Treponema pallidum delays the apoptosis of human polymorphonuclear neutrophils through the intrinsic and extrinsic pathways

Abstract

Treponema pallidum is a "stealth pathogen" responsible for infectious sexually transmitted diseases. Although neutrophils are usually present in skin lesions of early syphilis, the role of these cells in T. pallidum infection has barely been investigated. Neutrophils are short-lived cells that undergo constitutive apoptosis, and phagocytosis usually accelerates this process. Here, we demonstrated that human polymorphonuclear neutrophils (hPMNs) could phagocytose T. pallidum in vitro. An unexpected discovery was that T. pallidum inhibited hPMNs apoptosis markedly in an opsonin-independent manner. Furthermore, this phenomenon was not affected by bacterial viability, as detected by annexin V, morphology studies, and TUNEL staining. Exploration of the underlying mechanism showed that expression of the cleaved forms of caspase-3, -8, and -9 and effector caspase activity were diminished significantly in T. pallidum-infected hPMNs. T. pallidum also impaired staurosporine- and anti-Fas-induced signaling for neutrophil apoptosis. Of note, these effects were accompanied by inducing the autocrine production of the anti-apoptotic cytokine IL-8. Taken together, our data revealed that T. pallidum could inhibit the apoptosis of hPMNs through intrinsic and extrinsic pathways and provide new insights for understanding the pathogenicity mechanisms of T. pallidum.

Keywords: Apoptosis; IL-8; Neutrophil; Opsonization; Phagocytosis; Treponema pallidum.