CCL2 and obesity due to melanocortin 4 receptor deficiency: In contrast, obesity induces a shift from M2 to M1 macrophages, which secrete pro-inflammatory cytokines such as IL-1, IL-6, IL-12, TNF-α, and monocyte chemotactic protein-1 (MCP-1), and produce inducible nitric oxide synthase (iNOS).