Abstract

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.

Highlights

  • Subpopulations of human leukocyte antigen (HLA) class II molecules were studied in antigen presenting cells

  • CD4ϩ helper T cells recognize antigen-derived peptides bound to major histocompatibility complex (MHC)1 class II molecules exposed on the cell surface of antigen presenting cells (APC) such as monocytes, macrophages and B cells

  • The present study is focused on two subpopulations of HLA class II molecules that were detected on the surface of human monocytes and B lymphocytes

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Summary

EVIDENCE FOR SUPERDIMERS AND EMPTY MOLECULES ON HUMAN ANTIGEN PRESENTING CELLS*

(Received for publication, December 4, 1995, and in revised form, March 4, 1996). Corinne Roucard, Frederic Garban, Nuala A. The fact that Ii can be observed on the cell surface of B cells suggests that a minor fraction of the ␣␤Ii complexes reach the cell membrane via the secretory pathway [17,18,19] The internalization of such molecules directed by an endocytosis determinant in the cytoplasmic domain of Ii could give them access to a peptide-loading compartment [20]. In addition to SDS-stable dimers and superdimers, several studies on B cells have suggested the presence of MHC class II molecules on the cell membrane that are not stably associated with peptide. We provide evidence for the existence of superdimers of the DR isotype on human APC, and we have determined the exact number of empty class II molecules in relation to the total number of HLA class II DR, DQ, and DP molecules

EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
Monocytes Activated monocytes B cells Raji cells
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