Vesicular stomatitis virus M protein mutant stimulates maturation of Toll-like receptor 7 (TLR7)-positive dendritic cells through TLR-dependent and -independent mechanisms.

Wild-type (wt) vesicular stomatitis virus (VSV) strains stimulate plasmacytoid dendritic cells (pDC) through Toll-like receptor 7 (TLR7) and its adaptor molecule, MyD88. Granulocyte-macrophage colony-stimulating factor-derived DC (G-DC), which do not express TLR7, are unresponsive to wt VSV due to inhibition of cellular gene expression by the matrix (M) protein. In contrast to its recombinant wt (rwt) counterpart, an M protein mutant of VSV, rM51R-M virus, stimulates maturation of G-DC independently of MyD88. These results suggest that, as in the case of G-DC, rM51R-M virus may stimulate pDC by mechanisms distinct from that by rwt virus. Studies presented here demonstrate that both rwt and rM51R-M viruses induced maturation of TLR7-positive DC derived by culture in the presence of Flt3L (F-DC), with the subsequent expression of type I interferon (IFN). F-DC are a mixture of myeloid (CD11b(+)) and plasmacytoid (B220(+)) DC, both of which respond to TLR7 ligands. Separated CD11b(+) and B220(+) F-DC responded to both rwt and rM51R-M viruses. Both viruses were also defective at inhibiting host gene expression in F-DC, including the expression of genes involved in the antiviral response. The data from F-DC generated from IFN receptor knockout mice demonstrated that the maturation of F-DC induced by rwt virus was dependent on the type I IFN response, while maturation induced by rM51R-M virus was partially dependent on this molecule. Therefore, activation of the type I IFN pathway appears to be important for not only inducing an antiviral response but also for stimulating maturation of F-DC upon virus infection. Importantly, F-DC from TLR7 and MyD88 knockout mice did not undergo maturation in response to rwt virus, while maturation induced by rM51R-M virus was largely independent of both molecules. These results indicate that although both viruses induce F-DC maturation, F-DC detect and respond to rM51R-M virus by means that are distinct from rwt virus. Specifically, this mutant virus appears capable of inducing DC maturation in a wide variety of DC subsets through TLR-dependent and independent mechanisms.