Gregory A. Demopulos

Mannan binding lectin-associated serine protease-2 (MASP-2) critically contributes to post-ischemic brain injury independent of MASP-1

BackgroundComplement activation via the lectin activation pathway (LP) has been identified as the key mechanism behind post-ischemic tissue inflammation causing ischemia-reperfusion injury (IRI) which can significantly impact the clinical outcome of ischemic disease. This work defines the contributions of each of the three LP-associated enzymes—mannan-binding lectin-associated serine protease (MASP)-1, MASP-2, and MASP-3—to ischemic brain injury in experimental mouse models of stroke.MethodsFocal cerebral ischemia was induced in wild-type (WT) mice or mice deficient for defined complement components by transient middle cerebral artery occlusion (tMCAO) or three-vessel occlusion (3VO). The inhibitory MASP-2 antibody was administered systemically 7 and 3.5xa0days before and at reperfusion in WT mice in order to assure an effective MASP-2 inhibition throughout the study. Forty-eight hours after ischemia, neurological deficits and infarct volumes were assessed. C3 deposition and microglia/macrophage morphology were detected by immunohistochemical, immunofluorescence, and confocal analyses.ResultsMASP-2-deficient mice (MASP-2−/−) and WT mice treated with an antibody that blocks MASP-2 activity had significantly reduced neurological deficits and histopathological damage after transient ischemia and reperfusion compared to WT or control-treated mice. Surprisingly, MASP-1/3−/− mice were not protected, while mice deficient in factor B (fB−/−) showed reduced neurological deficits compared to WT mice. Consistent with behavioral and histological data, MASP-2−/− had attenuated C3 deposition and presented with a significantly higher proportion of ramified, surveying microglia in contrast to the hypertrophic pro-inflammatory microglia/macrophage phenotype seen in the ischemic brain tissue of WT mice.ConclusionsThis work demonstrates the essential role of the low-abundant MASP-2 in the mediation of cerebral ischemia-reperfusion injury and demonstrates that targeting MASP-2 by an inhibitory therapeutic antibody markedly improved the neurological and histopathological outcome after focal cerebral ischemia. These results contribute to identifying the key lectin pathway component driving brain tissue injury following cerebral ischemia and call for a revision of the presently widely accepted view that MASP-1 is an essential activator of the lectin pathway effector component MASP-2.

Lectin pathway effector enzyme mannan-binding lectin-associated serine protease-2 can activate native complement C3 in absence of C4 and/or C2.

All 3 activation pathways of complement—the classic pathway (CP), the alternative pathway, and the lectin pathway (LP)— converge into a common central event: the cleavage and activation of the abundant third complement component, C3, via formation of C3‐activating enzymes (C3 convertases). The fourth complement component, C4, and the second component, C2, are indispensable constituents of the C3 convertase complex, C4bC2a, which is formed by both the CP and the LP. Whereas in the absence of C4, CP can no longer activate C3, LP retains a residual but physiologically critical capacity to convert native C3 into its activation fragments, C3a and C3b. This residual C4 and/or C2 bypass route is dependent on LP‐specific mannan‐binding lectin‐associated serine protease‐2. By using various serum sources with defined complement deficiencies, we demonstrate that, under physiologic conditions LP‐specific C4 and/or C2 bypass activation of C3 is mediated by direct cleavage of native C3 by mannan‐binding lectin‐associated serine protease‐2 bound to LP‐activation complexes captured on ligand‐coated surfaces.—Yaseen, S., Demopulos, G., Dudler, T., Yabuki, M., Wood, C. L., Cummings, W. J., Tjoelker, L. W., Fujita, T., Sacks, S., Garred, P., Andrew, P., Sim, R. B., Lachmann, P. J., Wallis, R., Lynch, N., Schwaeble, W. J. Lectin pathway effector enzyme mannan‐binding lectin‐associated serine protease‐2 can activate native complement C3 in absence of C4 and/or C2. FASEB J. 31, 2210–2219 (2017). www.fasebj.org