IL6 and obesity due to melanocortin 4 receptor deficiency: 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.