Targeting lymphocyte activation through the lymphotoxin and LIGHT pathways

Abstract

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.

Fig. 1. Members and binding interactions of the immediate TNF family

Ligands shown at the top are depicted as trimers with transmembrane anchors. Cellular receptors are shown on the bottom. Arrows indicate the receptor-ligand binding specificity of the various members. Solid lines indicate high affinity interactions; dashed line refers to weak interactions.

Fig. 2. Microarchitecture of the spleen is regulated by LTβR-dependent chemokine expression

Chemokine circuits form between lymphocytes and stroma. Depicted are cellular interactions in the architecture of white pulp in the spleen dependent on LT/TNF signaling. The marginal zone contains marginal zone macrophages (MZM) and metallophilic macrophages (MMM). DCs require LT signaling for local proliferation in the spleen. B and T lymphocytes are compartmentalized in discrete areas in the white pulp (B-cell follicle and T-cell zone) through the reciprocal induction of LT expression on lymphocytes by chemokines and chemokine expression by stromal cells via the LTβR.

Fig. 3. TNFR and LTβR signaling pathways

Components in the TNFR1 and LTβR pathways for NF-κB activation. TNFR1 can access TRAF2 to activate the canonical NF-κB pathway RelA(p50/p65) via IκB degradation. This pathway controls many inflammatory genes and p100 synthesis. The LTβR induces both the canonical and the RelB NF-κB pathway via the NIK-IKKα-mediated processing of p100 and activation of p52/ RelB target genes.

Fig. 4. Structural models of LIGHT-HVEM and HVEM-BTLA complex

The molecular model of trimeric LIGHT in space filling mode was generated by SwissModel and encompasses amino acids Ser103 to Val240. Subunits are represented in red, blue, and gray (not fully visible); the transmembrane domains of each subunit would anchor in the top membrane. The LIGHT is adjacent to CRD2 and CRD3 in HVEM, predicted to contain the LIGHT binding site. The structure of the ectodomain of HVEM is in ribbon format showing CRD1 (blue), CRD2 (magenta), CRD3 (gray), and disulfide bonds (yellow) in all structures. The C-terminus is oriented towards the bottom, where it would transverse the membrane. The HVEM-BTLA complex is viewed from the side (middle panel) showing BTLA (green) (from 2AW2.pdb). Structures were drawn using PyMOL (http://www.pymol.org).

Fig. 5. The LIGHT-HVEM-BTLA switch

HVEM and BTLA interact when expressed in the same cell (cis) or between adjacent cells (trans). Conformation flexibility of HVEM or BTLA may be required when in cis, based on structural analysis (63). Induction of membrane LIGHT, which interacts in trans with HVEM, excludes BTLA from binding HVEM, turning off inhibitory signaling.

Fig. 6. The HVEM switch is targeted by herpesvirus

The depicted interactions involving HVEM that initiate positive cosignaling through LIGHT-HVEM interaction or inhibitory signaling through HVEM binding BTLA. LIGHT bound to HVEM activates (+) TRAF-dependent activation of NF-κB, whereas HVEM-BTLA acts through an immunoreceptor tyrosine-based inhibitory motif of BTLA to recruit the phosphatase SHP-2, attenuating kinases activated by TCR signaling. The HSV virion envelope protein gD attaches to HVEM, acting as an entry step for infection. The binding of gD to HVEM competitively blocks BTLA binding and non-competitively prevents LIGHT binding, inhibiting both intercellular communication pathways. UL144 of human CMV binds to BTLA but not LIGHT, selectively mimicking the inhibitory pathway of HVEM-BTLA. DcR3 is a soluble TNFR family member that binds to LIGHT, acting as a circulating inhibitor of LIGHT-HVEM signaling. The CRD1 of each protein is shown as a gray oval (78).

Fig. 7. Integrated signaling by LTαβ-LTβR and HVEM-BTLA pathways regulates DC homeostasis

LTαβ specifically regulates the proliferation of CD8α^- DC subsets in the spleen through a LTβR-NIK-RelB-dependent pathway during homeostasis. LIGHT expression in activated T cells can also increase DC proliferation through LTβR. Signaling through HVEM-BTLA provides inhibitory signaling that limits LTαβ-dependent proliferation of the CD8α^- DC subsets. HVEM-BTLA expression in DCs and in stromal cells contributes to limiting DC proliferation (99).