POS0418 SPLENIC EXTRAMEDULLARY HEMATOPOIESIS IS OMNIPRESENT AND CORRELATES WITH DISEASE SEVERITY IN THE LUPUS NZB/W F1 MURINE MODEL
Authors: S. Doumas, M. Grigoriou, A. Banos, C. Giakopoulou, I. Mitroulis, P. Verginis, D. Boumpas
Journal: Annals of the Rheumatic Diseases (2021)

Background: Extramedullary hematopoiesis (EMH) is increasingly recognized as an integral component of systemic inflammatory diseases; compared to their bone marrow counterparts, hematopoietic progenitors of EMH have an enhanced role in target organ damage 1,2 . We have found that β-glucan -a non-specific inducer of reprograming of innate immunity- results in dramatic EMH with marked increase in Long-Term (LT)-HSCs, massive splenomegaly and worsening of nephritis in the NZB/W F1 lupus murine model (unpublished data). Objectives: To investigate EMH’s time course and contribution to inflammatory target-organ damage (kidney) in the NZB/W F1 lupus murine model. Methods: Spleens and kidneys were isolated from female NZB/W F1, at pre-nephritic stage (3-month-old) and nephritic stage (&gt;6-month-old), and age/sex matched C57BL/6 (WT controls). Single-cell suspensions of spleens were analyzed by flow cytometry for Hematopoietic Stem and progenitor cells (HSPCs) phenotyping. Formalin-fixed and paraffin-embedded sections of spleens and kidneys were stained with conventional histological stains (H&amp;E, Silver, Trichrome Masson). Spleens were histologically assessed for the presence of ΕΜΗ and kidneys were assessed for activity and chronicity through the NIH Lupus nephritis scoring system. Results: Histological analysis revealed that NZW/B F1 mice at the nephritic stage display massive splenomegaly with concomitant expansion of the red pulp, increased presence of megakaryocytes and disorganized splenic architecture. This is further corroborated by the flow cytometry analysis which demonstrated a significant increase of all HSPCs subsets (Long-term/Short-term Hematopoietic Stem Cells and Multipotent progenitors) compared to the C57BL/6 WT controls at nephritic stage. The degree of HSPC expansion and splenic architecture disorganization correlates strongly with the activity of lupus nephritis as quantified by the NIH scoring system. Of note, evidence of splenic EMH were present even in 3-month-old animals before overt nephritis ensues. Conclusion: Extramedullary hematopoiesis is present before overt nephritis and is dramatically expanded at the nephritic stage of the NZW/B F1 mouse model. The degree of EMH positively correlates with the severity of lupus nephritis. These data support a pathogenic role of EMH, and spleen derived HSPCs, in driving lupus nephritis. References: [1]Regan-Komito, D., Swann, J.W., Demetriou, P., Cohen, E.S., Horwood, N.J., Sansom, S.N., Griseri, T., 2020. GM-CSF drives dysregulated hematopoietic stem cell activity and pathogenic extramedullary myelopoiesis in experimental spondyloarthritis. Nature Communications 11, 155. [2]Griseri, T., McKenzie, B.S., Schiering, C., Powrie, F., 2012. Dysregulated Hematopoietic Stem and Progenitor Cell Activity Promotes Interleukin-23-Driven Chronic Intestinal Inflammation. Immunity 37, 1116–1129. Acknowledgements: This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No 742390) Disclosure of Interests: None declared