CXCR3-Dependent CD4 ⁺ T Cells Are Required to Activate Inflammatory Monocytes for Defense against Intestinal Infection

Abstract

Chemokines and their receptors play a critical role in orchestrating immunity to microbial pathogens, including the orally acquired Th1-inducing protozoan parasite Toxoplasma gondii. Chemokine receptor CXCR3 is associated with Th1 responses, and here we use bicistronic CXCR3-eGFP knock-in reporter mice to demonstrate upregulation of this chemokine receptor on CD4 ⁺ and CD8 ⁺ T lymphocytes during Toxoplasma infection. We show a critical role for CXCR3 in resistance to the parasite in the intestinal mucosa. Absence of the receptor in Cxcr3 ^−/− mice resulted in selective loss of ability to control T. gondii specifically in the lamina propria compartment. CD4 ⁺ T cells were impaired both in their recruitment to the intestinal lamina propria and in their ability to secrete IFN-γ upon stimulation. Local recruitment of CD11b ⁺Ly6C/G ⁺ inflammatory monocytes, recently reported to be major anti- Toxoplasma effectors in the intestine, was not impacted by loss of CXCR3. However, inflammatory monocyte activation status, as measured by dual production of TNF-α and IL-12, was severely impaired in Cxcr3 ^−/− mice. Strikingly, adoptive transfer of wild-type but not Ifnγ ^−/− CD4 ⁺ T lymphocytes into Cxcr3 ^−/− animals prior to infection corrected the defect in inflammatory macrophage activation, simultaneously reversing the susceptibility phenotype of the knockout animals. Our results establish a central role for CXCR3 in coordinating innate and adaptive immunity, ensuring generation of Th1 effectors and their trafficking to the frontline of infection to program microbial killing by inflammatory monocytes.

Author Summary

Inflammatory monocytes have recently emerged as important effectors in intestinal defense against enteric pathogens, but requirements for their activation are poorly defined. Here we use the protozoan Toxoplasma gondii, an orally acquired Th1-inducing pathogen, to study the requirements for inflammatory macrophage activation in the intestinal mucosa. We find that CD4 ⁺ T lymphocytes, recruited in dependence upon their expression of the chemokine receptor CXCR3, mediate activation of intestinal mucosa inflammatory monocytes via secretion of IFN-γ, in turn resulting in control of infection. Thus, CXCR3 functions as a critical lynchpin coordinating anti-microbial communication between T lymphocytes and inflammatory monocyte effectors.