First Printed in R&D Systems 2003 Catalog
One of the most important events during a T cell response is activation.The discovery and characterization of the B7/CD28 interaction supports the "two-signal" model for lymphocyte activation.1
In this model, a lymphocyte requires two distinct signals in order for full activation to occur.1, 2 The first signal is provided by the interaction of the T cell receptor (TCR) on the lymphocyte with major histocompatibility class (MHC) antigens on the antigen-presenting cell (APC). The second, costimulatory, signal is required to avoid an apoptotic or anergic response by the lymphocyte. The interaction of CD28 on the lymphocyte with B7 proteins on the APC provides this necessary costimulatory second signal. If the lymphocyte only receives signal 1 (i.e. TCR engagement) from the APC, the lymphocyte becomes apoptotic or anergic, thus unable to respond to antigen.1, 2
Recently, several new members have been added to the B7 and CD28 superfamilies of immune costimulatory molecules.1, 3, 4, 5, 6 The original family members consisted of B7-1, B7-2, CD28 and CTLA-4.1, 3, 4, 5, 6 Newly identified family members include: B7-H1, B7-H2, B7-H3, PD-L2, ICOS and PD-1.1, 3, 4, 5, 6 The novel interactions between these new family members are proving to further enhance the complexity and expand the scope of our understanding of the immune response.
|Table 1. B7 & CD28 Family Members|
|Ligand||Alternative Names||Receptor||Alternative Names|
|B7-1||CD80, BB1||CD28||T44, Tp44|
|B7-2||CD86, B70||CD28||T44, Tp44|
|B7-H2||B7h, B7RP-1, GL50, ICOSL, LICOS||ICOS||AILIM, CRP-1, H4|
B7-1 (CD80) and B7-2 (CD86) are both type I proteins that are members of the immunoglobulin (Ig) superfamily.7, 8 B7-1 expression is found on activated B cells, activated T cells and macrophages.8 B7-2 is constitutively expressed on interdigitating dendritic cells, Langerhans cells, peripheral blood dendritic cells, memory B cells and germinal center B cells.7 Additionally, B7-2 is expressed at low levels on monocytes and is up-regulated through IFN-gamma stimulation. Human B7-1 and B7-2 share approximately 26% amino acid (aa) identity; mouse B7-1 and B7-2 share approximately 28% aa identity. Human and mouse B7-1 share approximately 44% aa identity.8 Human and mouse B7-2 share approximately 50% aa identity.7
Both B7-1 and B7-2 are capable of binding the receptors, CD28 and CTLA-4.1, 3, 4, 5, 6, 7, 8, 9, 10 Binding to CTLA-4 has been shown to have a 20-100 fold higher affinity than binding to CD28.11 Both human and mouse B7-1 and B7-2 can bind to either human or mouse CD28 and CTLA-4. This suggests that there are conserved amino acids that form the critical binding sites. CD28 plays a role in activation, whereas CTLA-4 functions as an inhibitory receptor important for down-modulating the immune response.1, 4, 5, 9, 12, 13 Although B7-2 is generally the first B7 molecule encountered, due to its constitutive expression on numerous APCs, there does not appear to be significant differences in the functions of B7-1 and B7-2. Rather, it depends on the type of APC encountered and its activation state.
CD28 and CTLA-4 (CD152) are also members of the immunoglobulin (Ig) superfamily. CD28 is expressed on nearly all CD4+ T cells and about half of the CD8+ T cells.1, 14 It is also expressed on developing thymocytes. CTLA-4 is not constitutively expressed; rather, it is rapidly up-regulated after CD28 ligation and T cell activation.15, 16 Cell surface expression of CTLA-4 peaks approximately 48 hours after activation. The genes encoding CD28 and CTLA-4 are closely linked on human chromosome 2 and mouse chromosome 1. Human and mouse CD28 share approximately 68% aa identity.14 Human and mouse CTLA-4 share approximately 76% aa identity.17 CD28 and CTLA-4 share approximately 30% aa identity.
