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CD27 and CD70 in T cell and B cell activation
CD27 and CD70 in T cell and B cell activation Jannie Borst, Jenny Hendriks and Yanling Xiao In vitro work has defined the TNF receptor family member CD27 as a T and B cell co-stimulatory molecule. Its activity is governed by the transient availability of its TNF-like ligand CD70 on lymphocytes and dendritic cells. Recent studies, enforcing or abrogating CD27 function by genetic or protein intervention in mouse models have revealed key contributions of the CD27–CD70 system to effector and memory T cell formation, which is probably based on improved cell survival. The stimulatory effects of CD27 on B cell function appear to oppose those of CD70, which also has a signaling role. Targeting CD27–CD70 for therapy is attractive but should take into account the fact that constitutive CD27 stimulation culminates in lethal immunodeficiency. Addresses Division of Immunology, The Netherlands Cancer Institute, 1066 CX Amsterdam, The Netherlands Corresponding author: Borst, Jannie ([email protected])
Current Opinion in Immunology 2005, 17:275–281 This review comes from a themed issue on Lymphocyte activation Edited by Gail A Bishop and Jonathan R Lamb Available online 13th April 2005 0952-7915/$ – see front matter # 2005 Elsevier Ltd. All rights reserved. DOI 10.1016/j.coi.2005.04.004
Introduction In peripheral lymphoid organs of naı¨ve mice, CD27 is found on a large proportion of NK cells and on most, if not all, CD4+ and CD8+ T cells [1,2]. CD27 is absent from naı¨ve B cells, but is induced upon priming [3]. The contribution of CD27 to the immune response is dependent upon CD70 expression, which is primarily controlled by antigen receptor and Toll-like receptor stimulation on T cells, B cells and dendritic cells (DCs) [4]. NK cells can also express CD70 [5]. The expression patterns of CD27 and CD70 in humans are very similar to those in mice, except that CD27 is expressed on primed B cells in higher frequency and more persistently in humans. In clinical practice, membrane CD27 and its soluble form are used as lymphocyte subset markers and disease markers in the case of lymphoid malignancies, autoimmunity and transplant rejection [6–8]. From in vitro studies on human cells, CD27 has long been known as a co-stimulator of T cell and B cell responses [6,8,9]; however, its exact mechanism of action and relative importance for immune www.sciencedirect.com
responsiveness in vivo are only now being revealed by studies in mouse models. This recent work will be subject of this review.
T cell priming and effector phases According to their expression patterns, CD27–CD70 interactions can come into play in the T cell priming phase during contact between T cells and DCs and in the expansion phase during contact between T cells. During the effector phase, interactions between CD27 on T cells and CD70 on DCs and B cells may be important, as CD70 is most abundant on these cell types at the effector site (J Hendricks et al., unpublished; Figure 1). CD27 / mice, intranasally infected with influenza virus, revealed that CD27 controls the accumulation of CD4+ and CD8+ effector T cells at the site of infection [10]. Focusing on CD8+ T cells, we found that CD27 promoted the survival of virus-specific T cells at sites of priming and infection and acted independent of and complementary to CD28, which also controls effector T cell accumulation. CD27 was in fact the principle determinant for T-cell accumulation at the tissue effector site (Figure 2a; [11]). Consistently, in a cardiac allograft model, accumulation of CD8+ effector T cells and their capacity to reject the graft depended on the collective contributions of CD27 and CD28, as shown by CD70 inhibition. CD4+ T cell accumulation and function, however, were much less affected [12]. The priming of regulatory T cells after intratracheal delivery of alloantigen may also depend on CD27–CD70 interactions [13]. Co-stimulatory effects of CD27 on both CD4+ and CD8+ T cells were also revealed in CD70 transgenic (tg) mice that constitutively express CD70 on B cells. In these mice, both subpopulations expand and turn over from a naı¨ve into an effector phenotype upon ageing. This process depends on CD27 and on T cell antigen receptor (TCR) stimulation, presumably by environmental or auto-antigens [14,15]. The phenotype of CD70tg mice essentially mirrors that of CD27 / mice. In CD70tg mice, CD27–CD70 interactions presumably enable T cells that have received low level TCR input to survive and develop into effector cells. CD8+ effector T cell accumulation upon challenge with influenza virus was enhanced in CD70tg mice (Figure 2b; [16]). Application of soluble recombinant (r)CD70 in vivo emphasized the potency of the CD27–CD70 system to increase the size of the effector T cell pool, in this case of ovalbuminspecific CD8+ T cells after immunization with peptide [17]. Deliberate expression of CD70 on tumor cells stimulated both NK and T cell immunity in lymphoma Current Opinion in Immunology 2005, 17:275–281
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Figure 1
Figure 2
Effector CD27 > 4-1BB CD28
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Priming Participation of CD27 and CD70 in formation and establishment of CD4+ and CD8+ tissue effector T cell pools. Cellular interactions and molecules dictating the contributions of CD27–CD70 interactions in T cell priming, expansion and effector phases are indicated. CD27–CD70 interactions promote the accumulation of effector T cells at the site of priming, primarily by improving activated T cell survival. Presumably this is accomplished during both priming and effector phases via the cell–cell interactions depicted. Evidence exists that CD27 also improves T cell survival at the effector site [11], possibly by interaction with CD70 on antigen presenting cells. Throughout the expansion phase, T cells also differentiate concomitantly under influence of CD27–CD70 interactions. Arrows indicate signals for induction of CD70, given by CD40, Toll-like receptors (TLR), the TCR and the BCR. The red star indicates antigen, which regulates CD70 expression via these receptors and determines its transient nature. According to our unpublished data, CD70 at the tissue effector site is predominant on DC and B cells, although effector T cells may also present CD70 to each other (not depicted).
and glioma models [18]. Also, the secretion of CD70 by tumor cells enhanced their rejection [19]. Moreover, CD70tg mice were protected against a dose of poorly immunogenic tumor cells that was lethal in wild-type mice [16]. All of these findings are consistent with the idea that CD27 stimulation promotes survival and expansion of primed CD8+ T cells and thereby even allows recruitment of new TCR repertoires into the responding population. Current Opinion in Immunology 2005, 17:275–281
Memory Time after antigenic challenge Current Opinion in Immunology
The impact of CD27–CD70 interactions on antigen-specific T cell numbers throughout the various phases of the primary CD8+ T cell response. (a) Relative quantitative impact of CD27, CD28, 4-1BB and OX40 according to studies in the intranasal influenza virus infection model, using recombinant mice lacking input of one or more receptors. = indicates equal to, indicates greater than or equal to. (Data taken from J Hendriks et al., unpublished; [11]). The relative impact of CD28 and 4-1BB has not been compared side-by-side in the same experiment and is, therefore, estimated in this figure. Impact on memory formation may result from effects during the contraction phase, but this remains to be proven. Pictures indicate lung-draining lymph nodes (DLN), lung and spleen. CD27 and 4-1BB only marginally affect CD8+ T cell expansion in the spleen. (b) Relative quantitative impact on antigenspecific T cell number throughout the primary response under conditions of deliberate CD27 stimulation by transgenic CD70, soluble recombinant CD70 or CD70 expression by tumor cells, or under conditions where CD27 input is lacking, due to genetic CD27 deficiency or blocking with CD70 antibody.
