EXPRESSION OF MHC CLASS-II MOLECULES ON HUMAN MONOCYTES IS REGULATED INDEPENDENTLY FROM EACH OTHER AFTER PHAGOCYTOSIS OF BACTERIA

Reactive arthritis is usually self-limiting polyarthritis which develo ps in HLA-B27 positive individuals after certain gastrointestinal or u rogenital infections. The pathogenesis of reactive arthritis is unknow n but T cells seem to have a crucial role. Most of the antigen-specifi c T cells isolated from the synovial fluid have been MHC class II rest ricted. The role of antigen presentation in the pathogenesis of reacti ve arthritis has been studied relatively little. In this work the auth ors studied the effect of arthritis-triggering bacterium (Yersinia ent erocolitica 0:3) on the expression of MHC class II molecules on human monocytes and found that the expression of different MHC class II mole cules was regulated independently from each other in half of the indiv iduals after certain incubation periods. In these cases the expression of HLA-DP was parallel to the expression of HLA-DQ, while HLA-DR expr ession went to the opposite direction or did not change at all. No dif ference between HLA-B27 negative and HLA-B27 positive healthy individu als was seen. The authors conclude that independent regulation of the expression of different MHC class II antigens on antigen-presenting ce lls is a more common phenomenon than usually thought.