Abstract Background Ischaemic heart disease is a leading cause of heart failure (HF) and despite advanced therapeutic options, morbidity and mortality rates remain high. Acute inflammation after myocardial infarction (MI) has been thoroughly studied in recent years and persistent immune-mediated damage has been suggested as a factor contributing to the development of HF. T and B cells are critical mediators of adaptive immune responses after MI, however a thorough analysis of their transcriptomic phenotypes in the heart at baseline and over time after injury are missing. Purpose Here, we aim to understand the transcriptional activity of B and T cells in the post-MI heart, in order to characterize their role in maintaining post-MI autoreactivity. Methods We obtained single cell RNA sequencing data of mouse and human hearts from Forte et al (2020), Martini et al (2019) and Litviňuková et al (2020) and performed differential expression analysis, gene regulatory inferences and trajectory-based differential expression analysis. Results Myocardial T cells subclustered into CD8+, CD4+, CD20+ and NK lineages with naïve, memory and effector functions (Fig. 1). The B cell population was divided into immature, plasma, memory, collagen-producing and B1 cell subgroups (Fig. 2). At baseline, despite a striking presence of effector populations, both B and T cells are governed by homeostatic and tolerance-promoting transcription factors (Deaf1, Ikzf1 and Nr1h3 for T cells and Spi-B as well as Bcl11 for B cells). Post-MI, the activity of transcription factors concerned with differentiation and proliferation increased. T cells express transcripts for proinflammatory chemokines and cytokines (Ccr7, Ccl5, Ifng and Tnf) as well as granzymes and perforin up until 28 days post-MI (Gzma and Prf1). Surprisingly, B cells upregulate collagen transcript levels 5-7 days post-MI (Col1a, Col5a, and Col4a) and regulon analysis supports that B cells increase the activity of transcription factors necessary for collagen production (Ets-1 and Krox20). Both B and T cells remain persistently activated post MI. 28 days post-MI T cells have increased activity of proinflammatory (Nfkb1) and cytokine producing (Crem and Rel) transcription factors, whereas B cells have increase maturation (Nfkb1), autoantibody production (Stat3 and Rel) and memory (Stat5b) transcription factor activity. Conclusion Collectively, our data shows that there are effector immune cell populations present at baseline, which are governed by homeostatic transcription factors. Post-MI, effector transcription factors are upregulated in both T cells and B cells, which promote proliferation, differentiation, cytokine production and in the case of B cells, collagen production. Even after resolution of acute inflammation, both the T and B cell population do not return fully to baseline but remain at a state of persistent activation, as effector transcription factors remain upregulated.
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