Delineating the Requirement for theBorrelia burgdorferiVirulence Factor OspC in the Mammalian Host

We previously demonstrated that outer surface protein C (OspC) ofBorrelia burgdorferiis essential for establishing mammalian infection. However, the role of OspC in mammalian infection is unknown. Here, we report experiments designed to distinguish between two models of OspC function in the mammalian host: (i) OspC fulfills an essential physiological role for growth and host adaptation or (ii) OspC provides a protective role for evasion of components of the innate immune response. We found that aB. burgdorferi ospCmutant, previously demonstrated to be noninfectious in both immunocompetent and SCID mice, could survive in the relatively immune-privileged environment of dialysis membrane chambers implanted within the peritoneum of a rat. TheospCmutant also adapts to the mammalian environment, as determined by the protein profiles of the chamber-cultivated spirochetes. Therefore, OspC does not appear to provide a physiological function for the survival ofB. burgdorferiwithin the mammalian host. The second model, evasion of the innate immune system, was tested by assessing the infectivity of theospCmutant in mice deficient for myeloid differentiation protein 88 (MyD88). Recent studies have shown thatB. burgdorferiis prevented from reaching high cell numbers in the mammalian host by MyD88-dependent signaling pathways. TheospCmutant was incapable of infecting MyD88-deficient mice, suggesting that the role of OspC cannot be related solely to evasion of MyD88-mediated innate immunity. These results reiterate the importance of OspC in mammalian infection and eliminate simple models of function for this enigmatic protein.