10 feb. 2025
Mroz et al. (BioRixv)
Keywords
Traumatic Brain Injury
Epilepsy
Thalamus
Neuroinflammation
Main Findings
This preprint investigates the role of lymphocytes in thalamic inflammation and seizure risk following traumatic brain injury (TBI). Secondary injuries can arise in the thalamus long after the initial impact to the head. Since the thalamus regulates sensory processing and cognition, inflammation in this region can lead to neurological complications, including epilepsy. While the contribution of myeloid cells to TBI-induced neuroinflammation has been extensively studied, the role of lymphocytes remains poorly understood. The authours use a controlled cortical impact mouse model of TBI that results in secondary injury to the thalamus. Applying flow cytometry to micro-dissected brain regions, the authours identify lymphocyte infiltration in the thalamus that peaks at 1-month post-injury. The authours show that infiltrates consist of CD8+ T cells, NK/ILC1s, and T helper 1 cells. Additionally, interferon-γ (IFNγ) levels are significantly elevated in the post-TBI thalamus compared to other brain regions. Using confocal microscopy, the authours show thalamic microglia are the likely responders to IFNγ. Specifically, Iba1+ microglia/macrophages upregulate known interferon-stimulated proteins, MHCII and STAT1, following TBI. To explore the role of T cells in TBI-induced seizures, the authours administer CD4+ T cell-depleting monoclonal antibody after TBI. Upon subsequent injection of pentylenetetrazole (PTZ), a pro-convulsant drug, the incidence and duration of seizures are lowered in CD4+ T cell-depleted mice. Interestingly, CD4+ T cell depletion post-TBI increases the frequencies of disease-associated microglia, CD8+ T cells, and NK/ILC1s in the thalamus. Considering the potential neuroprotective role of type 1 cytokines, the authors administer exogenous IFNγ after TBI. Remarkably, IFNγ treatment lowers mortality rates, as well as the incidence, duration, and severity of seizures following PTZ administration. Altogether, the authours conclude that type 1 lymphocytes enter the thalamus after TBI, producing IFNγ which acts on resident macrophages and mitigates subsequent seizure susceptibility.
Limitations
Including experiments that strengthen the rationale for CD4+ T cell depletion would significantly enhance the preprint. While the authors demonstrate that T-bet-expressing CD4+ T cells infiltrate the thalamus post-TBI, they also show that the majority of lymphocyte infiltrates consist of CD8+ T cells. It may be informative to utilize a CD4-cre inducible diphtheria toxin receptor (DTR) model, which would deplete both CD4+ and CD8+ T cells in the periphery.
A detailed characterization of the relationship between CD4+ T cells, disease associated microglia, and CD8+ T cells would be helpful to the study. The authours propose that ablation of CD4+ T cells could have “de-repressed” other type 1 lymphocytes that serve neuro-protective functions. Experimental exploration of this proposal – by intracerebroventricular injection of purified CD4+ T cells, for example – would provide great insights.
What is the role of interferon-responsive thalamic microglia and how do they influence seizure susceptibility post-TBI? Considering the upregulation of MHCII and STAT1 by this population, it may be helpful to conditionally delete these genes, or upstream regulators like Ifngr and assess the effect on TBI-induced changes in neuronal circuitry and subsequent seizure risks. Timed, conditional deletion on myeloid cells may further allow to distinguish the contributions of monocyte-derived macrophages and microglia to seizures post-TBI.
Despite the striking therapeutic effects of IFNγ, supplementary experiments with anti-IFNγ neutralizing antibody or IFNγ knockout mice that show reverse outcomes would strengthen the impact of this study even further.
Supplemental material that demonstrates how thalamic subregions were defined in microscopy images would be helpful to the readers. Moreover, time-course analysis of thalamic subregions following TBI may provide insightful contributions to the study.
Significance/Novelty
The brain is an undeniably complex organ with compartmentalized and highly specialized regions. This preprint sheds light on significant spatiotemporal changes that occur after injury, enhancing our understanding of region-specific immune responses in the brain. The authors also uncover a previously unrecognized, and surprisingly neuroprotective role of lymphocytes in TBI-induced neuroinflammation. Their demonstration of lymphocyte–microglia interactions and how these shape neurological outcomes is particularly novel.
The transition from primary to secondary injury in TBI patients represents a critical window for therapeutic intervention to prevent long-term neurological complications. This preprint highlights immune-mediated changes that occur during this period, deepening our understanding of the mechanisms that drive secondary injury progression. The authors also demonstrate a remarkable therapeutic effect of IFNγ in reducing seizure susceptibility after TBI. While additional preclinical studies are needed, this preprint suggests that IFNγ could be a promising therapeutic candidate for limiting neurological complications in TBI patients.
Credit
Reviewed by Jennifer Ahn as part of a cross-institutional journal club between the Icahn School of Medicine at Mount Sinai, the University of Oxford, the Karolinska Institute and the University of Toronto.
The author declares no conflict of interests in relation to their involvement in the review.