T lymphocyte-dependent IL-10 down-regulates a cytokine storm driven by
Toxoplasma gondii
GRA24
Authors: Doherty Claire M. , Patterson Paige R. , Emeanuwa Julie A. , Belmares Ortega Jessica , Fox Barbara A. , Bzik David J. , Denkers Eric Y. 
Published: October 2024

As a model organism in the study of immunity to infection,
Toxoplasma gondii
has been instrumental in establishing key principles of host anti-microbial defense and its regulation. Here, we employed an attenuated uracil auxotroph strain of Type I
Toxoplasma
designated OMP to further untangle the early immune response to this parasitic pathogen. Experiments using αβ T cell-deficient
Tcrb

-/-

mice unexpectedly revealed that an intact αβ T lymphocyte compartment was essential to survive infection with OMP. Subsequent antibody depletion and knockout mouse experiments demonstrated contributions from CD4
+
T cells and most predominantly CD8
+
T cells in resistance. Using transgenic knockout mice, we found only a partial requirement for IFN-γ and a lack of requirement for Toll-like receptor (TLR) adaptor MyD88 in resistance. In contrast to other studies on
Toxoplasma
, the ability to survive OMP infection did not require IL-12p40. Surprisingly, T cell-dependent IL-10 was found to be critical for survival, and deficiency of this cytokine triggered an abnormally high systemic inflammatory response. We also found that parasite molecule GRA24, a dense granule protein that triggers TLR-independent IL-12 production, acts as a virulence factor contributing to death of OMP-infected
Tcrb

-/-

and
IL-10

−/−

mice. Furthermore, resistance against OMP was restored in
Tcrb

-/-

mice via monoclonal depletion of IL-12p40, suggesting that GRA24-induced IL-12 underlies the fatal immunopathology observed. Collectively, our studies provide insight into a novel and rapidly arising T lymphocyte-dependent anti-inflammatory response to
T. gondii
which operates independently of MyD88 and IL-12 and that depends on the function of parasite-dense granule protein GRA24.

IMPORTANCE

As a model infectious microbe and an important human pathogen, the apicomplexan
Toxoplasma gondii
has provided many important insights into innate and adaptive immunity to infection. We show here that a low virulence uracil auxotrophic
Toxoplasma
strain emerges as a virulent parasite in the absence of an intact T cell compartment. Both CD4
+
and CD8
+
T lymphocytes are required for optimal protection, in line with previous findings in other models of
Toxoplasma
infection. Nevertheless, several novel aspects of the response were identified in our study. Protection occurs independently of IL-12 and MyD88 and only partially requires IFN-γ. This is noteworthy particularly because the cytokines IL-12 and IFN-γ have previously been regarded as essential for protective immunity to
T. gondii
. Instead, we identified the anti-inflammatory effects of T cell-dependent IL-10 as the critical factor enabling host survival. The parasite dense granule protein GRA24, a host-directed mitogen-activated protein kinase activator, was identified as a major virulence factor in T cell-deficient hosts. Collectively, our results provide new and unexpected insights into host resistance to
Toxoplasma
.