MyD88 mediates instructive signaling in dendritic cells and protective inflammatory response during rickettsial infection

Spotted fever group rickettsiae cause potentially life-threatening infections throughout the world. Several members of the Tolllike receptor (TLR) family are involved in host response to rickettsiae, and yet the mechanisms by which these TLRs mediate host immunity remain incompletely understood. In the present study, we found that host susceptibility of MyD88^-/- mice to infection with Rickettsia conorii or Rickettsia australis was significantly greater than in wild-type (WT) mice, in association with severely impaired bacterial clearance in vivo. R. australis-infected MyD88^-/- mice showed significantly lower expression levels of gamma interferon (IFN-γ), interleukin-6 (IL-6), and IL-1β, accompanied by significantly fewer inflammatory infiltrates of macrophages and neutrophils in infected tissues, than WT mice. The serum levels of IFN-γ, IL-12, IL-6, and granulocyte colonystimulating factor were significantly reduced, while monocyte chemoattractant protein 1, macrophage inflammatory protein 1α, and RANTES were significantly increased in infected MyD88^-/- mice compared to WT mice. Strikingly, R. australis infection was incapable of promoting increased expression of MHC-IIhigh and production of IL-12p40 in MyD88^-/- bone marrow-derived dendritic cells (BMDCs) compared to WT BMDCs, although costimulatory molecules were upregulated in both types of BMDCs. Furthermore, the secretion levels of IL-1β by Rickettsia-infected BMDCs and in the sera of infected mice were significantly reduced in MyD88^-/- mice compared to WT controls, suggesting that in vitro and in vivo production of IL-1β is MyD88 dependent. Taken together, our results suggest that MyD88 signaling mediates instructive signals in DCs and secretion of IL-1β and type 1 immune cytokines, which may account for the protective inflammatory response during rickettsial infection.