The HK5.3 monoclonal antibody specifically binds to CD275 which is also known as B7 homolog 2 (B7-H2 or B7h2), B7-related protein 1 (B7RP-1), B7-like protein Gl50, ICOS Ligand (ICOS-L), LICOS, or Ly115l. CD275 is a type I transmembrane glycoprotein that is encoded by Icoslg (icos ligand) which belongs to the B7 family of costimulatory molecules within the Ig gene superfamily. Two isoforms of mouse CD275 have been described. CD275 is variably expressed on antigen-presenting cells including B cells, macrophages, and dendritic cells. CD275 serves as a costimulatory molecule that binds to and signals through CD278 (ICOS) which is expressed on activated T cells to increase their proliferation, production of cytokines, and provision of help for B cells. CD275 expression is upregulated by TNF and can help mediate local tissue responses to inflammatory conditions. The HK5.3 antibody reportedly blocks the binding of CD275/ICOS Ligand to its receptor, CD278 (ICOS), both in vitro and in vivo, and thus inhibits its biological functions.