It is important to know whether CD27–CD70 interactions play a key role during T cell–DC communication. Taraban et al. [20] claim that there is a critical role for CD70 expressed on DCs in CD8+ T cell priming; www.sciencedirect.com
CD27 and CD70 in lymphocyte activation Borst et al. 277
however, application of CD70 antibody in vivo could not pinpoint the effect to a specific cell type. Bullock and Yagita [21] made a case for the importance of CD70 expression on DCs. They used MHC class II / DCs in adoptive transfer and showed that the capacity of CD40 to bypass CD4+ T-cell help in priming an anti-peptide CD8+ T cell response was partially inhibited by preincubation of DCs with CD70 antibody. In line with the idea that CD8+ T cell priming depends on the complementary effects of CD27 and CD28, antibodies to CD80 and CD86 further reduced the CD40 effect to almost undetectable levels. With regard to effector functions, CD27 might influence T-cell effector profiles in vivo by selective survival input into certain differentiated cell populations, or by directly delivering differentiation signals. Differentiation defects were not apparent in mice deficient for CD27 or treated with CD70 antibody [10,20]. In the cardiac allograft model, however, a shift from IFN-g to IL-5-producing cells was seen upon CD70 blocking [12]. Moreover, in CD70tg mice, CD8+ T cells produced more IFN-g and granzyme B on a per cell basis, although IL-2 and TNF were not increased in either CD4+ or CD8+ T cells [14,16]. Production of IFN-g by T cells is in fact responsible for profound B-cell depletion in CD70tg mice [14]. Whether CD27 directly regulates expression of differentiation-related genes or is permissive for differentiation remains to be proven.
T cell pool by improved CD27–CD70 interactions. Two unique patient studies indicate that also in human CD27 expression is beneficial for CD8+ memory T cell formation [23,24]. In vitro expansion of tumor-specific lymphocytes induced a CD27low/negative phenotype, similar to previous findings [7]. Upon adoptive transfer in vivo, however, these populations were selected for CD27+ cells in time, suggesting their advantage in long-term survival [23]. Essentially the same observation was made at the clonal level for HIV-specific CD8+ T cells [24]. Using adoptive transfer of standardized numbers of CD8+ memory T cells, we have shown that their secondary expansion is impaired if these memory cells are formed in absence of OX40 or 4-1BB receptor–ligand interactions. This indicates that OX40 and 4-1BB, which are the closest relatives of CD27 [9], imprint memory characteristics into T cells during the primary response. As CD27 / memory CD8+ T cells were also deficient in secondary expansion, CD27 may similarly contribute to memory cell differentiation (J Hendriks et al., unpublished). The role of CD27–CD70 signaling in CD4+ T cell memory is relatively unexplored. Accumulation of effector CD4+ T cells in spleen and lung after secondary challenge with influenza virus is impaired in CD27 / mice. [10]. However, these mice have no defects in CD4+ T helper-dependent B cell responses [3]. Further studies should reveal the importance of CD27–CD70 for CD4+ T cell responsiveness.
T cell memory
Upon infection with influenza virus [10,11], lymphocytic choriomeningitis virus (M Matter and A Ochsenbein, personal communication), or challenge with immunogenic tumor cells (Y Xiao, J Borst, unpublished), secondary CD8+ T cell responses are reduced in CD27 / mice. Also, CD70 blockade reduced secondary CD8+ T cell responses to cardiac allografts [12], or peptide plus CD40 antibody [20]. Conversely, deliberate CD27 stimulation by challenge with CD70-expressing tumor cells [18,19, 22], or infusion of soluble rCD70 [17] improved secondary CD8+ T cell responses. Secondary responsiveness is determined by the number of memory cells formed, as well as their capacity for secondary expansion. Many observations argue for a role of CD27–CD70 interactions in memory T cell formation. Formation of influenza virus-specific memory CD8+ T cells was reduced in CD27 / mice (J Hendriks et al., unpublished; Figure 2b), as was formation of ovalbuminspecific memory CD8+ T cells in mice treated with CD70 antibody [20]. Conversely, contraction of virus-specific CD8+ effector T cells was reduced in CD70tg mice, resulting in increased memory cell formation [16]. Moreover, soluble rCD70 could break non-responsiveness to ovalbumin peptide [17], suggesting that certain TCR repertoires are newly recruited into the memory CD8+ www.sciencedirect.com
The B cell response The presence of both CD27 and CD70 on primed T and B cells theoretically enables CD27–CD70 interactions to influence the B cell response in many ways (Figure 3). In vitro studies argue that CD27 on human B cells stimulates immunoglobulin production, which is at least in part due to promotion of plasma cell differentiation [25]. In the mouse, CD27 is acquired by a proportion of B cells at the centroblast stage and progressively lost upon their maturation. It is not a marker for somatically hypermutated B cells and present on only a few percent of memory B cells [3]. Germinal center (GC) formation is delayed in CD27 / mice, which is primarily due to deficient CD27 signaling into B cells. CD27 / mice have normal somatic hypermutation and normal immunoglobulin responses, both after primary challenge and recall [3]. Redundancy with OX40 or 4-1BB does not explain this lack of phenotype in CD27 / mice (J Hendriks et al., unpublished). T cells in the splenic microenvironment are relatively independent of CD27 [10,11], which may explain why CD27 deficiency has little effect on CD4+ T-cell help for immunoglobulin production. Apparently, in the mouse, CD27 accelerates centroblast expansion but has no significant impact on plasma cell differentiation. As, in humans, CD27 is expressed on a much higher frequency of GC B cells and maintained throughout B cell Current Opinion in Immunology 2005, 17:275–281
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this is not consistent with the phenotype or CD27 / mice and other data. Monitoring effects of soluble rCD70 and rCD27 should clarify the potential contributions of CD27 and CD70 to the B cell response. In addition, analysis of CD70 / mice (which are under development in our laboratory) should reveal whether reverse signaling by CD70 is triggered by CD27 or perhaps by a yet undefined ligand.
Figure 3
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Via a conserved motif of amino acids, PIQEDYR CD27 binds the adaptor proteins Traf-2 and Traf-5 and signals to the NF-kB and c-Jun kinase pathways (Figure 4: [29–31,32]). Traf signaling is connected to cell survival, but may also affect other cellular responses, such as Figure 4
Cell cycle BCR
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CD27 Participation of CD27 and CD70 in the germinal center B cell response. Cellular interactions and molecules dictating the contributions of CD27–CD70 interactions in B cell priming and expansion phases are indicated. Throughout the expansion phase, B cells differentiate concomitantly, possibly under the influence of CD27/CD70 interactions. Arrows indicate signals for induction of CD70, given by the BCR, CD40, Toll-like receptors (TLR), as well as the TCR in the case of helper CD4+ T cells. The red star indicates antigen, which, via these receptors, regulates CD70 expression. The role of CD27–CD70 interactions in expansion of helper CD4+ T cells is not depicted.
