Quantitative variation of C4 variant proteins associated with many MHC haplotypes.

C4 protein variants were analyzed in 64 individuals, of which 51 were either homozygous or heterozygous for an extended major histocompatibility complex (MHC) haplotype (a fixed combination of MHC alleles). The relative amount of each C4 variant was measured by densitometric scanning of stained immunofixed electrophoretic patterns of neuraminidase- and carboxypeptidase-treated samples. The relative concentrations of C4 variants on any haplotype were stable and inherited in families. In five of the eight extended haplotypes investigated, the amount of one of the C4 variants relative to others in the same pattern was increased: [HLA-B8, SC01, DR3] and [HLA-B7, SC31, DR2] produced an approximately doubled amount of C4B1; [HLA-B18, S042, DR2] an increased amount of C4B2; and [HLA-B44, SC30, DR4] a double amount of C4A3. The extended haplotype [HLA-Bw57, SC61, DR7] gave rise to two to three times as much C4B1 as C4A6. In the extended haplotypes [HLA-B44, FC31, DR7] and [HLA-Bw62, SC33, DR4], the results did not clearly indicate differences in expression of the C4 isotypes. DNA analysis possibly supported an actual gene duplication only for the haplotype [HLA-B7, SC31, DR2]. The results suggest that, in addition to variation in the number of structural genes, other MHC-linked mechanisms may be involved in the regulation of the relative amounts of C4A or C4B protein specified by any haplotype.