Pegifna Promotes Proliferation and Myeloid Differentiation In Human Hematopoietic Progenitors In Patients With Polycythemia Vera and Essential Thrombocythemia

The myeloproliferative neoplasms, polycythemia vera (PV) and essential thrombocythemia (ET), are characterized by clonal hematopoiesis that is often associated with a JAK2^V617F mutation, although this does not appear to be a disease-initiating event. Treatment of PV and ET with pegylated interferon-alpha (pegInfα) has been shown to lead to hematological remission, a decrease in the JAK2^V617F allelic burden in many cases, and even a reversion to polyclonal hematopoiesis. Despite promising therapeutic results, the mechanism of pegInfα-induced remission remains elusive. There are several potential mechanisms through which pegInfα may be acting, which include stimulating the immune system in order to more effectively suppress the aberrant PV clones, enhancing the activation of normal hematopoietic stem cells (HSCs), or by selectively suppressing the mutant clones. It has been previously reported that PV patients on pegInfα have an increased number of CD4+CD25+Foxp3+ T regulatory cells (Tregs) in the peripheral blood as compared to untreated or hydroxyurea treated patients (Riley Blood, 2011), which suggests that PegIFNa maybe altering immunity against the mutated clone. However, we have found that interferon treatment leads to increased proliferation of HSCs and myeloid-specific differentiation in mice (Baldridge Nature, 2010). If this finding is also true in humans, it suggests the return to polyclonality after pegInfα could also involve an increase in normal HSC proliferation.

In order to address this question, we are studying the effects of pegInfα treatment on the Tregs and HSCs of PV and EV patients, when compared to hydroxyurea or untreated patients. Previously we showed that pegInfα treatment reduced the JAK2^V617F allelic burden in 17 out of 32 patients. Of the 13 female patients for which clonality could be assessed, one developed polyclonal hematopoiesis with three-fold reduction of JAK2^V617F allelic burden, but one developed polyclonal hematopoiesis during therapy despite no reduction in the JAK2^V617F allelic burden, suggesting that pegInfα treatment is able to affect both pre-JAK2^V617F clones and JAK2^V617F-positive PV clones.

We have now assessed changes in the HSC population in response to pegInfα treatment. Upon analysis of bone marrow samples from these same pegInfα or hydroxyurea treated patients, we found that the number of HSCs (CD45+CD34+CD38-) was increased in patients treated with pegInfα. Further we saw a decrease in the percent of quiescent HSCs in the pegInfα treated samples, measured by the percentage of cells in G0, suggesting a more actively proliferating HSC population. In agreement with these data, our RNA analysis of the HSCs showed an increase in the expression of cell cycle genes in response to short-term pegInfα treatment. In addition to this apparent increase in HSC proliferation, we also saw an increase in the number of colonies formed in methocult media from the bone marrow samples of the pegInfα treated patients, suggesting an increase in myeloid specific differentiation. When we analyzed the RNA of patients who had received long-term pegInfα treatment, we saw a transcriptional profile that was indicative of cell death. Taken together, these data suggest a model in which pegInfα treatment is allowing for a return to polyclonal hematopoiesis by inducing cell division and differentiation of normal HSCs, while suppressing the pre-JAK2^V617F or JAK2^V617F-positive PV and ET clones, possibly by promoting apoptosis or inducing an immune-mediated cell death. Our findings do not exclude other potential mechanisms for salutary effects of pegInfα for treatment of PV and ET (see accompanying abstract by Swierczek et al).