Characterising the Heterogeneity of the Innate Immune Response in Elderly Individuals

Abstract

Global population ageing has made age-related immune dysfunction and higher infection risk a major public health challenge. Ageing is highly heterogeneous, following multiple trajectories, ranging from preserved function to normal age-related decline and, in some cases, accelerated decline with premature functional impairments. This ageing-related heterogeneity is reflected in immune responsiveness, which is shaped not only by genetic factors but also by lifelong exposures, including infections, vaccinations, comorbidities, and lifestyle choices. Understanding the determinants of immune diversity in the elderly is crucial for identifying vulnerable individuals and developing targeted prevention and intervention strategies. <br/> In this thesis, I investigated immune diversity in a cohort of elderly individuals aged 70 years and older at steady-state and in response to <em>ex vivo</em> and <em>in vivo</em> perturbation, applying high-dimensional flow cytometry and single-cell transcriptomics. Analysis of the circulating immune cell compartment confirmed established hallmarks of immunosenescence, including a reduction of CD56^bright NK cells and a shift of the T cell pool from naïve to differentiated memory phenotypes, particularly in CD8 T cells. Interestingly, higher frequencies of naïve CD4 T cells and CD56^bright NK cells correlated with lower frailty status. <br/> <em>Ex vivo</em> stimulation of whole blood with the AS01-adjuvanted recombinant protein subunit vaccine <em>Shingrix</em> for four hours strongly activated innate immune cells, including neutrophils, and upregulated Toll-like receptor and cytokine signaling pathway genes. This response was significantly reduced in individuals aged 85 years and older, exemplified by lower <em>IL1B</em> and <em>CCL4</em> expression. <br/> <em>In vivo</em> perturbation with <em>Shingrix</em> induced significant transcriptional changes one day post-vaccine challenge in circulating immune cells, particularly in neutrophils and monocytes, and revealed striking inter-individual variation in these early immune responses. I observed pronounced activation in a subgroup of elderly individuals, characterised by elevated expression of interferon response-related genes. These responses were independent of chronological age but associated with frailty and baseline immune phenotypes. <br/> Furthermore, I identified a neutrophil-derived <em>ex vivo</em> response signature that correlated with <em>in vivo</em> responsiveness, highlighting the potential of whole blood assays as predictive tools for immune function. <br/> In this thesis, I established an integrated experimental and computational framework to dissect inter-individual immune diversity in elderly individuals. This work demonstrates the potential of combining general practice medicine and systems immunological approaches to study immune diversity in elderly individuals as a potential contributor to differences in infection resilience, disease courses and age-dependent chronic diseases.