1086 Monoclonal Antibody Against

Immune response (Ir) loci that map in the I region of the H-2 gene complex regulate the capacity to generate an immune response against numerous foreign antigens (1). Loci that map in the I region also control polymorphic~cell surface antigens that are expressed in immunocompetent cells (I-region-associated antigens or Ia loci) (2) and antigens that stimulate T cell proliferative responses between I region disparate strains (lymphocyte-activating determinants or Lad loci) (3). Genetic recombination studies have shown that the I region can be divided into subregions, yet no crossover has yet been detected that allows separation of Ir loci from la loci or Lad loci. For example, antisera prepared between I-A-subregionincompatible strains react with Ia-I glycoproteins (4), Lad-I -controlled antigens that stimulate T cell proliferative responses (5), and specifically block T cell responses to antigens under control of the Ir-lA locus (6). These observations have led to the tentative conclusion that the same locus controls all three traits. However, because the I-A subregion represents a segment of chromosome potentially comprised of several loci, it could be argued that conventional anti-I-A-subregion sera contain multiple specificities. More recent studies with an I-A-subregion mutant strain suggest that the Ia-1, Lad-l, and H-2A loci are identical (7). Biochemical and functional studies have significantly advanced our understanding of the genetic control ofIa glycoproteins and support the notion that Ia loci, Lad loci, and Ir loci are equivalent. Antisera prepared between I-E-subregion-incompatible strains precipitate three glycoproteins, designated Ea (~33,000 mol wt), Eo (-30,000 tool wt), and Ii (~31,000 tool wt) (8). The Ea chain is controlled by a locus that maps in the I-E subregion, whereas the Ep chain is controlled by a locus that maps in the I-A subregion (9). We therefore prefer the designation A~ rather than Et~ for this lower molecular weight chain. Whether the invariant Ii chain is controlled by a locus mapping in the I region or elsewhere in the genome remains to be determined. Two forms of the ,% chain have been detected biochemically (9). One form is found in the cytoplasm of lymphocytes from strains in which no E~ chain is detected. The second