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differentiation [8], its impact on immunoglobulin production may be much greater than in the mouse. Interestingly, CD70 seems to play an opposing role to CD27 on B cells. This had already been suggested by studies on human B cells [26] and is confirmed in studies of CD70tg mice [27]. Deliberate CD70 triggering with antibody in CD27 / CD70tg mice reduced IgG production in response to hapten–protein conjugate. Also in IFNg / CD70tg mice with normal B cell numbers the IgG response was reduced. CD70 promotes cell cycle entry of activated B cells, but inhibits progression and possibly thereby impedes differentiation [27]. Infusion of CD27 antibody into mice has been reported to induce a shift of responding B cells from a plasma cell into memory cell differentiation pathway [28]. However, the activity of CD27 antibody in vivo was not defined. It was argued that the effect resulted from CD27 triggering on B cells, but Current Opinion in Immunology 2005, 17:275–281
c-Jun kinase
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Signal transduction pathways and cellular responses under control of CD27 and CD70. Although CD70 is an obligate trimer, trimerization of CD27 upon ligand binding as depicted here is hypothetical. Thick arrows indicate direct links that have been established in the endogenous pathway. Thin arrows indicate suspected, but not directly proven links. Cellular responses are underlined. While it is clear that CD27 can affect cell survival and cell differentiation, no direct connections between expression of specific gene products and these cellular responses have been established. Moreover, the signal transduction pathways depicted have not been implicated directly in these cellular responses. The transduction pathways that connect CD70 to its effect on cell cycle is undefined. That CD70 regulates cellular functions via gene expression is suspected from its capacity to affect activity of the NFAT transcription factor [38], but not depicted. NIK, NF-kB-inducing kinase; PI3K, phosphatidylinositol 3-kinase; PKB, protein kinase B; PLCg, phospholipase Cg. www.sciencedirect.com
CD27 and CD70 in lymphocyte activation Borst et al. 279
differentiation and migration [33]. Recently, an elegant study showed that CD27 activates both canonical and alternative NF-kB pathways via the NF-kB-inducing kinase (NIK), which is a serine/threonine kinase [32]. NF-kB transcription factors are associated with anti-apoptotic effects via targets such as c-Flip, inhibitory Bcl-2 family members and inhibitor of apoptosis proteins (IAPs). Comparing wild-type and CD27 / T cells, we found that CD27 protects T cells from apoptosis from the onset of TCR stimulation throughout successive cycles. Unlike CD28, CD27 did not appear to stimulate cell cycle entry or activity [11]. Soluble rCD70 strongly stimulated CD8+ T cell expansion in vivo with marginal effects on division, also indicating a clear pro-survival effect [17]. In CD70tg mice, numbers of cycling CD4+ and CD8+ T cells are greatly increased [15]. This probably represents a survival effect of CD27–CD70 on T cells that have entered cycle upon TCR stimulation, but may also include a proliferative effect. In Burkitt lymphoma cells, CD27 protected against B cell receptor-induced apoptosis and upregulated Bcl-2 and Bcl-xL [34]. Whether this is the molecular basis of its anti-apoptotic activity in vivo awaits validation. Some studies report that CD27 can induce apoptosis [35,36]. Prasad et al. [35] found Siva as a CD27 interactor in a yeast two-hybrid screen. Siva overexpression induced apoptosis and promoted the apoptotic effect of CD27 [35,37]. However, knock-down experiments are urgently needed to establish whether CD27, endogenous Siva and apoptosis are truly linked. The finding that Siva binds to NIK supports the idea that it participates in CD27 signaling [32]. A different connection between CD27 and apoptosis comes from the observation that CD27 can upregulate CD95 and sensitize T cells for CD95mediated apoptosis. This is part of a feedback mechanism, as effector T cell accumulation in CD70tg mice is greatly accelerated on a CD95-deficient background (R Arens and RAW van Lier, personal communication). When considering the impact of CD27–CD70 interactions on cellular responses, it needs to be taken into account that CD70 also has signaling properties. Two independent studies report that CD70 activates the phosphatidylinositol-3 kinase- and MAP kinase signaling pathways [27,38]. In primary B cells CD70 regulates cell cycle [27], whereas in NK, TCRgd+ and some TCRab+ T cell clones it triggers cytotoxicity [38]. The short cytoplasmic tail of CD70 has five residues that are conserved between species, as are two cysteines in the transmembrane segment. Further experiments should delineate their relevance for CD70 signaling. Moreover, connections between CD27 and CD70 signaling pathways, specific gene products and cellular functions need to be established. www.sciencedirect.com
Conclusions CD27–CD70 interactions govern the establishment of CD4+ and CD8+ effector T cell pools at tissue sites in primary and in particular in secondary responses. For CD8+ T cells, it is clear that CD27–CD70 interactions regulate expansion at the site of priming, maintenance at the effector site, contraction and memory formation, as well as secondary expansion. For CD4+ T cells, this remains to be explored. By controlling CD70 expression, antigen controls CD27 function. A major mechanism underlying the quantitative effects of CD27 on the T cell response is protection from apoptosis. However, CD27 also improves CD8+ effector T cell quality. With regard to B cell function, the mouse may suboptimally reflect the role of CD27–CD70 in humans. Unlike in humans, CD27 is expressed on only a subset of GC B cells and not maintained on memory B cells. It promotes GC expansion but does not detectably affect immunoglobulin production. The accumulated findings argue that the CD27–CD70 system is an important target for clinical intervention. In particular, blocking CD27–CD70 interactions (by small molecules preferably) will be an exciting option in cases of undesired immune activation. Indeed, positive results were obtained in models of heart transplantation [12] and experimental autoimmune encephalomyelitis [39]. Apart from incorporating the possibility to trigger CD27 in vaccines, other opportunities to exploit CD27–CD70 in cancer treatment present themselves. CD27 and CD70 are present on various lymphoid malignancies [6,40,41], whereas transformed cells outside the hematopoietic lineage can also express CD70. These include thymic [42], nasopharyngeal [43], breast [44], colon [45] and lung carcinoma [46], as well as brain tumors [36,47]. In these cases, CD70 might affect tumor progression directly, or indirectly by influencing the immune response. Further clarification of the mechanism of CD27–CD70 action is required to allow for rational intervention in human. The key goal is to identify CD27 and CD70 target genes and to implicate them in specific cellular functions. In vivo studies in various models of infection and disease should further unravel which cell types and which of their responses are under control of CD27– CD70 interactions and under exactly which circumstances this control is exerted.
Update The work referred to in the text as (R Arens and RAW van Lier, personal communication) is now in press [48].
