The context of HLA-DR/CD18 complex in the plasma membrane governs HLA-DR-derived signals in activated monocytes

HLA-DR-derived signals in activated monocytes mediate both pro-inflammatory cytokine production and caspase-independent death, and have been postulated to play a role in inflammation and in its resolution, respectively. Herein, using the monocytic/macrophagic human cell line THP-1 primed with IFNγ (IFNγ-primed THP-1), we investigated how HLA-DR may integrate both signals. Our inhibition studies demonstrated that if cell death is dependent on PKCβ activation, the induction of TNFα gene expression relies on PTK activation, in particular the Src family of kinases, but both cell responses implicate the β2-integrin CD18. Accordingly, sequential immunoprecipitation experiments demonstrated that following engagement of HLA-DR on IFNγ-primed THP-1 cells, the HLA-DR/CD18 complex physically associates with PKCβ and with PTK. Pharmacological disruption of lipid rafts microdomains abolished the assembly of HLA-DR/CD18/PTK signaling complex, HLA-DR-mediated tyrosine activation, and the PTK-dependent TNFα expression in IFNγ-primed THP-1 cells. In contrast, HLA-DR/CD18/PKCβ complex was still formed and able to mediate cell death after cholesterol depletion of these cells. These results indicate that while the integrity of lipid rafts is necessary for the transduction of cytokine gene expression through the HLA-DR/CD18 complex, it is not necessary for the induction of the HLA-DR/CD18-dependent cell death. Thus, our study provides experimental evidence indicating the compartmentalization of HLA-DR/CD18 complex within or outside lipid rafts as a mechanism through which HLA-DR can integrate both PTK and PKCβ signals leading to activation and death, respectively, of activated monocytes. This might provide new insights into how MHC class II signaling may regulate inflammatory response.