A human Ia cytoplasmic determinant located on multiple forms of invariant chain (gamma, gamma 2, gamma 3).

An antigenic determinant present in the cytoplasm, but not on the surface membrane of human Ia+ cells, is defined by a monoclonal antibody (VIC-Y1) and is shown by immunoprecipitation and by NEPHGE to be expressed by Ia oligomers. Immunoprecipitations of cellfree translates and of purified Ia subunits indicate that the VIC-Y1 determinant is located on the Ia gamma-(invariant) chain, as well as on two other related molecules, provisionally termed gamma 2 and gamma 3. Within our experimental conditions, the three forms of gamma-chains co-precipitate exclusively with Ia oligomers. As detected by VIC-Y1 and in the limits of our assays, gamma-chains could not be found at the cell surface; their tissue distribution, determined by cytoplasmic indirect immunofluorescence with VIC-Y1, closely resembles that of Ia antigens, with the possible exception of acute lymphatic leukemia cells (Ia+, gamma-chain-).