Impact of innate immune activation on T cell dynamics and functional recovery following traumatic brain injury

Background Traumatic brain injury (TBI) initiates a rapidly evolving neuroinflammatory response; however, the temporal relationship between early innate immune activation, T cell polarization, and neurobehavioural recovery remains poorly understood. Here, we hypothesize that interleukin-1β (IL-1β) is a critical upstream mediator that polarizes T cells towards pro-inflammatory and cytotoxic effector functions following TBI.

Using a controlled cortical impact model in adult male C57BL/6J mice, we mapped post-injury immune dynamics and investigated whether targeting key innate inflammatory compartments influenced subsequent T cell programming and neurological outcomes. We conducted longitudinal immune profiling by multiparameter spectral flow cytometry and quantitative polymerase chain reaction up to 10 days post-injury. Antibody-based immune depletion strategies were used to investigate neutrophil and monocyte contributions to the post-traumatic T cell response, while pharmacological inhibition of NLRP3 inflammasome by MCC950 treatment was used to investigate the contribution of IL-1β.

TBI elicited a structured early innate immune response, marked by rapid chemokine induction, followed by temporally distinct infiltration of neutrophils, monocytes, and dendritic cells. Neutrophils and monocytes were the predominant early IL-1β-producing infiltrating populations. This was followed by a delayed adaptive phase characterized by sustained recruitment of T cell subsets (CD4+, CD8+, γδ+), alongside dynamic effector cytokine production (IL-17, IFN-γ). Neutrophil depletion altered the early myeloid composition but did not result in durable improvements in T cell effector responses or neurobehavioral outcomes. Depletion of CCR2-dependent inflammatory monocytes reduced acute monocyte accumulation and attenuated early downstream T cell responses; however, these effects were not sustained and only resulted in modest neurobehavioural benefits. In contrast, inhibition of the NLRP3 inflammasome suppressed microglial IL-1β production, without significantly altering leukocyte recruitment or subacute T cell effector phenotypes. These phenotypic changes were associated with improvements in motor and cognitive function recovery.

We show that early monocyte IL-1β signalling actively regulates downstream T cell infiltration and effector function after TBI. In addition, inhibition of NLRP3 inflammasome after TBI attenuates microglial IL-1β-associated immune activation and results in behavioural improvement despite ongoing leukocyte recruitment, indicating that targeting the nature and cellular source of IL-1β signalling can dissociate immune cell burden from neurological outcomes. Collectively, our findings identify myeloid IL-1β-linked pathways as a viable bridge between innate and adaptive immunity post-TBI, and underscore cellular compensation as a critical design consideration for next-generation immunotherapies. Competing Interest Statement The authors have declared no competing interest. Footnotes Competing interest declaration: The authors declare no competing interests. Data availability declaration: Data are available upon request and can be accessed through the corresponding author. Ethics, consent to participate, and consent to publish declarations: Not applicable. Funding declaration: This work was supported by Irish Research Council Government of Ireland Postgraduate Scholarship (GOIPG/2021/1471) (ST) and Taighde Éireann - Research Ireland (Grant 17/FRL/4860) (DJL).