A Guide to Myeloid Cell Markers

Myeloid cells generally refer to a lineage of eukaryotic cells that includes monocytes, macrophages, dendritic cells, basophils, neutrophils, eosinophils, erythrocytes, and thrombocytes. The classical myeloid lineage progression begins from multipotent hematopoietic stem cells, giving rise to a common myeloid progenitor, before undergoing further myeloid fates as a myeloblast. Tissue-resident myeloid cells, including macrophages, have also been shown to derive from embryonic sources. The myeloid cell population is notably known for its role in the innate immune response, in contrast to the lyphoid cell lineage, which is commonly associated with the adaptive immune system. 

The myeloid cell compartment has gained increased recognition as a crucial element within the tumor microenvironment, consequently emerging as promising targets for anti-cancer therapies. Here, we aim to cover the common markers for studying myeloid cells, focusing on the protein biomarkers and targets in the context of cancer. 

General Myeloid Markers

Monocytes function as circulating phagocytes in the blood and act as a functional reservoir to replenish other myeloid cell subtypes. CD14 and CD16 are standard monocyte markers in humans, while Ly6C is used in mice. 

As the tissue-resident counterpart to monocytes, macrophages share similar phenotypic markers, such as CD14, CD16, and Ly6C. In addition, common macrophage markers also include CD68, CSF1R, CD11b, CD64, and CX3CR1.

Dendritic cells, by acting as antigen-presenting cells, can serve both innate and adaptive immune functions. Common dendritic cell markers include CD1A, CD141, CD1C, and CD103.

Granulocytes, which include neutrophils, basophils, eosinophils, and mast cells, feature cytoplasmic granules containing antimicrobial proteins. While each type of granulocyte possesses unique cellular markers and functions, there are a fair amount of overlap. Some notable markers shared by granulocyte lineage cells include CD11b, CD13, CD193, CD33, and IL5RA. Learn more about granulocyte markers here. 

Myeloid cells in the context of cancer

Cancer immunotherapy, including immune checkpoint inhibition and T cell-based immunotherapies, has shown promise in recent years, but is not without drawbacks. The tumor microenvironment (TME) exposes cells in the vicinity to harsh conditions, including hypoxia, restricted nutrition, and acidosis. Myeloid cells, which are among the many groups of tumor-infiltrating immune cells, are susceptible to such changes in phenotype to favor tumor growth, metastasis, and resistance to therapies. The presence of myeloid cell subtypes in several cancer types has been correlated with worse outcomes in patients. 

Inversely, myeloid cells may also play important supporting anti-tumor roles, such as in T cell antigen presentation, co-stimulatory signals, cytokine production, and the production of ROS or death signals. 

Recent studies, including those applying single-cell and multiomics approaches, have sought to understand the complexities of tumor-associated myeloid cell subsets, how they contribute to tumor survival, and how new treatments may be devised around this diverse immune cell population. 

This diagram highlights common markers for myeloid cells in the tumor microenvironment.

MDSC markers in cancer

A major tumor-associated cell type is the myeloid-derived suppressor cell (MDSC), whose populations can expand under chronic inflammatory conditions. MDSCs can inhibit the immune actions mediated by lymphocytes. In cancer, MDSCs suppress T cells via nitric oxide production. MDSCs are an important cell type for investigation due to their immunosuppressive functions in the tumor microenvironment. MDSCs have many markers in common with their monocyte progenitors, such as ADGRE1, CD62L, CD1A, and CXCR4. Markers that function in immunosuppression have also been associated with MDSCs, including IDO1 and ARG1. 

Macrophage markers in cancer

Tumor-associated macrophages (TAMs), while not always myeloid in lineage, are the most abundant immune cells within the TME. TAMs are known for inhibiting tumor cell clearance by T cells via direct contact or the production of proteins such as PD-L1 and arginase. In the clinical setting, an increased density of macrophages in tumor sites has been correlated with poor patient survival in many types of cancer. That TAMs can play a crucial role in immunosuppression and therapy resistance makes the cell type an ideal target for combination immunotherapy. Protein markers notably expressed by TAMs include CD14, CD64, HLA-DR, and CD163. 

Neutrophil markers in cancer

Tumor-associated neutrophils (TANs) are a heterogeneous myeloid population recruited into the TME by cytokine-producing tumors. By modulating the extracellular matrix with metalloproteases and neutrophil elastase, these neutrophils can promote cancer progression. They can also promote metastasis by producing VEGF and induce DNA damage by producing reactive oxygen (ROS) and nitrogen species (RNS). While TANs remain to be fully understood, they are believed to polarize into pro- and anti-tumor phenotypes, in a manner similar to macrophages.

Dendritic cells in cancer

Unlike TAMs and MDSCs, dendritic cells in the TME are considered to be anti-tumorigenic. A higher content of intratumoral DCs, especially the conventional DC (cDC) subtype, is associated with improved clinical outcomes for cancer immunotherapy. cDCs, which can be recognized as CD103+ (mouse) or CD141+ (humans), can cross-present tumor antigens to CD8+ T cells and stimulate CD4+ T cells. 

Strategies for targeting myeloid cells

The pro-tumor or anti-tumor functions of myeloid cells are ideal targets for immunotherapies, some of which are already underway. Preclinical studies of key signaling pathways in cancer-associated myeloid cells have led to several targeting strategies.

Myeloid cells can be altered in their differentiation, proliferation, and recruitment within the tumor microenvironment. Studies on this approach target signaling pathways for myeloid recruitment (CCL2–CCR2 and IL8–CXCR1/2), TAM polarization (CSF1–CSF1R), and cell growth (STAT3).

The immune suppression functions of myeloid cells can be blocked. One example is the use of CD24 by certain tumor cells to bind TAMs and escape macrophage clearance. Reportedly, an anti-CD24 monoclonal antibody has been used to construct an antibody-drug conjugate, a bispecific antibody to CD3, and a CAR-T therapy for potential clinical trials. Other immunosuppressive enzymes such as CD39 and CD73 are also candidate targets for inhibition. 

The reprogramming of myeloid cells to fulfill an anti-tumorigenic role is another actively pursued area of research. Examples of reprogramming include the modulation of activating receptors (such as TLR7 and CD40) and inhibition of key proteins (such as TREM2, LILRB4, and PI3Kγ).

Table of Myeloid Cell Markers

The table below highlights notable proteins expressed by myeloid cells in tumors compiled from recent literature. In addition, links to relevant antibodies and ELISA kits are provided as these immunodetection tools are routinely used for cell characterization via flow cytometry and immunostaining. The associated products are offered by a variety of manufacturers and, when used in combination with other phenotypic markers, can serve as a useful reference for studying myeloid cell populations. The marker types include tumor-associated myeloid cells (Tumor), macrophages (Mac), tumor-associated macrophages (TAM), tumor-associated neutrophils (TAN), monocytes (Mono), myeloid-derived suppressor cell (MDSC), granulocytes (Gran), dendritic cells (DC), and therapeutic target proteins (Target). 

Note: *Some markers are protein isoforms, multi-subunit protein complexes, or protein families composed of several distinct genes. Information on Protein Type, Localization, and Size (kDa) obtained from UniProt.org (for human genes only). 

References

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