After TCR ligation, B7 ligation of CD28 provides a critical costimulatory signal to the T cell. Without the CD28 signal, the T cell would either become apoptotic or anergic.1, 4, 14, 18 CD28 ligation can alter the threshold level of TCR ligation required for activation, reduce the time needed to stimulate naïve cells and enhance the magnitude of the T cell response.4, 19, 20 After ligation, CD28 cell surface expression is down-regulated. Although there has been active research in the signaling pathways activated following CD28 ligation, the actual mechanism by which CD28 signaling affects cell activation has not been fully elucidated. The observation that CD28-deficient mice are capable of suboptimal T cell activation suggests the presence of additional co-stimulatory receptors.1 CD28 also appears to possibly influence Th cell differentiation.4
In contrast to the stimulatory effects of CD28 ligation, CTLA-4 acts as an inhibitory receptor that is vital for down-modulation of the immune response.1, 4, 16, 18, 21 CTLA-4 binds both B7-1 and B7-2 with much higher affinity than CD28. CTLA-4-mediated inhibition results from the down-regulation of cytokine production, decreased IL-2 receptor expression and cell cycle arrest.1, 4, 16, 18, 21 It appears that the mechanism involves not only CTLA-4 intracellular signaling, but also B7 binding to CTLA-4 reduces B7 binding to CD28. The critical role of CTLA-4 in immune down-regulation is demonstrated in CTLA-4 deficient mice, which die by 3-5 weeks of age as a result of the development of a lymphoproliferative disease.22, 23 More recent research also suggests that CTLA-4 may play a role in T cell tolerance.4
B7-H1 (also called programmed death ligand 1, PD-L1) and PD-L2 (also called B7-DC) are two recently identified members of the B7 family. B7-H1 shares approximately 20% aa identity with B7-1 and 15% aa identity with B7-2. Human and mouse B7-H1 share approximately 69% aa identity.24, 25, 26 B7-H1 and PD-L2 share approximately 41% aa identity. Human and mouse PD-L2 share approximately 72% aa identity.1, 5, 27 B7-H1 and PD-L2 are both ligands for programmed death-1 (PD-1), a member of the CD28/CTLA-4 family.1, 5, 27, 28 Both B7-H1 and PD-L2 are widely expressed on normal tissues (e.g. liver, lung, pancreas, heart), but not on resting peripheral blood cells. Expression on monocytes and dendritic cells (DCs) is up-regulated with IFN-gamma stimulation or upon activation. B7-H1 and PD-L2 do not bind to CD28, CTLA-4 or ICOS.1, 24, 25, 26, 28
PD-1 (programmed death-1) is a member of the CD28/CTLA-4 family of Ig costimulatory immunoreceptors. PD-1 is most closely related to CTLA-4, sharing approximately 24% aa identity. Human and mouse PD-1 share approximately 63% aa identity in the extracellular domains.29 PD-1 expression is not found on unstimulated T cells, B cells or myeloid cells. PD-1 expression is up-regulated, however, on these cells after activation.1, 30, 31 To date, only B7-H1 and PD-L2 have been found to function as ligands for PD-1.
Similar to CTLA-4 ligation, PD-1 ligation appears to transmit a negative immunomodulatory signal. Ligation of PD-1 by B7-H1 or PD-L2 results in inhibition of TCR-mediated proliferation and cytokine production.1, 5, 27 In contrast to CTLA-4 deficient animals, PD-1 deficient mice display late lethality and incomplete penetrance.32, 33, 34 The observed defects are not as severe as those exhibited by CTLA-4 deficient animals. PD-1 deficient animals demonstrate defects in B cell proliferation, myeloid cell proliferation and mild defects in T cell proliferation. The PD-1 signaling pathways have not been fully elucidated and are under further investigation. The research to date, however, suggests that the B7-H1/PD-L2/PD-1 interactions are involved in the negative regulation of some immune responses and may play an important role in the regulation of peripheral tolerance.1, 5, 25, 27 Some experiments involving only B7-H1 and PD-L2 demonstrate a stimulatory rather than inhibitory effect. One of the main differences between the stimulatory and inhibitory experiments involves resting versus activated cells. There is not yet a full explanation for these contradictory results. Similar to CD28, there may be an as yet unidentified second receptor for B7-H1 and PD-L2 that delivers a stimulatory signal.1
B7-H2 (also called B7RP-1, B7h, LICOS and GL50) is another member of the growing B7 family of immune costimulatory proteins.1, 3, 5, 35, 36, 37, 38 As with other B7 family members, B7-H2 is a type I protein that is a member of the Ig superfamily. Approximately 20-30% aa identity is shared among the B7 family members. Human and mouse B7-H2 share approximately 49% aa identity.39 B7-H2 is expressed constitutively on resting B cells, DCs and at low levels on monocytes. IFN-gamma stimulation up-regulates expression on these cell types.37, 40 The mechanisms of up-regulation appear to be distinct from those for B7-1 and B7-2.40 Other data also demonstrate distinct regulation of B7-H2 expression compared to regulation of B7-1 and B7-2 expression.1 Analysis of these data suggests that B7-H2 expression could be regulated by inflammatory signals present in peripheral sites.37 B7-H2 is the ligand for ICOS and does not bind to CD28, CTLA-4 or PD-1.35
ICOS [inducible costimulator, also called AILIM (activation-inducible lymphocyte immunomodulatory molecule), CRP-1 (CD28-related protein-1) and H4] is another member of the growing CD28 family of immune costimulatory immunoreceptors.38, 41 ICOS shares approximately 39% aa similarity with CD28 and CTLA-4.42 Human and mouse ICOS share approximately 72% aa identity.42 To date, only B7-H2 has been found to be a ligand for ICOS.
