Characterisation of B cells in Acute Coronary Syndromes

This thesis aimed to characterise changes in B cell phenotypes associated with acute myocardial infarction (AMI). Current animal literature suggests that B cells have important roles in shaping the immune response to AMI, but systematic evaluation of how B cells change in AMI in humans is currently lacking. In order to commence the characterisation of B cells in humans, a 9-marker, 8-colour antibody panel was developed, which allowed for the identification of nine distinct B cell subsets from peripheral blood using flow cytometry. The developed AMI phenotyping panel was then utilised to compare B cell subset distribution in a cohort of 12 healthy subjects, 12 stable coronary artery disease (CAD) patients, and 12 AMI patients. Our panel successfully captured the established nine B cell subsets across each cohort in this study. Interestingly only limited differences were detected in B cell subset distribution between the three cohorts. Antibody-secreting cells (ASCs) were the only subset observed to be significantly lower in AMI patients compared with healthy participants. Next, this thesis examined B cell subsets over time in AMI patients in a longitudinal observational cohort of eight AMI patients, with peripheral blood samples taken at five different time points post-MI. A modest reduction was observed in the number of CD19+ B cell populations in AMI patients from days one to three immediately following the AMI. This was likely driven by the reduction of naïve B cells across the same timeframe. This finding suggests that naïve B cells are activating in response to the ischemic damage and are exiting the peripheral blood to enter lymphatic structures and potentially cardiac tissues, as described in the animal literature. Based on these findings, the next study was designed to examine B cells derived from central cardiac tissue. B cells have been identified in the adipose tissues surrounding the heart and myocardium, but the phenotype and function of these populations remain unknown. A primary aim of this chapter was the phenotypic characterisation of the previously explored B cell subsets derived from cardiac adipose tissue, compared with B cell subsets derived from the PBMCs within a severe CAD patient population. Samples were collected from patients with severe coronary artery disease from epicardial adipose tissues (EAT) derived from the right atrial appendage (RAA). Unfortunately, there was only a limited presence of B cells observable in this tissue source, with not enough cellular numbers to address the aim of this chapter, as it was not possible to phenotype beyond CD19+ B cell populations. Therefore, it is not feasible to continue to pursue this source of EAT in patients at this time. Lastly, we commenced exploration of B cell functional responses in human AMI, by developing in vitro models to investigate B cell activation. A pilot study was conducted in two in vitro stimulation conditions, using T cell-dependent and T cell-independent activation of B cells. This was applied to severe CAD primary B cell cultures to inform the feasibility of the culture conditions moving forward. From this work, we demonstrated exciting preliminary results for a human in vitro study of B cell function, designed for further pursuit in AMI. In conclusion, we observed limited changes in B cell subsets in patients with AMI but cannot exclude changes within the central cardiac tissue. Functional changes in B cells may be more important than phenotypic studies for exploring the role of B cells following AMI.