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

Toxoplasma gondii is an intracellular protozoan parasite that infects rodents as part of its natural transmission cycle and induces disease in humans, an end-stage host. As one of the natural hosts of T. gondii, the mouse has been used extensively for elucidating the cellular and molecular basis of immunity to this pathogen while relatively few studies have focused on the response of humans. In our recent work, we identified CD16⁺ monocytes and DC1 dendritic cells as the major myeloid cell populations that respond to T. gondii in human peripheral blood. Interestingly, these myeloid subsets represent the opposite counterparts from those triggered by the parasite in mice. Moreover, whereas the innate cytokine response to T. gondii in the mouse involves stimulation of Toll-like receptors by a soluble parasite ligand, the response of human cells instead requires phagocytosis of the live pathogen. We speculate that these marked distinctions in the pathways utilized for innate recognition of toxoplasma in mouse and man reflect the differing roles of the two hosts in the biology of this parasite.