COVID-19 myocarditis: quantitative analysis of the inflammatory infiltrate and a proposed mechanism

Abstract

COVID-19 has a significant effect upon the cardiovascular system. While a number of different cardiovascular histopathologies have been described at post-mortem examination, the incidence of typical viral myocarditis in COVID-19 positive patients appears very low [1-3]. In this study, we further characterize and quantify the inflammatory cell infiltrate in a COVID-19 study cohort and compare the findings to both an age and disease matched control cohort and a cohort of patients diagnosed with typical inflammatory myocarditis. All study and control cohorts had 1 or more of the comorbidities most commonly associated with severe disease (hypertension, type II diabetes, obesity, or known cardiovascular disease). The results demonstrate a skewed distribution of the number of CD68+ cells in COVID-19 hearts, with upper quantiles showing a significant increase as compared to both matched control hearts, and those with myocarditis. In contrast, hearts from typical inflammatory myocarditis contained increased numbers of CD4+, and CD8+ cells compared to both COVID-19 and control cohorts. In conclusion, the presence of an increased number of CD68+ cells suggests that COVID-19 may incite a form of myocarditis different from typical viral myocarditis, and associated with diffusely infiltrative cells of monocytes/macrophage lineage.

Keywords: CD68 cells; COVID-19; SARS-CoV-2; autopsy, myocarditis, heart, inflammation, macrophages.

Fig. 1

(A) Cardiac myocytes from control patient (H&E). (B) Myocarditis, characterized by patchy, dense inflammation within the myocardium (H&E). (C) Endotheliitis and diffuse, perivascular distribution of inflammation in COVID-19. (D) CD68 immunostaining highlighting the presence of CD68+ cells in a mild, diffuse intravascular and perivascular distribution in a case of COVID-19.

Fig. 2

(A) CD3 immunostaining, demonstrating a paucity of CD3+ lymphocytes in COVID-19. (B) CD68 immunostaining in COVID-19, highlighting a mild, diffuse distribution of CD68+ cells. (C) CD3 immunostaining in a case of myocarditis, demonstrating intense staining for CD3+ lymphocytes in a patchy distribution. (D) CD68 immunostaining in myocarditis, in a similar patchy distribution to that of lymphocytes, with example of region selection box for analysis.

Fig. 3

(A) Boxplots showing median and range of CD68, CD3, CD4, and CD8 staining cells for each Patient Group. A red bracket highlights the numerous outliers of CD68+ cells in the upper quantile of the COVID-19 group, which may represent a subset of COVID-19 patients with greater myocardial inflammation. (B) Population marginal means after correction for multiple comparisons within the nested ANOVA performed on CD68+ cells. The population marginal mean of CD68+ cells is significantly higher for the COVID-19 group as compared to controls. C) Q-Q plot of CD68+ cell quantiles in the COVID-19 myocardium samples as compared to control quantiles. The relationship is non-linear at higher quantiles, indicating a difference in distribution of CD68 positivity among the COVID-19 group, with higher values at the upper quantiles.