- Email: [email protected]
Germinal centers: Getting there is half the fun
Dispatch
R753
Germinal centers: Getting there is half the fun David Tarlinton
Several recent studies have provided new insights into how the infrequent B and T cells that are specific for a particular immunizing antigen come together to form the germinal centers that are crucial to the immune response. Address: The Walter and Eliza Hall Institute for Medical Research, PO Royal Melbourne Hospital, Victoria 3050, Australia. E-mail: [email protected] Current Biology 1998, 8:R753–R756 http://biomednet.com/elecref/09609822008R0753 © Current Biology Ltd ISSN 0960-9822
Germinal centers form in secondary lymphoid organs as a result of immunization [1]. While they are traditionally associated with the immune response to antigens that require T-cell help, known as T-dependent antigens, this connection is not absolute. The association of germinal centers with somatic mutation of immunoglobulin variable region genes and selection of high-affinity B cell variants is also no longer absolute, although germinal centers greatly enhance the efficiency with which these processes occur. Indeed, the very existence of germinal centers is a testament to the efficiency with which rare antigen-specific B and T lymphocytes are co-localized together with antigen. This raises the question of how on earth these infrequent B and T cells find each other among the billions of irrelevant lymphocytes, all whizzing around the lymphoid system? The answer, it appears, has much to do with what might be called instructive migration: the circumstances in which B and T cells are activated by antigen dictate to a certain extent the molecules they subsequently express on their surface and, as a consequence of this, the activation and migratory signals to which they are able to respond. The altered migration of B and T cells induced by antigen has been well documented using several different systems. Garside et al. [2], for example, used lymphocytes from two sets of transgenic mice, one carrying a transgene encoding a T-cell receptor specific for a peptide from ovalbumin (OVA) and the second carrying transgenes encoding a B-cell receptor specific for hen egg lysozyme (HEL). When small numbers of these donor cells were injected into recipients, they behaved like other naive lymphocytes: B cells entered the B-cell areas of lymph nodes called follicles, whereas T cells stayed outside in the T-cell-rich regions (Figure 1). When an antigen containing both the HEL and OVA epitopes was injected after lymphocyte transfer, the resultant movement of the antigen-specific B and T cells could be followed by histological analysis of the lymph
nodes draining the sites of immunization. This revealed a quite remarkable pattern of migration (Figure 1), showing the synchronized movements of both antigenspecific B and T cells. On day 1 after immunization, the OVA-specific T cells, still in the T-cell areas, were seen to have formed clusters resulting from their interaction with antigen-presenting dendritic cells in the paracortical region of the lymph nodes [3,4]. The B cells, still scattered throughout the follicles, showed reduced expression of surface immunoglobulin M, indicative of an interaction with antigen. It would therefore appear that, in this response, antigen priming of B and T cells occurs simultaneously, but separately. By day 2, OVA-specific T cells were found to be proliferating and, more importantly, to have migrated to the edge of the paracortical area, adjacent to the follicle. Thus the antigen-specific and activated T cells were now located adjacent to the B-cell area. By this time, the HEL-specific B cells, having restored surface immunoglobulin M expression, had also migrated to the edge of the follicle so as to abut the T-cell area. The antigen-specific T and B cells thus co-localize at the B-cell side of the boundary between their respective areas in the lymph node, and in this way the consummation necessary for T-cell-dependent antibody production occurs. In this particular response, Garside et al. [2] observed around 70% of OVA-specific T cells in physical contact with HEL-specific B cells, again showing the efficiency with which these two cell types find each other. Similar analyses of the immune response in the spleen ([3,5,6] for example) showed that, in this lymphoid organ, B cell–T cell interaction and proliferation occur in the T-cell areas, possibly reflecting differences in lymphocyte migration between spleen and lymph node. Like all meaningful relationships, that between the antigen-specific B and T cells continues long after the first assignation. By day 3 of the response, Garside et al. [2] found that many of the HEL-specific B cells and OVA-specific T cells were located within the follicles, where they continued to proliferate, resulting in histologically recognizable germinal centers by about day 4. Some of the HELspecific B cells, however, did not return to the depths of the follicle, but rather continued their migration on into the medulla of the lymph node. Garside et al. [2] suggest that, as foci of antibody-forming cells do not arise in this response, these B cells may be on their way to the bone marrow [2]. This may or not be the case, as it is now relatively clear that bone marrow antibody-forming cells derive
