In recent years, tumor immunotherapy, particularly anti-immune checkpoint antibody therapy and CAR-T therapy, has achieved remarkable clinical success. Natural killer (NK) cells, a key immune cell type, possess significant anti-cancer activity. NK cells possess diverse functions and pathways within the immune system, and NK cell activation can be promoted by triggering antibody-dependent cell-mediated cytotoxicity (ADCC) or targeting co-stimulatory receptors. CD16 is one of the most potent targets expressed by NK cells, contributing to potent anti-cancer, ADCC, and cytokine activities.

(Data source: Marofi F, et al. Stem Cell Res Ther. 2021)

CD16 composition:

CD16, also known as Fcγ receptor III (FcγRIII), is expressed from different genes in two forms: FcγRIIIa/CD16a and FcγRIIIb/CD16b. CD16a is an integral membrane protein primarily expressed on NK cells and monocytes; CD16b is primarily expressed on neutrophils (a subset of basophils) via a glycosylphosphatidylinositol anchor and displays inducible expression on eosinophils. Despite significant homology, CD16a differs in its antibody binding affinity by over 10-fold; the following description focuses on CD16a.

(Data source: Lajoie L, et al. J Immunol. 2014)

CD16a is a transmembrane receptor with a short C-terminal cytoplasmic tail and two extracellular Ig-like domains. Its intracellular part does not have any signaling components. In order to transduce signals, it requires two signaling chains with immunoreceptor tyrosine-based activation motifs (ITAMs).

CD16 signaling pathway and regulation:

NK cell cytotoxicity is regulated by a multitude of inhibitory and activating receptors. CD16a likely provides the strongest signal and can overcome inhibitory signals. CD16a lacks any ITAM domains in its cytoplasmic tail and therefore requires the assistance of two tandem intracellular chains bearing ITAM domains: CD3ζ and FcϵRIγ. Phosphorylation of CD16a promotes downstream signaling through cytokine production, while non-phosphorylation leads to enhanced degranulation. Upon CD16a engagement, the Src family kinase Lck is activated and phosphorylates the ITAM domains of CD3δ and/or FcϵRIγ. Phosphorylated ITAMs allow for the recruitment and phosphorylation of kinases from the Syk family, such as Syk and ZAP-70, which in turn are responsible for subsequent signaling.

(Data source: Meza Guzman LG, et al. Cancers. 2020)

Clinical value of CD16:

CD16 transduces signals by reacting with the immunoreceptor tyrosine-based activation motif (ITAM) linker, CD3δ, and Fc receptor common γ (FcRγ) chain in NK cells, triggering NK cell cytotoxicity ADCC and tumor cell apoptosis. For some therapeutic monoclonal antibodies, they exhibit stronger anti-cancer responses with the help of ADCC.

(Data source: Coënon L, et al. Front Immunol. 2022)

Currently aimed at enhancing therapeutic antibody-mediated CD16+NK cell activation are: (A) NK cell killer conjugates, recombinant chimeric molecules, such as trispecific killer conjugates (TriKEs), which are composed of a single-chain variable fragment (scFv) that recognizes CD16 and two antibodies targeting different tumor-associated antigens; (B) genetic modification of the CD16 gene to eliminate sensitivity to proteolytic cleavage or enhance affinity for the IgG-Fc region; (C) tumor-targeted monoclonal antibodies can be used in combination with agonistic monoclonal antibodies targeting activating receptors; blocking antibodies against HLA-I specific inhibitory receptors or immune checkpoints (ICIs); (D) Fc glycosylation modification to increase binding affinity to the CD16 receptor.

(Data source: Capuano C, et al. Cancers. 2021)