Obesity is frequently associated with an infiltration of macrophages into adipose tissue. Adipocyte dysfunction causes a phenotypic switch of macrophages from an alternatively activated M2-like phenotype towards a proinflammatory M1 phenotype. The cross talk between adipocytes and infiltrating immune cells, in particular macrophages, is thought to contribute to local and eventually systemic inflammation. Here, we tested the phenotypic impact of a lack of adipocytes on the inflammatory status of macrophages. We took advantage of the fat apoptosis through targeted activation of caspase-8 (FAT-ATTAC) mouse model that allows for the inducible system-wide elimination of adipocytes through a proapoptotic mechanism and followed the degree and type of inflammatory response upon ablation of live adipocytes. Analysis of depots 2wk after elimination of adipocytes resulted in markedly reduced levels of adipose tissue and a robust down-regulation of circulating adipokines. Quantitative PCR and immunohistochemistry on epididymal and inguinal fat depots revealed an increase of the macrophage markers F4/80 and CD11c. Using polychromatic flow cytometry, we observed an up-regulation of alternatively activated M2 macrophage markers (CD206 and CD301) on the majority of F4/80 positive cells. Apoptosis of adipocytes is sufficient to initiate a large influx of macrophages into the remnant fat pads. However, these macrophages are alternatively activated, antiinflammatory M2 macrophages and not M1 cells. We conclude that adipocyte death is sufficient to initiate macrophage infiltration, and live adipocytes are required to initiate and/or sustain a proinflammatory response within the infiltrating macrophages in adipose tissue.
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