Yonglian Sun

Administration of Agonistic Anti-4-1BB Monoclonal Antibody Leads to the Amelioration of Experimental Autoimmune Encephalomyelitis

4-1BB, a member of the TNFR superfamily, is a costimulatory receptor primarily expressed on activated T cells. It has been shown that the administration of agonistic anti-4-1BB Abs enhances tumor immunity and allogenic immune responses. Paradoxically, we found that the administration of an agonistic anti-4-1BB mAb (2A) dramatically reduced the incidence and severity of experimental autoimmune encephalomyelitis (EAE). Adoptive transfer of T cells from such treated mice failed to induce EAE, whereas anti-4-1BB treatment following adoptive transfer of encephalitogenic T cells did not prevent EAE pathogenesis. These results suggest that anti-4-1BB treatment during the induction phase inhibits autoreactive T cell immune responses rather than preventing T cell trafficking into the CNS. This was substantiated by the observations that draining lymph node cells from anti-4-1BB-treated mice failed to respond to Ag stimulation in vitro. In addition, we found that such treatment initially promotes the activation and proliferation of Ag-specific CD4+ T cells but subsequently increases their probability of undergoing activation-induced cell death, thereby inhibiting effector T cell responses. More importantly, 2A treatment also inhibits the relapse of EAE in a clinically relevant murine model of multiple sclerosis. This study indicates that the agonistic Ab against 4-1BB can potentially be used as a novel immunotherapeutic agent for treating autoimmune diseases.

Local expression of B7-H1 promotes organ-specific autoimmunity and transplant rejection

A number of studies have suggested B7-H1, a B7 family member, inhibits T cell responses. Therefore, its expression on nonlymphoid tissues has been proposed to prevent T cell-mediated tissue destruction. To test this hypothesis, we generated transgenic mice that expressed B7-H1 on pancreatic islet beta cells. Surprisingly, we observed accelerated rejection of transplanted allogeneic B7-H1-expressing islet beta cells. Furthermore, transgenic B7-H1 expression broke immune tolerance, as some of the mice spontaneously developed T cell-dependent autoimmune diabetes. In addition, B7-H1 expression increased CD8+ T cell proliferation and promoted autoimmunity induction in a T cell adoptive transfer model of diabetes. Consistent with these findings, B7-H1.Ig fusion protein augmented naive T cell priming both in vitro and in vivo. Our results demonstrate that B7-H1 can provide positive costimulation for naive T cells to promote allograft rejection and autoimmune disease pathogenesis.

The role of herpesvirus entry mediator as a negative regulator of T cell-mediated responses

Herpesvirus entry mediator (HVEM), a TNF receptor superfamily member, has been previously described as a T cell costimulatory receptor. Surprisingly, HVEM-/- T cells showed enhanced responses to in vitro concanavalin A (ConA) stimulation when compared with WT T cells. Consistent with these findings, HVEM-/- mice exhibited increased morbidity and mortality as compared with WT mice in a model of ConA-mediated T cell-dependent autoimmune hepatitis. HVEM-/- mice produced higher levels of multiple cytokines, which were dependent on the presence of CD4+ T cells. Furthermore, HVEM-/- mice were more susceptible to MOG peptide-induced experimental autoimmune encephalopathy, and they showed increased T cell proliferation and cytokine production in response to antigen-specific challenge. Taken together, our data revealed an unexpected regulatory role of HVEM in T cell-mediated immune responses and autoimmune diseases.

Dysregulated LIGHT expression on T cells mediates intestinal inflammation and contributes to IgA nephropathy

Whether and how T cells contribute to the pathogenesis of immunoglobulin A nephropathy (IgAN) has not been well defined. Here, we explore a murine model that spontaneously develops T cell-mediated intestinal inflammation accompanied by pathological features similar to those of human IgAN. Intestinal inflammation mediated by LIGHT, a ligand for lymphotoxin beta receptor (LTbetaR), not only stimulates IgA overproduction in the gut but also results in defective IgA transportation into the gut lumen, causing a dramatic increase in serum polymeric IgA. Engagement of LTbetaR by LIGHT is essential for both intestinal inflammation and hyperserum IgA syndrome in our LIGHT transgenic model. Impressively, the majority of patients with inflammatory bowel disease showed increased IgA-producing cells in the gut, elevated serum IgA levels, and severe hematuria, a hallmark of IgAN. These observations indicate the critical contributions of dysregulated LIGHT expression and intestinal inflammation to the pathogenesis of IgAN.

The complementation of lymphotoxin deficiency with LIGHT, a newly discovered TNF family member, for the restoration of secondary lymphoid structure and function.

Highly organized lymphoid structures provide the intricate microenvironment essential for the mediation of the effective immune responses. Compared with lymphotoxin β knockout mice (LTβ–/–), LTβ receptor knockout (LTβR–/–) mice present with more severely disorganized splenic structures, suggesting the potential involvement of another ligand. LIGHT, a newly identified TNF family member, is a costimulatory molecule for T cells and binds to LTβR and herpes virus entry mediator (HVEM) in vitro. Here, we show that the complementation of LTα–/– mice with a LIGHT transgene (LIGHT Tg/LTα–/–) leads to the restoration of secondary lymphoid tissue chemokine and T/B cell zone segregation. LIGHT Tg/LTα–/– mice also preserve dendritic cells, follicular dendritic cell networks, and germinal centers, though not the marginal zone. Consequently, IgG responses to soluble, but not particulate, antigens are restored, confirming the role of primary follicle and marginal zone in the responses to soluble and particulate antigens. The failure of the LIGHT transgene to rescue the defective splenic structures in LTβR–/– mice demonstrates that LIGHT can interact with LTβR in vivo. More severely disorganized splenic structures developed after blockade of endogenous LIGHT in LTβ–/– mice. These findings uncover the potential interaction between LIGHT and one of its receptors, LTβR, in supporting even in the absence of LT the development and maintenance of lymphoid microenvironment.

