H-2 hemizygous mutants from a heterozygous cell line: role of mitotic recombination

Variants that no longer express an entire H-2 haplotype were readily isolated, by immunoselection with antisera directed against the haplotype, from an H-2b/H-2d heterozygous Friend leukemia cell line carrying a Robertsonian translocation of the chromosomes bearing the H-2 genetic region. These variants can be denoted as being of the phenotype H-2b- H-2d+ or H-2b+ H-2d-. Some of the H-2b- H-2d+ variants: (1) lack the restriction enzyme fragments characteristic of the missing H-2b haplotype, as assessed by Southern blot analysis; (2) express more cell surface H-2d antigens than wild-type cells, as assessed by flow microfluorimetry; and (3) appear to have become homozygous for the more active H-2d-linked allele at the Glyoxalase I locus. These variants thus seem to have lost genetic material corresponding to the H-2b haplotype and may have gained genetic material corresponding to the H-2d haplotype. These results are consistent with the possibility that these variants were generated by mitotic recombination.