IL-2 promotes IL-5–producing ILC2s and induces eosinophilia. (A) HCV-induced vasculitis patients received IL-2 at 1.5 million international units (MIU)/day from days 1 to 5 (course1 [C1]), then at 3 MIU/day from days 15 to 19 (course 2 [C2]), 36 to 40 (course 3 [C3]), and 57 to 61 (course 4 [C4]). IL-5–fold increase (pg/mL) and eosinophil counts in Giga/L were measured just before and after 5 days of IL-2. Normal eosinophil counts in the local laboratory are 0 to 0.7 G/L for men and 0 to 0.5 G/L for women, and are showed as dashed lines. Statistical significance of the differences between the groups was assessed using the Mann-Whitney U test. (B) Correlation between increase in IL-5 and eosinophils for the same patients as in (A). Correlations between eosinophils and IL-5 concentrations were determined by Spearman's correlation coefficient; P = .0218. (C) IL-5–fold increase over the time of T1D patients received a 5-day course of placebo or of IL-2 at the doses of 0.33 MIU/day, 1 MIU/day, and 3 MIU/day. (D) Serum IL-5 concentration in Red5 heterozygous mice treated with phosphate-buffered saline (PBS) or IL-2/anti–IL-2 mAb complex administered every other day for 3 doses. (E) Fluorescence-activated cell sorter plots of IL-5– (Red5 tdtomato reporter) producing cells in the indicated tissues after PBS or IL-2 treatment. (F) Quantitation of CD45⁺ IL-5⁺ cells in tissues from IL-5 reporter mice after PBS or IL-2 treatment (VAT: perigonadal VAT). (G) Quantitation of eosinophils (CD45⁺ CD11b⁺ SiglecF⁺) in WT or Rag-deficient (RAG^−/−) animals from the indicated tissues after PBS or IL-2/anti–IL-2–treated mice. (H) Fluorescence-activated cell sorter plots of the lineage-defining markers negative subset expressing IL-5 (Red5) in the pancreas after PBS or IL-2 complex (top panel). Characterization of the Red5-producing cells in the pancreas, pre-gated on lineage-negative, CD45⁺ cells (bottom panels). Mean values ± standard error of the mean. All data were analyzed by comparison of means using unpaired 2-tailed Student t tests. Data are representative of 3 or more experiments or were pooled from 2 to 3 experiments. *P < .05; **P < .01; ***P < .001.