CTLA-4 checkpoint signalling modulates germinal centre dynamics and antibody avidity during Borrelia burgdorferi infection in mice.

Borrelia burgdorferi, the causative agent of Lyme borreliosis, persists in mammalian hosts despite robust activation of innate and adaptive immunity. A hallmark of murine infection is the production of low-avidity antibodies, yet the immunological mechanisms underlying this defective humoral response remain poorly understood. Here, we used single-cell RNA sequencing of draining lymph node cells from Borrelia-infected mice at days 7 and 14 post-infection, comparing transcriptional profiles with those from mice immunised with heat-killed Borrelia or with a potent germinal centre-inducing protein immunisation. Borrelia infection induced a rapid germinal centre response, however, light zone B cells displayed diminished expression of co-stimulatory and T-B cell interaction molecules while maintaining core germinal centre identity markers, suggesting impaired positive selection. T follicular helper cells upregulated multiple inhibitory checkpoint receptor genes, most prominently Ctla4, alongside an active germinal centre engagement programme. Foxp3? regulatory T cells upregulated genes associated with follicular-homing and suppressive function, including those implicated in CTLA-4-mediated trans-endocytosis. To test whether inhibitory signalling contributes to the impaired humoral response, infected mice were treated with anti-CTLA-4 blocking antibody. CTLA-4 blockade significantly increased plasma cell numbers and enhanced both the magnitude and avidity of Borrelia-specific IgG responses, including the induction of complement-fixing IgG2a. These findings reveal that CTLA-4-dependent inhibitory signalling contributes to defective antibody quality during Borrelia infection, identifying a mechanism by which dysregulated T cell responses may facilitate impaired humoral control of a persistent spirochaetal pathogen.