PDCD1 and obesity due to melanocortin 4 receptor deficiency: Chronic inflammation in the mammary glands may induce T cell exhaustion via cytokines that upregulate PD-1 expression (e.g., IL-2, IL-6, IL-7, IL-12, IL-15, IL-21, and VEGFa)17,35–37, and most of these cytokines are known to be associated with obesity-induced inflammation.