IFNG and Parkinson disease: Given that MHC-II expression increases upon cellular activation,8,37 we also measured disease- and genotype-specific effects on induction of this locus using interferon-γ (IFN-γ), a potent stimulator of MHC-II gene expression.37,38 In response to IFN-γ, the frequency of monocytes that became HLA-DR/DQ double-positive cells was significantly higher in the PD GG group relative to the PD AA group (Figure 1d).