Interferon α Has Varied Effects On CD34⁺ Cells From Patients With Polycythemia Vera

Prolonged therapy with pegylated interferon a (Peg-IFNα 2a) leads to hematological and complete molecular remissions in 70% and 17% of patients with polycythemia vera (PV) , respectively (Quintas-Cardama et al, Blood 2013). We have previously shown that PV CD34⁺ cells are more responsive to Peg-IFNα 2a than normal CD34⁺ cell. The type I IFN receptor 1 and 2 were shown to be expressed by a greater number of by PV CD34⁺ cells than normal(N) CD34⁺ cells (p=0.01 and p=0.002, respectively). The effects of Peg-IFNα 2a on PV hematopoietic stem cells(HSCs) was next evaluated by incubating PV CD34⁺ cell for 7 days with Peg-IFNα 2a (200ng/ml) followed by their transplantation into NSG mice. The degree of human cell chimerism following the transplantation of MPN CD 34⁺ cells was reduced by 50 -90% and the JAK2V617F allele burden by 40 -80%. Treatment of N CD34⁺ cells with Peg-IFNα 2a reduced donor chimerism by only 20%. We next examined the effects of increasing doses of Peg-IFNα 2a on CD34⁺ cells from 11 PV patients and 5 N controls. In 4 out of 10 PV cases the IC50 of Peg-IFNα 2a was less than 200ng/ml while in the remainder of cases the IC50 was greater. Low doses of IFNa were capable of eliminating JAK2V617F⁺ hematopoietic colonies in these IFNα sensitive patients while higher doses of IFNα were required to achieve the same effect in the other patients. PV and N CD34⁺ cells were then profiled using Illumina Gene expression arrays. In total, 32 intensity data files were generated, each containing 47,231 features, corresponding to 12,388 unique genes. At p-value <0.05 386 genes were down-regulated in PV; these genes were enriched for biological processes related to immune response including the IFN-γ-mediated signaling pathway (p=0.0002), the response to IFN-gamma (p=0.004), and the cellular response to IFN-γ (p=0.0004). The 715 up-regulated genes in PV were enriched for pathways involving glycolysis (p=9.4×10^-05), cellular response to stress (p=0.006), and catabolic processes. The gene expression patterns of CD34⁺ cells incubated with and without INFα were next analyzed. At pairwise t-test p-value <0.001, 315 genes were differentially expressed (223 up-regulated and 92 down-regulated by INFα). Up-regulated genes were enriched for INFα functions and immune response including: response to type I IFN (p=9.0×10^-49), innate immune response (2.6×10^-45), response to virus (7.5×10^-40). Among the 223 up-regulated genes, half were previously known as IFN regulated genes (IRGs). The individual response (IR) of genes to IFN was then defined as:

IR_i=log (exp ression_i @IFN/exp ression_i@control)

IR patterns were remarkably consistent within N samples while large inter-patient variations were observed within the PV samples. Significantly positive IRs were observed for 75 genes and negative IRs for 117 genes within PV as compared to N samples (p value<0.01). The 75 positively responsive genes to IFNa overlapped with 16 down-regulated PV signature genes (p=1.1×10^-10) while the negatively responsive of genes overlapped with 41 up-regulated PV signature genes (p=2.2×10^-24).These data indicate that the action of IFNa is associated with the alteration of the expression of specific PV signature genes.

The varied inhibitory effect of Peg-IFNα 2a on PV colony formation was then correlated with the IR of individual genes. The IRs of OAS2 and RPS24 showed particularly high variance and were related to colony formation. OAS2 (2'-5'-oligoadenylate synthetase 2) is an INF-induced, dsRNA-activated antiviral enzyme which plays a critical role in cellular innate antiviral response but also influences apoptosis, cell growth, differentiation and gene regulation. The IR of this gene was directly related to the inhibitory actions of IFNa (p=0.0011). By contrast, the IR of RPS24 (40S ribosomal protein S24), was inversely correlated to the IFNα response (p=0.0038). Mutations in RPS24 are associated with Diamond-Blackfan anemia. The strong correlation between the IR of these 2 genes with the inhibitory effects of IFNα suggests that their response ratio might be useful as therapeutic biomarker. These data indicate that the IFNα receptor is up-regulated in PV CD34⁺ cells and that IFNα treatment eliminates PV stem cells and its sensitivity against individual patient PV HSC/HPC varies. The patterns of differentially expressed genes following IFNα treatment may prove useful in determining its mechanism of action and predicting IFNα patient response.