Unexpected role of B and T lymphocyte attenuator in sustaining cell survival during chronic allostimulation

Targeting lymphocyte activation through the lymphotoxin and LIGHT pathways

SUMMARY

Cytokines mediate key communication pathways essential for regulation of immune responses. Full activation of antigen-responding lymphocytes requires cooperating signals from the tumor necrosis factor (TNF)-related cytokines and their specific receptors. LIGHT, a lymphotoxin-beta (LTbeta)-related TNF family member, modulates T-cell activation through two receptors, the herpesvirus entry mediator (HVEM) and indirectly through the LT-beta receptor. An unexpected finding revealed a non-canonical binding site on HVEM for the immunoglobulin superfamily member, B and T lymphocyte attenuator (BTLA), and an inhibitory signaling protein suppressing T-cell activation. Thus, HVEM can act as a molecular switch between proinflammatory and inhibitory signaling. The non-canonical HVEM-BTLA pathway also acts to counter LTbetaR signaling that promotes the proliferation of antigen-presenting dendritic cells (DCs) within lymphoid tissue microenvironments. These results indicate LTbeta receptor and HVEM-BTLA pathways form an integrated signaling circuit. Targeting these cytokine pathways with specific antagonists (antibody or decoy receptor) can alter lymphocyte differentiation and activation. Alternately, agonists directed at their cell surface receptors can restore homeostasis and potentially reset immune and inflammatory processes, which may be useful in treating autoimmune and infectious diseases and cancer.

Regulating the mucosal immune system: the contrasting roles of LIGHT, HVEM, and their various partners

LIGHT and herpes virus entry mediator (HVEM) comprise a ligand-receptor pair in the tumor necrosis factor superfamily. These molecules play an important role in regulating immunity, particularly in the intestinal mucosa. LIGHT also binds the lymphotoxin beta receptor, and HVEM can act as a ligand for immunoglobulin family molecules, including B- and T-lymphocyte attenuator, which suppresses immune responses. Complexity in this pivotal system arises from several factors, including the non-monogamous pairing of ligands and receptors, and reverse signaling or the ability of some ligands to serve as receptors. As a result, recognition events in this fascinating network of interacting molecules can have pro- or anti-inflammatory consequences. Despite complexity, experiments we and others are carrying out are establishing rules for understanding when and in what cell types these molecules contribute to intestinal inflammation.

Herpesvirus entry mediator-Ig treatment during immunization aggravates rheumatoid arthritis in the collagen-induced arthritis model

Previous studies attempting to influence the severity of collagen-induced arthritis (CIA) by modulating the LIGHT (lymphotoxin-related inducible ligand that competes for glycoprotein D binding to herpesvirus entry mediator (HVEM) on T cells)/lymphotoxin pathway have yielded conflicting results. To further clarify the role of LIGHT in autoimmune arthritis, a HVEM-Ig fusion protein was used. CIA was induced in DBA1 mice, which were injected i.p. with recombinant HVEM-Ig fusion protein and control Ig at different time points. Severity of clinical arthritis and histologic joint destruction were significantly increased in HVEM-Ig-treated mice compared with control-Ig-treated mice. Collagen II-induced in vitro T cell proliferation and IFN-gamma production was augmented in mice treated with HVEM-Ig, as was the production of IgG2a anti-collagen II Ab. Accordingly, serum concentrations of IFN-gamma and IL-6 were higher in mice treated with HVEM-Ig. In conclusion, HVEM-Ig aggravates autoimmunity in collagen-induced arthritis, which is possibly mediated by interaction with B and T lymphocyte attenuator (BTLA) or CD160, despite the blockade of LIGHT. Hence, HVEM-Ig seems not to be a valid therapeutic option in autoimmune arthritis.

Unconventional ligand activation of herpesvirus entry mediator signals cell survival

The herpesvirus entry mediator (HVEM; TNFRSF14) activates NF-kappaB through the canonical TNF-related cytokine LIGHT, serving as a costimulatory pathway during activation of T cells. HVEM also functions as a ligand for the Ig superfamily members B and T lymphocyte attenuator (BTLA) and CD160, both of which limit inflammatory responses initiated by T cells. Emerging evidence indicates BTLA also promotes T cell survival, but its structural differences from LIGHT intimate BTLA is unlikely to function as an activator of HVEM. We demonstrate here that BTLA, CD160, and herpes simplex virus envelope glycoprotein D (gD) function as activating ligands for HVEM, promoting NF-kappaB activation and cell survival. Membrane-expressed BTLA and CD160, as well as soluble dimeric receptor surrogates BTLA-Fc and gD-Fc specifically activated HVEM-dependent NF-kappaB. BTLA and CD160 engagement induced recruitment of TNF receptor-associated factor 2 (TRAF2), but not TRAF3, to HVEM that specifically activated the RelA but not the RelB form of NF-kappaB in a mucosal epithelial tumor cell line. Moreover, Btla(-/-) T cells survived poorly following activation but were rescued with BTLA-Fc, indicating HVEM-BTLA bidirectional signaling may serve as a critical cell-survival system for lymphoid and epithelial cells.

B and T lymphocyte attenuator regulates T cell survival in the lung

The initiation, intensity, and duration of T cell-directed inflammatory responses are dependent upon the coordination of both activating and inhibitory signals mediated by specific receptors on the T lymphocyte. The recently described receptor, B and T lymphocyte attenuator (BTLA), has been demonstrated to have an important role in limiting the duration of inflammation in a murine model of allergic asthma. In this study, we have examined the role of BTLA on the proliferation, recruitment, and survival of T cells in response to inhaled allergen. We find that there is decreased cell death in T cells from BTLA-deficient mice, whereas proliferation and recruitment to the lungs are unchanged. Thus, the regulation of cell death through BTLA signaling is a key determinant of the inflammatory response in the lung.