Acknowledgements We thank all cited investigators for making their unpublished data available to us. We are grateful to R Arens and K Schepers for valuable advise on manuscript and figures and to RAW van Lier and TNM Schumacher for critically reading the manuscript. The cited work from our laboratory was supported by grants from the Dutch Cancer Society and the Netherlands Organization for Scientific Research. Current Opinion in Immunology 2005, 17:275–281
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References and recommended reading Papers of particular interest, published within the annual period of review, have been highlighted as: of special interest of outstanding interest 1.
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Tesselaar K, Xiao Y, Arens R, van Schijndel GMW, Schuurhuis DH, Mebius R, Borst J, van Lier RAW: Expression of the murine CD27 ligand CD70 in vitro and in vivo. J Immunol 2003, 170:33-40.
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10. Hendriks J, Gravestein LA, Tesselaar K, van Lier RAW, Schumacher TNM, Borst J: CD27 is required for generation and long-term maintenance of T cell immunity. Nat Immunol 2000, 1:433-440. 11. Hendriks J, Xiao Y, Borst J: CD27 promotes survival of activated T cells and complements CD28 in generation and establishment of the effector T cell pool. J Exp Med 2003, 198:1369-1380. This study establishes that CD27 protects activated T cells from apoptosis and shows that, by this mechanism, it increments the size of the CD8+ effector T cell pool at the site of priming and the tissue effector site after influenza virus infection. Using single and double deficient mice to compare the roles of CD27 and CD28, it concludes that CD27 is more important than CD28 for establishment of CD8+ effector cells at the tissue site. 12. Yamada A, Salama AD, Sho M, Najafian N, Ito T, Forman J, Kewalramani R, Sandner S, Harada H, Clarckson MR et al.: CD70 signaling is critical for CD28-independent CD8+ T cellmediated alloimmune responses in vivo. J Immunol 2005, 174:1357-1364. This study explores the role of CD27–CD70 interactions in cardiac allograft rejection by CD70 blocking in wild-type and CD28 / mice. It is the first significant work to investigate the impact of CD27 on alloresponses in vivo and concludes that effector CD8+ T cells are more reliant on CD27–CD70 interactions than are CD4+ T cells Blockade of CD70 allowed indefinite graft survival in CD28 / animals. 13. Aramaki O, Shirasugi N, Akiyama Y, Shibutani S, Takayama T, Shimazu M, Kitajima M, Ikeda Y, Okumura K, Yagita H, Niimi M: CD27/CD70, CD134/CD134 ligand and CD30/CD153 pathways are independently essential for generation of regulatory cells after intratracheal delivery of alloantigen. Transplantation 2003, 76:772-776. Current Opinion in Immunology 2005, 17:275–281
14. Arens R, Tesselaar K, van Schijndel GMW, Baars PA, Pals ST, Krimpenfort P, Borst J, van Oers MHJ, van Lier RAW: Constitutive CD27/CD70 interaction induces expansion of effector-type T cells and results in IFNg-mediated B cell depletion. Immunity 2001, 15:801-812. 15. Tesselaar K, Arens R, van Schijndel GMW, Baars PA, van der Valk MA, Borst J, van Oers MHJ, van Lier RAW: Lethal T cell immunodeficiency induced by chronic costimulation via CD27/CD70 interactions. Nat Immunol 2003, 4:49-54. This study in CD70tg mice shows that constitutive delivery of co-stimulatory signals via CD27 exhausts the naı¨ve T cell pool and depletes T cells from lymph nodes. It is unresolved whether death of effector cells or their dissipation into tissues causes T cell depletion. Together with B cell depletion as result of IFN-g production by the increased pool of effector T cells, this results in profound immunodeficiency. CD70tg mice develop AIDS-like symptoms from week 20 onwards and die. It is postulated that these effects are also present in patients with chronic active viral infections that also lead to persistent CD70 expression. 16. Arens R, Schepers K, Nolte M, van Oosterwijk MF, van Lier RAW, Schumacher TNM, van Oers MHJ: Tumor rejection induced by CD70-mediated quantitative and qualitative effects on effector CD8+ T-cell formation. J Exp Med 2004, 199:1595-1605. This study demonstrates that deliberate CD70 expression (in CD70tg mice) counteracts CD8+ effector T cell contraction and augments memory cell formation. It also shows that CD27 triggering by transgenic CD70 permits the rejection of poorly immunogenic tumors. This appears to be accomplished by improvement of cytotoxic functions in CD8+ effector T cells, their increased number and possibly the recruitment of new TCR repertoires. 17. Rowley TF, Al-Shamkhani A: Stimulation by soluble CD70 promotes strong primary and secondary CD8+ cytotoxic T cell responses in vivo. J Immunol 2004, 172:6039-6046. These authors report on the potency of recombinant soluble CD70 to stimulate CD8+ T cell responses (to ovalbumin peptide) in vitro and in vivo. Consistent with findings in CD27 / and CD70tg mice, they find that deliberate CD27 stimulation increases the size of effector and memory T cell pools. Most importantly, they show that delivery of soluble CD70 at the moment of priming enables a significant secondary response to peptide, although this does not occur in untreated mice. This may be due to recruitment of normally unresponsive T cells into the memory T cell pool. 18. Kelly JM, Darcy PK, Markby JL, Godfrey D, Takeda K, Yagita H, Smyth MJ: Induction of tumor-specific T cell memory by NKmediated tumor rejection. Nat Immunol 2002, 3:83-90. 19. Cormary C, Gonzalez R, Faye J-C, Favre G, Tilkin-Mariame A-F: Induction of T-cell antitumor immunity and protection against tumor growth by secretion of soluble human CD70 molecules. Cancer Gene Ther 2004, 11:497-507. 20. Taraban VY, Rowley TF, Al-Shamkhani A: Cutting edge: a critical role for CD70 in CD8 T cell priming by CD40-licensed APC. J Immunol 2004, 173:6542-6546. 21. Bullock TNJ, Yagita H: Induction of CD70 on dendritic cells through CD40 or TLR stimulation contributes to the development of CD8+ T cell responses in the absence of CD4+ T cells. J Immunol 2005, 174:710-717. These authors address the role of CD70 on DCs. They bypass CD4+ T cell help for priming of ovalbumin-specific CD8+ T cells by stimulation of peptide-loaded MHC class II / DCs with CD40 antibody. Ex vivo incubation of DCs with CD70 and/or CD80/CD86 antibodies, followed by adoptive transfer, impeded their capacity to prime CD8+ T cells. The findings suggest that CD4+ T cell help for CD8+ T cell priming relies on invoking the collective contributions of CD27 and CD28 during T cell–DC interactions. 22. Douin-Echinard V, Peron J-M, Lauwers-Cances V, Favre G, Couderc B: Involvement of CD70 and CD80 intracytoplasmic domains in the co-stimulatory signal required to provide an anti-tumor response. Int Immunol 2003, 15:359-372. 23. Powell DJ Jr, Dudley ME, Robbins PF, Rosenberg SA: Transition of late-stage effector T cells to CD27+ CD28+ tumor-reactive effector memory T cells in humans after adoptive cell transfer therapy. Blood 2005, 105:241-250. 24. Ochsenbein AF, Riddell SR, Brown M, Corey L, Baerlocher GM, Lansdorp PM, Greenberg PD: CD27 expression promotes longterm survival of functional effector-memory CD8+ cytotoxic T lymphocytes in HIV-infected patients. J Exp Med 2004, 200:1407-1417. www.sciencedirect.com
