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Invasion by Toxoplasma gondii protects human-derived HL-60 cells from actinomycin D-induced apoptosis
Abstract:
Intracellular microorganisms have to rely on the integrity of their host cells to persist. We, therefore, investigated the effect of infections with different Toxoplasma gondii strains on apoptosis of human-derived HL-60 cells at the single cell level. Infection with either mouse-avirulent (NTE strain) or virulent parasites (RH strain) did not induce apoptosis of HL-60 cells as compared to uninfected controls. In contrast, treatment with actinomycin D (act D) led to apoptosis in 15-25% of the cells. However, concomitant infection with T. gondii clearly abrogated act D-induced apoptosis. This was especially apparent in those host cells that were actually infected; in these parasite-positive cells the rate of apoptosis decreased by 82.8+/-4.3% (mean+/-SEM, P=0.017, Student's t-test) and 91.7+/-3.4% (P= 0.024) after infection with either the NTE or the RH strain, respectively. Inhibition of host cell apoptosis was similarly observed in cells which had been invaded by UV-irradiated, non-replicating parasites (P=0.001, Student's t-test). However, incubation with heat-killed parasites or T. gondii lysates did not abrogate act D-induced apoptosis. In conclusion, inhibition of apoptosis by living, but not necessarily replicating T. gondii may facilitate parasite survival and persistence within its host cell.
Insights
Toxoplasma gondii infection prevents host cell death (apoptosis), even when cells are treated with a drug that normally induces it. This suggests living T. gondii actively inhibits apoptosis to survive within host cells.
Area of Science:
- Cell Biology
- Parasitology
- Immunology
Background:
- Intracellular microorganisms depend on host cell integrity for survival.
- Apoptosis, or programmed cell death, is a critical cellular process.
- Understanding host-pathogen interactions is key to controlling infections.
Purpose of the Study:
- To investigate the impact of Toxoplasma gondii infection on host cell apoptosis.
- To determine if different T. gondii strains affect apoptosis differently.
- To elucidate the mechanism by which T. gondii influences host cell survival.
Main Methods:
- Exposure of human HL-60 cells to virulent (RH) and avirulent (NTE) T. gondii strains.
- Induction of apoptosis using actinomycin D (act D).
- Single-cell analysis to quantify apoptosis rates in infected and uninfected cells, including those with UV-inactivated or heat-killed parasites.
Main Results:
- T. gondii infection did not induce apoptosis in HL-60 cells compared to controls.
- Actinomycin D induced apoptosis in 15-25% of untreated cells.
- T. gondii infection significantly abrogated act D-induced apoptosis, with reductions of 82.8% (NTE) and 91.7% (RH) in infected cells.
- Apoptosis inhibition was observed with both replicating and UV-inactivated T. gondii, but not with heat-killed parasites or lysates.
Conclusions:
- Living Toxoplasma gondii actively inhibits host cell apoptosis.
- This inhibition of apoptosis, even by non-replicating parasites, likely facilitates parasite survival and persistence.
- The findings highlight a key mechanism of host immune evasion by T. gondii.