Abstract Body (Do not enter title and authors here): Background: Myxomatous mitral valve dystrophy (MVD) is the first cause of mitral valve prolapse (MVP), a common disease affecting 2 to 3 % of the population. The first causal mutation in the FLNA gene (FLNA-P637Q), was associated with MVD in 2007. Recently, the FlnA-P637Q KI rat model was generated and phenotyped. Using multimodality imaging, MVD was diagnosed as early as 3 weeks (D21) and signature of chemotaxis and immune cell migration was described at the transcriptomic level.

Aims: This study aims to delineate the contribution of macrophages recruitment and activation to the pathophysiology of MVD.

Methods: New-born (D0), two days (D2) and seven days (D7) KI and WT animals were phenotyped. Anatomopathological score was used to assed MV morphological alterations. MV cell proportions were analyzed by flow cytometry (D7 and D21), and molecular phenotyping was done by bulk RNAseq (D7). WT and KI MVs were dissociated for valvular interstitial cells (VICs) primary culture and subsequent in vitro studies.

Results: At D21, a 2-fold increased proportion of myeloid cells in KI MV compared to WT was detected by flow cytometry (13% vs 7%, p<0.05). Interestingly, although no morphological differences were seen after birth (D0 and D2; p>0.05), MV remodeling, characteristic of MVD, was observed and quantified at D7 (histological score: 7 in KI vs 4 in WT; p<0.001). Consistently, transcriptomic data of D7-MV revealed enrichment of GO-Terms (p<0.05) related to morphogenesis and development, cytoskeleton and extracellular matrix, and cellular activation and differentiation. Chemotaxis and cytokine related pathways were also enriched (p<0.05), but no difference in myeloid cell proportions was seen by flow cytometry at D7 (7% in WT and KI, p>0.05). Furthermore, monocytes adhesion assay showed an increased monocytes adhesion to KI-VICs compared to WT-VICs (p<0.01). This could be partly mediated by ICAM1 over-expression observed in KI-VICs (p<0.05). Of note, TNFa pro-inflammatory stimulation exacerbated ICAM1 expression in KI-VICs and monocytes adhesion.

Conclusion: At seven days, the FLNA-KI rats model display MVD associated to a pro-inflammatory signature without increase in macrophage proportion, as observed at later stages. In vitro, KI-VICs have a higher expression of adhesion molecule ICAM1 and are more susceptible to inflammatory stimuli. This results in increased adhesion of monocytes to VICs.