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

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Abstract
Summary: 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.