Abstract
Monocytes are among the major myeloid cells that respond to Toxoplasma, a ubiquitous foodborne that infects ≥ 1 billion people worldwide, in human peripheral blood. As such, a molecular understanding of human monocyte-Toxoplasma interactions can expedite the development of novel human toxoplasmosis control strategies. Current molecular studies on monocyte-Toxoplasma interactions are based on average cell or parasite responses across bulk cell populations. Although informative, population-level averages of monocyte responses to Toxoplasma have sometimes produced contradictory results, such as whether CCL2 or IL12 define effective monocyte responses to the parasite. Here, we used single-cell dual RNA sequencing (scDual-Seq) to comprehensively define, for the first time, the monocyte and parasite transcriptional responses that underpin human monocyte-Toxoplasma encounters at the single cell level. We report extreme transcriptional variability between individual monocytes. Furthermore, we report that Toxoplasma-exposed and unexposed monocytes are transcriptionally distinguished by a reactive subset of CD14+CD16- monocytes. Functional cytokine assays on sorted monocyte populations show that the infection-distinguishing monocytes secrete high levels of chemokines, such as CCL2 and CXCL5. These findings uncover the Toxoplasma-induced monocyte transcriptional heterogeneity and shed new light on the cell populations that largely define cytokine and chemokine secretion in human monocytes exposed to Toxoplasma.
Highlights
A majority of lethal human pathogens spend a significant part of their life-cycle inside immune cells, mostly monocytes and m acrophages[1,2]
Monocytes are among the major myeloid cells that respond to the parasite by secreting interleukin 12 (IL12)[10], which is required to induce the production of the indispensable anti-Toxoplasma interferon-gamma (IFNγ) c ytokine[11]
Unsupervised graph-based clustering partitioned the unexposed monocytes into two clusters: hU1 and hU2, which correspond to the non-classical (CD14+CD16+) and classical (CD14+CD16-) monocyte subsets, respectively[27], (Fig. 1A)
Summary
A majority of lethal human pathogens spend a significant part of their life-cycle inside immune cells, mostly monocytes and m acrophages[1,2]. Host–pathogen interactions are mostly a single cell problem involving dynamic host and pathogen gene regulatory programs that often produce distinct outcomes in individual cells within a host. Usually studied in bulk host and/or parasite cells, when Toxoplasma interacts with monocytes from the same host, several possible outcomes can occur simultaneously to produce distinct monocyte and parasite subpopulations. Unlike IL12 secretion that required phagocytic parasite uptake by the monocytes[10], CCL2 secretion was dependent on a soluble e ffector[17] To resolve such discrepancies and characterize heterogeneous monocyte-Toxoplasma interactions, we need to go beyond population averages and define single cell responses that when combined represent the entire monocyte and parasite responses. We find that Toxoplasma-exposed and control monocytes are transcriptionally distinguished by non-classical monocytes and a novel subset of reactive classical monocytes
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