Heterogeneity of tumor-infiltrating myeloid cells in era of single-cell genomics

Tumor microenvironment (TME) is highly heterogeneous and composed of complex cellular components, including multiple kinds of immune cells. Among all immune cells in TME, tumor-infiltrating myeloid cells (TIMs) account for a large proportion and play roles as key regulators in a variety of functions, ranging from immune-mediated tumor killing to tumor immune evasion. Understanding the heterogeneity of TIMs will provide valuable insights for new therapeutic targeting of myeloid cells. Single-cell genomic technologies deciphering cell composition and gene expression at single-cell resolution have largely improved our understanding of the cellular heterogeneity of TIMs and highlighted several novel cell subtypes contributing to the variation of patient survival and treatment response. However, these cell subtypes were defined based on limited data without a concordant nomenclature, which makes it difficult to understand whether they exist in different studies. Thus, in this review, we comprehensively summarized the common agreements and current different opinions on the heterogeneity of TIMs gained from single-cell studies; evaluated the feasibility of current myeloid cell targets at single-cell level and proposed a uniform nomenclature for TIM subsets.