Expression of functional major histocompatibility complex class II molecules on HMC-1 human mast cells

Mast cells hold a key position in the defensive mechanisms against exogenou s intruders. In this study, we investigated whether human mast cells expres s functional major histocompatibility complex (MHC) class II molecules that can transduce endogenous signals and present staphylococcal enterotoxin A (SEA) to T cells. Similar to HMC-1 human mast cell line, umbilical cord blo od-derived mast cells express HLA-DR, -DP and -DQ molecules on their surfac e. MHC class II molecules expressed on HMC-1 cells bind significantly the S EA (a natural MHC class II ligand), and their ligation with specific mAbs o r with SEA, leads ultrastructural chages, suggesting their degranulation, R ecognition of SEA-bound MHC class II molecules on HMC-1 mast cells by the T cell receptor of K25 cells, an SEA-specific murine T cell hybridoma, trigg ers significant IL-2 secretion by these T cell hybridomas. Hence, our data point out the expression of functional MHC class II molecules on human mast cells, reinforcing the implication of these cells in the defense mechanism s of acquired immunity.