IFN-γ generates maturation-arrested dendritic cells that induce T cell hyporesponsiveness independent of Foxp3+ T-regulatory cell generation

Interferon-γ (IFN-γ) is a proinflammatory cytokine that induces the proliferation of T-helper 1 cells that contribute to allograft rejection. Surprisingly, allografts transplanted in IFN-γ deficient mice are rapidly rejected, suggesting that this cytokine has a paradoxical role in regulating alloimmune responses. Since dendritic cells (DC) play an essential role in initiating allograft rejection the effect of IFN-γ on DC differentiation, maturation and function in vitro were investigated. DC were differentiated with IL4/GMCSF and treated with IFN-γ at day 0 (IFN-γ-DC0) or day 5 (IFN-γ-DC5) during maturation and compared with untreated DC (UT-DC). Flow cytometric analysis of IFN-γ-DC0 demonstrated a downregulation in the DC maturation marker CD83 by 90% whereas the expression of the inhibitory molecules ILT2, ILT3 and ILT4 were upregulated. Inhibition of relB mRNA expression (79%; p = 0.01) and IL-12 (97%; p = 0.02) compared to UT-DC further confirmed that IFN-γ-DC0 were 'maturation-arrested'. Moreover, IFN-γ-DC0 inhibited allogeneic T cell proliferation by 33% (p = 0.02) compared to UT-DC. However, induction of T cell hyporesponsiveness by IFN-γ-DC0 was not regulated by the generation of CD4+Foxp3+ T cells nor due to IFN-γ induced inhibitory molecules, HLA-G and IDO. In contrast, IFN-γ-DC5 expressed higher levels of costimulatory molecules and MHC class II compared to UT-DC and did not cause T cell hyporesponsiveness. Thus, the timing of IFN-γ treatment of monocytes prior to their differentiation to DC is critical for generating DC that regulate T cell function. IFN-γ may therefore play a regulatory role in alloimmunity by acting on DC precursors.