Cell Subsets and Pathogen Recognition Requirements Involved in the Innate Response of Primary Human Monocytes and Dendritic Cells to Toxoplasma gondii

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. In this thesis project, I have shown that primary monocytes and dendritic cells from the peripheral blood of healthy donors produce IL-12 and other proinflammatory cytokines when exposed to toxoplasma tachyzoites. I also demonstrated that monocyte derived IL-12 is capable of driving primary human NK cell-IFNγ production in vitro. 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 and endosomal acidification, are required for cytokine induction. Furthermore, by utilizing small molecule inhibitors targeting critical kinases in PRR singling pathways, we demonstrate a TLR-independent, TAK1-dependent mechanism by which CD16+ monocytes respond to T. gondii.

Global transcriptome screening of uninfected and infected CD16+ and CD16- monocytes revealed that both are capable of detecting T. gondii, although only the CD16+ subset secretes proinflammatory cytokines. Additional analysis of the data suggested that the master regulator of translation and metabolic activity, mTORC1/2, controls this response. Metabolic analysis of total monocytes demonstrated a dramatic shift from oxidative phosphorylation to aerobic glycolysis shortly after infection with toxoplasma. Moreover, blockade of PI3K, an upstream regulator of mTOR, inhibited cytokine production, implicating the PI3K/Akt/mTOR pathway in the cellular response elicited by T. gondii. Together these studies reveal that the innate cytokine response of human myeloid cells is distinct from that of the mouse in both the specific subsets involved and that the pathogen sensing mechanism involved differs fundamentally from that utilized by mice. These differences may reflect the direct involvement of rodents, and not humans, in the parasite lifecycle.