Key Points

Perivascular deposition of IgA is not the only major trigger in IgA vasculitis.

Perivascular deposition needs to be supplemented with priming of circulating PMNs by IgA complexes.

IgA-primed PMNs undergo vast NETosis when adhering to vessels with perivascular IgA.

Abstract

In IgA vasculitis (IgAV) perivascular deposition of IgA1 immune complexes (IgA-ICs) is traditionally considered the fundamental trigger for polymorphonuclear neutrophil (PMN)–mediated damage. We propose that IgA-IC deposition, although mandatory, is not sufficient alone for IgAV. Serum IgA-IC levels and IgA-IC binding to PMNs were quantified in IgAV patients and controls. Activation of PMNs was evaluated by neutrophil extracellular trap (NET) release, adherence, and cytotoxicity assays and in a flow system to mirror conditions at postcapillary venules. In vitro results were related to findings in biopsies and a mouse vasculitis model. During acute IgAV flares we observed elevated serum levels of IgA-ICs and increased IgA-IC binding to circulating PMNs. This IgA-IC binding primed PMNs with consequent lowering of the threshold for NETosis, demonstrated by significantly higher release of NETs from PMNs activated in vitro and PMNs from IgAV patients with flares compared with surface IgA-negative PMNs after flares. Blocking of FcαRI abolished these effects, and complement was not essential. In the flow system, marked NETosis only occurred after PMNs had adhered to activated endothelial cells. IgA-IC binding enhanced this PMN tethering and consequent NET-mediated endothelial cell injury. Reflecting these in vitro findings, we visualized NETs in close proximity to endothelial cells and IgA-coated PMNs in tissue sections of IgAV patients. Inhibition of NET formation and knockout of myeloperoxidase in a murine model of IC vasculitis significantly reduced vessel damage in vivo. Binding of IgA-ICs during active IgAV primes PMNs and promotes vessel injury through increased adhesion of PMNs to the endothelium and enhanced NETosis.

Footnotes

This work was supported by the fund “Innovative Medical Research” of the University of Muenster Medical School (Grant PA111515). The work in Leicester was supported by the Mayer Family Fund.

C.S., J.M.E., J.R., K.I.P., S.M.-H., and T.V. designed and supervised the experiments. C.S., J.B., J.M.E., J.R., K.I.P., K.M., S.M.-H., and T.V. wrote and revised the manuscript. C.S., D.G., E.N., K.G., K.I.P., K.M., M.N., and S.M.-H. performed experiments and analyzed data. C.S. and K.I.P. achieved grants and provided funding. All authors discussed the results and contributed to the final manuscript.

The online version of this article contains supplemental material.

Abbreviations used in this article:

ANCAantineutrophil cytoplasmic AbC3complement component 3CTRLcontrolECendothelial cellEGMendothelial growth mediumH3Citcitrullinated histone 3ICimmune complexIgAVIgA vasculitisMPOmyeloperoxidaseNETneutrophil extracellular trapPADprotein arginine deiminasePMNpolymorphonuclear neutrophilReceived September 24, 2021.Accepted July 7, 2022.Copyright © 2022 by The American Association of Immunologists, Inc.