T-cell suppression by blood myeloid cells in melanoma

Studies have shown that tumour-bearing mice display expanded myeloid-derived suppressor cells. This observation contrasts with the characteristics of subsets of myeloid cells (monocytes/macrophages [CD14⁺], neutrophils [CD14⁻ CD15^hi], eosinophils [CD14⁻ CD15^int] and immature myeloid cells [CD14⁻ CD15⁻]), where no differences could be detected in the frequency and phenotype of these cells in the blood of patients with melanoma (n = 26) compared with healthy controls (n = 10). Interestingly, blood-derived monocytes and eosinophils display superior suppression of nonspecific T-cell proliferation compared with tumour-infiltrating myeloid cells. This finding suggests that the tumour suppressive function of these cells in patients with melanoma is weak and does not correspond to that of the mouse melanoma models. Thus, a reassessment of the usefulness of such models is warranted.