Mice Lacking Factor XIII Have Significant Protection from the Progression of Neuroinflammatory Disease

Multiple sclerosis (MS) is a chronic autoimmune inflammatory demyelinating disease of the central nervous system (CNS). Motor and sensory neurologic deficits associated with MS are secondary to autoimmune-mediated demyelination of neurons within the CNS. Autoreactive T cells that target myelin and local microglia/macrophage activation are both central to disease pathogenesis. Fibrin(ogen) deposition, the final product of the coagulation cascade, at sites of blood brain barrier (BBB) breakdown is an early key feature of MS. Growing evidence supports that fibrin(ogen) contributes to MS pathogenesis by serving as a local cue for microglial/macrophage activation through α_Mβ₂-CD11/CD18 interactions. However, the precise contributions of different molecular forms of fibrin(ogen) to the inflammation seen in MS have not been defined. In this study, we determined the roles of soluble fibrinogen, insoluble fibrin polymer and crosslinked fibrin matrix in neuroinflammatory disease by use of a standard murine model of MS, experimental autoimmune encephalomyelitis (EAE). We first sought to determine relative contributions of soluble fibrinogen versus fibrin polymer to MS. Fibrinogen^AEK (Fib^AEK) mice, which express a form of fibrinogen that is unable to be cleaved by thrombin and thus form a fibrin polymer, were challenged with EAE. Our results demonstrated that Fib^AEK mice challenged with EAE developed mildly attenuated clinical symptoms compared with wildtype mice. Notably, these Fib^AEK mice that are unable to form a fibrin polymer, but still express fibrinogen, were not as protected from EAE disease symptoms as mice that are deficient in fibrinogen. These data suggested that fibrinogen may contribute to the progression of EAE disease without polymerization of fibrinogen. To further explore the role of fibrin(ogen) structure in neuroinflammation, we utilized mice lacking factor XIII (fXIII), a transglutaminase that crosslinks fibrin monomers to form a stable fibrin matrix. FXIIIA^-/- mice, which are genetically deficient in the active subunit of fXIII, developed significantly ameliorated EAE clinical symptoms, including lower clinical scores, decreased weight loss, and decreased days of paralysis compared with wildtype animals. Similarly, pharmacological inhibition of transglutaminase activity by cystamine, a transglutaminase inhibitor, significantly ameliorated clinical symptoms in EAE mice. On examination of spinal cords from EAE challenged animals, a significantly reduced area of demyelination was present in the white matter of spinal cords from fXIIIA^-/- animals compared to wildtype mice. Further, significantly diminished areas of microglia/macrophage activation were observed in EAE-challenged fXIIIA^-/-spinal cords versus wildtype. Correspondingly, a low level of diffuse fibrin deposits was observed in fXIIIA^-/- spinal cord white matter. This was in marked contrast to the conspicuous, and significantly increased, fibrin deposition in the spinal cords of wildtype animals challenged with EAE. Further examination of the cytokine and chemokine expression profiles from the spinal cords of mice challenged with EAE suggest that the T helper 17 (Th17) cell responses, including expression of interleukin 17a (IL17a), interleukin 6 (IL6), transforming growth factor β (TGFβ) and chemokine (C-C motif) ligand 20 (CCL20), were decreased in animals lacking factor XIIIA. Together, our results suggest that fibrinogen, and not only polymerized fibrin, may contribute to EAE disease progression. Further, our data reveal that factor XIII-mediated crosslinking is a significant determinant of EAE disease severity. Targeting fibrin crosslinking may be a novel therapeutic strategy for MS/EAE.