Fibrosis independent atrial fibrillation in older patients is driven by substrate leukocyte infiltration: diagnostic and prognostic implications to patients undergoing cardiac surgery

Abstract

BackgroundThe objectives of the study were to characterize and quantify cellular inflammation and structural remodeling of human atria and correlate findings with molecular markers of inflammation and patient surrogate outcome.MethodsVoluntary participants undergoing heart surgery were enrolled in the study and blood samples were collected prior to surgery, and right atrium samples were harvested intraoperatively. Blood samples were analyzed by flow cytometry and complete blood counts. Atrial samples were divided for fixed fibrosis analysis, homogenized for cytokine analysis and digested for single cell suspension flow cytometry.ResultsA total of 18 patients were enrolled and samples assessed. Isolated cells from the atria revealed a CD45+ population of ~ 20%, confirming a large number of leukocytes. Further characterization revealed this population as 57% lymphocytes and 26% monocyte/macrophages (MoΦ), with the majority of the latter cells being classical (CD14++/CD16−). Interstitial fibrosis was present in 87% of samples and correlated significantly with patient age. Older patients (> 65) had significantly more atrial fibrosis and cellular inflammation. AFib patients had no distinguishing feature of atrial fibrosis and had significantly greater CD45+ MoΦ, increased expression of MMP9 and presented with a significant correlation in length of stay to CCL-2/MCP-1 and NLR (neutrophil-to-lymphocyte ratio).ConclusionAtrial fibrosis is correlated with age and not determinate to AFib. However, severity of atrial leukocyte infiltration and markers of matrix degradation are determinant to AFib. This also correlated with CCL2 (or MCP-1) and NLR-indicative of marked inflammation. These data show the potential importance of diagnostic and prognostic assessments that could inform clinical decision making in regard to the intensity of AFib patient management.

Increased mast cell density is associated with decreased fibrosis in human atrial tissue

“…No significant relationship was observed between atrial fibrosis and pre-or post-operative AF in the overall cohort or within CABG patients specifically. Our findings are in keeping with previous work in which atrial fibrosis was independent of AF [36]. It is possible that atrial fibrosis is not an exclusive driver of AF but rather a manifestation of aberrant myocardial processes at work in patients with cardiovascular disease [47].…”

“…Collagen content was quantified on the basis of SRFG staining as previously described [29,36] (Fig. 1D, F).…”

Leukocyte-Dependent Regulation of Cardiac Fibrosis

“…For instance, following a myocardial infarction (MI), the severe loss of cardiomyocytes triggers increased matrix production in an effort to replace lost and damaged tissue (Frangogiannis, 2019a). Further, other pathologies like pressure overload due to hypertension, volume overload, diabetes, obesity, and even aging can stress any chamber of the heart, therefore leading to fibrotic growth around the vasculature and within the interstitium (Aguiar et al, 2019;Frangogiannis, 2019a). Although this matrix growth initially serves as a form of repair, it can quickly become detrimental (Frangogiannis, 2019a).…”

“…Through flow cytometry and single cell RNA sequencing technologies, researchers have established the presence of many leukocyte subsets in the injured and healthy heart, with most of them being macrophages (Epelman et al, 2014;Swirski and Nahrendorf, 2018;Aguiar et al, 2019;Dick et al, 2019). Further studies using adult mouse hearts have allowed us to gain greater appreciation for the existence and diversity of these cardiac macrophage populations (Swirski and Nahrendorf, 2018).…”

“…Cardiac fibrosis is a typical feature of ageing and injury, both of which are known to trigger leukocyte expansion and inflammation within the heart (Epelman et al, 2015;Lu et al, 2017;Suetomi et al, 2018;Aguiar et al, 2019). For instance, a plethora of studies have revealed the rapid and continuous innate immune response that follows acute ischemic injuries.…”