The cat's out of the bag: Toxoplasma gondii provides further insight into myeloid-mediated host defense

Abstract

The obligate intracellular protozoan pathogen Toxoplasma gondii is estimated to infect a third of the world's population. Toxoplasmosis is considered a significant worldwide disease that can lead to morbidity or death in immunocompromised individuals. Host defense against T. gondii has been demonstrated to be dependent on a rapid myeloid cell and lymphocyte response working in concert to quickly eliminate the invading pathogen. Classically, T-bet-dependent group 1 innate lymphocytes (ILC1s), natural killer (NK) cells, and CD4+ T cell-derived interferon-γ (IFN-γ) are considered indispensable for host resistance against T. gondii. However, recent discoveries have illustrated that T-bet is not required for NK cell- or CD4+ T cell-derived IFN-γ. Yet, lack of T-bet still results in rapid mortality, pointing to a T-bet-dependent myeloid cell-mediated host defense pathway. This review summarizes the myeloid cell-mediated immune response against T. gondii and provides insights into the lesser known components of the T-bet-dependent myeloid cell-dependent host defense pathway for pathogen clearance.

Keywords: T-bet; immunoparasitology; innate immunity; myeloid cells.

Figure 1.

Myeloid cells involved in innate defense against T. gondii. Myeloid cells, such as neutrophils, monocytes, macrophages, DCs, and TRMs, employ a variety of defense mechanisms via cytokine production, chemoattractant release, and specialized antimicrobial mechanisms such as NETs, oxygen radicals, IRGs, and GBPs. This multifaceted functionality is critical for recruiting additional innate immune cells, initiating trained immunity, priming of the adaptive immune response, and ultimately, rapid parasite clearance. While the protective measures for each cell type vary greatly, overlap in overall function of such measures between multiple cell types highlights the need for redundant processes to ensure host survival. Emboldened text indicates pathways that have been established as directly relevant to host defense against T. gondii. Created in BioRender (https://biorender.com/c91w081).

Figure 2.

T-bet–mediated host defense during T. gondii infection. The transcription factor T-bet is critical for host survival. Here we illustrate the involvement of T-bet in the host immune response against T. gondii. Recognition of the parasite by myeloid cells, via TLRs and NLRs, contributes to a robust protective response, maintained through production and release of IL-12 and IL-18. The effector function of T-bet–dependent ILC1s and NK cells is dependent on myeloid cell–derived IL-12 and IL-18. T-bet orchestrates the innate immune response through expression in ILC1s, NK cells, Th1 cells, and a recently defined population of TMCs, that are mediated by IL-12. While additional innate immune cells are required for overall host survival, recent studies have demonstrated that mice conditionally lacking T-bet within CD11c⁺ cells succumb to infection at the same rate as Tbx21^−/− mice, suggesting that TMCs may play a crucial role in host defense. Upregulation of IFN-γ is another critical component of host defense; its classical production by ILC1s, NK cells, and Th1 cells. A novel finding of neutrophil-derived IFN-γ provides a method for production of this cytokine in a TLR- and IL-12–independent manner. IFN-γ also promotes TRM cell death, a protective measure aimed at limiting parasite replication within the phagocyte population. Created using BioRender (https://BioRender.com/x12o358).

Figure 3.

Frequency of T-bet expression within myeloid populations. C57BL/6J mice were i.p. infected with 20 cysts and peritoneal exudate cells were harvested on day 5 post-infection. CITE-seq was performed by using TotalSeq antibody cocktail (BioLegend) according to the manufacturer’s instructions. Raw FASTQ files were mapped to the GRCm39 reference genome using 10x Genomics Cell Ranger 9.0.0 to infer read counts of both gene expression (RNA) and antibody-derived tag per gene per cell. Quality control is performed using Seurat package v5.0 by removing cells with fewer than 500 or more than 7,500 detected genes, and cells fewer than 1,000 or more than 50,000 unique molecular identifiers, and cells greater than 20% mitochondrial content. Following quality control, SCTransform was used for normalizing the RNA read counts. Principal component analysis (PCA) is performed for dimension reduction on the normalized RNA read counts for the purpose of cell clustering. Specifically, the top 30 principal components were used with a resolution of 0.5. (A) t-Distributed stochastic neighbor embedding for cluster visualization. Antibody-derived tag counts were normalized using the centered log ratio method. Clusters were further annotated based on known marker genes or antibody levels. (B) Particularly, Tbx21 expression levels were subsequently visualized across annotated clusters. Through unpublished CITE-seq results, our group has identified a strong expression of Tbx21 within a subset of Ly6C^Lo-neg monocytes and DCs. While limited expression exists within other cell types, a majority of T-bet in myeloid populations is found within a distribution that we believe contains TMCs. Further analysis is being conducted to determine the qualities of this population and isolate them for further experiments to determine their nature of functionality.