Cytotoxic Signature and IFN-γ Production Dominate CD4⁺ T-Cell Response During Human Toxoplasmosis

Toxoplasma gondii is a highly versatile parasite that infects most warm-blooded animals and is a major cause of retinochoroiditis and uveitis in humans. The pathophysiology of these conditions remains poorly understood. Both parasite virulence and host inflammatory response contribute to the development of ocular disease. While CD4⁺ T cells play a critical role in host resistance to Toxoplasma infection, their kinetics and effector functions, as well as their contribution to the clinical outcome of the infection, including ocular involvement, remain poorly understood. To address this question, we investigated the immune response during acute and convalescent toxoplasmosis and stratified patients further based on the presence or absence of ocular disease. We found that T. gondii infection leads to decreased and increased proportions of central and effector memory CD4⁺ T cells, respectively. Applying unsupervised analysis, distinct CD4⁺ T-cell subsets were determined. Among 50 clusters, 10 produced cytotoxic proteins (granzyme B and perforin) and one produced cytokines upon antigen-specific stimulation. We observed that proportions of five CD4⁺ T-cell clusters out of 50 were different during acute disease between T. gondii-infected patients with and without ocular lesions. Interestingly, three of the five displayed a cytotoxic signature indicating their possible involvement in ocular immunopathology. Taken together, our results reveal that during T. gondii infection, CD4⁺ T cells not only develop a Th1 cytokine profile, but also acquire previously unappreciated cytotoxic capacity/function. These results, while underscoring the complexity of the CD4⁺ T-cell response to T. gondii, suggest that specific subsets may be involved in the development of pathology and provide possible targets for therapeutic intervention.