Lymphoma mutations in HVEM (TNFRSF14) disrupt bi-directional activation of BTLA and CD160 singaling within the tumor microenvironment

Herpesvirus entry mediator (HVEM; TNFRSF14) is frequently mutated in human cancer with a high degree of mutations in follicular and diffuse large B cell lymphoma. In lymphocytes, HVEM receptors are activated by its ligands LIGHT (TNFSF14) and Lymphotoxin-α, and engage in bi-directional signaling with the receptors B and T lymphocyte attenuator (BTLA) and CD160. We hypothesized that mutations in HVEM disrupted binding to either of its ligands resulting in altered cell-intrinsic HVEM signaling in lymphoma, or in altered activation of BTLA or CD160 in tumor infiltrating lymphocytes. Here, we show that point mutations identified in human lymphoma were localized to the extracellular domain and specifically target ligand binding, resulting in preferential loss of HVEM interactions with CD160 and BTLA compared to LIGHT (TNFSF14). Disrupted binding correlated with loss of receptor functionality resulting in abrogated cell-intrinsic HVEM activation of NF-κB, as well as reduced CD160 activation of ERK and AKT phosphorylation by HVEM ligation. We next identified HVEM-activated CD160 and BTLA signaling pathways through AP/MS and phospho-proteomic analysis in NK and T cells. Activation of CD160 versus BTLA resulted in contrasting phosphorylation status in a number of phospho-proteins, illustrating opposing functions of these receptors. In a mouse model of lymphoma we tested the impact of BTLA versus CD160 in regulating antigen-specific anti-tumor responses, demonstrating regulatory effects of these receptors in vivo. Together these data illustrate how selective pressures within the tumor microenvironment may drive cancer mutations that alter HVEM network signaling in vivo, and indicate potential targets for onco-immunotherapy.

A cognate interaction between T cell-expressed BTLA and B cell-expressed HVEM regulates mucosal immunity in the gut

Mucosal immunity in the gut is achieved in part through cellular connections between lymphoid cells such as regulatory-T cells (Treg), T follicular helper cells (Tfh), T follicular regulatory cells (Tfr) and Germinal Center B cells (GC B). Btla (B and T Lymphocyte Attenuator) is a receptor mainly expressed on B and T cells that is activated by its ligand Hvem (Herpesvirus entry mediator; Tnfrsf14) to inhibit cellular activation. However, the role of these specific immune checkpoints in the regulation of mucosal immunity in the gut is still unknown.

In mice containing genetic ablation of Btla in T cells, we observed an increased frequency of Tfh and GC B cells in Peyer’s Patches (PP), and a decreased frequency of Treg and Tfr cells in these tissues. Interestingly, in mice with a B-cell-specific deletion of the Btla ligand Tnfrsf14 (Hvem), we observe similar increases in GC B cells and Tfh cells and decreased in Treg. Moreover, these mice are characterized by an increase of mucosal IgA bound to bacteria in fecal pellets, indicating a functional outcome of Btla regulation of the GC. Finally, treating wild-type mice with an antibody agonist for Btla increases the frequency of regulatory T cells in PP demonstrating the potential therapeutic effect of activating Btla inhibitory signaling in mucosal immunity.

Together, we show that T cell-expressed Btla interacts with B cell-expressed Hvem to regulate GC responses in Peyer’s Patches. A disruption of the balance effector T cell/regulatory T cell is responsible for many inflammatory autoimmune diseases. It will be crucial to understand the role of Btla and its ligand in these populations in mucosal tissues to understand how immunity is regulated, and to develop potential novel therapeutics to treat disease.

Cancer Mutations Targeting TNFRSF14 alter Microenvironment Checkpoint Interactions to Limit Tumor Clearance by Cytotoxic Cells

The roles of many non-oncogenic mutations in cancer may influence tumor growth, survival, or how tumors interact with their surroundings. Here we characterize the functional relevance of missense mutations within the gene encoding the tumor necrosis receptor family member HVEM (TNFRSF14), a locus frequently targeted within human lymphoma and other cancers. We find that point mutations identified in human lymphoma were localized to the extracellular domain and specifically target ligand binding, resulting in preferential loss of CD160 and BTLA interactions compared to LIGHT (TNFSF14). Missense mutations were associated with alterations in cytotoxic effector cell signatures within tumor biopsies, while deletion mutations were associated with changes in myeloid cell signatures. Finally, we find that mutated HVEM proteins retained the capacity to inhibit T cell signaling through BTLA, while reducing costimulation of cytolysis in NK cells through CD160. Together, these data provide evidence for how immune selective pressures may drive mutation of TNFRSF14 resulting in greater tumor fitness.

