TLR2 Signaling in Myeloproliferative Neoplasms: Pathophysiological Roles, Mutation-Dependent Heterogeneity, and the Differential Efficacy of Ruxolitinib

Toll-like receptor 2 (TLR2) is a versatile pattern-recognition receptor of the innate immune system that recognizes both pathogen-associated molecular patterns (PAMPs) and endogenous danger-associated molecular patterns (DAMPs). Philadelphia chromosome-negative myeloproliferative neoplasms (MPNs)—clonal hematopoietic stem-cell disorders that include essential thrombocythemia (ET), polycythemia vera (PV), and myelofibrosis (MF)—are characterized by persistent inflammation and an increased risk of thrombosis. Dysregulated TLR2 signaling has emerged as a potentially important mediator of this chronic thrombo-inflammatory state. Findings from our group and others suggest that TLR2 may have a particularly prominent role in promoting inflammatory cytokine production, platelet–leukocyte interactions, endothelial dysfunction, and thrombotic complications in MPNs. Our findings are also consistent with evidence that NF-κB-mediated proinflammatory signaling, in addition to constitutive JAK/STAT activation, contributes substantially to MPN pathogenesis. In this review, we provide a comprehensive overview of TLR2 biology and examine its contribution to the thrombo-inflammatory milieu of MPNs, with particular emphasis on its potential utility as a biomarker and a therapeutic target. Ruxolitinib, a JAK1/2 inhibitor, markedly reduces symptom burden and splenomegaly in JAK2-mutant MPNs, particularly PV and MF; however, it has limited efficacy in CALR-mutant or triple-negative ET. This differential therapeutic response may reflect divergent downstream signaling pathways and may also be related to dysregulated TLR2 signaling. Furthermore, hyperactive TLR2 signaling in MPNs may help explain the incomplete ability of ruxolitinib to suppress thrombo-inflammatory responses. Therefore, combined TLR2-targeted therapy and ruxolitinib may represent a more effective therapeutic strategy for future MPN trials. Overall, however, we consider the TLR2-mediated thrombo-inflammatory response in MPNs to be an amplifier of inflammation rather than a primary disease driver.