JAK2V617F Impairs Lymphoid Differentiation in Myeloproliferative Neoplasms

Myeloproliferative neoplasms (MPNs) arise via the acquisition of a driver mutation in a single hematopoietic stem cell (HSC), often decades prior to the development of a clinical phenotype. The most common MPN driver mutation, JAK2V617F, activates aberrant JAK/STAT signaling via cytokine receptors critical for myelopoiesis. Over time, this MPN HSC clone outcompetes its normal counterparts, leading to excessive myeloid cell production and contributes to lymphopenia in patients with MPNs and leades to elevated neutrophil-to-lymphocyte ratio (NLR), which is predictive of disease-related complications including thrombosis and mortality. We conducted this study to learn how hematopoiesis from the JAK2V617F clone affects lymphopoiesis in patients with MPNs. Although myeloid proliferation via aberrant JAK2 signaling is the most apparent mechanistic link between JAK2V617F and MPN phenotypes, our findings demonstrate that impaired lymphoid differentiation is an additional feature of JAK2V617F hematopoiesis, leading to the rarity of JAK2V617F lymphocytes despite the dominance of JAK2V617F HSCs in patients with MPNs. The combination of prolific myelopoiesis and defective lymphopoiesis from the JAK2V617F clone is a potential connection between MPN pathology and the surrogate markers, including NLR and lymphopenia, which hold prognostic significance. Based on our data, we speculate that, defective JAK2V617F lymphopoiesis and the consequent increased burden of lymphopoiesis from residual normal HSC clones drives the appearance of abnormal lymphocyte subsets, lymphoproliferative disease or T cell exhaustion in MPNs. Further study of MPN lymphopoiesis provides an opportunity to define the immune deficits underlying the myriad complications that affect patients with MPNs.