2.11 HHLA2 (HERV-H LTR-associating 2)
HHLA2 (HERV-H LTR-associating 2; Human endogenous retrovirus-H Long repeat-associating 2; also called B7 homolog 5 [B7-H5]) is a member of the B7 family of immune checkpoint molecules that has garnered attention for its role in regulating immune responses, particularly in the context of cancer immunotherapy. It is a type I transmembrane glycoprotein that is expressed on various immune cells and tumor cells within the TME [250]. The discovery of HHLA2 as an immune checkpoint protein is relatively recent (1999) [251], and its precise functions are still being elucidated. However, studies have shown that HHLA2 plays a role in immune regulation by interacting with its receptors on T cells and other immune cells. It is constitutively expressed on the surface of human monocytes and can be induced on B cells [250]. HHLA2 molecule is also highly expressed in tumor associated macrophages (TAMs) [252].
HHLA2 interacts with KIR3DL3 (killer cell Ig-like receptor with three Ig domains and a long cytoplasmic tail, also known as  also known as KIRC1, KIR44, and KIR3DL,) and TMIGD2 (T cell membrane protein with immunoglobulin and ITIM domains 2) at different sites to exhibit both inhibitory and stimulatory functions, respectively. Despite the presence of TMIDG2, the inhibitory function mediated by the KIR3DL3-HHLA2 interaction predominates. [253, 254]. Therefore, tumors may escape immune surveillance through the KIR3DL3-HHLA2 pathway by suppressing CD4 and CD8 T-cell activation and effector functions in the presence of T-cell receptor signaling. This includes reducing T-cell proliferation, cytokine production, and cytotoxic activity against tumor cells. [254, 255]. KIR3DL3-HHLA2 pathway also inhibits the cytotoxicity of an NK-cell [254].
In cancer, HHLA2 expression has been observed on various types of tumors, including osteosarcoma [256], renal cell carcinoma [257], pancreatic cancer [258, 259], melanoma [260], hepatocellular carcinoma [261], ovarian cancer [262], gastric cancer [263], colorectal cancer [264] and lung cancer [265]. Elevated levels of HHLA2 have been associated with poorer prognosis, highlighting its role as a potential target for therapeutic intervention [250, 256, 257, 261, 263, 264, 265, 266]. However, better survival and prognosis is also reported with overexpression of HHLA2 in some studies [252, 258, 260, 262, ].
Targeting HHLA2 with specific inhibitors or blocking antibodies represents a novel strategy to enhance anti-tumor immunity. Considering the beneficial role of the TMIGD2 receptor, an ideal therapeutic approach would involve selectively blocking the interaction between HHLA2 and KIR3DL3 [255]. Monoclonal antibody targeting the HHLA2/KIR3DL3 pathway, which block the inhibitory activity of KIR3DL3 while preserving the immune-stimulatory effects of HHLA2 via TMIGD2 have shown promising results in pre-clinical studies [253]. A study by Wei et al. revealed that KIR3DL3 blockade inhibits tumor growth in multiple humanized mouse models [255].  However, study by Wang et al. showed (unexpected function ) targeting TMIGD2 signaling with anti-TMIGD2 monoclonal antibodies diminishes leukemia stem cell self-renewal and decreases leukemia burden in AML patient-derived xenograft models, while having minimal impact on normal hematopoietic stem and progenitor cells [267]. Clinical trials are underway to evaluate the safety, efficacy, and therapeutic potential of HHLA2 inhibitors in various cancers. In July 2023, a multicenter first-in-human study, (Phase I clinical trial, NCT06240728) commenced to evaluate NPX887, a antagonistic immunoglobulin G1 (IgG1) monoclonal antibody targeting HHLA2 (B7-H7)\ KIR3DL3 interaction. This antibody aims to reactivate exhausted T and NK cells in HHLA2-positive solid tumors. The trial focuses on recurrent or metastatic solid tumors, including renal cell carcinoma (RCC), non-small and small cell lung carcinoma, colorectal carcinoma (CRC), and TNBC.