Therapeutic activity of GARP:TGF-β1 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 pro-fibrotic 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.