Treatment of cell lines with type I IFNs activates the formation of ISGF3 (STAT1/STAT2/IRF9), which induces the expression of many genes. To study this response in primary cells, we treated fresh human blood with IFN-β and used flow cytometry to analyze phosphorylated STATs1, 3 and 5 in CD4+ and CD8+ T cells, B cells, and monocytes. The activation of STAT1 was remarkably different among these leukocyte subsets. In contrast to monocytes, CD4+ and CD8+ T cells, few B cells activated STAT1 in response to IFN-β, a finding that could not be explained by decreased levels of IFNAR2 or STAT1 or enhanced levels of SOCS1 or relevant protein tyrosine phosphatases in B cells. Micro-array and real-time PCR analyses revealed the induction of STAT1-dependent pro-apoptotic mRNAs in monocytes but not in B cells. These data show that ISGF3 or STAT1 homodimers are not the main activators of gene expression in primary B cells of healthy humans. Notably, in B cells and especially in CD4+ T cells, IFN-β activated STAT5 in addition to STAT3, with biological effects often opposite from those driven by activated STAT1. These data help to explain why IFN-β increases the survival of primary human B cells and CD4+ T cells, but enhances the apoptosis of monocytes, and also to understand how leukocyte subsets are differentially affected by endogenous type I IFNs during viral or bacterial infections, and by type I IFN treatment of patients with multiple sclerosis, hepatitis or cancer.
The mechanism of IFN-β therapy in relapsing-remitting multiple sclerosis (RRMS) is not well understood, but induction of apoptosis in specific leukocyte subsets is likely to be important. Enhanced expression of TNFSF10 or TNF-related apoptosis-inducing ligand (TRAIL) mRNA in unseparated leukocytes has been put forward as a therapeutic response marker, but it is unclear which leukocyte subsets express TRAIL. We investigated the basis of TRAIL expression in response to IFN-β by studying activation of STATs 1, 3, and 5, p38 MAPK, and NF-κB in different leukocyte subsets of patients with RRMS. Monocytes, B cells, and T cells showed substantial differences in the activation of p38 and the STATs in response to i.m. injection of IFN-β1a or stimulation in vitro. Induction of cellsurface TRAIL, analyzed in nine leukocyte subsets, was observed only on monocytes and granulocytes and correlated with the activation of p38 and/or NF-κB in these subsets only, in agreement with previous work in fibroblasts showing that the induction of TRAIL in response to IFN-β depends on the activation of p38 and NF-κB as well as STATs 1 and 2. We propose that, in myeloid cells, the differential activation of p38 and NF-κB and induction of TRAIL, which sensitizes cells to apoptosis, can help to explain differences in responsiveness to IFN-β therapy among patients with RRMS and, furthermore, that such differential patterns of activation and expression may also be important in understanding the therapeutic responses to IFN-α/β in hepatitis and cancer.humans | phosphoproteins | signal transduction | flow cytometry
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