CD27 and CD70 in lymphocyte activation Borst et al. 281
25. Morimoto S, Kanno Y, Tanaka Y, Tokano Y, Hashimoto H, Jacquot S, Morimoto C, Schlossman SF, Yagita H, Okunmura K, Kobata T: CD134L engagement enhances human B cell Ig production: CD154/CD40, CD70/CD27 and CD134/CD134L interactions coordinately regulate T cell-dependent B cell responses. J Immunol 2000, 164:4097-4104. 26. Kobata T, Jacquot S, Kozlowski K, Agamatsu K, Schlossman SF, Morimoto C: CD27-CD70 interactions regulate B-cell activation by T cells. Proc Natl Acad Sci USA 1995, 92:11249-11253. 27. Arens R, Nolte MA, Tesselaar K, Heemskerk B, Reedquist K, van Lier RAW, van Oers MHJ: Signaling through CD70 regulates B cell activation and IgG production. J Immunol 2004, 173:3901-3908. This work reveals that CD70 is capable of signal transduction and activates the PtdIns-3 kinase, protein kinase B and Erk MAP kinase pathways. It shows that CD70 signaling affects cell cycle entry and progression in activated B cells and impedes IgG production in response to hapten–protein conjugates in vivo. 28. Raman VS, Akondy RS, Rath S, Bal V, George A: Ligation of CD27 on B cells in vivo during primary immunization enhances commitment to memory B cell responses. J Immunol 2003, 171:5876-5881. 29. Akiba H, Nakano H, Nishinaka S, Shindo M, Kobata T, Atsuta M, Morimoto C, Ware CF, Malinin NL, Wallach D et al.: CD27, a member of the Tumor necosis Factor receptor superfamily, activates NF-kB and Stress activated protein kinase/c-Jun N-terminal kinase viaTRAF2, TRAF5, and NF-kB-inducing kinase. J Biol Chem 1998, 273:13353-13358. 30. Nakano H, Sakon S, Koseki H, Takemori T, Tada K, Matsumoto M, Munechika E, Sakai T, Shirasawa T, Akiba H et al.: Targeted disruption of Traf5 gene causes defects in CD40- and CD27mediated lymphocyte activation. Proc Natl Acad Sci USA 1999, 96:9803-9808. 31. Gravestein LA, Amsen D, Boes M, Revilla Calvo C, Kruisbeek AM, Borst J: The TNF receptor family member CD27 signals to Jun N-terminal kinase via Traf-2. Eur J Immunol 1998, 28:2208-2216. 32. Ramakrishnan P, Wang W, Wallach D: Receptor-specific signaling for both the alternative and the canonical NF-kB activation pathways by NF-kB-inducing kinase. Immunity 2004, 21:477-489. This is the most comprehensive study on CD27 signaling published thus far. Its particular merit is that it explores the endogenous pathway, using high quality biochemical analysis and RNA interference as major tools to implicate specific molecules in CD27 signaling. 33. Aggarwal BB: Signalling pathways of the TNF superfamily: a double-edged sword. Nat Rev Immunol 2003, 3:745-756. 34. Hase H, Kanno Y, Kojima H, Moromoto C, Okumura K, Kobata T: CD27 and CD40 inhibit p53-independent mitochondrial pathway in apoptosis of B cells induced by B cell receptor ligation. J Biol Chem 2002, 277:46950-46958. 35. Prasad KVS, Ao Z, Yoon Y, Wu MX, Rizk M, Jacquot S, Schlossman SF: CD27, a member of the tumor necrosis factor receptor family, induces apoptosis and binds to Siva, a proapoptotic protein. Proc Natl Acad Sci USA 1997, 94:6346-6351.
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36. Wischhusen J, Jung G, Radvanovic I, Beier C, Steinbach JP, Rimner A, Huang H, Schultz JB, Ohgaki H, Aguzzi A et al.: Identification of CD70-mediated apoptosis of immune effector cells as a novel immune escape pathway of human glioblastoma. Cancer Res 2002, 62:2592-2599. 37. Py B, Slomianny C, Auberger P, Petit PX, Benichou S: Siva-1 and an alternative splice form lacking the death domain, Siva-2, similarly induce apoptosis in T lymphocytes via a caspasedependent mitochondrial pathway. J Immunol 2004, 172:4008-4017. 38. Garcia P, De Heredia AB, Bellon T, Carpio E, Llano M, Caparros E, Aparicio P, Lopez-Botet M: Signalling via CD70, a member of the TNF family, regulates T cell functions. J Leukoc Biol 2004, 76:263-270. 39. Nakajima A, Oshima H, Nohara C, Morimoto S, Yoshino S, Kobata T, Yagita H, Okumura KJ: Involvement of CD70-CD27 interactions in the induction of experimental autoimmune encephalomyelitis. J Neuroimmunol 2000, 109:188-196. 40. Zhu Y, Hollmen J, Raty R, Aalto Y, Nagy B, Elonen E, Kere J, Mannila H, Franssila K, Knuutila S: Investigatory and analytical approaches to differential gene expression profiling in mantle cell lymphoma. Br J Haematol 2002, 119:905-915. 41. Kok M, Bonfrer JM, Korse CM, de Jong D, Kersten MJ: Serum soluble CD27, but not thymidine kinase, is an independent prognostic factor for outcome in indolent non-Hodgkin’s lymphoma. Tumour Biol 2003, 24:53-60. 42. Hishima T, Fukayama M, Hayashi Y, Fujii T, Ooba T, Funata N, Koike M: CD70 expression in thymic carcinoma. Am J Surg Pathol 2000, 24:742-746. 43. Agathanggelou A, Niedobitek G, Chen R, Nicholls J, Yin W, Young LS: Expression of immune regulatory molecules in Epstein-Barr virus-associated nasopharyngeal carcinomas with prominent lymphoid stroma. Evidence for a functional interaction between epithelial tumor cells and infiltrating lymphoid cells. Am J Pathol 1995, 147:1152-1160. 44. Sloan DD, Nicholson B, Urquidi V, Goodison S: Detection of differentially expressed genes in an isogenic breast metastasis model using RNA arbitrarily primed-polymerase chain reaction coupled with array hybridization (RAP-array). Am J Pathol 2004, 164:315-323. 45. Fan CW, Chan CC, Chao CC, Fan HA, Sheu DL, Chan EC: Expression patterns of cell cycle and apoptosis-related genes in a multidrug-resistant human colon carcinoma cell line. Scand J Gastroenterol 2004, 39:464-469. 46. Wolf K, Schultz C, Riegger GA, Pfeifer M: Tumor necrosis factoralpha induced CD70 and interleukin 7R mRNA expression in BEAS-2B cells. Eur Respir J 2002, 20:369-375. 47. Held-Feindt J, Mentlein R: CD70/CD27 ligand, a member of the TNF family, is expressed in human brain tumors. Int J Cancer 2002, 98:352-356. 48. Arens R, Baars PA, Jak M, Tesselaar K, van der Valk M, van Oers MH, van Lier RAW: CD95 maintains effector T cell homeostasis in chronic immune activation. J Immunol 2005, in press.
