can be recognized directly by TCRs and that they are especially important in causing rapid graft rejection. As foreign proteins, however, their peptides can also be processed and presented by the MHC molecules on recipient APCs, thereby generating an indirect response. Minor histocompatibility antigens, on the other hand, are peptides of donor proteins which can be processed, and presented by self MHC molecules and recognized as foreign by recipient T cells [1,2]. Individual minor histocompatibility antigens generally cause much slower graft rejection than do MHC antigens. These considerations raise a perplexing issue: if both MHC peptides and minor antigen peptides can be recognized indirectly, and if the indirect response is really the dominant pathway in graft rejection, then why should graft rejection be stronger when there are MHC antigenic differences? The early demonstration of cross-priming indicated that minor histocompatibility antigens can be recognized indi-
rectly [6]. Nonetheless, many of the recent studies of the indirect pathway have focused on recognition of peptides of MHC molecules, with the largely implicit assumption that these peptides are for some reason more important than minor peptides [19-21,22",23,24,25°-28",29-31]. No published studies to date, however, have actually compared the strength of indirect stimulation generated by peptides of minor compared to major histocompatibility antigens. Since matching for MHC antigens makes a difference in graft survival and if the indirect pathway is truly dominant, this assumption may be valid. It is, however, an assumption that requires further testing. Analysis of the endogenous peptides that have been eluted from MHC molecules have shown a surprisingly high frequency of peptides derived from MHC molecules. One likely explanation for this finding is simply that when self MHC molecules fail to fold or load peptides effectively (and are therefore degraded in the cytoplasm of self APCs) their peptides are the ones in the best location for subsequent presentation by newly forming self MHC molecules. This explanation for the frequent endogenous presentation of self MHC peptides, however, does not explain why peptides from exogenous MHC molecules should have a special advantage for presentation by the indirect pathway. Two other possibilities remain: first, allogeneic MHC molecules may be available in greater quantity than minor histocompatibility proteins (this could occur because MHC molecules are more frequently expressed, or because their location on the surface membrane makes them more easily available for uptake by self APCs, or because B cells are more likely to be stimulated by MHC than minor antigens and thus to serve as effective APCs for the presentation of MHC peptides); second, the T cell repertoire could be biased toward recognition of allogeneic MHC peptides presented by self MHC molecules (this might occur if the frequent endogenous presentation of self MHC peptides by self MHC molecules leads to positive selection in the thymus, especially for TCRs that recognize self MHC molecules presenting peptides of slightly different MHC proteins). Such explanations are speculative until clear evidence is obtained showing the dominance of MHC compared to minor peptides in presentation by the indirect pathway. Nonetheless, the evidence of the importance of the indirect pathway in graft rejection and the evidence that matching for MHC antigens matters in graft survival suggests a new concept: that MHC matching may matter primarily because of the importance of MHC peptides in stimulating indirect T cell responses rather than because of the importance of the direct recognition of intact MHC molecules.
Which APCs are important in the direct and indirect pathways? A significant difference between the direct and indirect pathways of sensitization is that the types of APCs
Antigen processingand presentationAuchinclossand Sultan
available in each case are likely to be different, a difference that may be important in determining the degree and character of T cell activation. Direct T cell activation is likely to involve two kinds of APCs that will vary with the type of graft. First, many tissues have their own specialized APCs (such as Langerhans' cells in the skin or Kupffer cells in the liver). Second, endothelial cells of vascularized grafts appear to have APC function, especially if activated [32,33]. T h e density of these donor APCs is different in different tissues [34] and also varies over time, as bone marrow derived donor cells are replaced by cells of recipient origin. Indirect T cell activation, on the other hand, depends on the APCs in the recipient's draining lymph nodes and spleen, including monocytes, lymphoid cells and dendritic cells. T h e availability of these APCs for indirect responses is probably less variable. Recent studies have indicated that dendritic cells [35"] and monocytes [32] have the capacity to stimulate indirect responses but that recipient B cells are not essential as APCs when recipient T ceils are sensitized to minor antigens [36°1. Activated B cells, however, do have APC function and may be especially important in stimulating some T cell responses [37]. Although so far few transplantation experiments have been performed to determine the different types of T-cell responses that might occur during graft rejection depending on the types of APCs available, many nontransplant experiments suggest that cytokine profiles of T cells do depend on the character of the APC. Furthermore, a reasonable hypothesis is that the APCs in draining lymph nodes are the ones best suited to stimulate immune responses, while the APCs in nonlymphoid organs are more likely to carry antigens away from that site and to downregulate local immune responses. Thus, an important issue in studying direct versus indirect responses is to determine the nature and the capacities of the different APCs involved.
