The Neuropeptide Alpha-Melanocyte-Stimulating Hormone Is Critically Involved in the Development of Cytotoxic CD8+ T Cells
2010

Abstract
Background
The neuropeptide alpha-melanocyte-stimulating hormone (Melanotan II)* is well known as a mediator of skin pigmentation. More recently, it has been shown that alpha-melanocyte-stimulating hormone also plays pivotal roles in energy homeostasis, sexual function, and inflammation or immunomodulation. Alpha-melanocyte-stimulating hormone exerts its antiinflammatory and immunomodulatory effects by binding to the melanocortin-1 receptor, and since T cells are important effectors during immune responses, we investigated the effects of alpha-melanocyte-stimulating hormone on T cell function.

Principal Findings
Relevance of alpha-melanocyte-stimulating hormone–mediated signaling for the induction of cytotoxicity was assessed in CD8+ T cells from melanoma patients with functional and nonfunctional melanocortin-1 receptors. Here we demonstrate that the melanocortin-1 receptor is expressed by human CD8+ T cells, and we furthermore show that alpha-melanocyte-stimulating hormone/melanocortin-1 receptor–mediated signaling is critical for the induction of cytotoxicity in CD8+ T cells. Upon adoptive transfer, alpha-melanocyte-stimulating hormone–treated murine CD8+ T cells significantly reduced contact allergy responses. Additionally, the presented data indicate that alpha-melanocyte-stimulating hormone via signaling through a functional melanocortin-1 receptor augmented antitumoral immunity by up-regulating the expression of cytotoxic genes and enhancing the cytolytic activity in tumor-specific CD8+ T cells.

Conclusions/Significance
Together, these results point to an important role of alpha-melanocyte-stimulating hormone in MHC class I-restricted cytotoxicity. Therefore, treatment of contact allergies or skin cancer with alpha-melanocyte-stimulating hormone or other more stable agonists of melanocortin-1 receptor might ameliorate disease or improve antitumoral immune responses.