Physical and molecular genetic analysis of Qa-2 antigen expression: multiple factors controlling cell surface levels

We have examined the surface expression of Qa-2 on lymphocyte subpopulations and on splenocytes from inbred mouse strains by a radioactive binding assay using purified anti-Qa-2 antibodies and antibody fragments (Fab). Quantitative measurements by Scatchard analysis revealed that spleen cells from Qa-2high mice express (4-5) x 10(4) Qa-2 molecules/cell, whereas T lymphocytes have as high as (7-8) x 10(4) molecules/cell. In addition, it was determined that B lymphocytes express (5-6) x 10(3) molecules on their cell surface. The Qa-2 levels on anti-CD3-activated T cells is 1.0 x 10(5) molecules/cell. Previous experiments have shown that the quantity of Qa-2 varies in a strain-specific fashion and may be classified into three groups: Qa-2high, Qa-2medium and Qa-2low. Our results indicated that Qa-2high strains express (4-5) x 10(4) Qa-2 molecules/cell, Qa-2medium strains (B6-K2, B10.A, A/J, BALB/c and DBA/2) express (1-1.7) x 10(4) molecules/cell, and Qa-2low strains (SWR/J and DBA/1) express no more than 6 x 10(3) molecules/cell. Detection of Q7 or Q9 mRNA by the polymerase chain reaction revealed that Qa-2high strains express two functional Qa-2 enconding genes, Q7 and Q9, whereas Qa-2medium and Qa-2low strains express either Q7 or Q9. These results strongly suggest that Qa-2 gene dosage contributes in part to the variation of Qa-2 levels on the cell surface.