Enterobacterial infection modulates major histocompatibility complex classI expression on mononuclear cells

Major histocompatibility complex (MRC) class I expression is reduced in sev eral viral infections, but it is not known whether the same happens during infections caused by intracellular enterobacteria. In this study, the expre ssion of MNC class I antigens on peripheral blood mononuclear cells (PBMC) from 16 patients with Salmonella, Yersinia, or Klebsiella infection was inv estigated. During or after the acute infection, the expression of MWC class I antigens was markedly decreased in eight patients, all with genotype HLA -B27, and six out of eight with reactive arthritis (ReA), A significant dec rease of monomorphic MHC class I was found in three patients, of HLA-B27 in eight (P < 0.05) and of HLA-A2 in two. However, patients negative for the HLA-B27 genotype, or healthy HLA-B27-positive individuals, did not have a s ignificant decrease of MHC class I antigens. During the decreased expressio n on the cell surface, intracellular retention of MHC class I antigens was observed, whereas HLA-B27 mRNA levels did not vary significantly. This is t he first evidence that enterobacterial infection may down-regulate expressi on of MHC class I molecules in vivo and that down-regulation is predominant in patients with the HLA-B27 genotype.