Signal Via Lymphotoxin-βR on Bone Marrow Stromal Cells Is Required for an Early Checkpoint of NK Cell Development

NK cells play an important role in the immune system but the cellular and molecular requirements for their early development are poorly understood. Lymphotoxin-α (LTα)−/− and LTβR−/− mice show a severe systemic reduction of NK cells, which provides an excellent model to study NK cell development. In this study, we show that the bone marrow (BM) or fetal liver cells from LTα−/− or LTβR−/− mice efficiently develop into mature NK cells in the presence of stromal cells from wild-type mice but not from LTα−/− or LTβR−/− mice. Direct activation of LTβR-expressing BM stromal cells is shown to promote to early NK cell development in vitro. Furthermore, the blockade of the interaction between LT and LTβR in adult wild-type mice by administration of LTβR-Ig impairs the development of NK cells in vivo. Together, these results indicate that the signal via LTβR on BM stromal cells by membrane LT is an important pathway for early NK cell development.

Progressive Depletion of Peripheral B Lymphocytes in 4-1BB (CD137) Ligand/I-Eα-Transgenic Mice

Interaction of 4-1BB (CD137) and its ligand (4-1BBL) is thought to positively regulate cell-mediated and humoral immune responses. We have prepared transgenic mouse strains that express 4-1BBL cDNA under the control of MHC class II I-Eα promoter. The 4-1BBL-transgenic mice show progressive splenomegaly and selective depletion of B220+ B cells accompanied with low levels of circulating IgG and defective humoral responses to Ag challenge. In addition, splenocytes from the transgenic mice fail to provide stimulation for allogeneic T cells in both lymphoproliferative and CTL responses in vitro, whereas their T cells remain functionally normal. Our results reveal unexpected functions of 4-1BBL in the regulation of humoral immune responses and Ag presentation.

Complementary Effects of TNF and Lymphotoxin on the Formation of Germinal Center and Follicular Dendritic Cells

The formation of germinal centers (GC) around follicular dendritic cells (FDC) is a critical step in the humoral immune responses that depends on the cooperative effects of B cells and T cells. Mice deficient in either TNF or lymphotoxin (LT) fail to form both GC and FDC network in B cell follicles. To test a potential complementary effect of TNF and LT, a mixture of bone marrow cells from TNF−/− mice and LTα−/− mice was transferred into irradiated LTα−/− mice or TNF−/− mice. Interestingly, the formation of both GC and FDC clusters in B cell follicles was restored in such chimeric mice, suggesting that TNF and LT from different cells could complement one another. To identify the exact contributions of each subset to the complementary effect of TNF and LT, different sources of T and B cells from LTα−/− mice or TNF−/− mice were used for reconstitution. Our study demonstrates that either T or B cell-derived TNF is sufficient to restore FDC/GC in the presence of LT-expressing B cells. However, TNF itself is not required for GC reactions if the FDC network is already intact. Thus, the development and maintenance of these lymphoid structures depend on a delicate interaction between TNF and LT from different subsets of lymphocytes.

B and T Lymphocyte Attenuator Tempers Early Infection Immunity

Coinhibitory pathways are thought to act in later stages of an adaptive immune response, but whether coinhibition contributes to early innate immunity is unclear. We show that engagement of the newly discovered coinhibitory receptor B and T lymphocyte attenuator (BTLA) by herpesvirus entry mediator (HVEM) is critical for negatively regulating early host immunity against intracellular bacteria. Both HVEM−/− and BTLA−/−, but not LIGHT−/−, mice are more resistant to listeriosis compared with wild-type mice, and blockade of the BTLA pathway promotes, while engagement inhibits, early bacterial clearance. Differences in bacterial clearance were seen as early as 1 day postinfection, implicating the initial innate response. Therefore, innate cell function in BTLA−/− mice was studied. We show that innate cells from BTLA−/− mice secrete significantly more proinflammatory cytokines upon stimulation with heat-killed Listeria. These results provide the first evidence that a coinhibitory pathway plays a critical role in regulating early host innate immunity against infection.

Cutting Edge: B and T Lymphocyte Attenuator Signaling on NKT Cells Inhibits Cytokine Release and Tissue Injury in Early Immune Responses

The role of coinhibition in an immune response is thought to be critical for the contraction of an adaptive immune response in its waning phases. We present evidence that B and T lymphocyte attenuator (BTLA) coinhibitory signaling is required to temper early inflammation. Using an in vivo Con A challenge model of acute hepatitis, we observed reduced survival and increased early serum cytokine secretion in BTLA−/− mice as compared with wild-type mice. In vitro, liver mononuclear cells from BTLA−/− mice are hyperresponsive to anti-CD3, Con A, and α-galactosylceramide stimulation and secrete higher levels of TNF-α, IFN-γ, IL-2, and IL-4. We found this was in part due to negative regulation of NKT cells by BTLA, as early cytokine inhibition from whole liver mononuclear cells or purified NKT cells depends upon BTLA signaling. Overall, our data demonstrate that coinhibition is active in early immune responses through BTLA regulation of NKT cells.