HHLA2 is a member of the B7 family and inhibits human CD4 and CD8 T-cell function

Abstract

T-cell costimulation and coinhibition generated by engagement of the B7 family and their receptor CD28 family are of central importance in regulating the T-cell response, making these pathways very attractive therapeutic targets. Here we describe HERV-H LTRassociating protein 2 (HHLA2) as a member of the B7 family that shares 10-18% amino acid identity and 23-33% similarity to other human B7 proteins and phylogenetically forms a subfamily with B7x and B7-H3 within the family. HHLA2 is expressed in humans but not in mice, which is unique within the B7 and CD28 families. HHLA2 protein is constitutively expressed on the surface of human monocytes and is induced on B cells after stimulation with LPS and IFN-γ. HHLA2 does not interact with other known members of the CD28 family or the B7 family, but does bind a putative receptor that is constitutively expressed not only on resting and activated CD4 and CD8 T cells but also on antigen-presenting cells. HHLA2 inhibits proliferation of both CD4 and CD8 T cells in the presence of T-cell receptor signaling. In addition, HHLA2 significantly reduces cytokine production by T cells including IFN-γ, TNF-α, IL-5, IL-10, IL-13, IL-17A, and IL-22. Thus, we have identified a unique B7 pathway that is able to inhibit human CD4 and CD8 T-cell proliferation and cytokine production. This unique human T-cell coinhibitory pathway may afford unique strategies for the treatment of human cancers, autoimmune disorders, infection, and transplant rejection and may help to design better vaccines. Interactions between members of the B7 ligand and CD28 receptor families generate positive costimulation and negative coinhibition, which are of central importance in regulating T-cell responses (1-3). B7-1/B7-2/CD28/CTLA-4 is the most extensively characterized of these pathways. Ligands B7-1 (CD80) and B7-2 (CD86) on antigen-presenting cells (APCs) bind to CD28 on naïve T cells and provide a major costimulatory signal to activate naïve T cells. After the initial activation, coinhibitory molecule cytotoxic T lymphocyte antigen-4 (CTLA-4, CD152) is induced on T cells and engages the same B7-1 and B7-2 ligands to restrain T-cell function. In contrast to the costimulatory activity of CD28, the interaction of B7-1 or B7-2 with CTLA-4 is essential for limiting the proliferative response of recently activated T cells to antigen and CD28-mediated costimulation.During the past decade, several new pathways in the B7 and CD28 families have been identified, including B7h/ICOS, PD-L1/PD-L2/PD-1, B7-H3/receptor, and B7x/receptor. B7h (4) (also called ICOS-L, B7RP-1 (5), GL50 (6), B7H2 (7), LCOS (8), and CD275) binds to the inducible costimulator (ICOS, CD278) on activated T cells (9), which induces strong phosphatidylinositol 3-kinase activity (10, 11) and leads to the expression of transcription factors involved in follicular helper CD4 T (Tfh) differentiation (12). Therefore, the B7h/ICOS pathway provides critical T-cell help to B cells. Deficiencies in this pathway result in substantially reduced numbers of mem...

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Immune Checkpoint Human Endogenous Retrovirus-H Long Terminal Repeat-Associating Protein 2 is Upregulated and Independently Predicts Unfavorable Prognosis in Bladder Urothelial Carcinoma

“…HHLA2 appears to be more closely related to B7-H3 and B7-H4 and shares 10-18% of identical amino acids and 23-33% of similarity with other members of the B7 family. As a T-cell co-suppressor molecule, HHLA2 inhibits the proliferation and cytokine production by human CD4 + and CD8 + T cells [15].…”

Emerging Targets in Cancer Immunotherapy: Beyond CTLA-4 and PD-1

“…HHLA2-Ig fusion protein bound resting and activated CD4 and CD8 T cells, as well as APC. It was shown to inhibit proliferation of CD4 and CD8 T cells in the presence of TCR signaling as well as T-cell cytokine production [66]. TMIGD2, also called CD28H or IGPR-1, is identified as one of the receptors for HHLA2 [67,68].…”

“…HHLA2 is another member of the B7 family that modulates T-cell function [66,67]. It is expressed on monocytes and induced on CD19 positive B cells.…”