Mouse ICOS ligand(B7-H2) is an approximately transmembrane glycoprotein in the B7 family of immune regulatory molecules. B7-H2 is expressed on antigen presenting cells such as B cells, macrophages, monocytes, and dendritic cells. It binds to ICOS on activated T cells, leading to both positive and negative effects on immune responses including its own down-regulation. The B7-H2 interaction with ICOS is costimulatory for T cell proliferation as well as the development of B cells, plasma cells, follicular helper T cells and germinal centers. B7-H2 contributes to the development of allergic asthma by enhancing Th2 biased immune responses, limiting Th17 responses, and promoting eosinophilic infiltration into the lung. Its activation of ICOS on Treg limits pulmonary inflammation and airway hyperresponsiveness, promotes the development of inhalational tolerance, and impairs antitumor immunity. In the thyroid, B7-H2 is up-regulated on thyrocytes during inflammation and promotes their proliferation and production of thryoid hormones.