Increased relative CD5<sup>+</sup>ILC2 counts in patients with rheumatoid arthritis

Innate lymphoid cells (ILCs) are the innate analogues of lymphocytes that do not express antigen-specific receptors and are primarily found in tissues and mucosa. ILCs are divided into three groups based on the transcription factors and cytokines they secrete. Group 1 ILCs produce interferon (IFN)-γ in response to IL-12 and are dependent on the transcription factor T-bet; group 2 ILCs (ILC2s) predominantly produce type 2 cytokines (IL-5, IL-4, IL-9, and IL-13) in response to IL-33, IL-25, and thymic stromal lymphopoietin (TSLP) and are dependent on GATA3. Group 3 ILCs include ILC3s and lymphoid tissue inducer cells (LTi). The latter group secretes IL-17 and IL-22 in response to IL-1β and IL-23 and functionally depends on RORγt. Recently, early ILC precursors were found in peripheral blood, which were defined by the CD5 marker and are likely to be of thymic origin. These cells can, on demand, enter the bloodstream (like monocytes), move with the blood flow into tissues for subsequent differentiation into a mature phenotype. In this work, we assessed the content of CD5+ILC2 in peripheral blood of patients with rheumatoid arthritis (RA), which is characterized by chronic inflammation in the joints and uncontrolled cell proliferation, thus maintaining the inflammatory events. In this work, we used peripheral blood from patients with RA (n = 7) and conditionally healthy donors (n = 13). The obtained peripheral blood mononuclear cells (PBMPs) were stained with the following panel of antibodies: anti-lineage (CD2/3/14/16/19/20/56/235a), antiCD11c and anti-FceR1 alpha-FITC, anti-CD294-PE, anti-CD127-PerCP/Cy5.5, antiCD117-APC, anti-CD5-BV-450. Innate lymphoid cells were defined as Lin-CD127+, CD294+ILCs were identified as ILC2. The proportion of CD5+ cells among ILC2 was also evaluated. The cell phenotype was analyzed by flow cytometry. We showed that the proportion of ILC2 among all PBMPs was significantly lower in patients with RA compared to the donor group, and the number of CD5+ILC2 among ILC2 was significantly higher than in the control group. The obtained results are unique and provide us with new data on the changing percentage of ILC2 among PC MNCs and CD5+ILC2 among ILC2.