Current Opinion in Immunology 2005, 17:275–281
Current Opinion in Immunology 2005, 17:275–281 This review comes from a themed issue on Lymphocyte activation Edited by Gail A Bishop and Jonathan R Lamb Available online 13th April 2005 0952-7915/$ – see front matter # 2005 Elsevier Ltd. All rights reserved. DOI 10.1016/j.coi.2005.04.004
Introduction In peripheral lymphoid organs of naı¨ve mice, CD27 is found on a large proportion of NK cells and on most, if not all, CD4+ and CD8+ T cells [1,2]. CD27 is absent from naı¨ve B cells, but is induced upon priming [3]. The contribution of CD27 to the immune response is dependent upon CD70 expression, which is primarily controlled by antigen receptor and Toll-like receptor stimulation on T cells, B cells and dendritic cells (DCs) [4]. NK cells can also express CD70 [5]. The expression patterns of CD27 and CD70 in humans are very similar to those in mice, except that CD27 is expressed on primed B cells in higher frequency and more persistently in humans. In clinical practice, membrane CD27 and its soluble form are used as lymphocyte subset markers and disease markers in the case of lymphoid malignancies, autoimmunity and transplant rejection [6–8]. From in vitro studies on human cells, CD27 has long been known as a co-stimulator of T cell and B cell responses [6,8,9]; however, its exact mechanism of action and relative importance for immune www.sciencedirect.com
responsiveness in vivo are only now being revealed by studies in mouse models. This recent work will be subject of this review.
T cell priming and effector phases According to their expression patterns, CD27–CD70 interactions can come into play in the T cell priming phase during contact between T cells and DCs and in the expansion phase during contact between T cells. During the effector phase, interactions between CD27 on T cells and CD70 on DCs and B cells may be important, as CD70 is most abundant on these cell types at the effector site (J Hendricks et al., unpublished; Figure 1). CD27 / mice, intranasally infected with influenza virus, revealed that CD27 controls the accumulation of CD4+ and CD8+ effector T cells at the site of infection [10]. Focusing on CD8+ T cells, we found that CD27 promoted the survival of virus-specific T cells at sites of priming and infection and acted independent of and complementary to CD28, which also controls effector T cell accumulation. CD27 was in fact the principle determinant for T-cell accumulation at the tissue effector site (Figure 2a; [11]). Consistently, in a cardiac allograft model, accumulation of CD8+ effector T cells and their capacity to reject the graft depended on the collective contributions of CD27 and CD28, as shown by CD70 inhibition. CD4+ T cell accumulation and function, however, were much less affected [12]. The priming of regulatory T cells after intratracheal delivery of alloantigen may also depend on CD27–CD70 interactions [13]. Co-stimulatory effects of CD27 on both CD4+ and CD8+ T cells were also revealed in CD70 transgenic (tg) mice that constitutively express CD70 on B cells. In these mice, both subpopulations expand and turn over from a naı¨ve into an effector phenotype upon ageing. This process depends on CD27 and on T cell antigen receptor (TCR) stimulation, presumably by environmental or auto-antigens [14,15]. The phenotype of CD70tg mice essentially mirrors that of CD27 / mice. In CD70tg mice, CD27–CD70 interactions presumably enable T cells that have received low level TCR input to survive and develop into effector cells. CD8+ effector T cell accumulation upon challenge with influenza virus was enhanced in CD70tg mice (Figure 2b; [16]). Application of soluble recombinant (r)CD70 in vivo emphasized the potency of the CD27–CD70 system to increase the size of the effector T cell pool, in this case of ovalbuminspecific CD8+ T cells after immunization with peptide [17]. Deliberate expression of CD70 on tumor cells stimulated both NK and T cell immunity in lymphoma Current Opinion in Immunology 2005, 17:275–281
276 Lymphocyte activation
Figure 1
Figure 2
Effector CD27 > 4-1BB CD28
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Priming Participation of CD27 and CD70 in formation and establishment of CD4+ and CD8+ tissue effector T cell pools. Cellular interactions and molecules dictating the contributions of CD27–CD70 interactions in T cell priming, expansion and effector phases are indicated. CD27–CD70 interactions promote the accumulation of effector T cells at the site of priming, primarily by improving activated T cell survival. Presumably this is accomplished during both priming and effector phases via the cell–cell interactions depicted. Evidence exists that CD27 also improves T cell survival at the effector site [11], possibly by interaction with CD70 on antigen presenting cells. Throughout the expansion phase, T cells also differentiate concomitantly under influence of CD27–CD70 interactions. Arrows indicate signals for induction of CD70, given by CD40, Toll-like receptors (TLR), the TCR and the BCR. The red star indicates antigen, which regulates CD70 expression via these receptors and determines its transient nature. According to our unpublished data, CD70 at the tissue effector site is predominant on DC and B cells, although effector T cells may also present CD70 to each other (not depicted).
and glioma models [18]. Also, the secretion of CD70 by tumor cells enhanced their rejection [19]. Moreover, CD70tg mice were protected against a dose of poorly immunogenic tumor cells that was lethal in wild-type mice [16]. All of these findings are consistent with the idea that CD27 stimulation promotes survival and expansion of primed CD8+ T cells and thereby even allows recruitment of new TCR repertoires into the responding population. Current Opinion in Immunology 2005, 17:275–281
Memory Time after antigenic challenge Current Opinion in Immunology
The impact of CD27–CD70 interactions on antigen-specific T cell numbers throughout the various phases of the primary CD8+ T cell response. (a) Relative quantitative impact of CD27, CD28, 4-1BB and OX40 according to studies in the intranasal influenza virus infection model, using recombinant mice lacking input of one or more receptors. = indicates equal to, indicates greater than or equal to. (Data taken from J Hendriks et al., unpublished; [11]). The relative impact of CD28 and 4-1BB has not been compared side-by-side in the same experiment and is, therefore, estimated in this figure. Impact on memory formation may result from effects during the contraction phase, but this remains to be proven. Pictures indicate lung-draining lymph nodes (DLN), lung and spleen. CD27 and 4-1BB only marginally affect CD8+ T cell expansion in the spleen. (b) Relative quantitative impact on antigenspecific T cell number throughout the primary response under conditions of deliberate CD27 stimulation by transgenic CD70, soluble recombinant CD70 or CD70 expression by tumor cells, or under conditions where CD27 input is lacking, due to genetic CD27 deficiency or blocking with CD70 antibody.
