MyD88L265P Mutation Impairs Bone Marrow Hematopoietic Stem Cell Function in a Cell-Autonomous Way Resulting in an MPN-like Phenotype

Background: The cytosolic adapter protein Myeloid Differentiation 88 (MYD88) plays a pivotal role in the innate and adaptive immune response by initiating the Toll-Like and Interleukin-1 Receptor (TLR and IL1R) signaling cascades. In humans, the somatic gain of function L265P mutation within the C-terminal Toll-IL1 receptor (TIR)-interaction domain of MYD88 has been associated to the development and progression of B-cell neoplasms. In the context of such malignancies, MyD88L265P leads to the constitutive activation of the Nuclear Factor Kappa-light-chain-enhancer of activated B-cells (NF-κB) and Janus Kinase2/Signal Transducer and Activator of Transcription (JAK2/STAT) pathways, fostering the secretion of inflammatory cytokines. Similarly, the spontaneous activation of NF-κB and JAK2/STAT pathways driven by the JAK2V617F gain of function mutation, and the subsequent overproduction of inflammatory cytokines, are central to the pathogenesis of myeloproliferative neoplasms (MPN). Nonetheless, a compelling link between MyD88L265P and MPN remains elusive. Using a model of severe inflammation induced by bacterial sepsis or lipopolysaccharide (LPS), our lab previously reported that “inflamed” hematopoietic stem cells (HSC) underwent dysfunctional expansion associated with a transient inability to differentiate into the downstream myeloid progenitors. Loss of MyD88 was able to abrogate myeloid dysregulation, suggesting that akin to JAK2, abnormal expression of MyD88 may significantly contribute to the breakdown of myeloid homeostasis. Therefore, we propose that MyD88L265P could emulate the inflammation-driven effects of JAK2V617F mutation, potentially resulting in a disease resembling MPN.

Objective: Investigate the impact of MyD88L265P-induced inflammation on bone marrow (BM) HSC cycling, differentiation, and reconstitution potential and explore the hypothesis that such gain of function mutation in mice results in a phenotype that mimics MPN in patients.

Results: Mutant MyD88L265P HSC displayed abnormal cycling and expansion, resulting in impaired differentiation, hyperproliferation and accumulation of granulocyte-monocyte progenitors (GMP) in the BM. As a consequence, the BM neutrophil compartment was exhausted while white blood cells massively mobilized into the peripheral blood. The latter also exhibited significantly increased immature neutrophils, along with anemia and profound thrombocytopenia. Concurrently, extramedullary hematopoiesis was observed in the liver and spleen (SP) of MyD88 mutant mice, leading to splenomegaly and hepatomegaly, which are hallmark clinical manifestations of MPN with primary myelofibrosis (PMF) in humans. A similar phenotype was observed after transplantation of mutant MyD88 HSC into healthy recipient mice, suggesting that MyD88L265P impairs the functionality of BM HSC in a cell-autonomous fashion. The expression of specific cytokines such as CXCL9, CXCL10, and CXCL12 associated with fibrosis and migration of clonal hematopoietic cells from the BM to the SP and liver are found altered in MPN patients with PMF. Notably, secretomics analysis identified this same abnormal cytokine profile in our MyD88 gain of function mouse model. Immunohistochemistry revealed a severe increase of STAT3 in the BM, SP, liver, and lymph nodes of mutant MyD88 animals. Moreover, while CD68+ monocytes/macrophages were barely detected, CD163+ monocytes/macrophages were expanded in all the tissues of the MyD88L265P mice. Similarly, increased CD163+ cells are found in PMF MPN patients. In addition, the BM of mutant mice showed hypercellularity with proliferation of pleomorphic megakaryocytes, while their SP, liver, and lymph nodes displayed disrupted tissue architecture and inflammatory infiltrates, again reflecting traits found in PMF MPN patients.

Conclusion: Our data show that MyD88L265P mutant mice exhibit hallmarks of human PMF MPN induced by the hyperactivation of the JAK2/STAT pathway.

Significance: Our MyD88L265P mouse model recapitulating the human PMF MPN phenotype could provide insight to elucidate the mutational landscape of patients with MPN, yielding further potential therapeutic avenues.

Disclosures: No relevant conflicts of interest to declare.