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Initial diagnosis of overt myeloproliferative neoplasms (MPNs) represents the juncture during clonal evolution when symptoms or complications prompt an afflicted individual to seek medical attention. In 30-40% of the MPN subgroups essential thrombocythemia (ET) and myelofibrosis (MF), somatic mutations in the calreticulin gene (CALR) are drivers of the disease resulting in constitutive activation of the thrombopoietin receptor (MPL). In the current study, we describe a healthy CALR mutated individual during a 12 year follow-up from initial identification of CALR clonal hematopoiesis of indeterminate potential (CHIP) to the diagnosis of pre-MF. The pre-diagnostic exponential development dynamics of the malignant clone demonstrated close correlation with the platelet counts, neutrophil-to-lymphocyte (NLR) ratio, and inversely correlated to hemoglobin and erythrocyte counts. Backward extrapolation of the growth rate indicated the potential for discovery of the malignant clone many years prior to presentation of overt disease, opening a window of opportunity for early treatment intervention. We did not find any additional mutations associated with MPNs and the current case report provides novel information regarding the development of a driver mutation and the association with blood cell counts prior to clinical manifestation of symptoms suggesting that pre-diagnostic dynamics may supplement future diagnostic criteria for early diagnosis and intervention in MPN patients.

Cell surface molecules of the B7/CD28 family play an important role in T-cell activation and tolerance. The relevance of the PD-1/PD-L1 pathway in cancer has been extensively studied whereas PD-L2 has received less attention. However, recently the expression of PD-L2 was described to be independently associated with clinical response in anti-PD1-treated cancer patients. Here, we investigated whether PD-L2 might represent a natural target that induces specific T cells. We identified spontaneous specific T-cell reactivity against two epitopes located in the signal peptide of PD-L2 from samples from patients with cancer as well as healthy individuals ex vivo. We characterized both CD8+ and CD4+ PD-L2-specific T cells. Interestingly, the epitope in PD-L2 that elicited the strongest response was equivalent to a potent HLA-A2-restricted epitope in PD-L1. Importantly, PD-L1-specific and PD-L2-specific T cells did not cross-react; therefore, they represent different T-cell antigens. Moreover, PD-L2-specific T cells reacted to autologous target cells depending on PD-L2 expression. These results suggested that activating PD-L2 specific T cells (e.g., by vaccination) might be an attractive strategy for anti-cancer immunotherapy. Accordingly, PD-L2 specific T cells can directly support anti-cancer immunity by killing of target cells, as well as, indirectly, by releasing pro-inflammatory cytokines at the microenvironment in response to PD-L2-expressing immune supressive cells.