KIR3DL3/HHLA2 Cell Screening Model

Tumor immunotherapy has fundamentally changed the mode of cancer treatment, from the traditional way of targeting the tumor itself to mobilizing the host's immune system to achieve the purpose of treating tumors. At present, the treatment scheme through immune checkpoint blockade has been widely used in patients with advanced tumors, and some patients can obtain durable responses after treatment, prolonging their survival cycle. For example, checkpoint inhibitors targeting cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) or programmed cell death receptor 1 (PD-1)/programmed cell death ligand 1 (PD-L1) have been approved for the treatment of multiple cancer types. Although more and more patients benefit from these drugs, at present, most cancer patients still cannot benefit from them. Therefore, it is very meaningful to find new immune checkpoints and develop corresponding treatment options to expand the beneficiary population of immune checkpoint therapy.

The success of CTLA4 and PD-1/L1 immune checkpoint inhibitors suggests that other members of the CD28 family and B7 family may also play similar important roles in immune regulation and are potential immune checkpoint therapeutic targets.

HHLA2 (also known as B7H5, B7H7, or B7y) is one of the B7 family members, which shares 10-18% homology and 23-33% similarity with other B7 family members at the amino acid level, and forms B7 family III group with B7x and B7H3 phylogenetically. HHLA2 was first reported as a new member of the immunoglobulin superfamily in 1999, and its coding gene is located on human chromosome 3q13. HHLA2 is expressed in different species, but not in mice and rats, which distinguishes it from other members of the B7 family. HHLA2 protein is normally expressed on antigen-presenting cells (APCs), with limited expression in normal human organs. In recent years, HHLA2 has been found to be highly expressed in a variety of solid tumors and blood tumors, and its high expression is positively correlated with the poor prognosis of most tumors. Paradoxically, some studies have shown that the higher the expression of HHLA2 in certain tumors, the higher the patient's survival rate. This contradiction can be partly explained by the fact that HHLA2 has dual functions, and there are different receptors in different immune environments, and the combination with different receptors can inhibit or enhance the function of immune cells.

TMIGD2 (also known as IGPR-1/CD28H) is the first discovered specific receptor of HHLA2, which is mainly expressed on naive T cells and NK cells, and the expression will decrease rapidly after being activated. TMIGD2 co-stimulates T cell and NK cell activity, thus, this does not explain the inhibitory effect of HHLA2. In recent years, studies have found that KIR3DL3 in the killer cell immunoglobulin-like receptor (KIR) family is another receptor of HHLA2. As the second receptor of HHLA2, it can inhibit the activation of T cell lines and the cytotoxicity of NK cell lines by binding to HHLA2. In the tumor microenvironment, the expressions of HHLA2, TMIGD2 and KIR3DL3 are all dynamically affected and changed. In general, when KIR3DL3 and TMIGD2 are co-expressed, the inhibitory function of KIR3DL3 is dominant, which is stronger than the stimulating function of TMIGD2. This mechanism is used by tumor cells to cause immune escape. Compared with PD-L1, the expression of HHLA2 is more common, and it tends to express HHLA2 on many PD-L1 negative tumor cells. Therefore, developing antibodies that block the binding of HHLA2 and KIR3DL3 may become a new immune checkpoint therapeutic strategy. In terms of development strategy, since HHLA2 has different binding sites with KIR3DL3 and TMIGD2, it is possible to develop HHLA2 antibodies that specifically block KIR3DL3 inhibitory signals but still maintain TMIGD2 stimulating signals. In addition, KIR3DL3 and PD-1 have a similar mechanism of regulating immune function, suggesting that people can try to use immunotherapy that jointly inhibits KIR3DL3 and PD-1 to exert a better anti-tumor effect and benefit more tumor patients.

For this potential new immune checkpoint target, we also developed a cell screening model for screening KIR3DL3/HHLA2 inhibitors in time, which can be used for functional screening and testing of KIR3DL3 & HHLA2 blocking antibodies to facilitate the development of related drugs.