H-2-linked murine factor B phenotypes.

A hemolytic assay has been developed which is specific for Factor B (B) activity in murine EDTA-plasma. Three discrete levels of B activity were observed among B10-congenic strains. Mice with standard H-2 haplotypes, b, d, k, r, f, q, s, and u, all exhibited the same mean level of activity. However, plasma from H-2v (B10.SM) mice contained only 0.25 of that level, and those with standard haplotype H-2ja (B10.WB) or wild haplotype H-2wr7 (B10.WR) exhibited 2.5 times the H-2b (B10) basal level of activity. These differences among B10 congenic lines suggested that the activity is H-2 controlled; further tentative mapping with intra-H-2 recombinants indicated that the gene is located in the S region. A fourth phenotype was found among progeny of backcross generations between B10.BR (H-2k) and mice of subspecies Mus musculus molossinus and M. m. bactrianus. This ultra-high activity was found also to be governed by a gene very closely linked to Ss, the primary S region marker. F1 generations between disparate phenotypes yielded progeny with activity levels intermediate between the parents; progeny of parents of different strains with the same phenotype expressed B hemolytic titres equal to those of the parental strains. No differences in antigenic levels of the protein among the strains of different phenotypes could be detected by radial immunodiffusion. In mixing experiments, resultant activity levels were intermediate between the higher and the lower phenotype, ruling out independent inhibitors or activators of the reaction. These studies indicate that an H-2-linked S region-located single gene governs structural differences in allelic B molecules that lead to differences in specific activities.