Phagocytosis is required for multiple innate immune responses to Treponema pallidum and Borrelia burgdorferi

Treponema pallidum (Tp) and Borrelia burgdorferi (Bb) are the etiologic agents of syphilis and Lyme disease respectively. Despite similarities in the inflammatory responses elicited by Tp and Bb, they elicit some striking clinical differences. For example, Bb induces marked constitutional symptoms early in the course of Lyme disease, despite achieving only low spirochete numbers in the blood. In contrast, Tp rarely induces systemic symptomatology in primary or secondary syphilis, when spirochetal burdens are believed to be at their highest. The goal of this study was to characterize initial spirochete-leukocyte interactions and to compare the early innate immune responses to Bb and Tp. We developed a human ex vivo system of freshly isolated peripheral blood mononuclear cells (PBMCs) cultured as a mixed cell system, and we stimulated these cells with live or lysed preparations of Bb or Tp. We found that live Bb induced dramatically greater cellular and cytokine responses than Bb lysates or live Tp. Specifically, live Bb induced a significantly greater level of macrophage activation and inflammatory cytokine production than Bb lysates. Live Bb also induced early T cell activation and an innate wave of IFNγ from NK cells whereas lysates did not. Interestingly, live Tp did not induce the robust inflammation seen in response to live Bb, nor did it elicit T cell activation or IFNγ production. We also found that Bb is internalized rapidly by human monocytes while Tp is not, prompting us to investigate the role of internalization in initiating the immune response. Blocking spirochete internalization with Cytochalasin D reduced the response elicited by live Bb and live Tp, but not spirochetal lysates, indicating that lysates activate immune cells via ligation of cell surface receptors. Furthermore, in the presence of opsonic antibody (human syphilitic serum), Tp was internalized by human monocytes and initiated an immune cascade resembling that seen with live Bb: high levels of macrophage activation and cytokine production, early T cell activation, and IFNγ production. Taken collectively, these results suggest that whether immune cells are activated by spirochetes via the cell surface or from within a phagolysosome has critical consequences on downstream immune events.