An antibody that targets cell-surface glucose-regulated protein-78 inhibits expression of inflammatory cytokines and plasminogen activator inhibitors by macrophages

Glucose-regulated protein-78 (Grp78) is an endoplasmic reticulum chaperone, which is secreted by cells and associates with cell surfaces, where it functions as a receptor for activated α₂ -macroglobulin (α₂ M) and tissue-type plasminogen activator (tPA). In Macrophages, α₂ M and tPA also bind to the transmembrane receptor, LDL receptor-related protein-1 (LRP1), activating a cell-signaling receptor assembly that includes the NMDA Receptor (NMDA-R) to suppress innate immunity. Herein, we demonstrate that an antibody targeting Grp78 (N88) inhibits NFκB activation and expression of proinflammatory cytokines in bone marrow-derived Macrophages (BMDMs) treated with the toll-like receptor-4 (TLR4) ligand, lipopolysaccharide, or with agonists that activate TLR2, TLR7, or TLR9. Pharmacologic inhibition of the NMDA-R or deletion of the gene encoding LRP1 (Lrp1) in BMDMs neutralizes the activity of N88. The fibrinolysis Protease inhibitor, plasminogen activator inhibitor-1 (PAI1), has been implicated in diverse diseases including metabolic syndrome, Cardiovascular Disease, and Type 2 Diabetes. Deletion of Lrp1 independently increased expression of PAI1 and PAI2 in BMDMs, as did treatment of wild-type BMDMs with TLR agonists. tPA, α₂ M, and N88 inhibited expression of PAI1 and PAI2 in BMDMs treated with TLR-activating agents. Inhibiting Src family Kinases blocked the ability of both N88 and tPA to function as anti-inflammatory agents, suggesting that the cell-signaling pathway activated by tPA and N88, downstream of LRP1 and the NMDA-R, may be equivalent. We conclude that targeting cell-surface Grp78 may be effective in suppressing innate immunity by a mechanism that requires LRP1 and the NMDA-R.