Enhanced induction of PMN class II expression by GM-CSF plus IFN-γ. PMN from both an HLA-DR^low responder and an HLA-DR^high responder were cultured in the presence of various concentrations of GM-CSF (♦), IFN-γ (▪), or GM-CSF/IFN-γ (•). After 44 hours of culture, the PMN were stained with either MoAb IVA12 (anti-MHC class II) or MoAb 22 (anti-CD64) before flow cytometric analysis. The level of PMN class II induction differed markedly for HLA-DR^low responder (A) and the HLA-DR^high responder (B) but was synergistically enhanced for both donors when compared to stimulation with either GM-CSF or IFN-γ alone. Regulation of PMN CD64 expression was similar for both donors (C and D). Surface expression levels of both class II and CD64 are presented as mean fluorescence intensity and represent the average of duplicate samples ± SD.