Background Innate lymphoid cells (ILC) are immune cells of the lymphoid lineage not expressing specific antigen receptors.They are classified into three subsets: ILC1 (including Natural Killer (NK) cells and ILC1) secrete IFN-γ and TNF-α; ILC2 secrete IL-4, IL-5, IL-9, IL-13, and amphiregulin; ILC3 (including Lymphoid Tissue Inducer (LTi) and NK cell activating receptor (NCR+ ILC3) secrete IL-17A and IL-22.
Objectives To enumerate the different subsets of ILCs in peripheral blood and synovial fluids of patients with inflammatory and non-inflammatory joint effusions.
Methods Patients with confirmed synovial effusion presenting at the Centre hospitalier universitaire de Sherbrooke (CHUS) signed an informed consent form. Cells from joint effusions were separated by Ficoll-density gradient centrifugation, stained for ILC, and analysed using a BD FACS Aria III flow cytometer. For cytokine detection, cells were stimulated for 6h with PMA/ionomycin prior to cell staining. The distribution of ILC subtypes according to various diagnoses is presented in cells/ml and ratios of ILC subtypes per ml of synovial fluid relative to peripheral blood. The protocol was approved by the CRC-CHUS ethics committee.
Results Synovial fluids and blood from 57 patients with various diagnoses were analyzed. The highest concentrations (cells/mL) of ILC cell subtypes found in the synovial fluids/peripheral blood were: ILC1: 7/4; ILC2: 70/3; LTi: 4x104 (in RA)/8x104 (in OA); NCR+ ILC3: 1.3x105 (in JIA)/5.8x104; NK: 5/1.
Synovial fluids relative to peripheral blood frequently presented ratios of ILC cell subtypes ≤1, suggesting little preferential homing in the joints. However, synovial fluids (relative to blood) were enriched 45 and 7 times for NCR+ ILC3 in Juvenile Idiopathic Arthritis (JIA) and spondylarthropathy patients, respectively, and 8 and 6 times for LTi in Psoriatic Arthritis and Rheumatoid Arthritis (RA) patients, respectively. We observed marked heterogeneity in ILC numbers within patients with the same inflammatory joint diseases. Part of this heterogeneity was associated with the presence of concomitant joint degenerative disease and low cell numbers in the synovial fluids.
Conclusions 1. LTi and NCR+ ILC3 subtypes are the ILC most abundant in synovial fluids.
ILC1 and NK cells are rare in synovial fluids and unlikely to be involved in pathogenesis; ILC2 remain infrequent, even when enriched in synovial fluid relative to peripheral blood (e.g. in JIA, RA and gout).
2. Relative to their concentrations in peripheral blood, LTi and NCR+ ILC3 subtypes are markedly enriched in synovial fluids of patients with autoimmune-mediated diseases, notably LTi in RA and Psoriatic Arthritis, and NCR+ ILC3 in spondylarthropathy and JIA. The abundance of these IL-17 secreting cells in synovial fluids from these diseases is especially intriguing.
3. We observed significant heterogeneity within patients with the same clinical diagnosis.
4. The pathophysiological implications of the differential distribution of subtypes of ILC cells across diseases and within clinical diagnoses remain unclear.
Disclosure of Interest None declared