Foxp3+ Regulatory T Cells Restrain Th1 Response Shielding the Brain from Lethal Inflammatory Damage during Cryptococcal Meningoencephalitis

Inflammatory brain damage is an important factor contributing to mortality or lasting neurological sequelae in CNS infections, such as cryptococcal meningoencephalitis (CM), but little is known about natural immunoregulatory mechanisms in the infected brain. Here we report that regulatory T cells (Tregs) are a central immunoregulatory component in CM. Tregs are present within the CNS in both human CM patients and in the experimental murine CM. Treg-depletion exacerbates Th1-driven brain inflammation and neurological symptoms, accelerating mortality, despite enhanced fungal clearance in mouse CM. Aligned brain NanoString, scRNA-seq, and flow cytometry analyses revealed that Tregs reduce brain inflammation, especially T-cell recruitment activation and differentiation, shielding the brain from neurological damage. The major CNS-Treg recruitment appears to be chemokine receptor CCR8-mediated, supporting the importance of the CCR8/CCL1 axis in Treg recruitment into the brain. Tregs in CM are major producers of anti-inflammatory IL-10 and the growth factor Amphiregulin (Areg), which is implicated in neuronal repair. IL-10 deletion in murine CM phenocopies Treg depletion. Areg deletion showed no survival effect; however, IFN-γ production by effector T cells in the brain was reduced, supporting Aregs as a potential internal regulator of Treg immunosuppressive function. Finally, a Treg-enhancing immunotherapy using low dose IL-2-immune complex treatment substantially improves mouse survival and neurological outcomes. Together, we identified that Tregs are crucial for neuronal protection in CM, predominantly via IL-10 production, the CCR8-CCL1 axis is an important CNS-Treg recruitment mechanism and Treg enhancement is a potential therapeutic strategy to mitigate immunopathology during CM, and possibly other forms of meningitis.