It is important to know whether CD27–CD70 interactions play a key role during T cell–DC communication. Taraban et al. [20] claim that there is a critical role for CD70 expressed on DCs in CD8+ T cell priming; www.sciencedirect.com
CD27 and CD70 in lymphocyte activation Borst et al. 277
however, application of CD70 antibody in vivo could not pinpoint the effect to a specific cell type. Bullock and Yagita [21] made a case for the importance of CD70 expression on DCs. They used MHC class II / DCs in adoptive transfer and showed that the capacity of CD40 to bypass CD4+ T-cell help in priming an anti-peptide CD8+ T cell response was partially inhibited by preincubation of DCs with CD70 antibody. In line with the idea that CD8+ T cell priming depends on the complementary effects of CD27 and CD28, antibodies to CD80 and CD86 further reduced the CD40 effect to almost undetectable levels. With regard to effector functions, CD27 might influence T-cell effector profiles in vivo by selective survival input into certain differentiated cell populations, or by directly delivering differentiation signals. Differentiation defects were not apparent in mice deficient for CD27 or treated with CD70 antibody [10,20]. In the cardiac allograft model, however, a shift from IFN-g to IL-5-producing cells was seen upon CD70 blocking [12]. Moreover, in CD70tg mice, CD8+ T cells produced more IFN-g and granzyme B on a per cell basis, although IL-2 and TNF were not increased in either CD4+ or CD8+ T cells [14,16]. Production of IFN-g by T cells is in fact responsible for profound B-cell depletion in CD70tg mice [14]. Whether CD27 directly regulates expression of differentiation-related genes or is permissive for differentiation remains to be proven.
T cell pool by improved CD27–CD70 interactions. Two unique patient studies indicate that also in human CD27 expression is beneficial for CD8+ memory T cell formation [23,24]. In vitro expansion of tumor-specific lymphocytes induced a CD27low/negative phenotype, similar to previous findings [7]. Upon adoptive transfer in vivo, however, these populations were selected for CD27+ cells in time, suggesting their advantage in long-term survival [23]. Essentially the same observation was made at the clonal level for HIV-specific CD8+ T cells [24]. Using adoptive transfer of standardized numbers of CD8+ memory T cells, we have shown that their secondary expansion is impaired if these memory cells are formed in absence of OX40 or 4-1BB receptor–ligand interactions. This indicates that OX40 and 4-1BB, which are the closest relatives of CD27 [9], imprint memory characteristics into T cells during the primary response. As CD27 / memory CD8+ T cells were also deficient in secondary expansion, CD27 may similarly contribute to memory cell differentiation (J Hendriks et al., unpublished). The role of CD27–CD70 signaling in CD4+ T cell memory is relatively unexplored. Accumulation of effector CD4+ T cells in spleen and lung after secondary challenge with influenza virus is impaired in CD27 / mice. [10]. However, these mice have no defects in CD4+ T helper-dependent B cell responses [3]. Further studies should reveal the importance of CD27–CD70 for CD4+ T cell responsiveness.
T cell memory
Upon infection with influenza virus [10,11], lymphocytic choriomeningitis virus (M Matter and A Ochsenbein, personal communication), or challenge with immunogenic tumor cells (Y Xiao, J Borst, unpublished), secondary CD8+ T cell responses are reduced in CD27 / mice. Also, CD70 blockade reduced secondary CD8+ T cell responses to cardiac allografts [12], or peptide plus CD40 antibody [20]. Conversely, deliberate CD27 stimulation by challenge with CD70-expressing tumor cells [18,19, 22], or infusion of soluble rCD70 [17] improved secondary CD8+ T cell responses. Secondary responsiveness is determined by the number of memory cells formed, as well as their capacity for secondary expansion. Many observations argue for a role of CD27–CD70 interactions in memory T cell formation. Formation of influenza virus-specific memory CD8+ T cells was reduced in CD27 / mice (J Hendriks et al., unpublished; Figure 2b), as was formation of ovalbuminspecific memory CD8+ T cells in mice treated with CD70 antibody [20]. Conversely, contraction of virus-specific CD8+ effector T cells was reduced in CD70tg mice, resulting in increased memory cell formation [16]. Moreover, soluble rCD70 could break non-responsiveness to ovalbumin peptide [17], suggesting that certain TCR repertoires are newly recruited into the memory CD8+ www.sciencedirect.com
The B cell response The presence of both CD27 and CD70 on primed T and B cells theoretically enables CD27–CD70 interactions to influence the B cell response in many ways (Figure 3). In vitro studies argue that CD27 on human B cells stimulates immunoglobulin production, which is at least in part due to promotion of plasma cell differentiation [25]. In the mouse, CD27 is acquired by a proportion of B cells at the centroblast stage and progressively lost upon their maturation. It is not a marker for somatically hypermutated B cells and present on only a few percent of memory B cells [3]. Germinal center (GC) formation is delayed in CD27 / mice, which is primarily due to deficient CD27 signaling into B cells. CD27 / mice have normal somatic hypermutation and normal immunoglobulin responses, both after primary challenge and recall [3]. Redundancy with OX40 or 4-1BB does not explain this lack of phenotype in CD27 / mice (J Hendriks et al., unpublished). T cells in the splenic microenvironment are relatively independent of CD27 [10,11], which may explain why CD27 deficiency has little effect on CD4+ T-cell help for immunoglobulin production. Apparently, in the mouse, CD27 accelerates centroblast expansion but has no significant impact on plasma cell differentiation. As, in humans, CD27 is expressed on a much higher frequency of GC B cells and maintained throughout B cell Current Opinion in Immunology 2005, 17:275–281
278 Lymphocyte activation
this is not consistent with the phenotype or CD27 / mice and other data. Monitoring effects of soluble rCD70 and rCD27 should clarify the potential contributions of CD27 and CD70 to the B cell response. In addition, analysis of CD70 / mice (which are under development in our laboratory) should reveal whether reverse signaling by CD70 is triggered by CD27 or perhaps by a yet undefined ligand.
Figure 3
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Via a conserved motif of amino acids, PIQEDYR CD27 binds the adaptor proteins Traf-2 and Traf-5 and signals to the NF-kB and c-Jun kinase pathways (Figure 4: [29–31,32]). Traf signaling is connected to cell survival, but may also affect other cellular responses, such as Figure 4
Cell cycle BCR
PKB Erk1/2 PI3K
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CD27 Participation of CD27 and CD70 in the germinal center B cell response. Cellular interactions and molecules dictating the contributions of CD27–CD70 interactions in B cell priming and expansion phases are indicated. Throughout the expansion phase, B cells differentiate concomitantly, possibly under the influence of CD27/CD70 interactions. Arrows indicate signals for induction of CD70, given by the BCR, CD40, Toll-like receptors (TLR), as well as the TCR in the case of helper CD4+ T cells. The red star indicates antigen, which, via these receptors, regulates CD70 expression. The role of CD27–CD70 interactions in expansion of helper CD4+ T cells is not depicted.
