The invention provides isolated nucleic acids molecules, designated B7-4 nucleic acid molecules, which encode novel B7-4 polypeptides. The invention also provides antisense nucleic acid molecules, recombinant expression vectors containing B7-4 nucleic acid molecules, host cells into which the expression vectors have been introduced, and nonhuman transgenic animals in which a B7-4 gene has been introduced or disrupted. The invention still further provides isolated B7-4 proteins, fusion proteins, antigenic peptides and anti-B7-4 antibodies. Diagnostic, screening, and therapeutic methods utilizing compositions of the invention are also provided.