CD14+ Monocytes are a Critical Component of the Innate Immune System

What are Monocytes?

Hematopoietic stem cells (HSCs), which reside in the bone marrow give rise to blood cells in a process called hematopoiesis. HSCs differentiate into the lymphoid and myeloid lineages, which further differentiate into mature blood cell types (Figure 1).

Monocytes are large agranular leukocytes of the myeloid lineage comprising between 5% – 10% of the circulating white blood cells¹. They are characterized by the expression of the CD14 cell surface receptor and play an integral role in inflammatory responses as part of the innate immune response to foreign pathogens. Once recruited to tissues, monocytes are capable of differentiating into macrophages and dendritic cells. Monocytes serve three key functions in the immune system: phagocytosis, antigen presentation, and cytokine production.

Monocyte Subsets

Three kinds of human monocytes are now described with phenotypic and functional heterogeneity observed within the monocyte subsets^1,2,3:

1. Classical monocytes (CD14⁺, CD16−)
2. Intermediate monocytes (CD14⁺, CD16+)
3. Non-classical monocytes (CD14^dim, CD16+)

Classical monocytes exhibit a more pro-inflammatory phenotype, able to secrete soluble cytokines and differentiate into dendritic cells to bridge innate and adaptive immune responses. Intermediate monocytes specialize in antigen presentation and play an important role in HIV infections and lastly, the non-classical monocytes are responsible for the antiviral responses of this lineage².

Research Applications

The ability to culture monocytes in vitro is critical not only for immunological research but for newer areas of research, like personalized medicine with cell and gene therapies². Monocytes and macrophages, integral to the host defense against pathogens, immune regulation, inflammation response, and tissue repair, are increasingly popular clinical candidates due to their spectrum of functionality³. Researchers are investigating their role in degenerative disease pathology and the current evidence reveals that these cells could be utilized as an effective therapeutic strategy.

More recently, a publication utilized genetically-engineered patient-derived monocytes as a strategy to treat relapsed acute myeloid leukemia (AML)⁴. CD14⁺ monocytes obtained from the AML patient or HSCT donor are enriched and transduced with high titer lentiviral vectors expressing GM-CSF, IFN-α, and a truncated WT1 antigen in a CliniMACS Prodigy system. These reprogrammed monocytes differentiated into induced dendritic cells (iDCs) in vivo, subsequently migrating to lymph nodes where they stimulated T and B cell development to produce an enhanced immune response against cancer cells.

Chimeric antigen receptor (CAR)-T adoptive cell therapy for solid tumor indications has been ineffective mainly because of the inability of T cells to penetrate solid tumors and the inhibitory tumor microenvironment (TME)⁵. Companies like Carisma Therapeutics^® are looking at modifying other immune cells that may be better equipped to deal with solid tumors like CAR-modified macrophage (CAR-M) cells. Their CT-0508 product aims to treat solid tumors that overexpress HER2. These CAR-Ms are able to infiltrate the TME and phagocytose HER2-overexpressing tumor cells in a targeted manner. As well, when the tumor cells are broken down, antigen processing and presentation to T-cells activate the adaptive immune system, broadening the immune response. Since macrophages lack TCR, the potential risk of graft vs. host disease (GvHD) is low, which makes them attractive candidates for allogeneic CAR-macrophage approaches. In July 2020, Carisma Therapeutics^® was granted FDA clearance to initiate the first in-human, phase 1, multicenter clinical trial of CT-0508.

AllCells CD14⁺ Monocytes

AllCells offers both positively and negatively selected CD14⁺ monocytes isolated from leukapheresis material collected from IRB-consented healthy human donors. Figure 2 shows the differences between the two separation methods. Peripheral blood is collected by leukapheresis using the Spectra Optia^® Apheresis System. AllCells products are quality controlled for cell count and viability using validated AO/PI protocols.

Positive immunomagnetic selection uses CD14⁺ antibodies to bind/isolate the monocytes from the heterogeneous cell population. Conversely, negative selection labels unwanted cell types for removal with antibodies or ligands targeting non-monocytes, leaving the CD14⁺ monocytes remain unbound by the particles. Although some research shows that positive and negative immunomagnetic isolation of CD14⁺ monocytes may result in cells that have slight functional differences, selecting the right isolation method depends on your intended downstream applications⁶. While positively selected monocytes are most useful in analytical assays (genomics, FACS, etc.), negatively selected monocytes are generally preferred for functional assays (cell-based functional assays).

AllCells has the capacity of selecting CD14⁺ cells from a whole Leukopak with great quality and competitive costs. Quality cells are key to attaining your research goals and having a dedicated partner like AllCells can minimize your risk and maximize your success.

AllCells CD14⁺ Portfolio:

Footnotes

1. Kapellos TS, Bonaguro L, Gemünd I, et al. Human Monocyte Subsets and Phenotypes in Major Chronic Inflammatory Diseases. Front Immunol. 2019;10:2035. Published 2019 Aug 30. doi:10.3389/fimmu.2019.02035
2. Sampath P, Moideen K, Ranganathan UD, Bethunaickan R. Monocyte Subsets: Phenotypes and Function in Tuberculosis Infection. Front Immunol. 2018;9:1726. Published 2018 Jul 30. doi:10.3389/fimmu.2018.01726
3. Wolf AA, Yáñez A, Barman PK, Goodridge HS. The Ontogeny of Monocyte Subsets. Front Immunol. 2019;10:1642. Published 2019 Jul 17. doi:10.3389/fimmu.2019.01642
4. Bialek-Waldmann JK, Heuser M, Ganser A, Stripecke R. Monocytes reprogrammed with lentiviral vectors co-expressing GM-CSF, IFN-α2 and antigens for personalized immune therapy of acute leukemia pre- or post-stem cell transplantation. Cancer Immunol Immunother. 2019;68(11):1891-1899.
5. Mukhopadhyay M. Macrophages enter CAR immunotherapy. Nat Methods. 2020;17(6):561. doi:10.1038/s41592-020-0862-4
6. Bhattacharjee J, Das B, Mishra A, Sahay P, Upadhyay P. Monocytes isolated by positive and negative magnetic sorting techniques show different molecular characteristics and immunophenotypic behaviour. F1000Res. 2017;6:2045. Published 2017 Nov 23. doi:10.12688/f1000research.12802.3