Targeting the HVEM-BTLA-CD160-LIGHT network in Psoriasis

Genetic studies have identified heritable linkages to psoriasis. However, complete identification of psoriasis susceptibility loci remains elusive. We have identified the HVEM-BTLA-CD160-LIGHT pathway as a critical regulator of innate and adaptive cell function in several autoimmune diseases, and recently in a mouse model of experimental psoriasis. Specifically, we have shown that the inhibitory receptor BTLA regulates the activity of innate γδ T cells, correlating with worsened dermatitis in BTLA-deficient animals, and attenuated disease in animals treated with agonistic BTLA antibodies. Additionally, our data indicates that the TNF receptor protein HVEM activates CD160 receptors in innate cells, promoting inflammatory signaling. More recently we have observed that skin inflammation is also worsened in animals lacking the TNF ligand for HVEM, LIGHT. In order to promote anti-inflammatory activities of HVEM, we sought to develop ligand specific biologics that could selectively engage inhibitory pathways through BTLA. Our initial studies showed that wild-type HVEM-Fc worsens disease. Additionally, we find that LIGHT-specific proteins that block HVEM-LIGHT interactions also worsen disease, consistent with our results in LIGHT-deficient animals, and identifying a unique regulatory role for LIGHT in skin inflammation. Finally, animals treated with proteins specific for BTLA and CD160 show reduced pathology compared to animals treated with wild-type HVEM-Fc. Through our efforts to synthesize ligand specific HVEM biologics, we have shown how select ablation of HVEM ligands can result in both pro- and anti-inflammatory signaling that can be targeted for the development of therapeutics for inflammatory diseases.

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

B and T lymphocyte attenuator (BTLA; CD272) can deliver inhibitory signals to B and T cells upon binding its ligand herpesvirus entry mediator. Because CD28, CTLA-4, programmed death-1, and ICOS regulate the development of acute graft-vs-host disease (GVHD), we wished to assess if BTLA also played a role in this T cell-mediated response. In the nonirradiated parental-into-F1 model of acute GVHD, BTLA+/+ and BTLA-/- donor lymphocytes showed equivalent engraftment and expansion during the first week of the alloresponse. Unexpectedly, BTLA-/- donor T cells failed to sustain GVHD, showing a decline in surviving donor cell numbers beginning at day 9 and greatly reduced by day 11. Similarly, inhibition of BTLA-herpesvirus entry mediator engagement by in vivo administration of a blocking anti-BTLA Ab also caused reduced survival of donor cells. Microarray analysis revealed several genes that were differentially expressed by BTLA-/- and BTLA+/+ donor CD4+ T cells preceding the decline in BTLA-/- donor T cells. Several genes influencing Th cell polarization were differentially expressed by BTLA+/+ and BTLA-/- donor cells. Additionally, the re-expression of the IL-7Ralpha subunit that occurs in BTLA+/+ donor cells after 1 wk of in vivo allostimulation was not observed in BTLA-/- donor CD4+ cells. The striking loss of BTLA-/- T cells in this model indicates a role for BTLA activity in sustaining CD4+ T cell survival under the conditions of chronic stimulation in the nonirradiated parental-into-F1 GVHD.

Evidence for carbohydrate recognition and homotypic and heterotypic binding by the TIM family

