Selective, autoantibody-immune complex mediated proportional and functional changes of specific NK-cell subsets in early seropositive but not seronegative rheumatoid arthritis

Background: Despite substantial data demonstrating NK-cell impairment in rheumatoid arthritis (RA), the exact role of NK-cells in RA immunopathogenesis remains unclear. Objectives: We studied the involvement of CD56dim and CD56bright NK-cells in the early stages of RA development to elucidate their role in the pathogenesis of RA. Methods: The study included newly diagnosed RA patients and seropositive arthralgia patients (SAP), representing subjects at risk of progression to RA. Whole blood samples obtained from disease modifying anti rheumatic drug (DMARD)-free RA patients at the time of diagnosis (autoantibody -positive [SP RA, n=45] and -negative [SN RA, n=12]); SAP (n=30) and healthy controls (HC, n=41), were studied to assess absolute numbers and proportions of T-cells, B-cells and NK-cells. Peripheral blood mononuclear cells (PBMC) were used for both phenotypical and functional analyses of NK-cells in the studied groups. Results: Patients (SP RA and SAP) positive for anti-cyclic citrullinated protein antibodies (ACPA) and/or rheumatoid factor (RF) showed decreased total NKcell numbers. Also, NK-cells from SP RA showed a decreased potency for IFN-γ production. NK-cell subset analysis demonstrated a specific decrease of CD56dim, but not CD56bright NK-cells in SP RA and SAP. Differential NK-cell alterations in seropositive and seronegative patients prompted the investigation of the role of CD16 (FcγRIIIa) triggering in apoptosis induction and cytokine expression by CD56dim and CD56bright NK-cell subsets. CD16 triggering in vitro induced apoptosis of CD56dim, but not CD56bright NK-cells, and was augmented by the addition of IL-2. Also, CD16 triggering in the presence of IL-2 stimulated IFN-γ expression by CD56dim NK-cells. Conclusions: The decline of CD56dim NK-cells is an early feature in the development of seropositive, but not seronegative RA. The decline of CD56dim NK-cells in SAP and SP RA and observed in vitro apoptosis of CD56dim NK-cells upon CD16 triggering, suggests a functional role of IgG-containing autoantibody (ACPA and/or RF)-immune complexes in CD16-mediated apoptosis. Moreover, CD16 triggering leads to an increased cytokine response by CD56dim NK-cells, which may contribute to the proinflammatory state in both SP RA and SAP