ICOS is not expressed on naïve T cells. ICOS expression is rapidly up-regulated, however, after TCR ligation.38, 41, 42, 43, 44 ICOS is expressed on most CD45RO+ cells. It appears that in addition to TCR ligation, CD28 ligation may also play a role in ICOS up- regulation.44 The role of ICOS/B7-H2 interaction in T cell-dependent Bcell activation was initially suggested by the expression of ICOS on germinal center T cells.41 The differential expression of ICOS on Th1 versus Th2 cells suggests that it may also play a role in Th cell differentiation.1
Like CD28, ICOS functions as a stimulatory co-receptor.38 It appears that the role of ICOS is more important in regulating cytokine production in effector T cells and recently activated T cells.41 This contrasts with CD28, which is more important in naïve T cell activation. Further, ICOS and CD28 stimulation differ in the pattern of cytokines up-regulated. For example, CD28 up-regulates IL-2 production to a much greater level than ICOS. Additionally, ICOS up-regulates IL-10 production to a greater extent than CD28.41 Thus, there appear to be distinct, yet overlapping and possibly complementary, roles for ICOS and CD28 in T cell activation. It also appears that CTLA-4 can down-modulate ICOS-mediated costimulation as well as CD28-mediated costimulation. Thus, the identification of ICOS appears to add a new level of regulation and complexity to immune costimulation. Further, the presence of B7-H2 ligand in the periphery may also suggest roles for B7-H2/ICOS interactions in peripheral sites.1, 35, 38
B7-H3 is a type I Ig superfamily transmembrane protein that is the most recently identified member of the B7 family of immune costimulatory proteins.1, 45, 46 B7-H3 shares approximately 24% sequence identity with B7-1, 26% with B7-2, 28% with B7-H1, 29% with PD-L2 and 29% with B7-H2. Human and mouse B7-H3 share 88% aa identity.46 There are two isoforms of the human B7-H3 protein. These differ in the N-terminal sequence, exhibit some aa substitutions and, most importantly, differ in the number of extracellular Ig domains.1, 46 It appears that the isoform with four, rather than two, Ig domains is most widely expressed in human tissues and has been designated as B7-H3b.46 This form is apparently the result of a gene duplication event that occurred in humans. The mouse homolog contains two extracellular Ig domains, as is usually observed in B7 family members.
B7-H3 expression is not found on resting peripheral blood cells or DCs. Expression on these cells, however, can be up-regulated by cytokine exposure or a phorbol myristate acetate (PMA)/ionomycin combination.1, 5, 45, 46 B7-H3 is widely expressed in other tissues. B7-H3 does not bind CD28, CTLA-4, ICOS or PD-1. A recombinant B7-H3/Ig fusion protein can bind activated T cells. This suggests that a novel putative counter-receptor is present on activated T cells. This putative receptor has not been identified as of yet. Data obtained with the recombinant fusion protein demonstrates that B7-H3 can mediate T cell proliferation and IFN-gamma production. Collectively, the data suggests that B7-H3 plays a regulatory role after initial T cell priming.1, 5, 45, 46
The expansion of the B7 and CD28 families is helping to further our understanding of the costimulatory signals required by activated and primed T cells. The presence of some of these family members in peripheral tissues may also help to elucidate mechanisms of peripheral tolerance. Although there appears to be some overlap in functions, each member exhibits some unique and complementary functions. Given the critical requirement of costimulation, it may not be surprising that some redundancies exist in the system.