Disruption of Gamma Interferon Signaling through the STAT1 Pathway Enhances Alloreactivity While Abrogating GvHD

BACKGROUND: Strategies that allow for sufficient alloreactivity to mediate graft-versus-tumor (GVT) effects after allogeneic bone marrow transplant (BMT) while minimizing graft-versus-host-disease (GVHD) remain elusive. Historically, gamma interferon (IFNγ) has been implicated as a critical mediator of both GVT and GVHD pathophysiology, and high levels of IFNγ have been implicated in immunosuppression. Given that immunosuppression is also a hallmark of GVHD, we hypothesized that modulation of IFNγ signaling could have important effects on both GVHD and GVT following BMT. We therefore compared immunocompetence to tumor challenge and tumor vaccination, in the presence or absence of IFNγ receptor (R1) and STAT1 signaling (a primary downstream mediator of IFNγ signaling) on donor bone marrow.

METHODS: We utilized a clinically relevant, minor histocompatibility antigen-mismatched, T cell-depleted BMT model, with delayed donor lymphocyte infusions (DLIs) into thymectomized mice to ensure that T cell recovery occurred exclusively from the DLI, as occurs in most clinical settings during the first 6 months post-BMT. Lethally irradiated female C57BL/6 (B6) × C3H.SW recipients were engrafted with normal B6, B6 IFNγR1-deficient, normal B6129, or B6129 STAT1-deficient bone marrow on day 0 and reconstituted with donor T cells on days 14 and 28 at a dose that induced sublethal GVHD, evidenced by weight loss, histology, and abrogated quantitative immune responses to an HY-directed tumor vaccine. Recipients were then challenged on day 42 with an HY-expressing tumor and followed for survival. Immune reconstitution was assessed by flow cytometry both prior to and after DLI administration.

RESULTS: Mild GVHD prevents vaccine-mediated immune responses and HY-expressing tumor protection in thymectomized recipients. Both IFNγR1 and STAT1-deficient marrow abrogate GVHD. STAT1-deficient marrow increased absolute numbers of splenic plasmacytoid dendritic cells (CD11c^int/Class II⁺/Gr-1⁺/B220⁺) prior to DLI administration. In addition, an increase in the percentage and absolute number of splenic regulatory T cells was observed at the time of tumor challenge. Surprisingly, despite the immunosuppressive milieu, recipients of IFNγR1-deficient marrow had enhanced vaccine-mediated tumor protection as measured by overall survival, similar to syngeneic BMT controls.

CONCLUSION: IFNγ signaling is a critical mediator of GVHD pathophysiology, including the immunosuppression associated with this condition. Disrupting IFNγ signaling through the JAK1/STAT1 pathway is a novel strategy to selectively modulate alloreactivity, and results in enhanced anti-tumor immune responses following allogeneic BMT without exacerbating GVHD.