LAUSR

Following myocardial infarction (MI), the left ventricle undergoes wound healing that begins withan intense inflammatory response to recruit leukocytes to the infarcted region. Infiltratedneutrophils degranulate, releasing a series of proteases including matrix metalloproteinase(MMP)-9 into the extracellular space. Macrophages also infiltrate into the infarct region, and wehypothesized that the macrophage may secrete factors that regulate neutrophil degranulation.Stimulating bone marrow derived neutrophils from C57BL/6J mice (3-6 month old male) withphorbol myristate acetate (PMA) induced degranulation, as evidenced by release of MMP-9.Co-stimulation with MI day 1 macrophage secretome (10% by volume) reduced PMA-induceddegranulation of MMP-9 by 8-fold (p<0.0014). Transcriptomic analysis of day 1 MI macrophagesrevealed galectin-3, vimentin, fibronectin, and murinoglobulin-1 were highly expressed.Examination of the day 1 MI macrophage secretome also showed high expression of vimentinand fibronectin. To further unmask signaling connections, neutrophils were co-stimulated withPMA and day 1 MI macrophage secretome, along with blocking antibodies for the 4 proteinsindividually. Galectin-3 inhibition promoted the macrophage secretome effect, whilemurinoglobulin-1 inhibition reversed the macrophage secretome effect. Neutrophil stimulationwith recombinant galectin-3 showed no additive effect with PMA on MMP-9 release, suggestingthat galectin-3 works through the protein kinase C signaling pathway to induce neutrophildegranulation. Neither Vimentin inhibition nor stimulation had an effect on neutrophildegranulation Overall, our results uncovered a role for galectin-3 and fibronectin in inducingneutrophil degranulation while murinoglobulin-1 may be a potent inhibitor of degranulation. Theday 1 MI macrophage therefore secretes both factors that promote as well as factors thattemper neutrophil degranulation. The balance of these factors determines the net effect of themacrophage secretome on neutrophil degranulation.