Toxoplasma gondii dense granule antigen 1 stimulates apoptosis of monocytes through autocrine TGF-β signaling

Monocyte/macrophages represent the first line of defense against protozoan parasites. Different mechanisms of monocyte suppression by Toxoplasma gondii that sustain parasite invasion and persistence have been described, including apoptosis. In the present study, we investigated the effect of microbial excretory–secretory polypeptides, namely the microneme protein MIC3 and the dense granule antigen GRA1, on apoptosis of monocytes from patients with congenital toxoplasmosis and healthy individuals. We found that GRA1 but not MIC3 could induce apoptosis of monocytes, observing the effect in samples from both Toxoplasma-infected and uninfected individuals, thus ruling out involvement of mechanisms of apoptosis linked to adaptive immunity or a cellular context related to infection. Selective inhibition of TGF-β type I receptors reversed GRA1-induced apoptosis, indicating that this apoptosis involved canonical TGF-β signaling. By using TGF-β-neutralizing antibodies, we showed that monocyte apoptosis required endogenous TGF-β and that GRA1 stimulation activated TGF-β transcription and expression in monocytes but not lymphocytes, suggesting involvement of an autocrine TGF-β-mediated mechanism in GRA1-induced apoptosis.