In vivo exposure to ruxolitinib differentially influences T-cell functions.Prf1^−∕− mice were infected with LCMV and treated with ruxolitinib as described. On day 9, splenocytes from these animals were restimulated ex vivo with MHC class I (gp33-41)- or class II (gp61-80)-restricted LCMV peptides. Percentages of CD8⁺CD44⁺ (A-B [left]) and CD4⁺CD44⁺ (C-D [left]) T cells producing both TNFα and IFNγ, or CD8⁺CD44⁺ T cells expressing LAMP1 (CD107a; E-F [left]) were determined by intracellular and surface staining and flow cytometry. Absolute number of CD8⁺CD44⁺ and CD4⁺CD44⁺ T cells producing both TNFα and IFNγ, and CD8⁺CD44⁺CD107⁺ are shown in panels B, D, and F, respectively (right panels). In panels B, D, and F, data are presented as mean ± SD. (G) Viral titer was determined in the liver samples of Prf1^−∕− mice in each group. (H) EL4 cells were pulsed with 20 μM gp33-41 peptide or left untreated and used as targets in an in vitro cytotoxicity assay. Cytolysis of gp33-loaded (filled symbols) or unloaded (open symbols) EL4 cells by splenic CD8⁺CD44⁺ cells of LCMV-infected or LCMV + ruxolitinib–treated (L+R) B6 mice. Specific lysis was determined by ⁵¹Cr release and plotted as the mean ± SD. Statistical significance in percent-specific lysis was determined by 2-way ANOVA. (I) Viral titers in the livers of B6 mice on day 8 (week 1) and day 15 (week 2) postinfection. Data in panels A through G and H and I are representative of 4 and 2 independent experiments, respectively. *P < .05.