P061 | MDS AND MESENCHYMAL STROMAL CELLS INTERACTIONS TO ENHANCE MONOCYTIC DIFFERENTIATION: DIRECT CONTACT AND LUSPATERCEPT RESPONSE

The bone marrow microenvironment plays a central role in MDS pathogenesis, with mesenchymal stromal cells (MSCs) regulating hematopoietic and immune cell behavior. How MSCs influence monocyte differentiation remains poorly understood. This study evaluates the impact of hematopoietic and stromal cells interactions, focusing on direct versus indirect contact, as well as on the expression of monocyte surface markers CD11b and CD14, in mononuclear cells (MNCs) obtained from 5 MDS-RS-MLD patients with low IPSS-R treated with Luspatercept [3 responsive: all 46,XY and PR(HI), aged 70, 75, 80; 2 non-responsive: one 46,XX aged 76 and one 46,XY, aged 79], in age-matched controls, and in THP-1 cells. Notably, at baseline, the CD11b and CD14 increased expression observed in MDS patients correlated with favourable clinical response, suggesting a potential link between immune microenvironment modulation and treatment efficacy. UMAP analysis of peripheral blood mononuclear cells from treated patients, generated using the percentage of cells positive for 15 different surface markers assessed by flow cytometry, supported this association, showing distinct clustering of responders and non-responders based on immunophenotypic profiles. In parallel, pulp-derived MSCs (DP-MSCs) elicited comparable effects to HS-5 cells in supporting hematopoietic cells, validating their use as an accessible alternative stromal model. Flow cytometry analyses revealed that direct co-culture with stromal cells—either the HS-5 bone marrow-derived line or dental DP-MSCs—significantly enhanced CD11b and CD14 expression compared to monoculture or transwell conditions, highlighting the importance of physical contact. Even Scanning Electron Microscopy imaging confirmed morphological changes consistent with flow cytometry results. All in all, our findings support a key role for stromal–immune interactions in modulating monocytic differentiation and treatment response in MDS and propose a practical model for further mechanistic studies. Ongoing studies are investigating the impact of Luspatercept on stromal–monocyte dynamics over multiple treatment cycles and aim to identify specific stromal-derived signals involved in MDS immune modulation. Additional mRNAseq experiments are investigating the differences between MDS Luspatercept responders and non-responders before and after co-culture, and the differences between healthy BM and DP-MSCs and MDS MSCs, to identify new possible dysregulated pathways.