Decreased expression of class II major histocompatibility complex (MHC) molecules on monocytes is found in open-heart surgery related immunosuppression.

The expression of class II major histocompatibility complex (MHC) molecules on monocytes and the relative proportion of immunoregulatory lymphocyte subpopulations were studied from the blood of eight patients undergoing a cardiac operation. The aim of the study was to reveal mechanisms responsible for depression of cellular immunity and B-lymphocyte activation associated with open-heart surgery. A decreased staining of HLA-DR and even more of HLA-DQ class II MHC molecules was found on the monocyte surface in samples taken 2 and 7 days after operation when compared to preoperative values. The relative proportion of monocytes in mononuclear cells increased after surgery, but within lymphocyte subpopulations only a decrease in total T cells and an increased number of activated (HLA-DR positive) T cells were found, whereas the rations between various T cells subsets remained relatively stable.