Can both class I as well as class II molecules present donor antigens indirectly? A basic tenant of classical immunology has been that peptides of exogenous antigens are presented by M H C class II molecules whereas peptides of endogenous antigens are presented by M H C class I molecules. Applied to transplantation immunology, this principle would suggest that indirect responses to exogenous donor antigens ought to be limited to CD4+ cells responding to modified self M H C class II molecules. Despite this prediction, transplantation studies involving minor antigen disparate grafts from M H C homozygous donors to M H C heterozygous recipients have shown that CD8 ÷ T cells can be sensitized to the minor antigens presented in association with recipient M H C class I molecules [6]. This 'cross-priming' phenomenon suggests that exogenous antigens can be presented by M H C class I molecules. Subsequently, nontransplantation experiments have also
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indicated that M H C class I molecules can present exogenous antigens under a variety of circumstances. Bevan has recently reviewed three major mechanisms by which this may occur [38°]: first, phagocytes can shunt protein from phagosomes into the cytosol for loading onto the surface M H C class I molecules; second, phagocytes can digest the ingested lysosomal content and then load peptides onto surface M H C class I molecules; and third, heat shock proteins which are released by dying cells and serve as peptide carriers for professional APCs. These heat shock proteins normally play a role in the transfer of peptides from proteasomes to M H C class I molecules and can gain access to the cytosol of professional APCs. While it is no longer in doubt that indirect presentation of donor antigens by recipient M H C class I molecules can occur, there is uncertainty over whether this form of indirect presentation is important in graft rejection. Except in cases where donor and recipient M H C class I antigens are matched, CD8 ÷ cells sensitized by peptides presented in the context of recipient M H C class I molecules, will not find such a determinant expressed by donor cells. Therefore, CD8 + cells sensitized indirectly can only participate in graft destruction by the production of cytokines, not by their direct interaction with or lysis of donor cells.
Are there limitations on the ability of indirect helpers to co-operate with direct effectors? Another central principle of immunology is that regulatory interactions between lymphocytes generally require physical proximity. In some cases this may be accomplished by simultaneous expression of helper and cytotoxic determinants for CD4 + and CD8 ÷ T cells respectively, on the same APC. This was described as a 'three cell' model by Mitchison and O'Mailey for the generation of cytotoxic T cells [39]. In other cases, physical linkage may be accomplished by a cognate interaction between the helper and effector cell (such as CD4 ÷ recognition of the modified M H C class II antigens of B ceils in the generation of antibody responses). In transplantation immunology, studies by Keene and Forman [40] have similarly suggested that helper and cytotoxic cells involved in graft rejection are best sensitized by APCs expressing both determinants, while studies by Rosenberg et al. [41] indicated that the two determinants had (at least) to be generated by the same donor graft. To the extent that the effector CD8 + T cells mediating graft rejection must recognize determinants expressed on the cells of the donor graft, the use of the indirect pathway for the sensitization of helper cells appears to violate the principal of physical linkage. If helper cells are sensitized by recipient APCs in the draining lymph node, but effector cells are sensitized by donor cells in the tissue graft, there are no obvious mechanisms that would tend to bring these populations together in a tight physical association.
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Lee et al. [16] addressed this issue using MHC class II deficient skin grafts which forced sensitization to occur through the indirect pathway. These investigators concluded that under some circumstances, sensitization of CD8 ÷ cells by antigens expressed on donor APCs could be shown to depend on help from a CD4+ indirect response. While these results (and the general effectiveness of the indirect pathway in producing graft rejection) suggest that a three cell linkage is not absolutely required for helper/effector interactions in graft rejection, it remains unlikely that helper and effector cells can be sensitized entirely independently. A degree of physical association might occur if direct and indirect sensitization take place within the same draining lymph node. It remains to be determined, however, exactly what constraints are involved when help is generated indirectly for effector cells sensitized by the direct pathway.
