Delineating the requirement for the Borrelia burgdorferi virulence factor OspC in the mammalian host

Abstract

We previously demonstrated that outer surface protein C (OspC) of Borrelia burgdorferi is 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 a B. burgdorferi ospC mutant, 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. The ospC mutant 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 of B. burgdorferi within the mammalian host. The second model, evasion of the innate immune system, was tested by assessing the infectivity of the ospC mutant in mice deficient for myeloid differentiation protein 88 (MyD88). Recent studies have shown that B. burgdorferi is prevented from reaching high cell numbers in the mammalian host by MyD88-dependent signaling pathways. The ospC mutant 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.

FIG. 1.

Survival and host adaptation of B. burgdorferi strains in dialysis membrane chambers. (A) Cell lysates of in vitro (IV)- and DMC-grown B31 A3 (wild type) were immunoblotted and incubated with pooled sera from mice infected with B. burgdorferi. (B) Silver-stained SDS-polyacrylamide gel comparing B. burgdorferi strains grown in vitro (IV) to those grown in DMCs. Protein bands correlating to OspC and other lower-molecular-weight proteins are induced in DMC-cultured B. burgdorferi cells (except in the mutant strain, which lacks OspC). Molecular mass standards (in kilodaltons) are indicated on the outside of both panels.

FIG. 2.

Two-dimensional NEPHGE immunoblot of host-adapted B. burgdorferi. The immunoblot was incubated with pooled sera from mice infected with B. burgdorferi. Acid ends are to the left. Presumptive identities of OspC and other proteins are indicated based on the molecular weight and position in NEPHGE, relative to previously identified proteins (32). Asterisks denote proteins that may be at lower concentrations in the OspC mutant than in wild-type and complemented strains. Positions of molecular mass standards (in kilodaltons) are indicated. P39 has also been referred to as BmpA (41).