(516) Breaking the Cytokine Dogma: A Novel HLA-DR3 Mouse Model with MS-like Brain Pathology Independent of IFNγ and IL-17A

Introduction/Rationale: Multiple sclerosis (MS) is a chronic CNS inflammatory disease strongly linked to HLA-DR alleles. HLA-DR restricted CD4⁺ T cells producing IFNγ and IL-17A are considered key drivers, yet their precise roles remain unclear. We used HLA-DR3 (DRB1*0301) transgenic mice lacking IFNγ or IL-17A in experimental autoimmune encephalomyelitis (EAE), a model of MS, and found that neither cytokine is essential for disease induction, although both play distinct roles. The absence of IFNγ causes atypical, brain-specific EAE and increased levels of IL-17A, while the absence of IL-17A results in classical, spinal cord-specific EAE with increased Treg cells and Treg-enriching gut microbiota.

Methods: We hypothesized that combined deficiency of IFNγ and IL-17A would prevent disease, defining their essential roles in CNS autoimmunity. We generated HLA-DR3.IFNγ-/-.IL-17A-/- double knockout (DKO) mice and analyzed clinical features, CNS pathology, immune cell infiltration, gene expression, and cytokine blockade (IL-17F).

Results: DKO mice developed atypical, non-paralytic EAE characterized by ataxia, brain inflammation, and demyelination, mirroring brain-specific MS pathology. Before onset, CD4⁺ T cells expressing CCR6 and upregulating Ccl20, Mmp3, Nfatc2, Tlr4, and Cd28 accumulated in the spleen and draining lymph nodes. At peak disease, brains exhibited CD4⁺RORγT⁺ (Th17IL-17A⁻/⁻) cell accumulation and robust activation of microglia and astrocytes, suggesting that alternative inflammatory circuits drive atypical EAE pathology.

Conclusion: Our data reveal that IFNγ and IL-17A are not required for CNS autoimmunity. This transgenic mouse model uniquely exhibits brain-specific pathology and provides a valuable tool to uncover mechanisms and therapeutic targets for brain-predominant MS. Current efforts are investigating whether blockade of GM-CSF, or TNFa can modulate disease severity.