Traf-2/5
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Apoptosis? Canonical NF-κB pathway Gene expression
differentiation [8], its impact on immunoglobulin production may be much greater than in the mouse. Interestingly, CD70 seems to play an opposing role to CD27 on B cells. This had already been suggested by studies on human B cells [26] and is confirmed in studies of CD70tg mice [27]. Deliberate CD70 triggering with antibody in CD27 / CD70tg mice reduced IgG production in response to hapten–protein conjugate. Also in IFNg / CD70tg mice with normal B cell numbers the IgG response was reduced. CD70 promotes cell cycle entry of activated B cells, but inhibits progression and possibly thereby impedes differentiation [27]. Infusion of CD27 antibody into mice has been reported to induce a shift of responding B cells from a plasma cell into memory cell differentiation pathway [28]. However, the activity of CD27 antibody in vivo was not defined. It was argued that the effect resulted from CD27 triggering on B cells, but Current Opinion in Immunology 2005, 17:275–281
c-Jun kinase
c-Jun Migration? Survival
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Signal transduction pathways and cellular responses under control of CD27 and CD70. Although CD70 is an obligate trimer, trimerization of CD27 upon ligand binding as depicted here is hypothetical. Thick arrows indicate direct links that have been established in the endogenous pathway. Thin arrows indicate suspected, but not directly proven links. Cellular responses are underlined. While it is clear that CD27 can affect cell survival and cell differentiation, no direct connections between expression of specific gene products and these cellular responses have been established. Moreover, the signal transduction pathways depicted have not been implicated directly in these cellular responses. The transduction pathways that connect CD70 to its effect on cell cycle is undefined. That CD70 regulates cellular functions via gene expression is suspected from its capacity to affect activity of the NFAT transcription factor [38], but not depicted. NIK, NF-kB-inducing kinase; PI3K, phosphatidylinositol 3-kinase; PKB, protein kinase B; PLCg, phospholipase Cg. www.sciencedirect.com
CD27 and CD70 in lymphocyte activation Borst et al. 279
differentiation and migration [33]. Recently, an elegant study showed that CD27 activates both canonical and alternative NF-kB pathways via the NF-kB-inducing kinase (NIK), which is a serine/threonine kinase [32]. NF-kB transcription factors are associated with anti-apoptotic effects via targets such as c-Flip, inhibitory Bcl-2 family members and inhibitor of apoptosis proteins (IAPs). Comparing wild-type and CD27 / T cells, we found that CD27 protects T cells from apoptosis from the onset of TCR stimulation throughout successive cycles. Unlike CD28, CD27 did not appear to stimulate cell cycle entry or activity [11]. Soluble rCD70 strongly stimulated CD8+ T cell expansion in vivo with marginal effects on division, also indicating a clear pro-survival effect [17]. In CD70tg mice, numbers of cycling CD4+ and CD8+ T cells are greatly increased [15]. This probably represents a survival effect of CD27–CD70 on T cells that have entered cycle upon TCR stimulation, but may also include a proliferative effect. In Burkitt lymphoma cells, CD27 protected against B cell receptor-induced apoptosis and upregulated Bcl-2 and Bcl-xL [34]. Whether this is the molecular basis of its anti-apoptotic activity in vivo awaits validation. Some studies report that CD27 can induce apoptosis [35,36]. Prasad et al. [35] found Siva as a CD27 interactor in a yeast two-hybrid screen. Siva overexpression induced apoptosis and promoted the apoptotic effect of CD27 [35,37]. However, knock-down experiments are urgently needed to establish whether CD27, endogenous Siva and apoptosis are truly linked. The finding that Siva binds to NIK supports the idea that it participates in CD27 signaling [32]. A different connection between CD27 and apoptosis comes from the observation that CD27 can upregulate CD95 and sensitize T cells for CD95mediated apoptosis. This is part of a feedback mechanism, as effector T cell accumulation in CD70tg mice is greatly accelerated on a CD95-deficient background (R Arens and RAW van Lier, personal communication). When considering the impact of CD27–CD70 interactions on cellular responses, it needs to be taken into account that CD70 also has signaling properties. Two independent studies report that CD70 activates the phosphatidylinositol-3 kinase- and MAP kinase signaling pathways [27,38]. In primary B cells CD70 regulates cell cycle [27], whereas in NK, TCRgd+ and some TCRab+ T cell clones it triggers cytotoxicity [38]. The short cytoplasmic tail of CD70 has five residues that are conserved between species, as are two cysteines in the transmembrane segment. Further experiments should delineate their relevance for CD70 signaling. Moreover, connections between CD27 and CD70 signaling pathways, specific gene products and cellular functions need to be established. www.sciencedirect.com
Conclusions CD27–CD70 interactions govern the establishment of CD4+ and CD8+ effector T cell pools at tissue sites in primary and in particular in secondary responses. For CD8+ T cells, it is clear that CD27–CD70 interactions regulate expansion at the site of priming, maintenance at the effector site, contraction and memory formation, as well as secondary expansion. For CD4+ T cells, this remains to be explored. By controlling CD70 expression, antigen controls CD27 function. A major mechanism underlying the quantitative effects of CD27 on the T cell response is protection from apoptosis. However, CD27 also improves CD8+ effector T cell quality. With regard to B cell function, the mouse may suboptimally reflect the role of CD27–CD70 in humans. Unlike in humans, CD27 is expressed on only a subset of GC B cells and not maintained on memory B cells. It promotes GC expansion but does not detectably affect immunoglobulin production. The accumulated findings argue that the CD27–CD70 system is an important target for clinical intervention. In particular, blocking CD27–CD70 interactions (by small molecules preferably) will be an exciting option in cases of undesired immune activation. Indeed, positive results were obtained in models of heart transplantation [12] and experimental autoimmune encephalomyelitis [39]. Apart from incorporating the possibility to trigger CD27 in vaccines, other opportunities to exploit CD27–CD70 in cancer treatment present themselves. CD27 and CD70 are present on various lymphoid malignancies [6,40,41], whereas transformed cells outside the hematopoietic lineage can also express CD70. These include thymic [42], nasopharyngeal [43], breast [44], colon [45] and lung carcinoma [46], as well as brain tumors [36,47]. In these cases, CD70 might affect tumor progression directly, or indirectly by influencing the immune response. Further clarification of the mechanism of CD27–CD70 action is required to allow for rational intervention in human. The key goal is to identify CD27 and CD70 target genes and to implicate them in specific cellular functions. In vivo studies in various models of infection and disease should further unravel which cell types and which of their responses are under control of CD27– CD70 interactions and under exactly which circumstances this control is exerted.
Update The work referred to in the text as (R Arens and RAW van Lier, personal communication) is now in press [48].
Acknowledgements We thank all cited investigators for making their unpublished data available to us. We are grateful to R Arens and K Schepers for valuable advise on manuscript and figures and to RAW van Lier and TNM Schumacher for critically reading the manuscript. The cited work from our laboratory was supported by grants from the Dutch Cancer Society and the Netherlands Organization for Scientific Research. Current Opinion in Immunology 2005, 17:275–281
280 Lymphocyte activation
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