Gradual Increase of FcγRIIIa/CD16a Expression and Shift toward IFN-γ Secretion during Differentiation of CD56dim Natural Killer Cells

Natural killer (NK) cell effector functions include cytotoxicity and secretion of cytokines such as interferon-γ (IFN-γ). The immature CD56^bright subset of human NK cells lacks expression of FcγRIIIa/CD16a, one of the low-affinity immunoglobulin G receptors, or exhibits low-density expression (CD56^brightCD16^-/dim) and produces IFN-γ in response to cytokine stimulation, whereas the mature CD56^dimCD16⁺ subset is the most cytotoxic one. A further differentiation/maturation of the latter subset according to the gradual loss of NKG2A and/or gain of KIR2DL (CD158a and CD158b) has been demonstrated and the ability to produce IFN-γ in response to activating receptor (AR) co-engagement is gradually acquired during terminal differentiation. In the course of flow cytometry analysis of CD56^dim NK cells, we noted a substantial intraindividual heterogeneity of expression of FcγRIIIa. FcγRIIIa is unique among ARs: it does not require the co-engagement of other ARs to induce substantial cytotoxicity or cytokine synthesis in CD56^dim cells. We, therefore, investigated whether individual differentiation/maturation of polyclonal CD56^dim NK cells defined by expression of NKG2A/KIR2DL is related to FcγRIIIa expression and to the heterogeneity of NK cell responses upon FcγRIIIa engagement. When we analyzed unstimulated CD56^dim cells by increasing level of FcγRIIIa expression, we found that the proportion of the more differentiated CD158a,h⁺ and/or CD158b,j⁺ cells and that of the less differentiated NKG2A⁺ cells gradually increased and decreased, respectively. FcγRIIIa engagement by using plate-bound murine anti-CD16 monoclonal antibody (mAb) or rituximab or trastuzumab (two therapeutic mAbs), resulted in donor-dependent partial segregation of IFN-γ-producing and/or degranulating CD56^dim cells. Importantly, the proportion of CD158a,h/b,j⁺ cells and that of NKG2A⁺ cells was increased and decreased, respectively, IFN-γ-producing cells, whereas these proportions were poorly modified in degranulating cells. Similar results were observed after engagement of ARs by a combination of mAbs targeting NKG2D, NKp30, NKp46, and 2B4. Thus, the gradual increase of FcγRIIIa expression is an important feature of the differentiation/maturation of CD56^dim cells and this differentiation/maturation is associated with a shift in functionality toward IFN-γ secretion observed upon both FcγRIIIa-dependent and FcγRIIIa-independent stimulation. The functional heterogeneity related to the differentiation/maturation of CD56^dim NK cells could be involved in the variability of the clinical responses observed in patients treated with therapeutic mAbs.

Human CD56dimCD16dim Cells As an Individualized Natural Killer Cell Subset

Human natural killer (NK) cells can be subdivided in several subpopulations on the basis of the relative expression of the adhesion molecule CD56 and the activating receptor CD16. Whereas blood CD56^brightCD16^dim/− NK cells are classically viewed as immature precursors and cytokine producers, the larger CD56^dimCD16^bright subset is considered as the most cytotoxic one. In peripheral blood of healthy donors, we noticed the existence of a population of CD56^dimCD16^dim NK cells that was frequently higher in number than the CD56^bright subsets and even expanded in occasional control donors but also in transporter associated with antigen processing-deficient patients, two familial hemophagocytic lymphohistiocytosis type II patients, and several common variable immunodeficiency patients. This population was detected but globally reduced in a longitudinal cohort of 18 HIV-1-infected individuals. Phenotypically, the new subset contained a high percentage of relatively immature cells, as reflected by a significantly stronger representation of NKG2A⁺ and CD57⁻ cells compared to their CD56^dimCD16^bright counterparts. The phenotype of the CD56^dimCD16^dim population was differentially affected by HIV-1 infection as compared to the other NK cell subsets and only partly restored to normal by antiretroviral therapy. From the functional point of view, sorted CD56^dimCD16^dim cells degranulated more than CD56^dimCD16^bright cells but less than CD56^dimCD16⁻ NK cells. The population was also identified in various organs of immunodeficient mice with a human immune system (“humanized” mice) reconstituted from human cord blood stem cells. In conclusion, the CD56^dimCD16^dim NK cell subpopulation displays distinct phenotypic and functional features. It remains to be clarified if these cells are the immediate precursors of the CD56^dimCD16^bright subset or placed somewhere else in the NK cell differentiation and maturation pathway.