Oncogenic Calreticulin Induces Immune Escape by Stimulating TGFβ Expression and Regulatory T-cell Expansion in the Bone Marrow Microenvironment

Increasing evidence supports the interplay between oncogenic mutations and immune escape mechanisms. Strategies to counteract the immune escape mediated by oncogenic signaling could provide improved therapeutic options for patients with various malignancies. As mutant calreticulin (CALR) is a common driver of myeloproliferative neoplasms (MPN), we analyzed the impact of oncogenic CALR^del52 on the bone marrow (BM) microenvironment in MPN. Single-cell RNA sequencing revealed that CALR^del52 led to the expansion of TGFβ1-producing erythroid progenitor cells and promoted the expansion of FoxP3⁺ regulatory T cells (Treg) in a murine MPN model. Treatment with an anti-TGFβ antibody improved mouse survival and increased the glycolytic activity in CD4⁺ and CD8⁺ T cells in vivo, whereas T-cell depletion abrogated the protective effects conferred by neutralizing TGFβ. TGFβ1 reduced perforin and TNFα production by T cells in vitro. TGFβ1 production by CALR^del52 cells was dependent on JAK1/2, PI3K, and ERK activity, which activated the transcription factor Sp1 to induce TGFβ1 expression. In four independent patient cohorts, TGFβ1 expression was increased in the BM of patients with MPN compared with healthy individuals, and the BM of patients with MPN contained a higher frequency of Treg compared with healthy individuals. Together, this study identified an ERK/Sp1/TGFβ1 axis in CALR^del52 MPNs as a mechanism of immunosuppression that can be targeted to elicit T-cell-mediated cytotoxicity.