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Current Biology, Vol 8 No 21
Figure 1
Day 0
Day 1 F
F
Day 2 F
Day 3 F
Day 4 F Germinal center
P
P
P
P
P
B cell OX40L
–
+
++
+++
+++
T cell OX40
–
+
++
+++
+++
T cell BLR1
–
–
+
+
–
T cell IL-4
–
+/–
+
++
+++ Current Biology
Early stages of an immune response. The figure shows the distribution of antigen-specific B and T cells in the lymphoid areas of a lymph node at various times after exposure to an immunizing antigen, with B cells (green) in follicles (F) and T cells (red) in the paracortex (P). Extrafollicular dendritic cells are also shown (blue). Immunization with antigen results in time-dependent changes in the distribution of these cells. B and T cells show evidence of priming by day 1 post immunization, but remain in their respective areas. By day 2, both cell
types migrate to the boundary of the B-cell and T-cell areas, and come in physical contact. Days 2, 3 and 4 are also periods of lymphocyte proliferation, culminating in the formation of germinal centers. Some antigen-specific B cells differentiate into antibody-forming cells and migrate away from the follicles. The changes in expression of molecules important for the initiation of the immune response are indicated. (Compiled from data in [2,6,8].)
primarily from the germinal center, rather than the pre-germinal center period described above [7]. In responses previously monitored in the spleen, some of the antigen-specific B cells remain in the T-cell area and do give rise to foci of antibody-forming cells, highlighting another potential difference between these two lymphoid organs.
ligand for OX40, imaginatively called OX40-ligand (OX40L), is upregulated on activated B cells [11] and expressed constitutively on some dendritic cells, at least in humans [12]. Blocking the OX40–OX40L interaction significantly inhibits production of immunoglobulin G antibodies during a T-cell dependent response, but does not affect the generation of either germinal centers or memory B cells in spleen [5]. Furthermore, OX40 is known to be upregulated on CD4 T cells early in an immune response, with kinetics to match the upregulation of OX40L [5,8]. It would appear that OX40 has many of the hallmarks of a molecule that is critical in defining the early stages of an immune response.
The rare antigen-specific B and T cells thus come together fairly soon after exposure to immunizing antigen. How are these lymphocytes instructed to follow the pathways that result in their meeting? And, of equal importance, how do they know what to do when they eventually meet? A partial answer to these questions is provided by the recent work of Flynn et al. [8]. They examined the effect of co-stimulating CD4 T cells — helper T cells as defined by expression of the T-cell coreceptor CD4 — through the cell-surface protein OX40 [8]. OX40 has had an interesting, albeit brief, history. It was originally identified as a member of the tumor necrosis factor (TNF) receptor family, expression of which is induced upon T-cell activation [9]. Antibodies to OX40 were found to co-stimulate activated T cells [10]. The
When CD4 T cells that have been primed by an initial activating stimulus are subsequently restimulated, for example with antibodies against the T-cell receptor complex (anti-CD3), they secrete cytokines. Flynn et al. [8] found that, if membrane-bound OX40L is present when T cells are initially activated by immobilized antiCD3 and soluble anti-CD28 antibodies, then on restimulation the cells secreted predominantly interleukin-4 (IL-4). In the absence of OX40L, interferon-γ (IFN-γ)
Dispatch
production dominated the cytokine response. OX40L thus biased the T-cell culture so that it took on the appearance of a so-called Th2 population. Th2 cells are predisposed to stimulate humoral — that is, antibody-based — rather than cell-mediated immunity. That the ligation of OX40 should preferentially have this outcome in vitro is made significant by the fact that the interaction of activated T and B cells presumably results in just such a stimulatory signal being delivered to the T cells by the activated B cells. Indeed, in situ tracking of IL-4 mRNA levels during an immune response showed that this cytokine is expressed with very similar kinetics to OX40 and OX40L [6]. Thus, T cell priming on an extrafollicular dendritic cell expressing OX40L may bias the responding population to