Ovarian cancer represents a major public health concern worldwide. High-grade serous ovarian cancer (HGSOC) is a primary epithelial ovarian cancer. Cisplatin resistance poses a substantial obstacle in the management of HGSOC, leading to unfavourable patient outcomes. The primary objective of this study was to investigate the mechanisms underlying cisplatin resistance in patients with HGSOC. TCGA data, GSE65819 dataset, and multiMiR package were used to identify 35 differentially expressed miRNAs (DE-miRNAs). Differentially expressed mRNAs (DE-mRNAs) are indicated using TCGA data. Further, weighted gene co-expression network analysis (WGCNA) was used to determine the correlation coefficients between the DE-mRNAs and DE-miRNAs. A network of miR-486-3p and TMIGD2 was constructed. Molecular biology experiments also indicated that low miR-486-3p or high TMIGD2 expression significantly increased the migratory rate and cisplatin resistance of both SK-OV3 and A2780 cells. In contrast, overexpression of miR-486-3p or downregulation of TMIGD2 decreased the migration rate and enhanced the sensitivity to cisplatin treatment, which provides insights for the development of novel therapeutic approaches. Moreover, RNA-binding protein immunoprecipitation experiment was used to determine the relationship between miR-486-3p and TMIGD2. Cell rescue assays were performed to further investigate these regulatory relationships. In TCGA and GSE65819 datasets, Benjamini and Hochberg false discovery rates (FDR) were selected for P-values. In the molecular biology experiments, one-way analysis of variance was employed to compare different groups, supplemented by Bonferroni post-hoc testing. Statistical significance was set at p < 0.05.

T cells and natural killer (NK) cells are essential components of the immune system and are regulated by coinhibitory and costimulatory molecules in which the B7 family and CD28 family play significant roles. Previous immune checkpoint studies on B7/CD28 family members, such as PD-1, have led to remarkable success in cancer immunotherapy. However, there is still a need to find new immune checkpoint molecules. Recent studies have demonstrated that HHLA2 exerts inhibitory and stimulatory functions on the immune system by binding to different receptors on different sites. However, the pathways between HHLA2 and its two receptors on T cells and NK cells remain controversial.
Here, we reviewed recent studies about HHLA2 ligand interactions with KIR3DL3 and TMIGD2. We focused on elucidating the pathways between KIR3DL3/TMIGD2 and HHLA2 as well as their function in tumour progression. We also addressed the relationship between HHLA2 expression and the clinical prognosis of cancer patients.
KIR3DL3/TMIGD2-HHLA2 may represent novel pathways within the tumour microenvironment and serve as crucial immune checkpoints for developing novel therapeutic drugs against human cancer.

Human endogenous retrovirus-H long terminal repeat-associating 2 (HHLA2) is a newly emerging immune checkpoint that belongs to B7 family. HHLA2 has a co-stimulatory receptor transmembrane and immunoglobulin domain containing 2 (TMIGD2) and a newly discovered co-inhibitory receptor killer cell Ig-like receptor, three Ig domains, and long cytoplasmic tail (KIR3DL3), which endows it with both immunostimulant and immunosuppression functions in cancer development. In this review, we summarize the HHLA2 expression profile in human cancers, its association with cancer prognosis and clinical features, and its dual roles in regulating cancer immune response through up-to-date literatures. Furthermore, we highlight that precision cancer immunotherapy through manipulating HHLA2-KIR3DL3/TMIGD2 interaction is a promising antitumour strategy.

HHLA2, a member of the B7 family of immune checkpoint players, has been implicated in various cancers. The study set to determine the expression and biological function of HHLA2 in hepatocellular carcinoma (HCC), and its connection to TMIGD2. First, after HHLA2 knockdown or overexpression in Huh-7 or HepG2 cells, we co-cultured T cells with HCC cells after transfection for 48 h. T cell proliferation and cytokine release were detected using flow cytometry and the FlowCytomix assay kit. Subsequently, we screened differentially expressed genes in cells overexpressing or under-expressing HHLA2 using GSEA database and analyzed the pathways enriched by them. We further detected the nuclear translocation of STAT3 and STAT2 using immunofluorescence. After that, we observed the subcellular localization of HHLA2 and TMIGD2 in HCC cells by laser confocal microscopy, followed by RIP and rescue experiments. We found that the proliferation of T cells and the release of cytokines were significantly reduced after co-culture with HCC cells overexpressing HHLA2, while co-culture with cells low in HHLA2 expression had the opposite results. HHLA2 bound to TMIGD2, thus inhibiting T cell proliferation and activation. Overexpression of HHLA2 significantly promoted the nuclear translocation of STAT2 and STAT3, thereby activating the JAK/STAT pathway. Subsequently, we showed that the immune tolerance of HCC cells was significantly attenuated after using a JAK/STAT signaling pathway antagonist. Aberrant overexpression of HHLA2 activates the JAK/STAT signaling pathway by binding to TMIGD2, thereby promoting immune tolerance in HCC cells.

The B7-CD28 family of ligands and receptors play important roles in T-cell co-stimulation and co-inhibition. Phylogenetically they can be divided into three groups. The recent discovery of the new molecules (B7-H3 [CD276], B7x [B7-H4/B7S1], and HHLA2 [B7H7/B7-H5]/TMIGD2 [IGPR-1/CD28H]) of the group III has expanded therapeutic possibilities for the treatment of human diseases. In this review, we describe the discovery, structure, and function of B7-H3, B7x, HHLA2, and TMIGD2 in immune regulation. We also discuss their roles in important pathological states such as cancers, autoimmune diseases, transplantation, and infection. Various immunotherapeutical approaches are emerging including antagonistic monoclonal antibodies and agonistic fusion proteins to inhibit or potentiate these molecules and pathways in cancers and autoimmune diseases.

We and others recently discovered HHLA2 as a new B7 family member and transmembrane and immunoglobulin domain containing 2 (TMIGD2) as one of its receptors. Based on a new study we propose that HHLA2 may represent a novel immunosuppressive mechanism within the tumor microenvironment and hence could be a target for cancer therapy. TMIGD2 may be another therapeutic target.