T-cell grit: large clonal expansions of virus-specific CD8⁺ T cells can dominate in the peripheral circulation for at least 18 years

Epstein-Barr virus (EBV) establishes a latent, life-long infection in more than 95% of the human adult population. Antigen-specific, cytolytic T lymphocytes (CTLs) are thought to be of central importance in controlling EBV in both primary and latent infection states. While a good deal is known about the CTL dynamics during primary infection and convalescence,^1-4  little is known about the CTL repertoire during the decades of infection after first contact. For example, do CTL clones persist indefinitely throughout an individual's lifetime or do new naive T-cell expansions emerge to help refresh the repertoire? We addressed this question by investigating the CTL dynamics toward immunodominant EBV epitopes up to 18 years apart, which is, to our knowledge, the longest follow-up study of T-cell responses conducted in humans.

The CTL response toward a latent HLA B^*0801-binding EBV epitope, FLRGRAYGL (FLR), has been described previously and was found to comprise large expansions of T cells bearing identical T-cell receptor (TcR) amino acid sequences in unrelated, B^*0801⁺ individuals.^5-8  In this follow-up study we investigated the FLR-specific T-cell repertoire of donor 1 using seven T-cell clones generated in 1987 and a pool of ex vivo, pentamer-sorted lymphocytes from 2005 (Figure 1A). Fifty-five TcR sequences derived from both time points revealed only a single TRBV7-8/TRAV26 clonal expansion, identical to the DNA level.

The CTL response toward another latent HLA B^*4405-binding EBV epitope, EENLLDFVRF (EEN), has also been described previously,¹⁰  but no tetramer or TcR sequence information is currently available. As a result, we sequenced 91 TcRs from the EEN-specific CTL repertoire from donor 2 using a pool of ex vivo, tetramer-sorted T cells from cryogenically stored lymphocytes from 1994 and 2005 (Figure 1B). Like the FLR-specific repertoire in donor 1, the donor 2 EEN-specific repertoire consisted of a single, nucleotide-identical TRBV7-8/TRAV26 expansion. In addition, the clonal donor 2 EEN-specific expansion appeared to increase in frequency with time (Figure 1C). This observation in the donor, aged 61 years in 2005, is consistent with a previous study showing virus-specific CTLs sometimes increasing with age.¹¹ 

This study follows the CTL response toward two immunodominant class I-binding EBV epitopes over a period of 18 or 11 years. Remarkably, we found no evidence of perturbations in the T-cell hierarchy over time, with both clonal CTL expansions remaining unchanged. Overall, the study provides an important advance in understanding the longevity and stability of the T-cell repertoire toward a persistent pathogen. Indeed, these findings suggest the likely situation of a single naive T-cell expansion facilitating the control of a persistent viral infection for the entire span of a human life.