Why JAK2-mutated neutrophils deserve to be on center stage in polycythemia vera

To The Editor

In polycythemia vera (PV), an increased neutrophil-to-lymphocyte ratio (NLR) is a risk factor for venous thrombosis [1]. Recent studies showed that reductions in the NLR correlated with suppression of JAK2^V617F variant-allele frequency (VAF) and were associated with improved event-free survival (EFS) in PV [2, 3]. It is likely that similar correlations also exist in essential thrombocythemia (ET), although there is limited published data of the impact of NLR on EFS and thromboses in ET. The results imply that NLR may be more than just a generic inflammatory marker, but rather a dynamic surrogate of clonal activity and prognosis in PV with therapeutic implications, particularly for interferon (IFN)-based strategies.

Despite its ubiquity and low cost, NLR has been relatively neglected in PV risk assessment compared with hematocrit and platelet counts. The emerging evidence base endorses leukocyte-based metrics as a powerful biomarker for disease related burden and risk; in ECLAP and other cohorts, higher NLR was associated with venous thrombosis and mortality in PV [2, 3]. Notably, contemporary analyses from the prospective REVEAL study identified sustained leukocytosis as an independent risk factor for thrombotic events even when hematocrit target was attained, underscoring that “white cells matter” in day-to-day practice [4]. This emphasizes the observation that therapies differ in their capacity to normalize NLR and, by extension, to modulate risk.

We believe the clinical relevance of NLR is based upon JAK2^V617F directly perturbing two immunological axes: neutrophil activation and lymphoid differentiation. Neutrophil extracellular trap (NET) formation is a normal phenomenon of innate immunity, a thrombogenic program attenuated by JAK/STAT inhibition, thereby providing a mechanistic path from clonal signaling to vascular events [5]. The JAK2^V617F pluripotent stem cells in PV preferentially augment myeloid differentiation compared to lymphopoiesis resulting in reduced JAK2^V617F lymphocytes compared to neutrophils [6, 7]. There is also evidence that JAK2^V617F impairs lymphoid differentiation, offering a genetic rationale for the lymphogenic denominator of elevated NLR in PV [8]. The NLR, therefore, may be viewed as a composite marker of both JAK2 mutation-driven myeloid overactivity alongside compromised adaptive immunity.

JAK2^V617F VAF is a key determinant of risk for thrombosis and molecular response in JAK2^V617F is associated with EFS [9]. The molecular response in PV is measured in peripheral blood leukocytes, not in enucleated red cells nor bone marrow cells. Therefore, leukocytes are the same cellular compartment in which JAK2^V617F VAF is quantified. This unity of measurement and mechanism strengthens the case for bringing leukocyte metrics into formal schemas for assessing treatment response and monitoring. While historical practice and guidelines have centered on hematocrit targets to mitigate cardiovascular events, newer data support explicit incorporation of leukocyte-derived markers into risk stratification and treatment goals.

JAK2-mutant neutrophils are not “normal” effectors caught in the crossfire, but rather mutation-transformed, peripheral neoplastic derivatives of the malignant clone [10]. In PV and ET, the mutated PV neutrophils are the major source of prothrombotic factors including tissue factor [11, 12]. Further, PV and ET neutrophils have decreased levels of antithrombotic transcription factor KLF2, and this decrease is ameliorated by IFN but not by hydroxyurea [13]. Recent work has also shown that senescent JAK2^V617F neutrophils evade homeostatic clearance via CD24 upregulation, accumulate in tissues, promote TGF-β activation, and drive myelofibrosis; antibody blockade of CD24 restores mutant neutrophil clearance and mitigates fibrosis in mouse models [14]. It may be argued that neutrophils should be placed along the causal chain of PV complications and progression. Concurrently, NET-primed neutrophils link clonal signaling to thrombosis, reinforcing the thesis that reducing the burden of JAK2 mutation-carrying neutrophils is both biologically and clinically meaningful.

The recent seminal work by Barbui et al. showed that ropeginterferon alfa-2b (ropeg) reduces NLR primarily by suppressing neutrophils with relative lymphocyte sparing, and that NLR decline correlates with JAK2^V617F VAF reduction and better EFS [2]. These observations corroborate randomized and prospective studies in which ropeg demonstrated superior, durable hematologic and molecular responses compared with hydroxyurea or phlebotomy-based strategies [15, 16]. Ropeg is a long-acting IFN-a-based therapy, which exerts clinical activities by activating type 1 IFN receptors. The binding of type 1 IFN receptors activates signal pathways and cause tumor cell apoptosis, cell cycle inhibition and senescence accompanied by a loss of tumorigenesis, and immune-stimulated antitumor responses [17‐20]. Clinically, IFN exposure is associated with improved myelofibrosis-free and overall survival [21‐23]. Together, these data align neatly: IFN selectively suppresses the JAK2-mutant hematopoietic clone favoring myelopoiesis including neutrophils, normalizes NLR and consequently JAK2^V617F VAF, and is associated with improved long-term outcomes. Further, only IFN therapy has been demonstrated to reactivate normal hematopoietic stem cells from dormancy as demonstrated by using X-chromosome expression-based clonality assays [24, 25]. Here, the conversion of PV clonal myelopoiesis to polyclonal takes place by demonstrating that neutrophils, platelets, and reticulocytes are converted from expressing only single same polymorphic X-chromosome allele, either maternal or paternal X-chromosome, but not both. However, after successful IFN therapy, some PV females convert to having mixture of cells, some expressing maternal while other paternal active X-chromosome, i.e. polyclonal hematopoiesis [24, 25].

There are possible practical implications with remaining questions. A relevant question is whether there is a need to further recognize the importance of JAK2-mutant neutrophils in PV and act on NLR routinely a readily available, prognostic marker in clinical practice. We support the notion regarding integrating NLR into regular PV monitoring. Given its demonstrated association with JAK2^V617F VAF dynamics and EFS, thresholds (e.g., ≥ 4–5) used in prior studies could be prospectively harmonized and validated across settings. Secondly, is there a need to normalize leukocyte-based inflammation? In patients with persistently elevated NLR and/or leukocytosis despite hematocrit control, it warrants a treatment selectively decreasing neutrophils. In this respect, ropeg is a suitable treatment option given the ability to suppress neutrophils, reduce NLR, and lower JAK2^V617F VAF in randomized trials. Furthermore, since JAK2^V617F VAF is assessed in peripheral leukocytes (mixture of clonal neutrophils and polyclonal lymphocytes [6]), it further justifies determining VAF in isolated clonal neutrophils, rather than determining VAF in polyclonal total leukocytes. The paired strategy of serial NLR and VAF could offer a viable alternative for assessing both inflammatory and clonal control in routine practice. Finally, NLR is a ratio. We suggest future analyses explicitly partition NLR dynamics into absolute-neutrophil-count- and absolute-lymphocyte-count-driven components and evaluate their independent associations with VAF and clinical outcomes. This could clarify how much prognostic power NLR derives from neutrophil suppression (clonal myeloid activity) versus lymphocyte restoration (immune competence) and may help define treat-to-target thresholds for each and improve our understanding of pathophysiology of PV.