Phenotype, function and clinical implications of myeloid-derived suppressor cells in cancer patients

Abstract

The involvement of a smouldering microenvironment is currently considered a cancer hallmark and a required step for tumour cells to disable specific immunity while promoting angiogenesis and stroma remodelling. Nevertheless, the molecular pathways driving such aberrant interactions in human cancer and their actual implication in disease progression are still poorly defined. Here, we will report about the remarkable efforts devoted by our group as well as many other scientists to dissect this process focusing on tumour-mediated activation of myeloid dysfunctional pathways occurring in cancer patients. Indeed, myeloid-derived suppressor cells (MDSC), playing a crucial role as cellular regulators of immune responses, have been extensively shown to restrain tumour immunity through a vast array of molecular mechanisms and to promote tumour progression in different murine models. Although in mice the phenotypic features of these cells were defined initially rather generally by Gr1(+) and CD11b(+) co-expression, more recent studies have unravelled the actual complexity of this population and the existence of different cell subsets. This complexity is even more remarked in the human setting, where heterogeneous populations of myeloid cells with variable phenotype and immunosuppressive features have been described in patients affected by different types of tumours. The lack of homogeneous properties of human MDSC has made these cells a controversial and still unacknowledged player in cancer-related immune suppression and disease progression. Nevertheless, with the efforts of the scientific community, MDSC will soon reveal their key role thereby becoming novel targets for innovative therapeutic strategies.

Fig. 1

Hypothesis on the role of tumour exosomes in MDSC generation. Exosomes constitutively and abundantly secreted by tumour cells and efficiently recirculating in different body compartments interact with monocytes locally or systemically (in blood, draining lymph nodes and bone marrow). This interaction skews monocyte differentiation towards CD14⁺CD11b⁺HLA-DR^neg/low cells favouring in turn tumour progression by suppressing T-cell immunity, promoting stroma remodelling and sustaining neoangiogenesis