produce Th2 cytokines. When such T cells encounter B cells activated by the same antigen, this bias in cytokine phenotype would presumably be reinforced, and thus enhance the differentiation of B cells into antibody-forming cells. Consideration of the relationship between stimulation of CD4 T cells by OX40 and the development of a Th2 phenotype leads directly into the morass of T-cell commitment. Whether the OX40–OX40L interaction induces the differentiation of CD4 T cells into a Th2 type is not entirely clear from the work of Flynn et al. [8]. It is apparent that the favored outcome is an IL-4-secreting population, and that OX40 stimulation is sufficient to counteract the Th1 bias usually imposed by the presence of IL-12 [8]. The bias induced through OX40 might result from the direct induction of IL-4, a possibility favored by the authors, or it might be due to the preferential growth of a contaminating population already expressing IL-4. Hence the morass. With respect to alterations in T-cell proliferation, recent work of Gett and Hodgkin [13] has revealed a relationship between division number and cytokine production in a population of polyclonally activated T cells. They found that cytokine production changes in a predictable, division-dependent manner, indicating that altering the rate of proliferation of a T-cell culture can significantly alter the cytokine profile secreted. While Flynn et al. [8] did not measure the effect of OX40L on T-cell proliferation, others have noted the co-stimulatory effect of antiOX40 ligation on CD3-stimulated T cells [10]. Interestingly, in the experiments of Flynn et al. [8], some IL-4 mRNA was apparent in the presumptive naive CD4 T cells stimulated by anti-CD3 plus anti-CD28 alone, albeit much less than when OX40L was present [8]. That is, OX40 stimulation might also amplify, rather than induce, cytokine expression. The vexed issue of Th1/Th2 commitment lives on. A feature of the OX40L stimulation of CD4 T cells that is critical for T cell–B cell interaction is the upregulation of
R755
BLR1 (Burkitt’s lymphoma receptor-1) [8]. BLR1 is a chemokine receptor on naive B cells necessary for the Bcell migration into follicles in spleen and Peyer’s patches [14]. BLR1’s ligand has been identified recently in both humans and mice, and called B-lymphocyte chemoattractant (BLC) [15] or B-cell-attracting chemokine 1 (BCA-1) [16]. BLC/BCA-1 is probably secreted by follicular dendritic cells, resulting in a gradient up which naive B cells migrate. It may well be that CD4 T cells come under this siren’s spell after their expression of BLR1 is upregulated in response to stimulation through OX40, and thus migrate towards the B-cell area. Activated B cells, however, move counter to the BLC/BCA-1 gradient, out of the follicle towards the T-cell area. Another chemokine at work? Almost certainly. Ngo et al. [17] have cloned a chemokine expressed by dendritic cells that is able to attract naive T cells and activated, but not naive, B cells. Thus, the differential regulation of chemokine receptors on activated B and T cells leads to their positioning within the lymphoid organ, thereby orchestrating the B cell–T cell interaction that is critical for humoral immunity. Signaling through OX40/OX40L not only promotes the differentiation of B and T cells into antibody and cytokine secreting effector cells, but also promotes the expression of a set of molecules that allows the rare antigen-specific lymphocytes to find each other among a sea of strangers. It would not be science if some questions did not remain. Among the more curious aspects of the role of OX40 is that blocking this receptor in vivo blocks antibody production but not germinal center formation [5]. This suggests redundancy in the pathways capable of altering the migration of B and T cells, but a unique role for OX40L in the differentiation of B cells into antibody-forming cells. In mice lacking BLR1, the spleen, Peyer’s patches and inguinal lymph nodes are seriously disrupted [14], suggesting that the remaining lymph nodes may use a different chemokine system to establish B and T cell areas. That is, there may well be combinations of chemokines and receptors, other than those described here, which are important for the immune response in different lymphoid organs. Finally, the small population of antigen-specific B cells that differentiate into antibody-forming cells and migrate to the bone marrow represent a particularly interesting piece of the immune response puzzle. What is it that first induces the differentiation of these cells into antibodyforming cells, and what is it that localizes them in the Tcell area or directs them to the bone marrow? As these cells are critical for protective humoral immunity, these are very important questions. Acknowledgements I would like to thank Anne Kelso and Andrew Lew for helpful discussions.
