Factor XIII Deficiency Improves Motor Function in Mice Challenged with Experimental Autoimmune Encephalomyelitis, An Established Model of Multiple Sclerosis

Abstract 858

Multiple sclerosis (MS) is an inflammatory disease characterized by autoimmune demyelination of neurons and axonal damage within the central nervous system, leading to relapsing/remitting neurological deficits, including sensory and motor deficits. MS plaques are characterized by blood-brain barrier disruption leading to perivascular deposition of fibrin that correlates with areas of microglia activation and myelin damage. Consistent with multiple studies showing that hemostatic factors can serve as important modulators of the inflammatory response in vivo, fibrin(ogen) has been identified as a novel regulator of microglial activation and differentiation. Furthermore, fibrin(ogen) has been found, via the α_Mβ₂ binding motif on the γ chain, to play a role in microglial activation in experimental autoimmune encephalomyelitis (EAE), a murine model of MS. However, previous work by our lab, as well as others, suggests that thrombin-mediated proteolysis plays a greater role in driving neuroinflammatory disease than by merely supporting fibrin formation. Our working hypothesis is that thrombin-mediated proteolysis drives autoimmune neuroinflammation through both fibrin(ogen)-dependent and fibrin(ogen)-independent pathways. To test this hypothesis, we have initiated studies of EAE in mice lacking selected thrombin substrates, including protease activated receptors (PARs) and factor XIII (fXIII). Clinical evaluation of loss of motor function in EAE-challenged cohorts of PAR-1-deficient, PAR-4-deficient and WT control mice revealed a similar outcome in all three cohorts, suggesting PARs may have either a modest or a secondary role in EAE-induced neuroinflammatory disease. However, mice with a constitutive deficiency of another thrombin substrate, the fibrin cross-linking transglutaminase fXIII, were found to have a significant amelioration in motor function loss when challenged with EAE. To further explore this phenotype, we have utilized cystamine, a transglutaminase inhibitor, as a pharmacologic complement to the fXIII knockout mice. Mice with cystamine placed in their drinking water during the EAE challenge period also experienced significantly less disease than control mice that did not receive cystamine. This data suggests that (pro)thrombin contributes neuroinflammatory disease by both supporting local fibrinogen polymerization and by activation of fXIII leading to fibrin stabilization. Furthermore, this line of research suggests that more detailed studies of fXIII may reveal novel strategies for limiting or reversing the devastating pathologies associated with multiple sclerosis.