GM-CSF stimulates the growth and differentiation of hematopoietic progenitors and also affects mature cell function. These effects have led to the use of GM-CSF as a vaccine adjuvant with promising results; however, the mechanisms underlying GM-CSF-mediated immune potentiation are incompletely understood. In this study, we investigated the hypothesis that the immune stimulatory role of GM-CSF is in part due to effects on class II MHC Ag presentation. We find that, in primary human monocytes treated for 24-48 h, GM-CSF increases surface class II MHC expression and decreases the relative level of the invariant chain-derived peptide, CLIP, bound to surface class II molecules. GM-CSF also increases expression of the costimulatory molecules CD86 and CD40, but not the differentiation marker CD1a or CD16. Furthermore, GM-CSF-treated monocytes are better stimulators in a mixed leukocyte reaction. Additional analyses of the class II pathway revealed that GM-CSF increases total protein and RNA levels of HLA-DR, DM, and DOalpha. Expression of class II transactivator (CIITA) types I and III, but not IV, transcripts increases in response to GM-CSF. Furthermore, GM-CSF increases the amount of CIITA associated with the DR promoter. Thus, our data argue that the proinflammatory role of GM-CSF is mediated in part through increased expression of key molecules involved in the class II MHC pathway via induction of CIITA.