Immunomodulatory role of megakaryocytes in the hematopoietic niche of myeloproliferative neoplasms

Myeloproliferative neoplasms (MPN) are clonal stem cell disorders characterized by dysregulated megakaryopoiesis and expansion of neoplastic hematopoietic stem cells (HSC). Megakaryocytes (MK) not only regulate HSC function, but also shape immune responses within the marrow niche. Using an aging murine model of MPN with MK-restricted JAK2^V617F expression, we investigated the immunomodulatory roles of mutant MK. Compared to wild-type MK, aged mutant MK exhibit enhanced antigen uptake and MHC I presentation, secretion of pro-inflammatory cytokines (PF4, TGFβ, IL-Ιβ), and induction of T-cell dysregulation in the marrow niche. In chimeric murine models with co-existing wild-type and JAK2^V617F mutant hematopoietic cells, enhanced MK immune activity correlates with mutant cell expansion and MPN development. Single-cell RNA sequencing revealed that aging amplifies JAK2^V617F MK-driven immune remodeling. Notably, aged mutant MK showed marked upregulation of long-interspersed element-1 (LINE-1) retrotransposon transcripts alongside elevated innate immune sensors cyclic GMP-AMP synthase (cGAS) and stimulator of interferon genes (STING), implicating retrotransposon activity in niche inflammation. In human MPN marrow, immunohistochemistry detected LINE-1-encoded protein ORF1p in MK from 12 of 13 MPN patients, but not in orthopedic controls (N=5). These findings identify MK as active immune regulators in MPN, with JAK2^V617F mutation and aging synergizing to reprogram MK into inflammatory, immune-modulatory niche cells. LINE-1 activation emerges as a potential driver of chronic marrow inflammation, and a targetable mechanism in clonal hematopoiesis and MPN progression.