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Background: Macrophage apoptosis plays a critical role in progression of atherosclerosis. Previous studies suggest that M1 and M2 macrophage phenotypes dominate atherosclerosis. Recently, we showed that advanced lesions in the aortic root of Apoe -/- mice transplanted with bone marrow deficient in the calcium-permeable channel Transient Receptor Potential Canonical 3 (TRPC3) are characterized by reduced areas of necrosis and less apoptotic macrophages. However, the donor mice used in these studies had global deficiency of TRPC3, raising the question whether the observed phenotype was also contributed by TRPC3-deficient non-myeloid cells which could undermine the true impact of macrophage deletion of TRPC3. To address this important question, we generated mice with macrophage-specific loss of TRPC3 function (MacTrpc3 -/- ). Methods & results: 13 six week-old female Ldlr -/- mice were irradiated and transplanted with Ldlr -/- (control) or MacTrpc3 -/- Ldlr -/- (experimental) bone marrow and kept on high fat diet for 14 weeks. At the end of the diet period, aortic roots were sectioned and processed for atherosclerotic lesion analysis. Total lesion size (H&E), neutral lipid (Oil Red O) and macrophage content (CD68 staining) were not different between groups. However, we found a 1.7 fold decrease (P=0.01) in percent necrotic area in advanced lesions of MacTrpc3 -/- Ldlr -/- mice (23.12 ± 2.07%, n=10) compared to controls (39.63 ± 5.93%, n=10). Using in situ TUNEL we found that MacTrpc3 -/- Ldlr -/- lesions have less apoptotic cells compared to controls, and these overlapped with CD68 + areas. Using iNOS and mannose receptor as markers for M1 and M2 macrophages, respectively, we found that both subsets overlapped with CD68 + and TUNEL + positive areas, with no differences between groups (n=5). Previously, we showed that M1, but not M2 macrophages derived from MacTrpc3 -/- mice, had reduced apoptosis. This suggests that reduced plaque necrosis of MacTrpc3 -/- Ldlr -/- mice may be due to reduced apoptosis of M1 macrophages. In sum, these in vivo studies indicate that macrophage-specific deficiency of TRPC3 has a genuine beneficial effect on advanced atherosclerotic plaques, reducing apoptosis and necrosis, probably due to a selective effect of TRPC3 on M1 macrophages.