The T cell immunoglobulin mucin (TIM) proteins regulate T cell activation and tolerance. Here we showed that TIM-4 is expressed on human and mouse macrophages and dendritic cells, and both TIM-4 and TIM-1 specifically bound phosphatidylserine (PS) on the surface of apoptotic cells but not any other phospholipid tested. TIM-4(+) peritoneal macrophages, TIM-1(+) kidney cells, and TIM-4- or TIM-1-transfected cells efficiently phagocytosed apoptotic cells, and phagocytosis could be blocked by TIM-4 or TIM-1 monoclonal antibodies. Mutations in the unique cavity of TIM-4 eliminated PS binding and phagocytosis. TIM-4 mAbs that blocked PS binding and phagocytosis mapped to epitopes in this binding cavity. These results show that TIM-4 and TIM-1 are immunologically restricted members of the group of receptors whose recognition of PS is critical for the efficient clearance of apoptotic cells and prevention of autoimmunity.
Many chronic viral infections are marked by pathogen persistence and a generalized immunosuppression. The exact mechanisms by which this occurs are still unknown. Using a mouse model of persistent lymphocytic choriomeningitis virus (LCMV) infection, we demonstrate viral targeting of fibroblastic reticular cells (FRC) in the lymphoid organs. The FRC stromal networks are critical for proper lymphoid architecture and function. High numbers of FRC were infected by LCMV clone 13, which causes a chronic infection, whereas few were infected by the acute strain, LCMV Armstrong. The function of the FRC conduit network was altered after clone 13 infection by the action of CD8(+) T cells. Importantly, expression of the inhibitory programmed death ligand 1, which was up-regulated on FRC after infection, reduced early CD8(+) T cell-mediated immunopathology and prevented destruction of the FRC architecture in the spleen. Together, this reveals an important tropism during a persistent viral infection. These data also suggest that the inhibitory PD-1 pathway, which likely evolved to prevent excessive immunopathology, may contribute to viral persistence in FRC during chronic infection.
MRL/MpJ-Tnfrsf6lpr (MRL-Faslpr) mice develop a spontaneous T cell-dependent autoimmune disease that shares features with human lupus, including fatal nephritis, systemic pathology, and autoantibodies (autoAb). The inducible co-stimulator (ICOS) is upregulated on activated T cells and modulates T cell-mediated responses. To investigate whether ICOS has an essential role in regulating autoimmune lupus nephritis and the systemic illness in MRL-Faslpr mice, ICOS null (-/-) MRL Faslpr and ICOS intact (+/+) MRL-Faslpr strains (wild-type [WT]) were generated and compared. It was determined that in ICOS-/- MRL-Faslpr as compared with the WT strain, (1) there is a significant reduction in circulating IgG and double-stranded DNA autoantibody isotype titers, and (2) there is an amplification of the frequency of intrarenal T cells generating IFN-gamma and TNF-alpha in ICOS-/- versus WT mice. Of note, eliminating ICOS in the MRL-Faslpr strain does not alter renal pathology or function. Despite the reduction in circulating IgG and autoantibody isotypes (G1, G2a, and G2b), the amount of these IgG isotypes depositing in kidneys is similar. Furthermore, the systemic illness (skin, salivary and lacrimal glands, lungs, lymphadenopathy, and splenomegaly) is equivalent in ICOS-/- MRL-Faslpr and WT mice. These findings highlight the danger of relying on individual parameters, such as quantitative serum Ig levels and T cell functions, as prognostic indicators of lupus.