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Current Biology, Vol 8 No 21
References 1. Tarlinton D: Germinal centers: form and function. Curr Opin Immunol 1998, 10:245-251. 2. Garside P, Ingulli E, Merica RR, Johnson JG, Noelle RJ, Jenkins MK: Visualization of specific B and T lymphocyte interactions in the lymph node. Science 1998, 281:96-99. 3. Luther SA, Gulbranson-Judge A, Acha-Orbea H, MacLennan IC: Viral superantigen drives extrafollicular and follicular B cell differentiation leading to virus-specific antibody production. J Exp Med 1997, 185:551-562. 4. Ingulli E, Mondino A, Khoruts A, Jenkins MK: In vivo detection of dendritic cell antigen presentation to CD4(+) T cells. J Exp Med 1997, 185:2133-2141. 5. Stuber E, Strober W: The T cell-B cell interaction via OX40–OX40L is necessary for the T cell-dependent humoral immune response. J Exp Med 1996, 83:979-989. 6. Toellner KM, Luther SA, Sze DM, Choy RK, Taylor DR, MacLennan ICM, Acha-Orbea H: T helper 1 (Th1) and Th2 characteristics start to develop during T cell priming and are associated with an immediate ability to induce immunoglobulin class switching. J Exp Med 1998, 187:1193-1204. 7. Smith KG, Light A, Nossal GJ, Tarlinton DM: The extent of affinity maturation differs between the memory and antibody-forming cell compartments in the primary immune response. EMBO J 1997, 16:2996-3006. 8. Flynn S, Toellner KM, Raykundalia C, Goodall M, Lane P: CD4 T cell cytokine differentiation: the B cell activation molecule, OX40 ligand, instructs CD4 T cells to express interleukin 4 and upregulates expression of the chemokine receptor, blr-1. J Exp Med 1998, 188:297-304. 9. Mallett S, Fossum S, Barclay AN: Characterization of the MRC Ox40 antigen of activated CD4 positive T lymphocytes — a molecule related to nerve growth factor receptor. EMBO J 1990, 9:1063-1068. 10. Godfrey WR, Fagnoni FF, Harara MA, Buck D, Engleman EG: Identification of a human OX-40 ligand, a costimulator of CD4+ T cells with homology to tumor necrosis factor. J Exp Med 1994, 180:757-762. 11. Stuber E, Neurath M, Calderhead D, Fell HP, Strober W: Crosslinking of OX40 ligand, a member of the TNF/NGF cytokine family, induces proliferation and differentiation in murine splenic B cells. Immunity 1995, 2:507-521. 12. Ohshima Y, Tanaka Y, Tozawa H, Takahashi Y, Maliszewski C, Delespesse G: Expression and function of OX40 ligand on human dendritic cells. J Immunol 1997, 159:3838-3848. 13. Gett AV, Hodgkin PD: Cell division regulates the T cell cytokine repertoire, revealing a mechanism underlying immune class regulation. Proc Natl Acad Sci USA 1998, 95:9488-9493. 14. Forster R, Mattis AE, Kremmer E, Wolf E, Brem G, Lipp M: A putative chemokine receptor, BLR1, directs B cell migration to defined lymphoid organs and specific anatomic compartments of the spleen. Cell 1996, 87:1037-1047. 15. Gunn MD, Ngo VN, Ansel KM, Ekland EH, Cyster JG, Williams LT: A B-cell-homing chemokine made in lymphoid follicles activates Burkitt’s lymphoma receptor-1. Nature 1998, 391:799-803. 16. Legler DF, Loetscher M, Roos RS, Clark-Lewis I, Baggiolini M, Moser B: B cell-attracting chemokine 1, a human CXC chemokine expressed in lymphoid tissues, selectively attracts B lymphocytes via BLR1/CXCR5. J Exp Med 1998, 187:655-660. 17. Ngo VN, Lucy Tang H, Cyster JG: Epstein-barr virus-induced molecule 1 ligand chemokine is expressed by dendritic cells in lymphoid tissues and strongly attracts naive T cells and activated B cells. J Exp Med 1998, 188:181-191.
