TCR repertoire: size, diversity, and function

National Institute On Aging

Investigators

Linked publications, trials & patents

Abstract

The collection of all unique TCRs within an individual is referred to as the TCR repertoire. However, the vast number of T cells in a human adult presents significant technical challenges in accurately measuring its size. Current TCR repertoire analyses primarily focus on TCRβ sequences. Recent advances in single-cell TCR sequencing (scTCRseq) now enable the assessment of αβ-paired TCR repertoires, albeit on a limited scale (10,000–100,000 T cells). Two common measurements of the TCR repertoire include: (1) the total number of TCR clonotypes (species richness) and (2) diversity indices, such as Shannon entropy and the Simpson index, which assess TCR clonal distribution and expansion. Experimental analyses estimate the lower bound of the human TCR repertoire to be approximately 10⁸. Additionally, TCR repertoire composition is highly individualized, shaped by genetic differences in major histocompatibility complex (MHC) polymorphisms, antigenic epitope selection, and available TCR clonotypes. Cross-sectional studies of blood T cells indicate a relatively linear decline in TCRβ repertoire size with age, from newborns to centenarians. Here, we aimed to measure the αβ-paired TCR repertoire using a longitudinal human cohort. To investigate changes in the αβ paired TCR repertoire over time, we analyzed a longitudinal cohort of 48 adults (equal sex distribution), each with two blood samples collected approximately 15 years apart. At the first visit, participants were aged 41–78 years, and at the second visit, they were 56–93 years old. We employed single-cell TCR sequencing (scTCRseq) to profile CD4+ and CD8+ T cells from both time points for all 48 donors. The αβ paired TCR repertoire diversity was quantified using the Inverse Simpson Index. To assess the impact of aging, we applied a mixed-effects linear regression model to determine the overall trajectory of repertoire changes (Fig. 3a). Our analysis revealed that the αβ paired TCR repertoire in CD4+ T cells was larger than that of CD8+T cells (average repertoire sizes: 68 and 27 as first and second visit for CD4+ T cells, and 33 and 11 as first and second visit for CD8+T cells). However, CD4+T cells exhibited a greater decline in repertoire diversity over time compared to CD8+T cells. The annual reduction slopes were -2.4/year for CD4+ T cells and -1.1/year for CD8+ T cells, indicating a more pronounced loss of diversity in the CD4+compartment. These findings confirm a significant decline in the αβ paired TCR repertoire with advancing age, with CD4+ T cells experiencing a more substantial reduction than CD8+ T cells.