Hepatitis C virus-specific Th17 cells are suppressed by virus-induced TGF-β

IL-17-secreting T (Th17) cells play a protective role in certain bacterial infections, but they are major mediators of inflammation and are pathogenic in organ-specific autoimmune diseases. However, human Th17 cells appear to be resistant to suppression by CD4⁺CD25⁺FoxP3⁺ regulatory T cells, suggesting that they may be regulated by alternative mechanisms. Herein we show that IL-10 and TGF-β suppressed IL-17 production by anti-CD3-stimulated PBMC from normal individuals. TGF-β also suppressed IL-17 production by purified CD4⁺ T cells, whereas the inhibitory effect of IL-10 on IL-17 production appears to be mediated predominantly by its effect on APC. An examination of patients infected with hepatitis C virus (HCV) demonstrated that Ag-specific Th17 cells are induced during infection and that these cells are regulated by IL-10 and TGF-β. PBMC from HCV Ab-positive donors secreted IL-17, IFN-γ, IL-10, and TGF-β in response to stimulation with the HCV nonstructural protein 4 (NS4). Furthermore, NS4 induced innate TGF-β and IL-10 expression by monocytes from normal donors and at higher levels from HCV-infected patients. Neutralization of TGF-β, and to a lesser extent IL-10, significantly enhanced NS4-specific IL-17 and IFN-γ production by T cells from HCV-infected donors. Our findings suggest that both HCV-specific Th1 and Th17 cells are suppressed by NS4-induced production of the innate anti-inflammatory cytokines IL-10 and TGF-β. This may represent a novel immune subversion mechanism by the virus to evade host-protective immune responses. Our findings also suggest that TGF-β and IL-10 play important roles in constraining the function of Th17 cells in general.