Ligand for the T-cell-specific cell surface receptor ICOS. Acts as a costimulatory signal for T-cell proliferation and cytokine secretion (PubMed:11007762, PubMed:11023515, PubMed:30498080). Also induces B-cell proliferation and differentiation into plasma cells. Could play an important role in mediating local tissue responses to inflammatory conditions, as well as in modulating the secondary immune response by co-stimulating memory T-cell function (By similarity). In endothelial cells, required for proper neutrophil transmigration in response to chemoattractants, such as CXCL8/IL8 or N-formyl-methionyl peptides (fMLP) (PubMed:30498080). {ECO:0000250, ECO:0000269|PubMed:11007762, ECO:0000269|PubMed:11023515, ECO:0000269|PubMed:30498080}. This is the function of ENSG00000160223 (ICOSLG, inducible T cell costimulator ligand).