Abstract 875: Hematopoietic deficiency of Macrophage Inhibitory Cytokine-1 Affects CCR2 function and Attenuates Plaque Leukocyte Accumulation in LDL Receptor Knockout Mice
Macrophage inhibitory cytokine-1 (MIC-1) is a distant member of the transforming growth factor β family with alleged anti-inflammatory activity that acts through an unknown receptor. Recently it was shown that elevated MIC-1 serum levels are an independent risk factor for atherothrombosis. In this study we have therefore addressed the involvement of hematophoietic MIC-1 in atherogenesis. LDLr−/− mice were lethally irradiated and 24 hours later transplanted with either MIC1−/− or littermate bone marrow. The animals were allowed to recover for 6 weeks and placed on a high cholesterol diet for 12 weeks after which lesions were analyzed. Contrary to our expectations, plaque burden in the aortic sinus of MIC1−/− chimeras was similar to that in littermate controls. However plaque cellularity was markedly decreased in MIC1−/− chimeras (−31%, P = 0.003), which was completely attributable to reduced macrophage infiltration (−32%, P = 0.047). As MIC1−/− chimeras also displayed reduced circulating monocytes levels (−37%; P = 0.04) we turned our attention to CCR2. Expression of CCR2 was significantly decreased in PBMC and peritoneal leukocytes, possibly explaining the reduced chemotactic capacity of these subsets in MIC-1 deficiency. In keeping with these in vivo findings, in vitro macrophage migration confirmed the chemotactic properties of MIC-1(+102%; P = 0.001). Interestingly MIC-1 did not augment MCP-1 induced migration (P = 0.7), nor did it induce chemotaxis in CCR2 deficient macrophages (P = 0.3). These findings point to a direct signaling of MIC-1 to CCR2. Besides chemotaxis, MIC-1 deficiency also resulted in a relative enrichment in circulating regulatory T cells in MIC1−/− chimeras. In conclusion, we are the first to demonstrate a regulatory role of MIC-1 in macrophage migration to sites of inflammation, such as the atherosclerotic plaque, possibly via modulating CCR2 function. Moreover MIC-1 appears to inhibit regulatory T cell responses, thereby prolonging already established immunogenic responses. While further characterization of the underlying pathways is still awaited, our data suggest that focal inhibition of MIC-1 signaling will improve plaque stability by impairing CCR2 activity and re-enforcing regulatory T cell responses.