Changes in inflammatory T cell subsets - the missing link underlying immune remodelling in atrial fibrillation

Background Atrial fibrillation (AF) is the most prevalent sustained arrhythmia, it associates with significant morbidity and is the leading cause of death worldwide. Although inflammation is a well-known driver in AF, the underlying immune mechanisms involved in the development and maintenance of AF are yet to be deciphered. Elucidating the cellular and molecular mechanisms driving AF is critical for effective and targeted treatment. Recently, a large-scale single cell atlas of the adult human heart revealed that immune cells are twice as many in atria compared with ventricles and account for 10% of all atrial cells. T cells are the main orchestrators of inflammation and immune responses. Specialised subsets of T cells either promote or prevent inflammation. We are investigating a unique subset of T cells, the CD4+CD28null (CD28null) T cells that expand in patients with chronic inflammation. These cells secrete high levels of pro-inflammatory cytokines and have cytolytic function. CD28null T cells are normally maintained under control by regulatory T cells (Treg), a specialised subset of T cells with suppressive function that maintain immune homeostasis and prevent pathogenic immune responses. The role of CD28null and Treg cells has not been investigated in AF. Purpose To assess whether AF patients show changes in circulating T cell subsets compared to healthy individuals and to investigate the mechanisms underlying these changes. Methods Fresh whole blood samples from AF patients (n=60) and healthy controls (n=40) were used to quantify circulating CD4+ T cell subsets (pro-inflammatory CD28null T cells and anti-inflammatory Treg cells) using multicolour flow cytometry. Moreover, the frequency of CD28null and Treg cells was assessed in patients with paroxysmal AF (n=28) or with persistent AF (n=31). Multiplex proteomics with the Olink Target 96 Immuno-oncology panel was used to carry out a comprehensive analysis of potential factors driving changes in CD4+ T cell subsets. Results We found a significant increase in pro-inflammatory CD28null T cells in AF patients compared to healthy controls (Fig 1A, p<0.01), whilst anti-inflammatory Treg cells were decreased (Fig 1B, p<0.01). Of note, a higher proportion of patients with persistent AF had an increased CD28null T cells than patients with paroxysmal AF (45% vs 26%, respectively) (Figure 2). No differences in Treg cells were present in persistent compared to paroxysmal AF. Interestingly, the plasma proteome of AF patients with increased CD28null T cells revealed a unique signature consisting of proteins involved in cytolytic function, T cell production and migration. Conclusions Our novel data indicate immune remodelling of the CD4+ T cell compartment that may sustain inflammation and contribute to AF persistence. Further studies in this field are likely to provide effective therapies targeting AF immunopathogenesis.Figure 1Figure 2