R753
Germinal centers: Getting there is half the fun David Tarlinton
Several recent studies have provided new insights into how the infrequent B and T cells that are specific for a particular immunizing antigen come together to form the germinal centers that are crucial to the immune response. Address: The Walter and Eliza Hall Institute for Medical Research, PO Royal Melbourne Hospital, Victoria 3050, Australia. E-mail: [email protected] Current Biology 1998, 8:R753–R756 http://biomednet.com/elecref/09609822008R0753 © Current Biology Ltd ISSN 0960-9822
Germinal centers form in secondary lymphoid organs as a result of immunization [1]. While they are traditionally associated with the immune response to antigens that require T-cell help, known as T-dependent antigens, this connection is not absolute. The association of germinal centers with somatic mutation of immunoglobulin variable region genes and selection of high-affinity B cell variants is also no longer absolute, although germinal centers greatly enhance the efficiency with which these processes occur. Indeed, the very existence of germinal centers is a testament to the efficiency with which rare antigen-specific B and T lymphocytes are co-localized together with antigen. This raises the question of how on earth these infrequent B and T cells find each other among the billions of irrelevant lymphocytes, all whizzing around the lymphoid system? The answer, it appears, has much to do with what might be called instructive migration: the circumstances in which B and T cells are activated by antigen dictate to a certain extent the molecules they subsequently express on their surface and, as a consequence of this, the activation and migratory signals to which they are able to respond. The altered migration of B and T cells induced by antigen has been well documented using several different systems. Garside et al. [2], for example, used lymphocytes from two sets of transgenic mice, one carrying a transgene encoding a T-cell receptor specific for a peptide from ovalbumin (OVA) and the second carrying transgenes encoding a B-cell receptor specific for hen egg lysozyme (HEL). When small numbers of these donor cells were injected into recipients, they behaved like other naive lymphocytes: B cells entered the B-cell areas of lymph nodes called follicles, whereas T cells stayed outside in the T-cell-rich regions (Figure 1). When an antigen containing both the HEL and OVA epitopes was injected after lymphocyte transfer, the resultant movement of the antigen-specific B and T cells could be followed by histological analysis of the lymph
nodes draining the sites of immunization. This revealed a quite remarkable pattern of migration (Figure 1), showing the synchronized movements of both antigenspecific B and T cells. On day 1 after immunization, the OVA-specific T cells, still in the T-cell areas, were seen to have formed clusters resulting from their interaction with antigen-presenting dendritic cells in the paracortical region of the lymph nodes [3,4]. The B cells, still scattered throughout the follicles, showed reduced expression of surface immunoglobulin M, indicative of an interaction with antigen. It would therefore appear that, in this response, antigen priming of B and T cells occurs simultaneously, but separately. By day 2, OVA-specific T cells were found to be proliferating and, more importantly, to have migrated to the edge of the paracortical area, adjacent to the follicle. Thus the antigen-specific and activated T cells were now located adjacent to the B-cell area. By this time, the HEL-specific B cells, having restored surface immunoglobulin M expression, had also migrated to the edge of the follicle so as to abut the T-cell area. The antigen-specific T and B cells thus co-localize at the B-cell side of the boundary between their respective areas in the lymph node, and in this way the consummation necessary for T-cell-dependent antibody production occurs. In this particular response, Garside et al. [2] observed around 70% of OVA-specific T cells in physical contact with HEL-specific B cells, again showing the efficiency with which these two cell types find each other. Similar analyses of the immune response in the spleen ([3,5,6] for example) showed that, in this lymphoid organ, B cell–T cell interaction and proliferation occur in the T-cell areas, possibly reflecting differences in lymphocyte migration between spleen and lymph node. Like all meaningful relationships, that between the antigen-specific B and T cells continues long after the first assignation. By day 3 of the response, Garside et al. [2] found that many of the HEL-specific B cells and OVA-specific T cells were located within the follicles, where they continued to proliferate, resulting in histologically recognizable germinal centers by about day 4. Some of the HELspecific B cells, however, did not return to the depths of the follicle, but rather continued their migration on into the medulla of the lymph node. Garside et al. [2] suggest that, as foci of antibody-forming cells do not arise in this response, these B cells may be on their way to the bone marrow [2]. This may or not be the case, as it is now relatively clear that bone marrow antibody-forming cells derive
