Human Tumor-Infiltrating Myeloid Cells: Phenotypic and Functional Diversity

Abstract

Our current understanding of human tumor-resident myeloid cells is, for the most part, based on a large body of work in murine models or studies enumerating myeloid cells in patient tumor samples using immunohistochemistry (IHC). This has led to the establishment of the theory that, by and large, tumor-resident myeloid cells are either "protumor" M2 macrophages or myeloid-derived suppressor cells (MDSC). This concept has accelerated our understanding of myeloid cells in tumor progression and enabled the elucidation of many key regulatory mechanisms involved in cell recruitment, polarization, and activation. On the other hand, this paradigm does not embrace the complexity of the tumor-resident myeloid cell phenotype (IHC can only measure 1 or 2 markers per sample) and their possible divergent function in the hostile tumor microenvironment. Here, we examine the criteria that define human tumor-infiltrating myeloid cell subsets and provide a comprehensive and critical review of human myeloid cell nomenclature in cancer. We also highlight new evidence characterizing their contribution to cancer pathogenesis based on evidence derived from clinical studies drawing comparisons with murine studies where necessary. We then review the mechanisms in which myeloid cells are regulated by tumors in humans and how these are being targeted therapeutically.

Keywords: immune cell phenotyping; macrophages; monocytes; myeloid derived suppressor cells; neutrophils; oncoimmunology; tumor microenvironment.

Figure 1

Overview of human myeloid cells identified in the bone marrow, blood, and tissue of cancer patients. Cancer-associated inflammation upregulates the production of myeloid cells from hematopoietic progenitors in the bone marrow. This figure illustrates the network of myeloid cells that have been identified in the blood and tumor tissue in human cancer. Cell surface markers expressed by the various myeloid cell types are listed portraying the huge degree of phenotypic similarity between the cell subsets. The thick curved black line depicts a pathway of cell differentiation that has been suggested but has not yet been proven.

Figure 2

Suggested gating strategy for the identification of tumor-infiltrating myeloid cells in humans. To dissect the main infiltrating myeloid cells, we propose a 12 color flow cytometry panel and progressive gating strategy. Gating on the CD45⁺ population identifies the leukocyte population. Within the HLA-DR^hiCD11c^hi population, tumor-associated macrophages (TAMs) can be distinguished from DCs based on CD14⁺CD64⁺ expression. The CD11b^hiCD15^hi population identifies tumor-infiltrating neutrophils. CD66b is used to confirm the identity of neutrophils. Tumor-associated neutrophils (TANs) express CD33 and arginase at varying levels. It is important to note that low levels of CD64 and CD14 can be expressed on TANs, whereas TAMs can express low levels of CD15. Eosinophils are CD15^int and CD16^lo. It is important to use the appropriate controls such as fluorescence minus one controls and normal or uninvolved tissue where possible.