CD16: A Key Marker for Immune Cell Subsets and Disease Diagnosis

What is CD16 ?

CD16, also known as FcγRIII, is a cluster of differentiated molecules found on the surface of natural killer cells, neutrophils, monocytes, macrophages, and certain T cells. CD16 is the best-studied membrane receptor involved in triggering NK cell lysis, a molecule of the immunoglobulin superfamily (IgSF), involved in antibody-dependent cellular cytotoxicity (ADCC). It can be used to isolate specific immune cell populations by fluorescence-activated cell sorting (FACS) or magnetic-activated cell sorting using antibodies against CD16.

The molecular weight of the CD16 antigen is 50-70 kD, and it is an Fc receptor with low affinity for the IgG-Fcγ receptor III complex. CD16 is a biomarker associated with monocytes and natural killer (NK) cells in the lymphatic system. Immunohistochemical detection of CD16 can be used in the differential diagnosis of γ-δ T-cell lymphoma of the liver and spleen and γ-δ T-cell large granular lymphocytic leukemia. It has been reported that about 58% of γ-δ T-cell lymphomas of the liver and spleen and 86% of γ-δ T-cell large granular lymphocytic leukemia express CD16, while skin and mucosal γ-δ T-cell lymphomas do not express CD16.

Expression and Function of CD16

CD16 is involved in antibody-dependent cell-mediated cytotoxicity and is expressed on large granular lymphocytes (LGLs), both NK and T cell types. Approximately 15-20% of peripheral blood lymphocytes and a small fraction (5%) of bone marrow lymphocytes express CD16^dim. CD16 is also expressed at moderate levels in granulocytes, tissue macrophages and monocyte subsets, eosinophils, and dendritic cells.

NK cells are a type of natural lymphocytes that are very effective in destroying stressed cells, such as virus-infected or tumor-transformed cells. Normally, the ADCC effect mainly activates NK cells through antibodies that bind to FCR. The most characteristic FcR on the NK cell membrane is FcγRIII (CD16), in which NK cells mainly express CD16a, while CD16b is limited to neutrophils. Human NK cells are divided into two major subsets: CD56^bright and CD56^dim. CD56^bright NK cells are potent cytokine producers but lack CD16a, whereas CD56^dim are highly cytotoxic and express CD16a.

Upon recognition of IgG-conditioned targets via CD16a, NK cells release different cytotoxic molecules that trigger the death of the target cell. This mechanism relies on the formation of immune synapses and degranulation of lysed particles containing perforin and granzyme. In addition to degranulation, NK cells can remove target cells by binding target death receptors (e.g. DR4, DR5, or Fas) to their death receptor ligands (e.g. FasL and TRAIL).

CD16 as a Key Marker for Immune Cell Subsets

CD16 is often used as an additional marker to identify distinct subsets of human immune cells. Several other CD molecules, such as CD11b and CD33, have traditionally been used as markers for human myeloid-derived suppressor cells (MDSCs). However, since these markers are also expressed in NK cells and all other myeloid-derived cells, additional markers such as CD14 and CD15 are required. Neutrophils were found to be CD14^low and CD15^high, while monocytes were found to be CD14^high and CD15^low. While these two markers were sufficient to distinguish neutrophils from monocytes, eosinophils showed similar CD15 expression to neutrophils. Therefore, CD16 is used as a further marker to identify neutrophils: mature neutrophils are CD16^high, while eosinophils and monocytes are both CD16^low. CD16 allows the distinction between these two types of granulocytes. Furthermore, CD16 expression was differential between different stages of neutrophil development: differentiation-competent neutrophil progenitors were CD16^low, and CD16 expression was increased in postmyeloid, banded, and mature neutrophils.

References

1. Coënon L.; Villalba M. From CD16a biology to antibody-dependent cell-mediated cytotoxicity improvement. Frontiers in Immunology. 2022, 13: 913215.
2. Capuano C.; et al. Harnessing CD16-mediated NK cell functions to enhance therapeutic efficacy of tumor-targeting mAbs. Cancers. 2021, 13(10): 2500.