HLA-DR expression in B-lymphocytes in vitro is not suppressed by the absence of exogenous antigens

The proper loading of exogenous peptide antigens affects the transport and cell surface expression of MHC class II molecules. In the present study, th e goal was to determine to what extent this step determines the cell surfac e expression level of MHC class II molecules, such as the HLA-DR. EBV-trans formed B-cells, were cultured either in a serum- and protein-free medium, o r in a medium that contained different concentrations of exogenous antigens . Using HLA-DR-specific antibodies, the induction of the MHC class II expre ssion was observed in cells that were cultured under serum- and protein-fre e conditions, when compared to those cultured with exogenous protein antige ns. This upregulation was completely suppressed to the normal level by the addition of a high concentration of hen egg lysozyme to the serum- and prot ein-free medium. This indicates that exogenous proteins regulate the HLA-DR expression. To further examine whether this modulation is controlled at th e transcription level, the expression of the HLA-DR beta -chain mRNA was an alyzed by reverse transcription-PCR and Northern blots. The same levels of HLA-DRB mRNA were detectable in both culture conditions, indicating that th e present observation is dependent on some regulatory mechanisms at the pos t-transcriptional level. This might include a different pathway for traffic king of HLA-DR molecules to the cell surface, since peptide-binding assays revealed that a high proportion of cell surface HLA-DR molecules under the serum- and protein-free condition were transported to the cell surface with out associated peptide antigens.