The IL-12 Response of Primary Human Dendritic Cells and Monocytes to Toxoplasma gondii Is Stimulated by Phagocytosis of Live Parasites Rather Than Host Cell Invasion

Abstract

As a major natural host for Toxoplasma gondii, the mouse is widely used for the study of the immune response to this medically important protozoan parasite. However, murine innate recognition of toxoplasma depends on the interaction of parasite profilin with TLR11 and TLR12, two receptors that are functionally absent in humans. This raises the question of how human cells detect and respond to T. gondii. Here, we show that primary monocytes and dendritic cells from peripheral blood of healthy donors produce IL-12 and other proinflammatory cytokines when exposed to toxoplasma tachyzoites. Cell fractionation studies determined that IL-12 and TNFα secretion is limited to CD16+ monocytes and the CD1c+ subset of dendritic cells. In direct contrast to their murine counterparts, human myeloid cells fail to respond to soluble tachyzoite extracts and instead require contact with live parasites. Importantly, we found that tachyzoite phagocytosis, but not host cell invasion, is required for cytokine induction. Together these findings identify CD16+ monocytes and CD1c+ dendritic cells as the major myeloid subsets in human blood producing innate cytokines in response to T. gondii and demonstrate an unappreciated requirement for phagocytosis of live parasites in that process. This form of pathogen sensing is distinct from that utilized by mice, possibly reflecting a direct involvement of rodents and not humans in the parasite life cycle.

NLRP3 and Potassium Efflux Drive Rapid IL-1β Release from Primary Human Monocytes during Toxoplasma gondii Infection

“…In addition, primary monocytes are significantly more phagocytic than THP-1 cells, and in our experiments, ~30% of the intracellular T. gondii in primary monocytes are the result of phagocytosis events (data not shown). Indeed, phagocytosis of T. gondii by primary monocytes likely contributes to some of the IL-1β signal observed, as phagocytosis induces a stronger cytokine response than does invasion by the parasite (48). …”

Innate recognition of Toxoplasma gondii in humans involves a mechanism distinct from that utilized by rodents

“…Moreover, we observed that direct parasite-host cell contact is critical for TNF and IL-12 induction. 42 These findings are at odds with a recent study by Salazar Gonzalez et al , 43 who showed that T. gondii profilin stimulates human monocytes to secrete IL-6 and IL-12 in a TLR5-dependent manner. The basis of this discrepancy is unclear but may relate to the purity of the profilin or cell populations employed by the two laboratories.…”

“…42 Because of its major role in Th1 immunity, we decided to use IL-12 as the primary read-out in our subsequent experiments. The bioactivity of the IL-12 produced was confirmed in co-culture experiments in which stimulated monocytes as well as their supernatant were shown to trigger IFN-γ production from purified NK cells in an IL-12-dependent manner.…”

“…The above experiments established a model for studying innate recognition of T. gondii by human myeloid cells. 42 …”

“…The failure of the mDC2 and CL myeloid cells to produce IL-12 following T. gondii stimulation could not be accounted for by a general defect in the synthesis of the cytokine by these subsets as they were fully responsive when stimulated with LPS and R848. 42 …”