Background:
Aortic stenosis (AS) is driven by progressive inflammatory and fibro-calcific processes regulated by circulating inflammatory and valve resident endothelial (VEC) and interstitial cells (VIC). The impact of platelets, platelet-derived mediators, platelet-monocyte interactions, and monocytes on the acceleration of local valvular inflammation and mineralization is presently unknown.

Methods:
We prospectively enrolled 475 consecutive patients with severe symptomatic AS undergoing aortic valve (AV) replacement. Patients were stratified into slow (SP-) and fast progressive (FP-) AS by repetitive transthoracic echocardiography. Cardiac workup included physical examination, echocardiography, blood sampling for laboratory parameters, chemokine profiling, and phenotyping of platelets and monocytes as well as collection of explanted valve tissue for further analysis of protein and gene expression.

Results:
Macroscopic and immuno-histological analysis of AVs revealed substantially enhanced infiltrating inflammatory cells and platelet accumulation predominantly in the lamina fibrosa on the aortic side (p<0.001) with less calcification in FP-AS compared to SP-AS (p<0.0001). Furthermore, pro-inflammatory macrophage inhibitory factor (MIF)-associated gene expression was significantly enhanced in FP-AS. Tissue expression of MIF correlated with tissue infiltration of macrophages, monocytes, and platelets (p<0.05). This was accompanied by significantly elevated MIF plasma levels (p<0.05) and increased platelet activation in FP-AS (p<0.05) with decreased intracellular MIF expression indicating an enhanced MIF release upon platelet activation (p<0.0001). Moreover, intracellular MIF expression was increased in intermediate and non-classical monocytes in patients with FP- compared to SP-AS (p<0.05) and correlated with the degree of platelet activation (p<0.0001) and inversely with MIF expression in platelets (p<0.001). In-depth phenotyping uncovered disease-specific changes in platelet and monocyte marker profiles in FP- and SP-AS (p<0.05). Finally, linear regression analysis confirmed that plasma and cell-based MIF markers are strongly associated with FP-AS. Thus, patients with FP-AS are characterized by significant alterations of systemic MIF expression comprising plasma, circulating platelets and monocytes which can be linked to accelerated inflammation of AV tissue in FP-AS.

Conclusion:
Our findings suggest a key role for the platelet-derived mediator MIF and its interplay with circulating and valve resident monocytes/macrophages in local and systemic thrombo-inflammation associated with fast progression of AS. “MIF-biomarkers” in peripheral blood may help to identify patients at risk for fast progressive disease.