Borrelia burgdorferi clinical isolates induce human innate immune responses that are not dependent on genotype

Borrelia burgdorferi can be categorized based on restriction fragment length polymorphism analysis into ribosomal spacer type (RST) 1, 2 and 3. A correlation between RST type and invasiveness of Borrelia isolates has been demonstrated in clinical studies and experimental models, and RST 1 isolates are more likely to cause disseminated disease than RST 3 isolates. We hypothesized that this could partially be due to increased susceptibility of RST 3 isolates to killing by the innate immune system early in infection. Thus, we investigated the interaction of five RST 1 and five RST 3 isolates with various components of the human innate immune system in vitro. RST 3 isolates induced significantly greater upregulation of activation markers in monocyte-derived dendritic cells compared to RST 1 isolates at a low multiplicity of infection. However, RST 1 isolates stimulated greater interleukin-6 production. At a high multiplicity of infection no differences in dendritic cell activation or cytokine production were observed. In addition, we observed no differences in the ability of RST 1 and RST 3 isolates to activate monocytes or neutrophils and all strains were phagocytosed at a comparable rate. Finally, all isolates tested were equally resistant to complement-mediated killing, as determined by dark-field microscopy and a growth inhibition assay. In conclusion, we demonstrate that the RST 1 and 3 isolates showed no distinction in their susceptibility to the various components of the human immune system studied here, suggesting that other factors are responsible for their differential invasiveness.