Structure of Ia antigens from the rat. Mouse alloantisera demonstrate at least two distinct molecular species.

Ia antigens isolated from spleen cells of rats and mice are composed of two polypeptide chains, designated alpha and beta. Mouse alloantisera specific for the I-Ak and I-Ek subregions react with two distinct groups of rat Ia antigens, designated A-like and E-like, respectively. Two-dimensional gel electrophoresis and peptide map analysis demonstrate that the A-like antigens of rat are distinct from the E-like antigens. Both rat Ia antigens react with alloantiserum produced in rats congenic for the major histocompatibility complex (MHC). These results demonstrate for the first time that two distinct Ia antigens are present in the rat. Accordingly, the rat, like the mouse, may have Ia antigens encoded by at least two subregions of the rat MHC. The existence of multiple Ia gene products in rats is revealed by chemical techniques even in the absence of formal genetic evidence of more than one I subregion in the rat.