MSC-derived IL-6 promotes expansion of myeloid progenitors and mature myeloid cells (138, 139), and TLR4 signaling on MSCs directly leads to secretion of the monocyte chemokine MCP-1 (also known as CCL2) to promote the extravasation of inflammatory Ly6chi monocytes into circulation (140), another mechanism that could be exacerbated during obesity. This evidence concerns the gene CCL2 and obesity due to melanocortin 4 receptor deficiency.