TMEM119+ microglia MHC class I restricted antigen presentation impacts CD8 T cell memory, effector status, and blood-brain barrier disruption during neurotropic virus infection

Technical Summary: TMEM119+ Microglia MHC Class I Restricted Antigen Presentation in Neurotropic Virus Infection

Problem Statement
The role of microglia as antigen-presenting cells (APCs) in the central nervous system (CNS) during neurotropic viral infections remains incompletely defined. While microglia are known to upregulate antigen processing and presentation molecules, it is unclear how their specific MHC class I restricted antigen presentation influences acute antiviral CD8+ T cell responses, effector functions, and long-term memory recall. Previous studies utilizing broad myeloid depletion strategies (e.g., CSF1R inhibitors) or CX3CR1-Cre lines have been limited by off-target effects on perivascular macrophages and infiltrating monocytes, making it difficult to isolate the specific contribution of parenchymal microglia. Furthermore, the distinct roles of individual MHC class I alleles (specifically H-2Kb vs. H-2Db) in shaping CD8+ T cell proliferation, perforin expression, and blood-brain barrier (BBB) integrity within the brain parenchyma have not been dissected using microglia-specific tools.

Methodology
The authors employed a novel, microglia-specific conditional knockout (cKO) approach to investigate the impact of discrete MHC class I molecules.

• Mouse Models: The study utilized Tmem119-CreERT2 mice crossed with H-2Kb^LoxP or H-2Db^LoxP mice to generate Tmem119 Kb cKO and Tmem119 Db cKO strains. Tamoxifen administration was used to induce Cre-mediated recombination specifically in TMEM119+ microglia, ablating H-2Kb or H-2Db expression while sparing other myeloid populations.
• Infection Model: Mice were infected intracranially with Theiler's murine encephalomyelitis virus (TMEV) or a recombinant TMEV encoding the OVA antigen (TMEV-OVA). TMEV-OVA allows for the tracking of H-2Kb-restricted CD8+ T cells, while wild-type TMEV infection tracks H-2Db-restricted responses.
• Experimental Interventions:
  • Proliferation Assays: BrdU incorporation and FTY720 treatment were used to sequester T cells in secondary lymphoid organs, isolating in situ proliferation within the brain.
  • Parabiosis: Surgical conjoining of cKO mice with Cre-negative littermates was performed to determine if observed phenotypes were driven by peripheral factors or local CNS events.
  • Peptide-Induced Fatal Syndrome (PIFS): To assess BBB disruption, mice were challenged with the immunodominant VP2121–130 peptide post-infection.
  • Memory Recall: Virus clearance was allowed (30–45 days), followed by secondary peptide challenge to evaluate CD8 tissue-resident memory (TRM) cell reactivation.
• Analysis: High-dimensional spectral flow cytometry, MRI (gadolinium leakage), and FITC-albumin leakage assays were used to quantify immune cell populations, effector molecules (perforin), and BBB integrity.

Key Results

1. Validation of Specificity: Tamoxifen-induced deletion successfully ablated H-2Kb and H-2Db expression specifically in TMEM119+ microglia without affecting expression in peripheral blood immune cells or disrupting T cell development in the thymus.
2. H-2Kb and CD8+ T Cell Proliferation: In Tmem119 Kb cKO mice infected with TMEV-OVA, there was a marked reduction in the frequency and absolute numbers of Kb:OVA-specific CD8+ T cells in the brain at 7 days post-infection. Parabiosis experiments confirmed this reduction was not due to peripheral priming defects but resulted from a lack of local proliferation within the brain parenchyma. BrdU labeling confirmed that microglial H-2Kb presentation is required for the expansion of infiltrating CD8+ T cells.
3. H-2Db and Effector Function: In Tmem119 Db cKO mice infected with TMEV, the total number of Db:VP2121–130-specific CD8+ T cells remained unchanged compared to controls. However, these cells exhibited significantly reduced perforin expression (both frequency and mean fluorescence intensity).
4. BBB Disruption: The reduction in perforin in Tmem119 Db cKO mice correlated with preserved BBB integrity. In the PIFS model, Tmem119 Db cKO mice showed significantly less vascular leakage (FITC-albumin and gadolinium) compared to controls. Parabiosis confirmed that this protection was driven by the local absence of microglial H-2Db, not peripheral factors.
5. Memory Recall and TRM Reactivation: While the initial generation of CD8 TRM cells (30 days post-infection) was unaffected by microglial H-2Db deletion, the recall response was impaired. Upon secondary peptide challenge in the presence of FTY720 (to block peripheral influx), Tmem119 Db cKO mice failed to expand their brain-resident CD8 TRM populations and exhibited reduced overall immune cell infiltration and BBB permeability compared to controls.

Significance and Claims
The paper claims to provide the first direct evidence that microglia function as critical APCs for CD8+ T cells via MHC class I restriction within the CNS, with distinct roles for different alleles:

• Discrete Allelic Roles: The study demonstrates that H-2Kb and H-2Db on microglia regulate different aspects of the CD8+ T cell response. H-2Kb is essential for the proliferation of antigen-specific CD8+ T cells, whereas H-2Db is critical for enhancing the effector status (perforin expression) and subsequent recall expansion of TRM cells.
• Mechanism of Neuropathology: The authors establish a direct link between microglial antigen presentation and BBB disruption. They propose that microglial H-2Db presentation boosts the perforin-mediated cytotoxicity of CD8+ T cells, which drives vascular leakage and neuropathology.
• Localization: Through parabiosis, the study confirms that these regulatory mechanisms are localized to the CNS and are not governed by peripheral immune dynamics.
• Therapeutic Implications: The findings suggest that microglia are key regulators of CD8+ T cell-mediated pathology in neurotropic infections. The authors posit that targeting specific MHC class I interactions on microglia could offer a strategy to modulate CD8+ T cell responses and BBB integrity in diseases such as cerebral malaria, multiple sclerosis, and Alzheimer's disease, though they note that further studies in other disease models are required.

The study emphasizes the necessity of using precise, cell-type-specific tools (like Tmem119-CreERT2) to avoid the confounding variables associated with broader myeloid depletion strategies, thereby clarifying the specific contribution of parenchymal microglia to adaptive immunity in the brain.