Immune cell populations in human atrial myocardium from patients with atrial fibrillation

Introduction:

The processes underlying the initiation and development of atrial fibrillation (AF) are still not sufficiently explored. One of the generally recognized factors contributing to persistency of AF is structural remodeling of the myocardium. There is alteration in both cardiomyocyte morphology as well as changes affecting endomysium. It is hypothesized that some of these changes are caused by an inflammatory processes. Aim: In this study we focused on morphological and functional changes in endomysium of atrial myocardium with special attention to CD45+- cells, representing cells of bone marrow origin. In addition, we characterize different types of resident or infiltrating immune cells.

Methods:

We analyzed atrial biopsies obtained from patients undergoing bypass or mitral valve surgery. The patients had a regular sinus rhythm or were suffering from AF. The atrial samples were fixed with 4 % paraformaldehyde and embedded into paraffin. Sections from atrium were histologically examined using routine hematoxylin-eosin staining. For detection of different types of immune cells the following markers were detected immunohistochemically: CD45 as a pan-leukocyte marker, CD3 for T-lymphocytes, CD68 for monocyte/macrophages, mast cell tryptase for mast cells and DC-SIGN for immature dendritic cells. Three-step immunoperoxidase detection was performed on paraffin sections after antigen retrieval and images were taken using Leica DMLB microscope equipped with DC300 camera. For double labeling we used immunofluorescence approach and images were taken using a confocal laser scanning microscope (FV1000, Olympus)

Results:

In all atrial samples from both groups we detected CD45+-cells in atrial myocardium as well as in endocardium and epicardium. The frequency of these cells was variable. Atria from patients with atrial fibrillation had tendency for a higher infiltration with CD45+ cells. Apparently, CD45+ cells formed a heterogeneous group of cells. We also detected CD68+cells, CD3+cells, mast cells and DC-SIGN -positive dendritic cells. A dual immunostaining showed CD45+/CD3+, CD45+/CD68+ and CD68+/DC-SIGN double positive cells. There were CD45+ and CD68+ cells with elongated shape sending out long cellular processes, these corresponded mainly to DC-SIGN -positive dendritic cells. We performed a semiquantitative analysis of CD45+ cells with elongated cell bodies and found that they are more frequent in auricular samples from patients with atrial fibrillation. The difference reached statistical significance in the right atrial samples.

Conclusion:

Our results document that CD45+ cells are a heterogeneous cell population in atrial myocardium from patients undergoing open heart surgery and these cells can be detected regardless of the heart rhythm. The finding of higher frequency of CD45+ leukocytes with elongated processes in the AF samples suggests the activation of inflammatory cells in this arrhythmia. It has to be confirmed whether all these cells are immature dendritic cells or they represent more diverse population.

This work was supported by the Research Program of Charles University – PRVOUK -P25/LF1/2.