Conformation of human leukocyte antigen class II molecules. Evidence for superdimers and empty molecules on human antigen presenting cells

Subpopulations of human leukocyte antigen (HLA) class II molecules were studied in antigen presenting cells. We present evidence for double dimers or "superdimers" of HLA class II molecules that were stable in an SDS solution at room temperature but dissociated when heated to 50 degrees C into 60-kDa alphabeta heterodimers. Development of an immunofluorescence assay allowed us to quantify the expression of HLA antigens as reflected by the number of bound isotype-specific monoclonal antibodies per cell. The total expression of class II (DR, DQ, and DP) augmented 6-fold after a 36-h interferon-gamma (IFNgamma) treatment of freshly isolated monocytes. Next, we used a recombinant and fluorescein-conjugated form of the class II-associated invariant chain as a quantitative probe for empty peptide-binding sites. The fraction of empty class II molecules was 0.73-2.9% in resting monocytes but was reduced to 0. 12-0.5% of the total after IFNgamma treatment. The fraction of empty sites in B lymphocytes was 0.09-0.36%. The mean number of empty sites per cell were: 6.3 x 10(3) (monocytes), 7.2 x 10(3) (IFNgamma-activated monocytes), 5.2 x 10(2) (B lymphocytes), and 3.6 x 10(3) (Raji B cells). A minor population (4.3-7.4% of total cells), which expressed a much higher number of empty sites, was consistently present in all cell types studied.