2B4 belongs to the CD2 subset of the IgG family of receptors. Members in this family have been shown to function as coreceptors via homophilic or heterophilic interactions. Both 2B4 and CD2 bind to CD48, another member of this family. Because all 3 molecules are expressed on natural killer (NK) cells, it raises a possibility that the binding of 2B4 and CD2 to CD48 among NK cells may have functional consequences. Using specific monoclonal antibodies and gene-deficient NK cells, we found that 2B4/CD48, but not CD2/CD48, interaction is essential for IL-2-driven expansion and activation of murine NK cells. In the absence of 2B4/CD48 interaction, NK cytotoxicity and IFN-gamma secretion on tumor target exposure is severely impaired. Impaired activation of NK cells in 2B4-deficient mice was also demonstrated by poor NK-mediated clearance of syngeneic tumor cells in these mice. Functional impairment of NK cells in the absence of 2B4/CD48 interactions was accompanied by defective calcium signaling, suggesting that the early signaling pathway of NK receptors is inhibited. Finally, homotypic interactions among NK cells through 2B4/CD48 was visualized by specific localization of GFP-tagged 2B4 onto NK-NK conjugation sites. Thus, these data identify a novel mechanism whereby NK effector function is regulated via homotypic 2B4/CD48 interactions.
Naturally occurring CD4+ regulatory T cells (T(R)) that express CD25 and the transcription factor FoxP3 play a key role in immune homeostasis, preventing immune pathological responses to self and foreign Ags. CTLA-4 is expressed by a high percentage of these cells, and is often considered as a marker for T(R) in experimental and clinical analysis. However, it has not yet been proven that CTLA-4 has a direct role in T(R) function. In this study, using a T cell-mediated colitis model, we demonstrate that anti-CTLA-4 mAb treatment inhibits T(R) function in vivo via direct effects on CTLA-4-expressing T(R), and not via hyperactivation of colitogenic effector T cells. Although anti-CTLA-4 mAb treatment completely inhibits T(R) function, it does not reduce T(R) numbers or their homing to the GALT, suggesting the Ab mediates its function by blockade of a signal required for T(R) activity. In contrast to the striking effect of the Ab, CTLA-4-deficient mice can produce functional T(R), suggesting that under some circumstances other immune regulatory mechanisms, including the production of IL-10, are able to compensate for the loss of the CTLA-4-mediated pathway. This study provides direct evidence that CTLA-4 has a specific, nonredundant role in the function of normal T(R). This role has to be taken into account when targeting CTLA-4 for therapeutic purposes, as such a strategy will not only boost effector T cell responses, but might also break T(R)-mediated self-tolerance.
BACKGROUND: T-cell-mediated immunity contributes to the pathogenesis of atherosclerosis, but little is known about how these responses are regulated. We explored the influence of the inducible costimulatory molecule (ICOS) on atherosclerosis and associated immune responses.
METHODS AND RESULTS: Bone marrow chimeras were generated by transplanting ICOS-deficient or wild-type bone marrow into irradiated atherosclerosis-prone, LDR receptor-deficient mice, and the chimeric mice were fed a high-cholesterol diet for 8 weeks. Compared with controls, mice transplanted with ICOS-deficient marrow had a 43% increase in the atherosclerotic burden, and importantly, their lesions had a 3-fold increase in CD4+ T cells, as well as increased macrophage, smooth muscle cell, and collagen content. CD4+ T cells from ICOS-deficient chimeras proliferated more and secreted more interferon-gamma and tumor necrosis factor-alpha than T cells from control mice, which suggests a lack of regulation. FoxP3+ regulatory T cells (Treg) were found to constitutively express high ICOS levels, which suggests a role for ICOS in Treg function. ICOS-deficient mice had decreased numbers of FoxP3+ Treg and impaired in vitro Treg suppressive function compared with control mice.
CONCLUSIONS: ICOS has a key role in regulation of atherosclerosis, through its effect on regulatory T-cell responses.
