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-2^high mice express (4-5) × 10⁴ Qa-2 molecules/cell, whereas T lymphocytes have as high as (7-8) × 10⁴ molecules/cell. In addition, it was determined that B lymphocytes express (5-6) × 10³ molecules on their cell surface. The Qa-2 levels on anti-CD3-activated T cells is 1.0 × 10⁵ 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-2^high, Qa-2^medium and Qa-2^low. Our results indicated that Qa-2^high strains express (4-5) × 10⁴ Qa-2 molecules/cell, Qa-2^medium strains (B6-K2, B10.A, A/J, BALB/c and DBA/2) express (1-1.7) × 10⁴ molecules/cell, and Qa-2^low strains (SWR/J and DBA/I) express no more than 6 × 10³ molecules/cell. Detection of Q7 or Q9 mRNA by the polymerase chain reaction revealed that Qa-2^high strains express two functional Qa-2 enconding genes, Q7 and Q9, whereas Qa-2^mediumand Qa-2^low 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.