Date Published: March 9, 2017
Publisher: Public Library of Science
Author(s): Shruthi Krishnamurthy, Eleni K. Konstantinou, Lucy H. Young, Daniel A. Gold, Jeroen P. J. Saeij, Laura J Knoll.
Toxoplasma gondii is an obligate intracellular parasite that can infect a broad range of warm-blooded animals. In humans, it can cause serious disease in immune-compromised individuals or if contracted congenitally. In North America and Europe, strains belonging to the clonal types I, II, and III haplogroups predominate, while in South America a large variety of other “atypical” strains exist. These strains differ enormously in virulence in mice and there is evidence that different strains can also cause variable pathology in humans. In mice, the cytokine interferon gamma (IFNγ) induces multiple toxoplasmacidal mechanisms and therefore plays a crucial role in immunity to Toxoplasma. Compared to mice, humans are more resistant to Toxoplasma. This is surprising because humans lack the Toll-like receptors 11/12 (TLR11/12) found in mice that bind to the Toxoplasma protein profilin and trigger a signaling cascade leading to the production of interleukin 12 (IL12), a key cytokine that stimulates IFNγ production. Humans also lack the multitude of murine immunity-related GTPases (IRGs) that are induced upon IFNγ stimulation and play a crucial role in the destruction of the membrane surrounding the parasitophorous vacuole (PV) in which Toxoplasma resides in the host cytoplasm. Thus, the mechanisms involved in the production of IFNγ in humans (Fig 1) differ from those in mice and the pathways that mediate parasite clearance are less well understood (reviewed in ). In this review, we focus on two mechanisms of Toxoplasma clearance by human cells: autophagy and cell death.
Autophagy is a degradation process that clears cytoplasmic material such as misfolded proteins and damaged organelles. Canonical autophagy involves the formation of a double membrane structure, the phagophore, which elongates to engulf cytoplasmic material. During autophagy, cytosolic microtubule-associated protein light chain 3 (LC3), a ubiquitin-like protein, is processed and conjugated to the lipid phosphatidylethanolamine (PE) to form LC3-II on the autophagosome membrane by a set of autophagy-related proteins (ATGs). Ubiquitinylated cargo bound to ubiquitin-binding proteins such as p62 and nuclear dot protein 52 kDa (NDP52) then gets recruited through these proteins’ LC3-interacting regions . The double membrane structure eventually closes to form the autophagosome, which ultimately fuses with lysosomes leading to degradation of the ubiquitinylated cargo.
The costimulatory protein cluster of differentiation 40 (CD40) is expressed on the surface of antigen presenting cells (APCs), such as macrophages, and on some nonhematopoietic cells. In human and murine cells, CD40 interacts with its CD40L ligand, which is expressed on the surface of T cells, leading to T cell production of IFNγ, which is only partially dependent on macrophage production of IL12 (Fig 1). CD40–CD40L interactions lead to autocrine production of Tumor Necrosis Factor (TNFα), which is needed for CD40–CD40L-signaling to induce autophagy-mediated clearing of Toxoplasma-containing PVs by fusion of LC3-decorated PVs with lysosomes (Fig 1) . The importance of CD40–CD40L-mediated Toxoplasma destruction in humans is supported by the fact that patients with HyperIgM syndrome—where the communication between T cells and APCs is impaired due to defective CD40L expression—are more susceptible to Toxoplasma . Furthermore, CD4 T cells from HIV patients, which are defective in CD40L induction, display impaired cell-mediated immunity against Toxoplasma infection . A more dominant role for CD40–CD40L-mediated immunity in humans compared to rodents may explain why IL12/IFNγ pathway-deficient human patients are not more susceptible to Toxoplasma, while mice deficient in IL12/IFNγ pathway genes are extremely susceptible to Toxoplasma . Indeed, in vitro studies revealed that residual IFNγ-responsiveness in patients with partial IFNγ receptor 1 deficiency in the presence of TNFα is sufficient to inhibit Toxoplasma but not Salmonella proliferation, possibly explaining why these patients are more susceptible to bacteria but not to Toxoplasma . It is therefore likely that in humans, contrary to mice, CD40–CD40L-dependent toxoplasmacidal mechanisms can compensate for the lack of IL12 and IFNγ. CD40(-/-) mice control acute infection but are susceptible to cerebral and ocular toxoplasmosis , likely because CD40–CD40L interactions have a role in mediating long-term CD8 T cell immunity and in preventing CD8 T cell exhaustion (reviewed in ).
Host cell death inhibits Toxoplasma proliferation, which requires a host cell to replicate and survive (Fig 1). Thus, it is not surprising that Toxoplasma strongly inhibits apoptosis, possibly through inhibition of caspases 3/7/8, which are proteolytic proenzymes that can trigger cell death upon activation (reviewed in ). However, inhibition of caspase-8 sensitizes cells towards necroptosis, a programmed form of inflammatory cell death, because caspase-8 is an inhibitor of receptor-interacting serine/threonine protein kinase (RIPK3), a key mediator of necroptosis . It is therefore likely that upon IFNγ, TNFα, or TLR3 activation, which can all activate RIPK3, Toxoplasma-infected cells can die from necroptosis. HFFs normally support the growth of Toxoplasma; however, IFNγ-stimulated HFFs rapidly die through an unknown mechanism upon infection with the type I strain, triggering premature parasite egress . Human cytotoxic T cells can kill Toxoplasma-infected target cells by secreting pore-forming perforins into the infected cells and subsequently secreting granzymes through these pores, which can kill the infected cell by activating caspases. Human, but not rodent, T cells also secrete the antimicrobial peptide granulysin through these pores, which can destroy the Toxoplasma PVM, allowing granzymes to enter the parasite where they kill the parasite through generation of reactive oxygen species (Fig 1).
Different cell lines from the same species or similar cell types from distinct species can vary enormously in the expression of indoleamine 2,3-dioxygenase , inducible nitric oxide synthase, RIPK3 , NLRs, and IRGs, likely explaining cell type and species differences in response to Toxoplasma. Furthermore, different Toxoplasma strains not only vary in their ability to block the variety of toxoplasmacidal mechanisms but they also differ in the activation of host signaling pathways that can lead to differences in expression of host toxoplasmacidal effectors. For example, type II parasites activate the NFκB transcription factor (predominantly the p50/p65 heterodimer) via the secreted protein GRA15. This eventually leads to enhanced secretion of IL1β and higher expression of CD40 by human monocytes (reviewed in ). Because IL1β can enhance the IL12-mediated stimulation of IFNγ production  and CD40–CD40L interaction can activate Toxoplasma destruction through autophagy, it is likely that GRA15 plays a role in human toxoplasmosis. The timing and cellular context for IFNγ-dependent and -independent toxoplasmacidal mechanisms in humans remain unclear and warrant further research.