JAK1/JAK2 inhibitors block the IFNγR-CXCR3 axis both in mouse and human T cells and mitigates GVHD. (A) Shown are WT pan T cells (both CD4⁺ and CD8⁺ T cells; CD4⁻ T cells are CD8⁺ T cells) after drug treatment. (B) Tissue sections of skin, liver, and intestine were graded by a veterinary pathologist in blinded fashion on day 21 after allo-HSCT for acute GVHD according to the Lerner grading system (see “Methods” for details).²¹  (C) Effect of INCB018424 on GVHD. Allo-HSCT (B6 (H-2^b) → Balb/c (H-2^d) was performed as follows. Five × 10⁶ T cell–depleted bone marrow cells (TCD BM) and 5 × 10⁵ Tconvs were injected into lethally irradiated (925cGy) Balb/c recipient mice. INCB018424 was injected intraperitoneally into recipients daily from day 0 through 20 (D0: 100 μg twice daily for the first 7 days and 100 μg once daily for the following 14 days) or day 3 through 23 (D3: 100 μg twice daily for the first 4 days and 100 μg once daily for the following 17 days). (D) In vivo BLI was performed to specifically track T cells (0.5 × 10⁶ cells) obtained from FVB-Tg(CAG-luc,-GFP)L2G85Chco/J mice (H-2^q) after allo-HSCT. FVB/NJ mice (H-2^q) were used as TCD BM donors (5 × 10⁶ cells) and Balb/c as recipients. INCB018424 or 10% DMSO was administered twice a day from days 3 to 6 and once daily from days 7 to 23. BLI images of dissected mice (n = 10 each) at day 31 after allo-HSCT were analyzed. Spleens, livers, and GI tracts were separated from the body cavities. Photon flux (photons/s) was measured from whole body (right panel) and the ratio of signal intensities (photons/s/cm²/sr) from spleen, liver, and GI tract and the rest of body were compared (left panel). (E-G) INCB018424 does not inhibit donor engraftment and reduce neither platelet (PLT) counts nor white blood cell (WBC) counts. Day 21 after allo-HSCT, BM+PBS (n = 2), BM+INCB (n = 5), PBS (n = 4), INCB (D0; n = 18), and INCB (D3; n = 15). (H) Shown are human pan T cells 5 days after drug treatment in the presence of anti-CD3/CD28 antibody coated beads (cell:bead = 1:1). Mean and SD of activated human T cells are as follows (n = 2); 0μM of INCB018424: CD8+CXCR3+: 6.0% ± 2.7%, CD8+CXCR3−: 6.3% ± 2.8%, CD4⁺CXCR3⁺: 49.6% ± 1.6%, CD4⁺CXCR3⁻: 38.1% ± 1.5%. 0.2μM of INCB018424: CD8⁺CXCR3⁺: 0.4% ± 0.2%, CD8⁺CXCR3⁻: 8.7% ± 0.3%, CD4⁺CXCR3⁺: 10.2% ± 4.2%, and CD4+CXCR3−: 80.8% ± 3.7%.