Fig 2: NIX Is Critical for Formation of Effector Memory in Ova-Specific CD8+ T CellsSpleens from OT-I mice (A–D) or wild-type (WT) and T/NIX-/- mice (E–K) were collected at designated time points.(A) Kinetics of Nix expression in Ova-specific CD8+ T cells (Ova-CD8+) after VSV-Ova immunization.(B) Gene expression of Nix in Ova-CD8+ 24 h after addition of IL-15. CD8+ T cells from naive OT-I mice were activated with anti-CD3 and anti-CD28 for 72 h, followed by IL-15 addition.(C) Kinetics of Nix expression in Ova-CD8+ after CD3-stimulation, followed by IL-15 addition.(D) Kinetics of Il15ra expression in Ova-CD8+ after VSV-Ova immunization. Ova-CD8+ from mice within the same experimental group in (A)–(D) were pooled before analysis.(E) Representative dot plot showing percentage of Ova-EM in WT or T/NIX-/- spleens on day 30 p.i. with 104 plaque-forming units (PFU) of VSV-Ova.(F) Mean frequencies of Ova-EM from (E).(G) Experimental model for adoptive transfer experiment performed in (H).(H) Left: representative plot showing percentages of CD45.1+ WT and CD45.2+ T/NIX-/- Ova-EM in CD45.2+ T/NIX-/- mice 30 days after VSV-Ova immunization. Right: mean frequencies of CD45.1+ WT and CD45.2+ T/NIX-/- Ova-EM from experiment performed in the left panel.(I) Kinetics of effector memory formation in Ova-CD8+ in vivo in WT or T/NIX-/- mice after VSV-Ova immunization.(J) Gene expression of Nix in day 0 naive, day 6 Ova-activated, day 10 Ova-CD8+ MPECs, day 30 Ova-EM, and day 30 Ova-CM in WT mice after VSV-Ova immunization.(K and L) Gene expression of Foxo1 (left panel) and Tcf7 (right panel) in day 10 Ova-CD8+ MPECs (K) or day 30 Ova-EM (L) harvested from WT or T/NIX-/- spleens after VSV-Ova immunization.(M) In vitro differentiation of Ova-EM. Left: representative plot for percentage of WT or T/NIX-/- Ova-EM on day 8. Right: mean frequency of Ova-EM from left panel.In (E) and (M), CD8+Ova_tetramer+ population (Ova-EM) was gated on CD3+CD8+CD43-CD62L-CD44+ population. Data are representative of two or more independent experiments (n = 3–10). Data were analyzed using one-way ANOVA with Bonferroni’s posttest (mean ± SEM) in (A)–(D); two-tailed Student’s t test (mean ± SEM) in (F), (H, right), (J)–(L), and (M, right); and two-way ANOVA with Bonferroni’s posttests (mean ± SEM) in (I). *p < 0.05, **p < 0.01, ***p < 0.001, and ****p < 0.0001. See also Figures S1 and S2.

Fig 3: IL-15 stimulates proliferation of FAPs both in vivo and in vitro. a A schematic showing the experiment in vivo: IL-15 (or combination with Jak inhibitor) was administered from 3dpi to 5 dpi and samples were sectioned on 7 dpi (top). Activated FAPs were detected by staining Ki67 (bottom). Scale bars, 20 μm. b Quantifications of percentage of PDGFRα+Ki67+ FAPs in total FAPs. c The ability of proliferation of FAPs in the presence of IL-15 (or combination with Jak inhibitor) detected by BrdU staining. d Quantifications of percentage of BrdU+ FAPs in total FAPs. e Western blots for activation of Jak-STAT pathways after stimulated by IL-15 with/without inhibitor and (f) quantity analysis. g FITC-Annexin-V/PI assay for apoptosis of FAPs after stimulated by IL-15 for 48 h. Values not sharing a common small letter differ significantly (p < 0.05). Abbreviations: Gly, glycerol; I, Inhibitor

Fig 4: Administration of IL-15 enhances collagen deposition in injured muscles. a A schematic showing the experiment in vivo: IL-15 (or combination with Jak inhibitor) was administered from 1dpi to 3 dpi and samples were sectioned on 5 dpi. b Immunofluorescence for collagen I after administration of IL-15 in injured muscle 5 dpi. Scale bar, 20 μm. c Quantification of collagen deposition area (shown in percentage). d qPCR analysis of fibrosis-associated biomarkers, Fn1 and collagen I, in injured muscle with glycerol injection 5 dpi. Abbreviations: Gly, glycerol; Colla 1, Collagen I

Fig 5: The expression of IL-15 is positively correlated with number of FAPs and collagen deposition in subjects with rotator cuff tear. a Immunofluorescence for PDGFRα and Laminin in muscles from subjects with RCT. b Quantification of number of FAPs in muscles from subjects with RCT. c Immunofluorescence for Collagen Iin muscles from subjects with RCT. d Quantification of percentage of collagen deposition area in muscles from subjects with RCT. e qPCT analysis of mRNA expression of IL-15 in samples from patients with RCT. f Pearson’s correlation analysis for mRNA level of IL-15 and number of FAPs in samples from patients with RCT. g Pearson’s correlation analysis for mRNA level of IL-15 and percentage of area of collagen deposition in samples from patients with RCT