https://orcid.org/0000-0001-9318-3368
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This editorial refers to ‘Single cell RNA sequencing reveals profound changes in circulating immune cells in patients with heart failure’, by W.T. Abplanalp et al., pp. 484–494.
Next-generation sequencing (NGS) technologies have been invaluable for the study of complexity in biological systems in recent years.1 Gene expression can now be analysed at the single-cell level through a variety of techniques, among which the most commonly used is RNA sequencing [single-cell RNA sequencing (scRNAseq)]. This is a revolutionary advancement, as now several thousand genes can be identified in a single cell, allowing for cell type identification at an unprecedented level. scRNAseq has also led to the uncovering of intra- and inter-cellular relationships, tracking trajectories of distinct cell lineages in development and disease.2
The cardiovascular field has benefitted dramatically from scRNAseq: recent achievements include the identification of novel cellular subpopulations with specific roles and tissue-specific characteristics in the heart and blood vessels,3 as well as the identification of cell-to-cell interactions between different cell types and cell fate trajectories in different physiopathological settings.4,5 Moreover, myocardial cell types have been studied through scRNAseq during normal development6 and in heart failure (HF),7 a condition in which tissue remodelling is accompanied by gene expression changes in the cell types populating the heart. The stunning variety and gene expression modifications of immune cells in the myocardium—including macrophages and lymphocytes, but also less commonly studied cell types such as mast cells and neutrophils—have also been explored using scRNAseq in mouse models and human HF.8,9
Abplanalp et al.10 apply scRNAseq to describe circulating immune cells in human HF (Figure 1). They collected circulating cells expressing CD31+ [the platelet–endothelial cell adhesion molecule expressed on monocytes (Mo) and on other immune cells] from eight patients diagnosed with HF with reduced ejection fraction and eight healthy controls, utilizing the 10X Genomics platform. Their analysis on healthy individuals brought to the identification of 16 different clusters, including Mo, T cells, and a small number of NK cells, B cells, and megakariocytes. Mo were re-clustered and divided into the three known Mo subsets based on relative expression of CD14 and CD16. The effect of HF was then assessed: while a similar number of Mo was found within each group, there were significant differences in the gene expression profile. Mo overexpressed genes related to the inflammatory response, wound healing, and angiogenesis, such as interleukin-1b, endogenous danger-associated molecular pattern genes, S100 calcium binding protein A8, and the matricellular antiangiogenic protein thrombospondin-1. Furthermore, in HF patients, gene expression modification was different in each group, defining a specific signature for each sub-population: in the classical Mo subset, enriched genes included those in metabolic processes and stress response pathways, among which fatty acid binding protein 5 (FABP5), a lipid carrier that promotes inflammation;11,12 in the non-classical Mo category, enriched genes included ones involved in antigen processing and presentation, the inflammatory response, and negative regulation of apoptosis; lastly, in the intermediate Mo category, epithelial-to-mesenchymal transition, and cell proliferation genes were enriched. By comparing the relative abundance of the different circulating populations, the authors also describe an increase of the Mo-to-T cell ratio, a finding they confirm in a validation cohort.
Scheme of the results from Abplanalp et al. In heart failure, there is an increased monocyte-to-lymphocyte ratio, with modification of the monocyte gene expression ‘signature’.
This report opens a window on the transcriptional profile modifications occurring in circulating immune cells during HF, underscoring the changes in activation state of the immune system in the context of human disease. This involves a complex network of cell subsets with specific functions and interactions. Interestingly, Abplanalp et al.10 confirm an increased circulating Mo-to-T cell ratio, shown previously to be altered in HF and ischaemic heart disease.13,14 Results also confirm the close correlation between metabolism and inflammation in HF, since expression of FABP5 was found elevated in Mo in this condition. However, an aspect that needs to be further investigated is the relevance of pre-existing comorbidities, such as diabetes mellitus and dyslipidemia, on circulating Mo activation. A ‘metabolic’ activation of macrophages associated with metabolic syndrome has been shown in both experimental models and in human disease, determining an upregulation of genes involved in lipid metabolism, such as Plin2 and Abpc1.15 Pre-existing inflammatory activation linked to metabolic dysfunction could therefore regulate and influence the development of HF.
Despite being observational, this work highlights the power of new NGS technologies in generating insight on yet unclear mechanisms of cardiovascular disease. This potentially allows us to exploit them for diagnostic and therapeutic purposes. The future could not be more exciting for HF research.
Conflict of interest: none declared.
Funding
G.C. is supported by Fondazione Regionale per la Ricerca Biomedica (INTERSTAD-CAD project) and Italian Ministry of Health (‘Plaque’ project); C.P. is supported by 5 × 1000 funds (Immuno-Heart).
