BTN3A1 has the structure of a type I receptor of the Ig superfamily and is part of a family of seven BTN receptors encoded by genes in the MHC. BTN molecules are composed of two Ig domains (IgV, IgC2), a single transmembrane domain, and a large carboxyl-terminal domain termed B30.2 (or PRYSPRY) located in the cell cytoplasm. There are three human BTN3A loci, BTN3A1, BTN3A2, and BTN3A3, and clear orthologs of BTN3A molecules, now called CD277, are absent from the mouse genome. Despite its similarity to B7 molecules, BTN3A1 was proposed to act not as a coreceptor or costimulatory molecule, but rather to directly present pAg to the γδ TCR in a manner analogous to MHC-restricted peptide presentation. However, this model of BTN3A1 function has been challenged by conflicting data, which show pAg binding to a positively charged pocket in the cytosolic B30.2 domain, and that BTN3A1 does not directly engage the γδ TCR. This contradictory picture has emerged as a result of the complexity of the system and in particular by the use of endogenous and exogenous routes of Ag delivery in in vitro assays.