The T cell Ig domain and mucin domain (TIM) proteins form a conserved family of transmembrane cell-surface glycoproteins expressed by a variety of tissues. Each TIM protein contains a single V-type Ig domain, a glycosylated mucin-like domain, a transmembrane domain and a cytoplasmic domain. TIM proteins recognize a diverse array of ligands, including H-ferritin, galectin-9 as well as other TIM family members. In this study, we demonstrate that the Ig domains of murine TIM-1, -3 and -4 display calcium-dependent binding to ligands expressed by murine splenocytes and several non-murine cell lines, indicating non-species-specific ligand recognition. Further, the intrafamilial interaction of various TIM family Ig domains with surface-expressed TIM-1 and TIM-4 requires an intact TIM-1 and TIM-4 glycosylated mucin stalk. Importantly, we also uncovered the previously unrecognized potential for homotypic TIM interactions in forming ligand-receptor pairs. Using a glycan array screen, we identified the novel capacity of the TIM-3 Ig domain to recognize specific carbohydrate moieties, suggesting a role for carbohydrate modification along with protein epitopes in TIM ligand recognition. Identification of the carbohydrate-binding capacity of TIM proteins helps explain the diversity of ligands recognized by this family and adds to our understanding of homotypic and heterotypic interactions between TIM family members.

B and T Lymphocyte Attenuator Exhibits Structural and Expression Polymorphisms and Is Highly Induced in Anergic CD4+T Cells

B and T lymphocyte attenuator (BTLA) was initially identified as expressed on Th1 cells and B cells, but recently reported to be expressed by macrophages, dendritic cells, and NK cells as well. To address this discrepancy we generated a panel of BTLA-specific mAbs and characterized BTLA expression under various activation conditions. We report the existence of three distinct BTLA alleles among 23 murine strains, differing both in Ig domain structure and cellular distribution of expression on lymphoid subsets. The BALB/c and MRL/lpr alleles differ at one amino acid residue, but C57BL/6 has nine additional differences and alters the predicted cysteine bonding pattern. The BALB/c BTLA allele is also expressed by B cells, T cells, and dendritic cells, but not macrophages or NK cells. However, C57BL/6 BTLA is expressed on CD11b+ macrophages and NK cells. Finally, in CD4+ T cells, BTLA is expressed most highly following Ag-specific induction of anergy in vivo, and unlike programmed death-1 and CTLA-4, not expressed by CD25+ regulatory T cells. These results clarify discrepancies regarding BTLA expression, suggest that structural and expression polymorphisms be considered when analyzing BTLA in various murine backgrounds, and indicate a possible role in anergic CD4+ T cells.

B and T lymphocyte attenuator regulates T cell activation through interaction with herpesvirus entry mediator

B and T lymphocyte attenuator (BTLA) provides an inhibitory signal to B and T cells. Previously, indirect observations suggested that B7x was a ligand for BTLA. Here we show that BTLA does not bind B7x; instead, we identify herpesvirus entry mediator (HVEM) as the unique BTLA ligand. BTLA bound the most membrane-distal cysteine-rich domain of HVEM, distinct from regions where the ligands LIGHT and lymphotoxin-alpha bound HVEM. HVEM induced BTLA tyrosine phosphorylation and association of the tyrosine phosphatase SHP-2 and repressed antigen-driven T cell proliferation, providing an example of reverse signaling to a non-tumor necrosis factor family ligand. The conservation of the BTLA-HVEM interaction between mouse and human suggests that this system is an important pathway regulating lymphocyte activation and/or homeostasis in the immune response.

BTLA is a lymphocyte inhibitory receptor with similarities to CTLA-4 and PD-1

During activation, T cells express receptors for receiving positive and negative costimulatory signals. Here we identify the B and T lymphocyte attenuator (BTLA), an immunoglobulin domain-containing glycoprotein with two immunoreceptor tyrosine-based inhibitory motifs. BTLA is not expressed by naive T cells, but it is induced during activation and remains expressed on T helper type 1 (T(H)1) but not T(H)2 cells. Crosslinking BTLA with antigen receptors induces its tyrosine phosphorylation and association with the Src homology domain 2 (SH2)-containing protein tyrosine phosphatases SHP-1 and SHP-2, and attenuates production of interleukin 2 (IL-2). BTLA-deficient T cells show increased proliferation, and BTLA-deficient mice have increased specific antibody responses and enhanced sensitivity to experimental autoimmune encephalomyelitis. B7x, a peripheral homolog of B7, is a ligand of BTLA. Thus, BTLA is a third inhibitory receptor on T lymphocytes with similarities to cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) and programmed death 1 (PD-1).