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Current Biology, Vol 8 No 21
Figure 1
Day 0
Day 1 F
F
Day 2 F
Day 3 F
Day 4 F Germinal center
P
P
P
P
P
B cell OX40L
–
+
++
+++
+++
T cell OX40
–
+
++
+++
+++
T cell BLR1
–
–
+
+
–
T cell IL-4
–
+/–
+
++
+++ Current Biology
Early stages of an immune response. The figure shows the distribution of antigen-specific B and T cells in the lymphoid areas of a lymph node at various times after exposure to an immunizing antigen, with B cells (green) in follicles (F) and T cells (red) in the paracortex (P). Extrafollicular dendritic cells are also shown (blue). Immunization with antigen results in time-dependent changes in the distribution of these cells. B and T cells show evidence of priming by day 1 post immunization, but remain in their respective areas. By day 2, both cell
types migrate to the boundary of the B-cell and T-cell areas, and come in physical contact. Days 2, 3 and 4 are also periods of lymphocyte proliferation, culminating in the formation of germinal centers. Some antigen-specific B cells differentiate into antibody-forming cells and migrate away from the follicles. The changes in expression of molecules important for the initiation of the immune response are indicated. (Compiled from data in [2,6,8].)
primarily from the germinal center, rather than the pre-germinal center period described above [7]. In responses previously monitored in the spleen, some of the antigen-specific B cells remain in the T-cell area and do give rise to foci of antibody-forming cells, highlighting another potential difference between these two lymphoid organs.
ligand for OX40, imaginatively called OX40-ligand (OX40L), is upregulated on activated B cells [11] and expressed constitutively on some dendritic cells, at least in humans [12]. Blocking the OX40–OX40L interaction significantly inhibits production of immunoglobulin G antibodies during a T-cell dependent response, but does not affect the generation of either germinal centers or memory B cells in spleen [5]. Furthermore, OX40 is known to be upregulated on CD4 T cells early in an immune response, with kinetics to match the upregulation of OX40L [5,8]. It would appear that OX40 has many of the hallmarks of a molecule that is critical in defining the early stages of an immune response.
The rare antigen-specific B and T cells thus come together fairly soon after exposure to immunizing antigen. How are these lymphocytes instructed to follow the pathways that result in their meeting? And, of equal importance, how do they know what to do when they eventually meet? A partial answer to these questions is provided by the recent work of Flynn et al. [8]. They examined the effect of co-stimulating CD4 T cells — helper T cells as defined by expression of the T-cell coreceptor CD4 — through the cell-surface protein OX40 [8]. OX40 has had an interesting, albeit brief, history. It was originally identified as a member of the tumor necrosis factor (TNF) receptor family, expression of which is induced upon T-cell activation [9]. Antibodies to OX40 were found to co-stimulate activated T cells [10]. The
When CD4 T cells that have been primed by an initial activating stimulus are subsequently restimulated, for example with antibodies against the T-cell receptor complex (anti-CD3), they secrete cytokines. Flynn et al. [8] found that, if membrane-bound OX40L is present when T cells are initially activated by immobilized antiCD3 and soluble anti-CD28 antibodies, then on restimulation the cells secreted predominantly interleukin-4 (IL-4). In the absence of OX40L, interferon-γ (IFN-γ)
Dispatch
production dominated the cytokine response. OX40L thus biased the T-cell culture so that it took on the appearance of a so-called Th2 population. Th2 cells are predisposed to stimulate humoral — that is, antibody-based — rather than cell-mediated immunity. That the ligation of OX40 should preferentially have this outcome in vitro is made significant by the fact that the interaction of activated T and B cells presumably results in just such a stimulatory signal being delivered to the T cells by the activated B cells. Indeed, in situ tracking of IL-4 mRNA levels during an immune response showed that this cytokine is expressed with very similar kinetics to OX40 and OX40L [6]. Thus, T cell priming on an extrafollicular dendritic cell expressing OX40L may bias the responding population to produce Th2 cytokines. When such T cells encounter B cells activated by the same antigen, this bias in cytokine phenotype