Role of immune cells in the development of mitral valve dystrophy

Myxomatous valve dystrophy (MVD) is characterized by an elongated, and thickened mitral valve (MV) with extracellular matrix (ECM) accumulation. Using the FilaminA P637Q Knock-In (FLNA-KI) rat model of MVD, we previously demonstrated that it recapitulates the human pathology. RNA-Seq analysis revealed important contribution of immune cells. Cytometry experiments showed increase proportion of macrophages at D21 in KI valves. The objective is to decipher how macrophages contribute to the development of MVD. Seven days, two days and newborn FLNA-KI rats (D7, D2 and D0) were phenotyped through a quantitative histological analysis. Cytometry, to assess valvular macrophages population, as well as qPCR for expression of the key molecular targets was performed. At D7, FLNA-KI rats exhibit thickened and elongated mitral valve leaflets, and presented a higher histological score as compared to WT rats (P = 0.0007). qPCR experiments revealed dysregulation of genes involved in ECM regulation (HAS1 FC = 8 and HYAL1 FC = 0.63 vs. WT, both P < 0.01) and endothelial dysfunction (ESM1 FC = 2.2 vs. WT, P = 0.005) concordant with the results at D21. Preliminary results at D0 and D2 showed upregulation of genes related to ECM remodeling (HAS1 FC = 2.6 and 3.2 vs. WT, respectively, P < 0.01). Morphological analysis at D0 and D2 is still ongoing. Although a pro-inflammatory environment was present at D7 (CCL7 FC = 2.6 and S100A8 FC = 4.2 vs. WT, P < 0.5), no changes in the proportion of macrophages was detected by cytometry (6% in both WT and KI MV, P > 0.5). The expression of inflammatory cytokines was not different at D0 and D2 in KI rats’ mitral valves (all P > 0.5). Our study indicates that MVD is present in KI as soon as birth. The ECM remodeling is probably a basis for the pro-inflammatory environment that promotes macrophages recruitment in the valvular leaflets. This suggests that macrophages infiltration is not causal of MVD but rather participates to the evolution of the disease. Further investigations are ongoing to understand how macrophages contribute to the pathophysiology of MVD.