Recovery from Friend disease in mice with reduced major histocompatibility complex class I expression.

Mice homozygous for the b allele of the MHC gene, H-2D, have a high incidence of recovery from Friend virus infections, while mice heterozygous for the b allele at H-2D have a very low incidence of recovery. Previous experiments indicated that the low recovery rates associated with heterozygosity at H-2D might be related to a gene dosage effect requiring the expression of two H-2Db alleles for high recovery. We investigated the effects of reduced H-2Db expression on recovery from Friend disease by using H-2b homozygous mice carrying a single beta 2-microglobulin gene disruption. These mice had reductions in cell surface H-2Db expression comparable to those of H-2Da/b heterozygotes. Numerous cell types with various levels of H-2Db expression were examined, and in each case, the expression levels in the beta 2-microglobulin mutants closely reflected those observed in the H-2Da/b heterozygotes. We found, however, that reduced expression did not affect recovery from Friend disease, indicating that heterozygous levels of H-2Db expression are sufficient for the high-recovery phenotype previously associated only with H-2Db homozygotes.