Molecular basis of Qa-11 antigen and paradoxical Qa-gene expression in an H-2 recombinant

The Qa-11 Ag expressed in certain strains with the B₂-microglobulin-b allele, apparently maps into the Tla region as well as into the Qa-2 region. Moreover Qa-11 has been shown to be biochemically indistinguishable from Qa-2. Genetic complementation studies combining the right Qa and Tla regions failed to lead to Qa-11 expression. To elucidate the molecular basis of this apparent paradox, we examined the expression of Qa-11 on products of transfected Q-region class I genes. Immunochemical analysis has shown that the Qa-11 Ag is expressed on class I molecules encoded by the Q7 gene from both C57BL/10 (Q7^b) and BALB/c (Q7(d)), but not on the protein product of the Q9 gene isolated from the C57BL/10 strain (Q9^b). Inasmuch as the predicted protein products of the Q7^b and Q9^b genes would differ at a single amino acid, a residue critical for Qa-11 expression has been identified. Based on these results it is proposed that among the β-2-m^b strains, the Qa-11⁺/Qa-2⁺ mice are likely to express at least the Q7 gene, whereas Qa-11^-/Qa-2⁺ mice express only Q9. In support of this model, the Qa-2⁺/Q-11^- recombinant B6.K2, essential for the apparent mapping of Qa-11 into the Tla region, expresses only Q9 but not Q7 encoded molecules on the cell surface, and only Q9 and no processed Q7 mRNA is detected in the cytoplasm. This expression pattern in B6.K2 cannot be explained on the basis of a single crossing-over event.