T cell receptor CDR3 loop length repertoire is determined primarily by features of the V(D)J recombination reaction

The third complementarity-determining region (CDR) of the TCR alpha and beta chains forms loops that engage amino acid residues of peptides complexed with MHC. This interaction is central to the specific discrimination of antigenic-peptide-MHC complexes by the TCR. The TCRbeta chain CDR3 loop is encoded by the Dbeta gene segment and flanking portions of the Vbeta and Jbeta gene segments. The joining of these gene segments is imprecise, leading to significant variability in the TCRbeta chain CDR3 loop length and amino acid composition. In marked contrast to other pairing antigen-receptor chains, the TCR beta and alpha chain CDR3 loop size distributions are relatively narrow and closely matched. Thus, pairing of TCR alpha and beta chains with relatively similar CDR3 loop sizes may be important for generating a functional repertoire of alpha beta TCR. Here we show that the TCRbeta chain CDR3 loop size distribution is minimally impacted by TCRbeta chain or alpha beta TCR selection during thymocyte development. Rather, this distribution is determined primarily at the level of variable-region gene assembly, and is critically dependent on unique features of the V(D)J recombination reaction that ensure Dbeta gene segment utilization.

T-bet is a STAT1-induced regulator of IL-12R expression in naïve CD4+ T cells

T helper type 1 (T(H)1) cell development involves interferon-gamma (IFN-gamma) signaling through signal transducer and activator of transcription 1 (STAT1) and interleukin-12 (IL-12) signaling through STAT4 activation. We examined here T-bet regulation and evaluated the actions of T-bet in STAT1- and STAT4-dependent T(H)1 development processes. We found that T-bet expression during T cell activation was strongly dependent on IFN-gamma signaling and STAT1 activation, but was independent of STAT4. Ectopic T-bet expression strongly increased IFN-gamma production in T(H)2 cells activated by PMA-ionomycin, but weakly increased IFN-gamma production in T(H)2 cells stimulated by IL-12 IL-18 or OVA peptide antigen-presenting cell stimulation. In contrast, IL-12 IL-18 induced IFN-gamma production remained STAT4-dependent despite ectopic T-bet expression. Ectopic T-bet expression selectively induced expression of IL-12Rbeta2, but not IL-18Ralpha, in wild-type and STAT1(-/-) T(H)2 cells, but did not extinguish expression of GATA-3 and T(H)2 cytokines. Finally, ectopic T-bet did not directly induce expression of endogenous T- bet independently of IFN-gamma or STAT1. Thus, T-bet is induced by IFN-gamma and STAT1 signaling during T cell activation. In addition, T-bet mediates STAT1-dependent processes of T(H)1 development, including the induction of IL-12Rbeta2.

TCR signaling induces selective exclusion of CD43 from the T cell-antigen-presenting cell contact site

CD43, a large highly glycosylated molecule, is arguably the most abundant molecule on the surface of T cells. Nevertheless, the function of CD43 remains unclear. Utilizing fluorescence microscopy, we find that CD43 is excluded from the T cell-APC contact site. This exclusion is Ag dependent since optimal CD43 exclusion requires Ag-pulsed APC, and since signaling through CD3, in the absence of any other receptor ligand interactions, can induce the modulation of CD43. These data suggest that CD43 may function as a barrier to nonspecific T cell-APC interactions that is removed as a result of T cell activation. Exclusion from the interaction site is a unique feature of CD43 and not universally found for all large highly glycosylated molecules since CD45 is not excluded. Thus, CD43 may represent a novel regulatory molecule on the T cell surface that can direct T cell interactions by changing its location on the cell surface.

Cutting Edge: TCR Signaling Induces Selective Exclusion of CD43 from the T Cell-Antigen-Presenting Cell Contact Site

CD43, a large highly glycosylated molecule, is arguably the most abundant molecule on the surface of T cells. Nevertheless, the function of CD43 remains unclear. Utilizing fluorescence microscopy, we find that CD43 is excluded from the T cell-APC contact site. This exclusion is Ag dependent since optimal CD43 exclusion requires Ag-pulsed APC, and since signaling through CD3, in the absence of any other receptor ligand interactions, can induce the modulation of CD43. These data suggest that CD43 may function as a barrier to nonspecific T cell-APC interactions that is removed as a result of T cell activation. Exclusion from the interaction site is a unique feature of CD43 and not universally found for all large highly glycosylated molecules since CD45 is not excluded. Thus, CD43 may represent a novel regulatory molecule on the T cell surface that can direct T cell interactions by changing its location on the cell surface.