IL6 and obesity due to melanocortin 4 receptor deficiency: Our previous studies showed upregulation of 4-1BB and its ligand in inflamed adipose tissue/skeletal muscle with infiltrated macrophages in obese condition [15, 16], and the interaction of the receptor/ligand augmented obesity-induced production of inflammatory cytokines (TNFα, IL-6, and MCP-1), which was reduced in the absence of 4-1BB [3, 15].