Blockade of checkpoint ILT3/LILRB4/gp49B binding to fibronectin ameliorates autoimmune disease in BXSB/Yaa mice

Abstract Extracellular matrix (ECM) not only provides a supporting framework for each tissue, but also elicits cellular responses by providing an inductive substrate. Fibronectin (FN), a major ECM macromolecule, is composed of multiple modules that interact with ECM-associated proteins, plasma proteins, and cell-surface proteins to ensure its modulatory roles. Here we revealed that FN is identified as a physiological ligand of the immunoglobulin (Ig)-like transcript 3/leukocyte Ig-like receptor B4 (ILT3/LILRB4) which serves as an immune checkpoint for autoimmune and cancer diseases. Human LILRB4 and the murine ortholog, gp49B, bound FN with sub-micromolar affinities as assessed by bio-layer interferometry. The major LILRB4-binding site in FN was located at the N-terminal 30-kDa module (FN30). Blockade of B4–FN binding with gp49 antibody or a recombinant FN30-Fc fusion protein reduced the secretion of pathogenic IgG autoantibodies, thereby reducing PAS-positive glomerular deposition in lupus-prone BXSB/Yaa mice. Compared with the control group, activation-induced cytidine deaminase (AID) expression in splenic activated B cells of FN30-Fc-treated BXSB/Yaa mice was decreased, indicating gp49B-FN30 signaling contributes in the class switch recombination (CSR) of Ig genes. These results suggested that gp49B–FN interplay is critical for pathogenic autoantibodies-mediated glomerulonephritis in the BXSB/Yaa lupus mouse model.