Therapeutic activity of GARP:TGF-b1 blockade in murine primary myelofibrosis

Primary myelofibrosis (PMF) is a myeloproliferative neoplasm characterized by clonal expansion of myeloid cells, notably megakaryocytes (MKs), and aberrant cytokine production leading to bone marrow (BM) fibrosis and insufficiency. Current treatment options are limited. TGF-b1, a profibrotic and immunosuppressive cytokine, is involved in PMF pathogenesis. While all cell types secrete inactive, latent TGF-b1, only a few activate the cytokine via cell type-specific mechanisms. The cellular source of the active TGF-b1 implicated in PMF is not known. Transmembrane protein GARP binds and activates latent TGF-b1 on the surface of regulatory T lymphocytes (Tregs) and MKs or platelets. Here, we found increased expression of GARP in BM and spleen of mice undergoing PMF and tested the therapeutic potential of a monoclonal antibody that blocks TGF-b1 activation by GARP-expressing cells. GARP:TGF-b1 blockade reduced not only fibrosis, but also clonal expansion of transformed cells. Using mice carrying a genetic deletion of Garp in either Tregs or MKs, we found that the therapeutic effects of GARP:TGF-b1 blockade in PMF imply targeting GARP on Tregs. These therapeutic effects, accompanied by increased IFN-g signals in the spleen, were lost upon CD8 T cell depletion. Our results suggest that selective blockade of TGF-b1 activation by GARP-expressing Tregs increase a CD8 T cell-mediated immune reaction that limits transformed cell expansion, providing a novel approach that could be tested to treat patients with myeloproliferative neoplasms. Overall design: RNAseq of spleens collected from non-treated MPLW508A mice on days 13 and 20 were compared to establish signatures of PMF progression. RNAseq of spleens collected on day 34 from MPLW508A mice responding to 58A2 were compared to non-treated controls to analyze effects of treatment with 58A2.