To compare the roles of programmed death 1 ligand 1 (PD-L1) and PD-L2 in regulating immunity to infection, we investigated responses of mice lacking PD-L1 or PD-L2 to infection with Leishmania mexicana. PD-L1(-/-) and PD-L2(-/-) mice exhibited distinct disease outcomes following infection with L. mexicana. In comparison to susceptible WT mice, PD-L1(-/-) mice showed resistance to L. mexicana, as demonstrated by reduced growth of cutaneous lesions and parasite burden. In contrast, PD-L2(-/-) mice developed exacerbated disease with increased parasite burden. Host resistance to L. mexicana is partly associated with the development of a Th1 response and down-regulation of the Th2 response. Both PD-L1(-/-) and PD-L2(-/-) mice produced levels of IFN-gamma similar to WT mice. However, the development of IL-4-producing cells was reduced in PD-L1(-/-) mice, demonstrating a role for PD-L1 in regulating Th cell differentiation. This inadequate Th2 response may explain the increased resistance of PD-L1(-/-) mice. Although no alterations in Th1/Th2 skewing were observed in PD-L2(-/-) mice, PD-L2(-/-) mice exhibited a marked increase in L. mexicana-specific antibody production. Increased Leishmania-specific IgG production may suppress the healing response through FcgammaR ligation on macrophages. Taken together, our results demonstrate that PD-L1 and PD-L2 have distinct roles in regulating the immune response to L. mexicana.
The programmed death (PD)-1-PD-1 ligand (PD-L) pathway, which is part of the B7-CD28 family, consists of the PD-1 receptor and its two ligands PD-L1 and PD-L2. Engagement of PD-1 by its ligands inhibits immune responses, and recent work has shown that PD-1 is highly expressed on exhausted T cells during chronic lymphocytic choriomeningitis virus (LCMV) infection in mice. Blockade of this pathway reinvigorates the exhausted T cells, allowing them to expand and produce effector cytokines, raising the issue of whether this pathway has been exploited by a variety of viruses during chronic infection. New studies now extend these observations to HIV infection and human disease.
Functional impairment of antigen-specific T cells is a defining characteristic of many chronic infections, but the underlying mechanisms of T-cell dysfunction are not well understood. To address this question, we analysed genes expressed in functionally impaired virus-specific CD8 T cells present in mice chronically infected with lymphocytic choriomeningitis virus (LCMV), and compared these with the gene profile of functional memory CD8 T cells. Here we report that PD-1 (programmed death 1; also known as Pdcd1) was selectively upregulated by the exhausted T cells, and that in vivo administration of antibodies that blocked the interaction of this inhibitory receptor with its ligand, PD-L1 (also known as B7-H1), enhanced T-cell responses. Notably, we found that even in persistently infected mice that were lacking CD4 T-cell help, blockade of the PD-1/PD-L1 inhibitory pathway had a beneficial effect on the 'helpless' CD8 T cells, restoring their ability to undergo proliferation, secrete cytokines, kill infected cells and decrease viral load. Blockade of the CTLA-4 (cytotoxic T-lymphocyte-associated protein 4) inhibitory pathway had no effect on either T-cell function or viral control. These studies identify a specific mechanism of T-cell exhaustion and define a potentially effective immunological strategy for the treatment of chronic viral infections.
Programmed death 1 (PD-1), an inhibitory receptor expressed on activated lymphocytes, regulates tolerance and autoimmunity. PD-1 has two ligands: PD-1 ligand 1 (PD-L1), which is expressed broadly on hematopoietic and parenchymal cells, including pancreatic islet cells; and PD-L2, which is restricted to macrophages and dendritic cells. To investigate whether PD-L1 and PD-L2 have synergistic or unique roles in regulating T cell activation and tolerance, we generated mice lacking PD-L1 and PD-L2 (PD-L1/PD-L2(-/-) mice) and compared them to mice lacking either PD-L. PD-L1 and PD-L2 have overlapping functions in inhibiting interleukin-2 and interferon-gamma production during T cell activation. However, PD-L1 has a unique and critical role in controlling self-reactive T cells in the pancreas. Our studies with bone marrow chimeras demonstrate that PD-L1/PD-L2 expression only on antigen-presenting cells is insufficient to prevent the early onset diabetes that develops in PD-L1/PD-L2(-/-) non-obese diabetic mice. PD-L1 expression in islets protects against immunopathology after transplantation of syngeneic islets into diabetic recipients. PD-L1 inhibits pathogenic self-reactive CD4+ T cell-mediated tissue destruction and effector cytokine production. These data provide evidence that PD-L1 expression on parenchymal cells rather than hematopoietic cells protects against autoimmune diabetes and point to a novel role for PD-1-PD-L1 interactions in mediating tissue tolerance.
