Strain B6.C-H-Zbm13 has a mutation in the I-A subregion of the mouse H-2 gene complex, which causes skin graft rejection, mixed lymphocyte reaction (MLR), and alterations in the expression of Ia antigens. The mutation affects the expression of Ia.3, 8, 9, 15, and 20 on normal spleen cells. When the spleen cells were stimulated with lipopolysaccharide (LPS), the expression of all Ia specificities were found except Ia.8. Ia molecules when internally labeled with3H-leucine can be precipitated with antisera directed against Ia.3, 9, 15, and 20, but not Ia.8. When F1s are made between the mutant bm12 and unrelated haplotypes, Ia.3, 9, 15 and 20 can be detected by microcytotoxic assay on normal spleen cells, but not Ia.8. These studies suggest: (1) The mutation affects either the amount of Ia molecules expressed on normal spleen cell surfaces or the molecule is anchored improperly in the cell surface such that it is not accessible for cytotoxicity and radioiodination. (2) Specificity Ia.8, which may be a combinatorial determinant, is absent in the mutant because of a structural alteration in one of the chains, probably the b chain. (3) The mutation does not involve the Ae chain. The significance of this finding in relation to I region-mediated allorecognition and antigen presentation is discussed.
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