Abstract
Myeloproliferative neoplasms (MPN) are myeloid malignancies characterized by overproduction of mature blood cells, hyperplastic bone marrow and tendency to evolve into acute myeloid leukaemia. In solid tumours, calreticulin (CALR) overexpression produces a pro-phagocytic signal and is counteracted by concomitant expression of anti-phagocytic CD47, reflecting an apoptosis vs survival mechanism. Increases of both CALR and CD47 on the cell membrane have been observed in response to chemotherapy, however their role in myeloid malignancies is poorly understood.
Aims: Investigate the expression and cellular localisation of CALR and CD47 in untreated and treated patients with essential thrombocythemia (ET), polycythemia vera (PV) myelofibrosis (MF), in comparison with healthy controls.
Methods: Mononuclear cells were collected by Ficoll separation, from peripheral blood of 30 MPN (8 PV, 16 ET, 6 MF); 18 MPN patients received cyto-reductive therapies (Hydroxyurea, Anagrelide or Ruxolitinib); and 4 controls. Cells were fractionised into 4 compartments: membrane, cytoplasm, cytosol and nucleus. Proteins were extracted using TRIzol, with CALR and CD47 protein expression analysed by western blotting.
Results: Total CALR and CD47 protein expression increased in MPN samples compared with controls (CALR- 7.9 vs 5.1; CD47- 2.7 vs 2.2 fold, respectively). CD47 showed higher expression of its overall protein on MPN cell membranes when compared with CALR (22% vs 13.9%). We observed a significant reduction of CALR expression in all MPN subtypes when patients were treated with cyto-reductive agents (ET- untreated 43.3% vs treated 2%, PV- 3.6% vs 2.2%, ET- 21% vs 11%). Interestingly we have observed a significant increase in CD47 cell membrane expression after treatment in MF and PV (CD47 in MF- untreated 11.8% vs treated 34.3%, PV-11.4% vs 35.9%), suggesting an anti-phagocytic effect induced by cytotoxic drugs. In ET cell membranes however, CD47 expression is reduced after cyto-reductive treatment (22% vs 16.6%), suggesting instead a prophagocytic effect.
Summary/Conclusion: CD47, but not CALR, is overexpressed on the membrane of patients with MPN, suggesting a role for CD47 as a strong antiphagocytic signal responsible for immune survival in MPN. We observed a significant difference in CD47 expression across different MPN subtypes with a significant increase in CD47 expression in PV and MF but not ET when patients were exposed to cytoreduction. The use of anti-CD47 antibodies could represent a new strategy to enhance the treatment response in particular in PV and MF.
No relevant conflicts of interest to declare.
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