The inability to reproducibly induce robust and durable transplant tolerance using CD28-B7 pathway blockade is in part related to the persistence of alloreactive effector/memory CD8(+) T cells that are less dependent on this pathway for their cellular activation. We studied the role of the novel T cell costimulatory pathway, CD27-CD70, in alloimmunity in the presence and absence of CD28-B7 signaling. CD70 blockade prolonged survival of fully mismatched vascularized cardiac allografts in wild-type murine recipients, and in CD28-deficient mice induced long-term survival while significantly preventing the development of chronic allograft vasculopathy. CD70 blockade had little effect on CD4(+) T cell function but prevented CD8(+) T cell-mediated rejection, inhibited the proliferation and activation of effector CD8(+) T cells, and diminished the expansion of effector and memory CD8(+) T cells in vivo. Thus, the CD27-CD70 pathway is critical for CD28-independent effector/memory CD8(+) alloreactive T cell activation in vivo. These novel findings have important implications for the development of transplantation tolerance-inducing strategies in primates and humans, in which CD8(+) T cell depletion is currently mandatory.
Inducible costimulator (ICOS), a CD28/cytotoxic T lymphocyte antigen 4 (CTLA-4) family member, is expressed on activated T cells. ICOS ligand, a B7 family member, is constitutively expressed on B cells, macrophages, and dendritic cells and is up-regulated on antigen-presenting cells (APCs) and some nonlymphoid tissues by tumor necrosis factor alpha (TNFalpha) or lipopolysaccharide (LPS). Thus, ICOS: ICOS ligand (ICOSL) blockade could reduce alloreactive T cell-APC interactions responsible for graft-versus-host disease (GVHD) and bone marrow (BM) graft rejection. ICOS blockade, achieved with ICOS-/- mice or anti-ICOS monoclonal antibody (mAb) administration, resulted in significant inhibition of GVHD in multiple strain combinations whether mediated by CD4+ and/or CD8+ T cells, alloantigen-specific T-cell receptor (TCR) transgenic (Tg) T cells, or CD28-, T helper 1 (Th1)-, or Th2-deficient T cells. Anti-ICOS significantly delayed GVHD mortality even when mAb infusions were delayed until day 5 after transplantation. ICOS blockade reduced the number of alloantigen-specific effector cells but did not prevent their activation. Imaging of green fluorescent protein-positive (GFP+) effectors indicated that ICOS blockade inhibited expansion of GVHD-causing effector T cells in secondary lymphoid and GVHD target organs. Engraftment rates were significantly higher in ICOS-/- versus wild-type (WT) mice receiving allogeneic BM, and ICOS blockade significantly inhibited expansion of host antidonor alloantigen-specific BM graft-rejecting T cells. These data suggest that the ICOS pathway may be a beneficial therapeutic target for GVHD inhibition, GVHD therapy, and BM graft promotion.
