Recipient-derived IL-6 induces Th17-dependent IPS. Splenocytes from G-CSF mobilized BALB/c.WT donors were transplanted into B6.IFN-γR^−/− recipients. Control or anti–IL-6R mAb (500 μg/dose) was administered on days −1 and +3. Lung and spleen tissue were harvested between days 7 to 11 after SCT. (A) Absolute number of donor CD4⁺ IL-17A⁺ T cells were calculated for both lung and spleen. Data from 2 replicate experiments is shown (n = 9 to 10 per group). Lung: ****P < .0001, Spleen: *P = .04, control vs anti–IL-6R mAb groups. (B) Donor CD4⁺ populations were isolated from lung and spleen tissues and cultured ex vivo with plate bound CD3/28 mAbs. IL-17A production in supernatants was measured using cytokine bead arrays. Data from 2 replicate experiments is shown ( n = 8 to 11 per group). Lung: *P = .03, Spleen: ****P < .0001, control vs anti–IL-6R mAb groups. (C) Splenocytes from G-CSF mobilized B6.IL-17-eYFP fate map reporter donors were transplanted into B6D2F1 recipients. Representative TNF production in fate reporter eYFP⁺ Th17 cells and eYFP^neg T cells 7 days after SCT, mean ± SEM, n = 10 per group from 2 experiments, P < .0001. (D-F) G-CSF mobilized BALB/c.IFN-γ^−/− grafts were transplanted into allogeneic B6.IL-6^−/− recipients. (D) Representative plots of lung tissue showing intracellular IL-17A expression in donor CD4⁺ T cells in B6.WT or B6.IL-6^−/− recipients are shown. (E) Absolute number of donor CD4⁺ IL-17A⁺ T cells in lung and spleen were calculated and data from 2 combined replicate experiments is shown (n = 9 to 12 per group). Lung: *P = .049, B6.WT vs B6.IL-6^−/− recipients. (F) IL-17A and TNF production by ex vivo stimulated donor CD4⁺ populations isolated from lung and spleen (n = 5 to 7 per group). Lung: IL-17A, **P = .003; TNF, **P = .005, B6.WT vs B6.IL-6^−/− recipients. (G) BALB/c.WT grafts (T-cell replete or TCD) were transplanted into B6.IFN-γR^−/− recipients. Control or anti–IL-17A mAb (M210, 100 μg/dose) was administered every alternate day posttransplant starting from day 0 with lung tissue harvested on day 9. Data from 3 combined replicate experiments is shown (n = 13 to 16 per group). Lung pathology: **P = .002, control vs anti–IL-17A mAb treated groups. (H) BALB/c.WT or BALB/c.IL-17RA^−/− donor grafts (T-cell replete or TCD) were transplanted into allogeneic B6.IFN-γR^−/− recipients and lung tissue was harvested on days 9 to 11 post-SCT. Data from 3 combined replicate experiments is shown (n = 15 to 16 per T-cell replete group). Lung pathology: **P = .003, WT vs IL-17RA^−/− grafts. (I) Mixing experiment utilizing grafts comprised of either BALB/c.IL-17RA^−/− BM or T cells in combination with BALB/c.WT BM or T cells transplanted into allogeneic B6.IFN-γR^−/− recipients. Lung tissue was harvested on days 7 and 8, digested, and T cells and macrophages enumerated. Data from 2 combined replicate experiments is shown (n = 8 to 10 per group). T cells: **** P < .0001, WT recipients (white bars) vs IFN-γR^−/− recipients (black bars) of WT BM + WT T-cell grafts; *P = .01, WT BM + IL-17RA^−/− T cell grafts (blue bars) vs WT BM + WT T-cell grafts (black bars) into IFN-γR^−/− recipients; *P = .02, IL-17RA^−/− BM + IL-17RA^−/− T-cell grafts (gray bars) vs WT BM + WT T-cell grafts (black bars) into IFN-γR^−/− recipients. Macrophages: ***P = .0004, WT recipients (white bars) vs IFN-γR^−/− recipients (black bars) of WT BM + WT T-cell grafts; * P = .02, IL-17RA^−/− BM + WT T-cell grafts (orange bars) vs WT BM + WT T-cell grafts (black bars) into IFN-γR^−/− recipients.