PD-1/PD-L1, PD-1/PD-L2, and other co-inhibitory signaling pathways in transplantation

Transplantation of cells, tissues and vascularized solid organs is a successful therapeutic intervention for many end-stage chronic diseases. The combination of co-stimulatory blockade with the delivery of negative signals to T cells through co-inhibitory receptors would provide a robust approach to modulating T-cell receptor signaling and improving alloantigen-specific control of transplant rejection. This approach based on fundamental knowledge of APC/T-cell interactions may complement conventional therapies in the near future to reinforce long-term allograft survival, and permit minimal immunosuppression. The focus of this review was primarily on two major co-inhibitory signaling pathways, namely PD-1/PD-L1/PD-L2 and BTLA/CD160/HVEM/LIGHT that have been thoroughly characterized in murine models of transplantation using genetically modified mice, specific monoclonal antibodies and fusion proteins.

The TNF receptor and Ig superfamily members form an integrated signaling circuit controlling dendritic cell homeostasis

Dendritic cells (DC) constitute the most potent antigen presenting cells of the immune system, playing a key role bridging innate and adaptive immune responses. Specialized DC subsets differ depending on their origin, tissue location and the influence of trophic factors, the latter remain to be fully understood. Myeloid-associated lymphotoxin-beta receptor (LTbetaR) signaling is required for the local proliferation of lymphoid tissue DC. This review focuses on the LTbetaR signaling cascade as a crucial positive trophic signal in the homeostasis of DC subsets. The noncanonical coreceptor pathway comprised of the immunoglobulin (Ig) superfamily member, B and T lymphocyte attenuator (BTLA) and TNFR superfamily member, herpesvirus entry mediator (HVEM) counter regulates the trophic signaling by LTbetaR. Together both pathways form an integrated signaling circuit achieving homeostasis of DC subsets.

A crucial role for HVEM and BTLA in preventing intestinal inflammation

The interaction between the tumor necrosis factor (TNF) family member LIGHT and the TNF family receptor herpes virus entry mediator (HVEM) co-stimulates T cells and promotes inflammation. However, HVEM also triggers inhibitory signals by acting as a ligand that binds to B and T lymphocyte attenuator (BTLA), an immunoglobulin super family member. The contribution of HVEM interacting with these two binding partners in inflammatory processes remains unknown. In this study, we investigated the role of HVEM in the development of colitis induced by the transfer of CD4(+)CD45RB(high) T cells into recombination activating gene (Rag)(-/-) mice. Although the absence of HVEM on the donor T cells led to a slight decrease in pathogenesis, surprisingly, the absence of HVEM in the Rag(-/-) recipients led to the opposite effect, a dramatic acceleration of intestinal inflammation. Furthermore, the critical role of HVEM in preventing colitis acceleration mainly involved HVEM expression by radioresistant cells in the Rag(-/-) recipients interacting with BTLA. Our experiments emphasize the antiinflammatory role of HVEM and the importance of HVEM expression by innate immune cells in preventing runaway inflammation in the intestine.

Ligation of B and T lymphocyte attenuator prevents the genesis of experimental cerebral malaria

B and T lymphocyte attenuator (BTLA; CD272) is a coinhibitory receptor that is predominantly expressed on T and B cells and dampens T cell activation. In this study, we analyzed the function of BTLA during infection with Plasmodium berghei ANKA. Infection of C57BL/6 mice with this strain leads to sequestration of leukocytes in brain capillaries that is associated with a pathology resembling cerebral malaria in humans. During the course of infection, we found an induction of BTLA in several organs, which was either due to up-regulation of BTLA expression on T cells in the spleen or due to infiltration of BTLA-expressing T cells into the brain. In the brain, we observed a marked induction of BTLA and its ligand herpesvirus entry mediator during cerebral malaria, which was accompanied by an accumulation of predominantly CD8+ T cells, but also CD4+ T cells. Application of an agonistic anti-BTLA mAb caused a significantly reduced incidence of cerebral malaria compared with control mice. Treatment with this Ab also led to a decreased number of T cells that were sequestered in the brain of P. berghei ANKA-infected mice. Our findings indicate that BTLA-herpesvirus entry mediator interactions are functionally involved in T cell regulation during P. berghei ANKA infection of mice and that BTLA is a potential target for therapeutic interventions in severe malaria.

B and T lymphocyte attenuator interacts with CD3zeta and inhibits tyrosine phosphorylation of TCRzeta complex during T-cell activation

B and T lymphocyte attenuator (BTLA) is an important negative regulator of T-cell activation. T-cell activation involves partitioning of receptors into discrete membrane compartments known as lipid rafts and the formation of an immunological synapse (IS) between the T cell and antigen-presenting cell (APC). Here we show that after T-cell stimulation, BTLA co-clusters with the CD3zeta and is then involved in IS, as determined by a two-photon microscope. BTLA can interact with the phosphorylated form of T-cell receptor (TCR) within the lipid raft, which is associated with the T-cell signaling complex. Coligation of BTLA with the TCR significantly decreased the amount of phosphorylated TCR-related signal accumulation in the lipid raft during T-cell activation. These results suggest that BTLA functions to regulate T-cell signaling by controlling the phosphorylated form of TCRzeta accumulation in the lipid raft.