Immunotherapeutic targeting of calreticulin mutant myeloproliferative neoplasms.

Calreticulin (CALR) frameshift mutations drive the majority of JAK2/MPL-wild-type cases of essential thrombocythemia and myelofibrosis, producing a shared novel C-terminus that activates the thrombopoietin receptor and leads to constitutive Janus kinase (JAK)/signal transducer and activator of transcription (STAT) signaling. CALR mutations transform the multifunctional endoplasmic reticulum (ER)-resident protein into an oncogenic driver that aberrantly traffics to the cell surface, activates the thrombopoietin receptor, MPL, and promotes MF megakaryocytic proliferation and hemopoietic stem cell fitness. Clinically, CALR-mutated MPNs affect younger patients, exhibit thrombocytosis and progressive anemia, and, in MF patients, are associated with a superior survival compared with JAK2- or MPL-mutated disease, yet responses to hydroxyurea and ruxolitinib remain inferior. The C-terminal motif shared between CALR mutations is presented extracellularly on major histocompatibility complex (MHC) Class I molecules and in complex with the thrombopoietin receptor enabling targeting with vaccination and antibody based therapeutic platforms, respectively. Mutant CALR peptide vaccines induce T cell responses but have failed to result in hematologic or molecular responses. Mutant CALR-specific monoclonal antibodies, such as Fc-silent antagonists, can block mutant-CALR-MPL signaling, suppress hematopoietic stem and progenitor cells (HSPC) proliferation and megakaryopoiesis, and achieve rapid, durable hematologic remissions with minimal toxicity in early-phase trials. Preclinical antibody-drug conjugates, bispecific T-cell engagers, and chimeric antigen receptor T cell (CAR-T) cells also show potent, selective mutant-cell killing. Here we review the basis and ongoing translational and clinical efforts in the development of CALR-targeted immunotherapies that offer a potential shift from symptom management to disease-modifying treatment in this molecularly defined MPN subset.