Is there an indirect effector mechanism? One way to avoid the physical linkage problem in the case of graft rejection would be to drop the assumption that the effector cells responsible for graft destruction must recognize determinants expressed on donor cells. If cells stimulated by the indirect pathway can themselves destroy grafts, then there would be no need for them to cooperate with effector cells that have been sensitized directly. T h e idea that there might be an indirect effector mechanism would seem contrary to the results of studies by Rosenberg and Singer [42] using grafts from semiallogeneic tetraparental donors which are composed of cells of both donor and recipient origin. The apparent selectivity of the tissue destruction in these experiments suggested that effector mechanisms involve cells with donor specificity. On the other hand, studies by Winn and collegues [43] using skin grafts in which only the APCs were foreign to the recipient, indicated that rejection of an entire graft could occur when only some of the cells within it expressed foreign determinants. Wecker et al. [44"'] addressed this issue by placing MHC class II deficient skin grafts onto severe combined immunodeficiency mice that had been reconstituted only with CD4+ T cells. T h e y found that the MHC class II deficient skin was rejected very slowly in the absence of effector cells which could recognize the remaining MHC class I alloantigens. Similarly, Markmann et al. [45°,46,47] found that MHC deficient cardiac and skin allografts were not rejected by normal recipients, suggesting that an indirect response was not sufficient to cause destruction of these tissues. Overall, the evidence at this point suggests that an indirect effector mechanism can participate in graft destruction, albeit inefficiently in many circumstances. Indirect effector mechanisms are likely to be most effective as donor APCs are replaced by recipient APCs over time, when there are a large number of antigenic disparities and when certain types of tissues are involved, such as islets. Therefore,
indirect effector mechanisms may be especially important in chronic rejection and in xenogeneic transplantation.
Indirect responses and tolerance induction The ability to prolong graft survival in some cases by depleting donor APCs [48] and the subsequent demonstration that cells stimulated by non-APCs become anergic [49], suggested that graft survival and tolerance induction might be achieved by manipulating donor APCs to render them incompetent. Many strategies to accomplish this (for example, by blockade ofcostimulatory molecules) have been used successfully in rodent models [50",51]. The current emphasis on the importance of the indirect pathway, however, suggests that the mechanisms underlying these strategies probably involve more than simply disrupting donor APC function. In the first place, simple depletion of donor APCs would not generally be expected to be effective if indirect presentation of donor antigens can effectively sensitize T cells. Therefore, if an indirect response is important, strategies to prevent APC function must be aimed equally at recipient APCs, with the limitation that recipient APCs must be allowed to regain their normal function at some point in order to restore immunocompetence. It would appear, therefore, that strategies to induce tolerance by manipulation of APC function must generate a capacity to maintain tolerance even after normal APC function returns. The currently popular idea that tolerance can be induced by immune deviation of cytokine responses is especially attractive precisely because it includes a self-reinforcing component. Once deviated toward Th2 production, the Th2 cytokines both inhibit subsequent T h l responses and promote Th2 production. Nonetheless, the perpetration of a capacity to generate an indirect response after graft rejection suggests that tolerance induced by the temporary manipulation of APC function is likely to become unstable, to some degree, over time.
A second implication stemming from the emphasis on the indirect pathway is that it is not necessarily as important to introduce donor cells or intact donor antigens when manipulating the immune system toward tolerance as it is to introduce the peptides of donor antigens. Given the possibility that MHC peptides are particularly important in indirect sensitization (discussed earlier), several investigators have suggested that MHC peptides could be used [131 or are effective [17,27",28"] in promoting tolerance induction.
Conclusion: why did the direct response appear so important for so long if indirect recognition is so important? The strongest evidence for the special importance of the direct response to allogeneic MHC antigens in allograft rejection came from the powerful in vitro data. The existence of a primary in vitro response and the dramatically higher precursor frequency for T cells responding by direct stimulation was hard to ignore. On the other hand, the
Antigen processing and presentation Auchincloss and Sultan
survival time for skin grafts in mice is not very different for grafts mismatched only for MHC antigens compared to those mismatched only for multiple minor antigens, despite the vigorous primary mixed lymphocyte reaction in the first case and its complete absence in the other. Thus, evidence that in vitro assays of alloreactivity poorly predict the strength of graft rejection in vivo has been available for many years. T h e evidence that indirect responses may be more important than direct ones in vivo simply reemphasizes the limitations of our in vitro assays.
