Abstract
Toxoplasma gondii is an intra-cellular protozoan parasite that can infect almost all nucleated cells, eliciting host immune responses against infection. Host tissue damage is mainly caused by cellular lysis when T. gondii egresses from infected cells. However, the effects of cytokines released by host immune cells on egression of T. gondii remain elusive. This study aimed to investigate the role of tumor necrosis factor-alpha (TNF-α) on the egress of T. gondii from infected human foreskin fibroblast (HFF) cells and to elucidate the underlying mechanisms that regulate TNF-α-induced egress. Using flow cytometry to count tachyzoites of T. gondii released into cell culture medium, we found that egress of T. gondii from infected HFF cells could be induced by 10 ng/mL TNF-α in a time-dependent manner. Pre-treatment of infected HFF cells with BAPTA-AM to chelate intra-parasitic calcium could greatly inhibit TNF-α-induced egress. Similar results were obtained when using cytochalasin D to block parasite motility before the TNF-α-induced egress assay. In addition, blocking host apoptosis by Z-VAD-FMK could decrease TNF-α induced egress, while blocking necroptosis by necrostatin-1 has little impact on TNF-α-induced egress. The egressed tachyzoites displayed a normal growth rate and lost no virulence. Our results suggest that host cytokines could influence the cellular lytic processes of T. gondii, providing new insights into the relationship between host TNF-α and T. gondii pathogenesis.
Highlights
Toxoplasma gondii is an obligate intra-cellular parasite that can infect almost all kinds of nucleated cells in warmblooded animals, leading to toxoplasmosis [6]
To test whether exogenous tumor necrosis factor-alpha (TNF-a) could trigger early egress of T. gondii, human foreskin fibroblast (HFF) infected with tachyzoites for 36 h were treated with 10 ng/mL tumor necrosis factor (TNF)-a for various time periods
Previous studies showed that various chemicals, such as DTT, calcium ionophore A23187 and potassium ionophore nigericin, can trigger early egress of T. gondii from infected cells [2,5,20]
Summary
Toxoplasma gondii is an obligate intra-cellular parasite that can infect almost all kinds of nucleated cells in warmblooded animals, leading to toxoplasmosis [6]. Parasitic motility plays a critical role in T. gondii egress [3]. Toxoplasma perforin-like protein 1 (TgPLP1) displays structural features necessary for pore formation, which facilitates the natural egress of tachyzoites from infected cells [10]. Other parasitic proteins, such as Toxoplasma calciumdependent protein kinase 1 (TgCDPK1) and TgCDPK3 [11,13], have been reported as molecular regulators for egress of T. gondii, both of which function in a calcium (Ca2+)-dependent manner. In a Staphylococcus aureus a-toxin-induced egress assay and a nigericininduced egress assay [5,16], it was found that T. gondii detect potassium (K+) efflux from host cells before moving out of infected cells
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