Monocyte subset accumulation in the human heart following acute myocardial infarction and the role of the spleen as monocyte reservoir

Abstract

Aims: Monocytes are critical mediators of healing following acute myocardial infarction (AMI), making them an interesting target to improve myocardial repair. The purpose of this study was a gain of insight into the source and recruitment of monocytes following AMI in humans.

Methods and results: Post-mortem tissue specimens of myocardium, spleen and bone marrow were collected from 28 patients who died at different time points after AMI. Twelve patients who died from other causes served as controls. The presence and localization of monocytes (CD14(+) cells), and their CD14(+)CD16(-) and CD14(+)CD16(+) subsets, were evaluated by immunohistochemical and immunofluorescence analyses. CD14(+) cells localized at distinct regions of the infarcted myocardium in different phases of healing following AMI. In the inflammatory phase after AMI, CD14(+) cells were predominantly located in the infarct border zone, adjacent to cardiomyocytes, and consisted for 85% (78-92%) of CD14(+)CD16(-) cells. In contrast, in the subsequent post-AMI proliferative phase, massive accumulation of CD14(+) cells was observed in the infarct core, containing comparable proportions of both the CD14(+)CD16(-) [60% (31-67%)] and CD14(+)CD16(+) subsets [40% (33-69%)]. Importantly, in AMI patients, of the number of CD14(+) cells was decreased by 39% in the bone marrow and by 58% in the spleen, in comparison with control patients (P = 0.02 and <0.001, respectively).

Conclusions: Overall, this study showed a unique spatiotemporal pattern of monocyte accumulation in the human myocardium following AMI that coincides with a marked depletion of monocytes from the spleen, suggesting that the human spleen contains an important reservoir function for monocytes.

Keywords: Acute myocardial infarction; Bone marrow; Inflammation; Monocytes; Spleen.

Figure 1

Identification of the infarct area by LDH decolouration, and the microscopical infarct core and border zone within the infarcted myocardial tissue. (A) Photograph of the LDH coloured myocardium. The dotted line outlines the LDH decoloured infarct area. The square indicates the centre of the infarct area (including the microscopical infarct core and border zone), wherefrom tissue was sampled. (B) Haematoxylin and eosin staining of infarcted myocardial tissue (post-acute myocardial infarction proliferative phase) was performed to identify two areas within the infarct area: the microscopical infarct core and border zone (×50 magnifications). BZ, border zone; IC, infarct core.

Figure 2

CD14⁺ cells infiltrate distinct regions of the infarct area in different phases of healing after acute myocardial infarction. (A) Histology images of haematoxylin and eosin stainings (top row) and CD14 immunostainings (bottom row) of the infarcted area (×100 magnifications). In the inflammatory phase after acute myocardial infarction, CD14⁺ cells predominantly infiltrate the infarct border zone, surrounding the necrotic infarct core. In the subsequent proliferative phase after acute myocardial infarction, CD14⁺ cells are clustered in the infarct core, consisting of granulation tissue. (B) Magnification of the infarct border zone in the inflammatory phase after acute myocardial infarction (×400 magnifications), showing infiltrated CD14⁺ cells adjacent and adherent to cardiomyocytes (arrow head). (C) Quantification of CD14⁺ cells in different healing phases following acute myocardial infarction. Data are presented as box plot with median and 25th–75th percentiles (boxes) and 5th–95th percentiles (whiskers). BZ, border zone; h, hours; IC, infarct core.

Figure 3

Co-localization of CD14⁺ cells in different regions of the infarct area. In the inflammatory phase after acute myocardial infarction, CD14⁺ cells (red) infiltrate the microscopical border zone, which is the area directly adjacent to the infarct core, consisting of the necrotic myocardium indicated by C3d (yellow). In the proliferative phase after acute myocardial infarction, CD14⁺ cells (red) are located within the infarct core, consisting of granulation tissue, and surrounded by viable cardiomyocytes in the border zone, indicated by α-actinin (green). BZ, border zone; h, hours; IC, infarct core.

Figure 4

CD14⁺ cell subsets in the myocardium following acute myocardial infarction. (A) Images of CD14 (red) and CD16 (green) double immunostainings of the infarcted myocardium at different phases of healing after acute myocardial infarction (×400 magnifications). (B) Quantification of CD14⁺, CD14⁺CD16^–, and CD14⁺CD16⁺ cells in different phases of healing after acute myocardial infarction. Data are presented as box plot with median and 25th–75th percentiles (boxes) and 5th–95th percentiles (whiskers). H, hours.

Figure 5

Presence of CD14⁺ cells in the bone marrow and spleen after acute myocardial infarction. Histology images of CD14 immunostainings and quantification of CD14⁺ cells in (A) bone marrow and (B) spleen in different phases of healing after acute myocardial infarction. Data are presented as box plot with median and 25th–75th percentiles (boxes) and 5th–95th percentiles (whiskers). HPS, high-power field; h, hours.