Postdoctoral Fellow Indiana University School of Medicine Indianapolis, Indiana, United States
Disclosure(s):
Daisy Woellner-Santos, PhD, DVM: No financial relationships to disclose
Introduction/Rationale: Protective immunity to the intracellular parasite Toxoplasma gondii relies on a rapid and robust CD4+ Th1 effector response. The transcription factor T-bet is classically regarded as the master-regulator of IFN-γ in CD4⁺ Th1s; however, while T-bet–deficient CD4⁺ T cells can still produce IFN-γ, global T-bet-deficient (Tbx21⁻/⁻) mice rapidly succumb to infection. Therefore, we investigated if T-bet-dependent programming is required for long-term immunity against T. gondii.
Methods: Using a cyst-forming strain, ME49, we compared wild-type (WT), Tbx21⁻/⁻, and CD4-restricted T-bet-deficient mice (CD4-Cre × Tbx21flox/flox; CD4-Tbx21⁻/⁻).
Results: As expected, Tbx21⁻/⁻ mice succumbed to infection by day 10 post-infection (p.i.), coinciding with high parasite burden. In contrast, CD4-Tbx21⁻/⁻ mice survived acute infection, but eventually succumbed by 25 days p.i., revealing a selective requirement for CD4-intrinsic T-bet during chronic disease. Across all infected groups, IFN-γ production was confined to early apoptotic CD4⁺ T cells. However, in CD4-Tbx21⁻/⁻ mice, this apoptotic subset lacked Ly6C but retained CD44, supporting a requirement for T-bet in the generation of IFN-γ⁺Ly6C⁺ CD4⁺ effector cells. Although CD4-Tbx21⁻/⁻ mice showed WT-like parasite burden at day 8 p.i., they developed markedly increased peripheral burden by day 14. Furthermore, we observed that CD4-Tbx21⁻/⁻ mice had reduced CD4⁺ T cells in the spleen and the brain at day 14 p.i., suggesting that T-bet expressing CD4+ T cells in the CNS are essential for long-term host resistance. Simultaneously, parasites failed to convert into cysts, and IFN-γ-inducible GTPases essential for parasite clearance were significantly reduced in CD4-Tbx21⁻/⁻ brains, likely due to diminished CNS T-bet-dependent CD4+ T cell-derived IFN-γ.
Conclusion: Mechanistically, we demonstrate T-bet-expressing CD4⁺ T cells in the CNS are critical for efficient conversion of T. gondii from the fast-growing tachyzoite stage to life-long cysts, ensuring host survival.
