Mouse B7-H2, also known as ICOS ligand (ICOSL) or CD275, is a member of the B7 family of costimulatory molecules and plays a critical role in regulating T cell–dependent immune responses. Expressed primarily on antigen-presenting cells such as dendritic cells, B cells, macrophages, and certain non-hematopoietic cells, B7-H2 binds to its receptor ICOS (Inducible T cell Co-Stimulator) on activated T cells to promote T cell proliferation, cytokine production, and differentiation. In mice, the B7-H2–ICOS pathway is particularly important for T follicular helper (Tfh) cell development, germinal center formation, and antibody class switching, thereby supporting effective humoral immunity. Dysregulation of B7-H2 signaling has been implicated in autoimmune disease models, chronic inflammation, transplant rejection, and tumor immunity. In biomedical research, mouse B7-H2 is widely studied in models of systemic lupus erythematosus (SLE), rheumatoid arthritis, asthma, infectious disease, and cancer immunology, where modulation of the ICOS–ICOSL pathway influences T cell activation, B cell help, and adaptive immune regulation.