would presumably be reinforced, and thus enhance the differentiation of B cells into antibody-forming cells. Consideration of the relationship between stimulation of CD4 T cells by OX40 and the development of a Th2 phenotype leads directly into the morass of T-cell commitment. Whether the OX40–OX40L interaction induces the differentiation of CD4 T cells into a Th2 type is not entirely clear from the work of Flynn et al. [8]. It is apparent that the favored outcome is an IL-4-secreting population, and that OX40 stimulation is sufficient to counteract the Th1 bias usually imposed by the presence of IL-12 [8]. The bias induced through OX40 might result from the direct induction of IL-4, a possibility favored by the authors, or it might be due to the preferential growth of a contaminating population already expressing IL-4. Hence the morass. With respect to alterations in T-cell proliferation, recent work of Gett and Hodgkin [13] has revealed a relationship between division number and cytokine production in a population of polyclonally activated T cells. They found that cytokine production changes in a predictable, division-dependent manner, indicating that altering the rate of proliferation of a T-cell culture can significantly alter the cytokine profile secreted. While Flynn et al. [8] did not measure the effect of OX40L on T-cell proliferation, others have noted the co-stimulatory effect of antiOX40 ligation on CD3-stimulated T cells [10]. Interestingly, in the experiments of Flynn et al. [8], some IL-4 mRNA was apparent in the presumptive naive CD4 T cells stimulated by anti-CD3 plus anti-CD28 alone, albeit much less than when OX40L was present [8]. That is, OX40 stimulation might also amplify, rather than induce, cytokine expression. The vexed issue of Th1/Th2 commitment lives on. A feature of the OX40L stimulation of CD4 T cells that is critical for T cell–B cell interaction is the upregulation of
R755
BLR1 (Burkitt’s lymphoma receptor-1) [8]. BLR1 is a chemokine receptor on naive B cells necessary for the Bcell migration into follicles in spleen and Peyer’s patches [14]. BLR1’s ligand has been identified recently in both humans and mice, and called B-lymphocyte chemoattractant (BLC) [15] or B-cell-attracting chemokine 1 (BCA-1) [16]. BLC/BCA-1 is probably secreted by follicular dendritic cells, resulting in a gradient up which naive B cells migrate. It may well be that CD4 T cells come under this siren’s spell after their expression of BLR1 is upregulated in response to stimulation through OX40, and thus migrate towards the B-cell area. Activated B cells, however, move counter to the BLC/BCA-1 gradient, out of the follicle towards the T-cell area. Another chemokine at work? Almost certainly. Ngo et al. [17] have cloned a chemokine expressed by dendritic cells that is able to attract naive T cells and activated, but not naive, B cells. Thus, the differential regulation of chemokine receptors on activated B and T cells leads to their positioning within the lymphoid organ, thereby orchestrating the B cell–T cell interaction that is critical for humoral immunity. Signaling through OX40/OX40L not only promotes the differentiation of B and T cells into antibody and cytokine secreting effector cells, but also promotes the expression of a set of molecules that allows the rare antigen-specific lymphocytes to find each other among a sea of strangers. It would not be science if some questions did not remain. Among the more curious aspects of the role of OX40 is that blocking this receptor in vivo blocks antibody production but not germinal center formation [5]. This suggests redundancy in the pathways capable of altering the migration of B and T cells, but a unique role for OX40L in the differentiation of B cells into antibody-forming cells. In mice lacking BLR1, the spleen, Peyer’s patches and inguinal lymph nodes are seriously disrupted [14], suggesting that the remaining lymph nodes may use a different chemokine system to establish B and T cell areas. That is, there may well be combinations of chemokines and receptors, other than those described here, which are important for the immune response in different lymphoid organs. Finally, the small population of antigen-specific B cells that differentiate into antibody-forming cells and migrate to the bone marrow represent a particularly interesting piece of the immune response puzzle. What is it that first induces the differentiation of these cells into antibodyforming cells, and what is it that localizes them in the Tcell area or directs them to the bone marrow? As these cells are critical for protective humoral immunity, these are very important questions. Acknowledgements I would like to thank Anne Kelso and Andrew Lew for helpful discussions.
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Current Biology, Vol 8 No 21
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