Integrin-dependence of extramedullary erythropoiesis in the spleen of Jak2-V617F positive myeloproliferative neoplasm in mice

The molecular mechanisms driving splenomegaly in myeloproliferative neoplasms (MPN) remain poorly understood. Utilizing the Jak2-V617F knock-in mouse model, we investigated the role of β1 and β2 integrins in regulating spleen volume and spleen weight. The response to neutralizing antibodies against VLA-4 and the β2 integrin chain, as well as to isotype controls, was evaluated by serial intra-individual MRI, by assessment of spleen weight and by analysis of the cellular composition of spleens. Short-term anti-VLA-4/β2-integrin treatment (applied on day 1 and evaluated at day 8) significantly reduced the spleen volume by 30% compared to the IgG control. At the cellular level, anti-integrin treatment led to a substantial 30% decrease in erythroblast counts and a 23% reduction in basophilic erythroblasts within the spleen, as compared to the isotype control. Furthermore, immunohistochemistry analysis of spleen sections revealed that CD71 (= Transferrin receptor protein 1) expression in spleen remained largely unchanged, while there was a clear reduction in Ter119 expression upon anti-integrin treatment. This data suggests that the substantial decrease in erythroblasts following anti-integrin treatment is a primary factor contributing to the overall reduction in spleen size. To study the spleen architecture, Multi-Epitope Ligand Cartography (MELC) analysis of spleen sections was applied. This demonstrated that the spatial distribution of the marginal zone, red pulp and white pulp remained unaltered upon anti-integrin treatment in JAK2-V617F knock-in mice. In summary, the present study identified a previously unrecognized role of the β1-integrin VLA-4 and of β2-integrin chains in extramedullary erythropoiesis of the spleen in JAK2-V617F-induced disease.