References and recommended reading
T h e greater strength of MHC antigen disparities compared with isolated minor antigen disparities in causing graft rejection also suggested the importance of direct recognition. If, however, MHC peptides are better at stimulating indirect responses than peptides of minor antigens, then the importance of MHC antigen matching is equally consistent with the suggestion that the indirect pathway predominates. T h e ability in some cases to prolong graft survival by depleting donor APCs probably represents the most puzzling challenge to the model emphasising indirect responses. There are at least three possibilities to explain this discrepancy. First, donor APCs may be important, not as the stimulating cells in graft rejection, but rather as the vehicles required to bring donor proteins to draining lymph nodes in order to stimulate an indirect response. This is a well recognized function of APCs in peripheral tissues [52]. Second, APCs may represent one of the critical targets for the effector mechanism of graft destruction as suggested by the studies of Winn and collegues [43]. Thus donor APC depletion may remove, at least temporarily, one of the important components in graft destruction. Third, even if the stimulation of helper responses occurs primarily through the indirect pathway, the stimulation of direct effector cells may still require the presence of donor APCs. This last consideration highlights a critical aspect of any discussion of direct versus indirect pathways in graft rejection. As we emphasize the newly recognized importance of indirect T cell stimulation, it is currently far too early to discard completely the long-standing idea that direct recognition of donor antigens plays a critical role in graft rejection. More likely, we will come to realize that both indirect and direct pathways contribute to graft rejection and that our effort should be to understand the interaction between these two responses. It is, in fact, the availability of two different sets of APCs which represents the fundamental difference between aJlograft rejection and all other immune responses.
Acknowledgements Some of the work reported by the authors was supported in part by National Institutes of Health grants HL36372, AI-38397 and HL18646. Heena Suhan was a Medical Research Fellow supported by the Howard Hughes Medical Institute.
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Gallon L, Watschinger B, Murphy B, Akalin E, Sayegh MH, Carpenter CB: The indirect pathway of allorecognition: the occurrence of self-restrictad T cell recognition of a l I o - M H C peptides early in acute renal ellograft rejection and its inhibition by conventional immunosuppression. Transplantation 1995, 59:612-616. This paper illustrates several points regarding the indirect pathway: first, the accelerated rejection of rat renal allografts subsequent to sensitization with alIo-MHC peptides; second, the susceptibility of the indirect pathway to conventional immunosuppressive therapy; and third, the role of recipient genotype in allorecognition. 23.
Benham AM, Fabre JW: Elucidation of key peptide determinants involved in an indirect T-cell allorecegnition pathway of rat kidney allogreft rejection. Transplant Proc 1995, 27:547-548.
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35. •
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Terness P, Dufter C, Otto G, Opelz G: Induction of acute rejection by indirect recognition of donor MHC antigens. Transplant Proc 1995, 27:457-458. This group, like [25"], demonstrates the role of indirect recognition of donor MHC antigens in induction of acute cardiac allograft rejection in an otherwise weakly incompatible donor-recipient rat combination. 27. •
Chowdhury NC, Jin M, Hardy MA, Oluwcle SF: Donor-specific unresponsiveness to murine cardiac allografts induced by intrathymic-soluble alloantigens is dependent on alternate pathway of antigen presentation. J Surg Res 1995, 59:91-96. This article, along with [17], is another example in which donor-specific unresponsiveness is achieved by the induction of intrathymic soluble alloantigens via the indirect pathway. 28. •
Saltovitch D, Morris PJ, Wood KJ: Donor-type single MHC locus products combined with anti-CD4 mAb can induce tolerance to cardiac allogrefts which is dependent on active suppression. Transplant Proc 1995, 27:119-120. This article demonstrates the induction of tolerance via an active suppression mechanism utilizing donor-type single MHC locus products. The model uses donor specific transfusions and anti-CD4 + treatment. The results suggest again that tolerance induced by indirect responses may be suftficient to prevent graft rejection. 29.
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44. •.
Wecker H, Grusby MJ, Auchincloss H Jr: Effector cells must recognize antigens expressed in the graft to cause efficient skin graft rejection in SCID mice. Transplantation 1995, 59:1223-1227. In this article, the authors place MHC class II deficient skin grafts on severe combined immunodeficiency mice that are reconstituted only with CD4 + ceils. The results demonstrate that skin graft rejection only occurs very slowly when effector cells cannot recognize antigens expressed in the graft itself. 45.
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46.
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