Negative costimulatory signals mediated via cell surface molecules such as CTLA-4 and programmed death 1 (PD-1) play a critical role in down-modulating immune responses and maintaining peripheral tolerance. However, their role in alloimmune responses remains unclear. This study examined the role of these inhibitory pathways in regulating CD28-dependent and CD28-independent CD4 and CD8 alloreactive T cells in vivo. CTLA-4 blockade accelerated graft rejection in C57BL/6 wild-type recipients and in a proportion of CD4(-/-) but not CD8(-/-) recipients of BALB/c hearts. The same treatment led to prompt rejection in CD28(-/-) and a smaller proportion of CD4(-/-)CD28(-/-) mice with no effect in CD8(-/-)CD28(-/-) recipients. These results indicate that the CTLA-4:B7 pathway provides a negative signal to alloreactive CD8(+) T cells, particularly in the presence of CD28 costimulation. In contrast, PD-1 blockade led to accelerated rejection of heart allografts only in CD28(-/-) and CD8(-/-)CD28(-/-) recipients. Interestingly, PD-1 ligand (PD-L1) blockade led to accelerated rejection in wild-type mice and in all recipients lacking CD28 costimulation. This effect was accompanied by expansion of IFN-gamma-producing alloreactive T cells and enhanced generation of effector T cells in rejecting allograft recipients. Thus, the PD-1:PD-L1 pathway down-regulates alloreactive CD4 T cells, particularly in the absence of CD28 costimulation. The differential effects of PD-1 vs PD-L1 blockade support the possible existence of a new receptor other than PD-1 for negative signaling through PD-L1. Furthermore, PD-1:PD-L1 pathway can regulate alloimmune responses independent of an intact CD28/CTLA-4:B7 pathway. Harnessing physiological mechanisms that regulate alloimmunity should lead to development of novel strategies to induce durable and reproducible transplantation tolerance.
The discovery of new functions for the original B7 family members, together with the identification of additional B7 and CD28 family members, have revealed new ways in which the B7:CD28 family regulates T cell activation and tolerance. B7-1/B7-2:CD28 interactions not only promote initial T cell activation but also regulate self-tolerance by supporting CD4+CD25+ T regulatory cell homeostasis. CTLA-4 can exert its inhibitory effects in both B7-1/B7-2 dependent and independent fashions. B7-1 and B7-2 can signal bidirectionally by engaging CD28 and CTLA-4 on T cells and by delivering signals into B7-expressing cells. The five new B7 family members, ICOS ligand, PD-L1 (B7-H1), PD-L2 (B7-DC), B7-H3, and B7-H4 (B7x/B7-S1) are expressed on professional antigen-presenting cells as well as on cells within nonlymphoid organs, providing new means for regulating T cell activation and tolerance in peripheral tissues. The new CD28 families members, ICOS, PD-1, and BTLA, are inducibly expressed on T cells, and they have important roles in regulating previously activated T cells. PD-1 and BTLA also are expressed on B cells and may have broader immunoregulatory functions. The ICOS:ICOSL pathway appears to be particularly important for stimulating effector T cell responses and T cell-dependent B cell responses, but it also has an important role in regulating T cell tolerance. In addition, the PD-1:PD-L1/PD-L2 pathway plays a critical role in regulating T cell activation and tolerance. In this review, we revisit the roles of the B7:CD28 family members in regulating immune responses, and we discuss their therapeutic potential.
Host defense is dependent on the appropriate induction of immune responses. A central concept in immunology is the ability of the immune system to differentiate foreign from self-antigens. The failure of the immune response to recognize foreign pathogens can result in infection and disease in the host. The inappropriate response of the immune system to self-antigens is equally problematic, leading to autoimmune disease. Central and peripheral tolerance mechanisms control self-reactive T-cell responses and protect peripheral tissues from autoimmune attack. This review examines the roles of B7/CD28 family members, which can augment or antagonize T-cell receptor signaling, in the regulation of central and peripheral T-cell tolerance. We also discuss how B7/CD28 pathways influence both T-cell-intrinsic and -extrinsic mechanisms of regulation.
