MC1 receptors are constitutively expressed on leucocyte subpopulations with antigen presenting and cytotoxic functions

The expression of melanocortin MC1 receptors on human peripheral lymphocyte subsets was analysed by flow cytometry using rabbit antibodies selective for the human MC1 receptor and a panel of monoclonal antibodies against lymphocyte differentiation markers. The MC1 receptor was found to be constitutively expressed on monocytes/macrophages, B-lymphocytes, natural killer (NK) cells and a subset of cytotoxic T-cells. Interestingly T-helper cells appeared to be essentially devoid of MC1 receptors. The results were confirmed by RT-PCR which indicated strong expression of MC1 receptor mRNA in CD14(+), CD19(+) and CD56(+) cells. However, only a faint RT-PCR signal was seen in CD3(+) cells, in line with the immuno-staining results that indicated that only part of the CD3(+) cells (i.e. some of the CD8(+) cells) expressed the MC1 receptor. The MC1 receptors' constitutive expression on immune cells with antigen-presenting and cytotoxic functions implies important roles for the melanocortic system in the modulation of immune responses.