Both CD28 and ICOS are important costimulatory molecules that promote Ag-specific cellular and humoral immune reactions. Whereas CD28 is generally thought to be the most important molecule in the initiation of a T cell response, ICOS is considered to act during the effector phase. We have investigated the contribution of ICOS to T cell responses in the absence of CTLA-4-mediated inhibition. Mice lacking CTLA-4, which show spontaneous CD28-mediated CD4(+) T cell activation, expansion and differentiation, were treated with antagonistic alphaICOS antibodies. Blocking the interaction between ICOS and its ligand B7RP-1 significantly reduced this aberrant T cell activation and caused a reduction in T cell numbers. In vitro analysis of CD4(+) T cells from treated mice revealed that ICOS blockade significantly reduced Th1 differentiation, while Th2 differentiation was only moderately inhibited. Further in vitro stimulation experiments demonstrated that ICOS is able to induce proliferation of murine CD4(+) and CD8(+) T cells but only in the presence of IL-2. These results indicate that ICOS is not only important for T cell effector function but also contributes to the expansion phase of a T cell response in the presence of CD28 signaling.
The ICOS molecule stimulates production of the immunoregulatory cytokine IL-10, suggesting an important role for ICOS in controlling IL-10-producing regulatory T cells and peripheral T cell tolerance. In this study we investigate whether ICOS is required for development of oral, nasal, and high dose i.v. tolerance. Oral administration of encephalitogenic myelin oligodendrocyte glycoprotein (MOG) 35-55 peptide to ICOS-deficient (ICOS-/-) mice did not inhibit experimental autoimmune encephalomyelitis (EAE), T cell proliferation, or IFN-gamma production, in striking contrast to wild-type mice. Similarly, intranasal administration of MOG(35-55) before EAE induction suppressed EAE and T cell responses in wild-type, but not in ICOS-/-, mice. In contrast, ICOS-/- mice were as susceptible as wild-type mice to high dose tolerance. These results indicate that ICOS plays an essential and specific role in mucosal tolerance and that distinct costimulatory pathways differentially regulate different forms of peripheral tolerance. Surprisingly, CD4+ cells from MOG-fed wild-type and ICOS-/- mice could transfer suppression to wild-type recipients, indicating that functional regulatory CD4+ cells can develop in the absence of ICOS. However, CD4+ T cells from MOG-fed wild-type mice could not transfer suppression to ICOS-/- recipients, suggesting that ICOS may have a key role in controlling the effector functions of regulatory T cells. These results suggest that stimulating ICOS may provide an effective therapeutic approach for promoting mucosal tolerance.
Although previous studies have shown that altered B7 costimulation plays a critical role in UV irradiation-induced regulation of immunity, the individual roles of the B7 receptors (CD28 and CTLA-4) or the B7 family members (CD80 and CD86) have not been explored. Thus, we investigated CTLA-4 signaling during photocarcinogenesis of chronically UV-B-exposed mice using an antagonistic anti-CTLA-4 Ab. Anti-CTLA-4-treated mice developed significantly fewer UV-induced tumors. Moreover, anti-CTLA-4 treatment induced long-lasting protective immunity because progressively growing UV tumors inoculated into anti-CTLA-4- and UV-treated mice that had not developed tumors were rejected. Next, we used mice deficient for CD80, CD86, or both in photocarcinogenesis studies to assess the relative contributions of these CTLA-4 ligands. Double-deficient mice showed significantly reduced UV-induced skin tumor development, whereas CD86(-/-) mice produced skin cancer earlier compared with CD80(-/-) and control mice. The growth of UV-induced tumors appears to be controlled by UV-induced suppressor T cells, because CD80(-/-)/CD86(-/-) mice had strongly reduced numbers of UV-induced CD4(+)CD25(+) suppressor T cells. In vitro, CTLA-4 blockade inhibited the suppressor activity of UV-induced CD4(+)CD25(+) T cells, suggesting that reduced photocarcinogenesis might be due to decreased numbers or function of suppressor T cells. Together, these data indicate that blocking CD80/86-CTLA-4 signaling induced immune protection against the development of UV-induced skin tumors. Furthermore, CD86-mediated costimulation appears to play a more critical role in the protection against photocarcinogenesis than CD80.