Multifunctional cytomegalovirus (CMV)‐specific CD8⁺ T cells are not restricted by telomere‐related senescence in young or old adults

Abstract

Antigen-specific multifunctional T cells that secrete interferon-γ, interleukin-2 and tumour necrosis factor-α simultaneously after activation are important for the control of many infections. It is unclear if these CD8+ T cells are at an early or late stage of differentiation and whether telomere erosion restricts their replicative capacity. We developed a multi-parameter flow cytometric method for investigating the relationship between differentiation (CD45RA and CD27 surface phenotype), function (cytokine production) and replicative capacity (telomere length) in individual cytomegalovirus (CMV) antigen-specific CD8+ T cells. This involves surface and intracellular cell staining coupled to fluorescence in situ hybridization to detect telomeres (flow-FISH). The end-stage/senescent CD8+ CD45RA+ CD27− T-cell subset increases significantly during ageing and this is exaggerated in CMV immune-responsive subjects. However, these end-stage cells do not have the shortest telomeres, implicating additional non-telomere-related mechanisms in inducing their senescence. The telomere lengths in total and CMV (NLV)-specific CD8+ T cells in all four subsets defined by CD45RA and CD27 expression were significantly shorter in old compared with young individuals in both a Caucasian and an Asian cohort. Following stimulation by anti-CD3 or NLV peptide, similar proportions of triple-cytokine-producing cells are found in CD8+ T cells at all stages of differentiation in both age groups. Furthermore, these multi-functional cells had intermediate telomere lengths compared with cells producing only one or two cytokines after activation. Therefore, global and CMV (NLV)-specific CD8+ T cells that secrete interferon-γ, interleukin-2 and tumour necrosis factor-α are at an intermediate stage of differentiation and are not restricted by excessive telomere erosion.

The (gradual) rise of memory inflation

“…Further, it has been shown that CX3CR1 mid cells can de‐differentiate over time and these could further reseed the central memory pool—likely the majority part which is mainly found in lymphoid tissue 60. Likewise, CD45RA+ T cells in humans can “de‐differentiate” and proliferate despite being commonly labeled as terminally differentiated 81, 82…”

Known unknowns: how might the persistent herpesvirome shape immunity and aging?

“…In mouse models, there is some evidence that CMV-specific memory T cell inflation is the result of continued recruitment of T cells into the immune response [39], although that issue is not fully settled. In contrast to chronic infections where antigen load is high and T cells are driven to functional exhaustion (Hepatitis C virus, HIV; reviewed in [40]), the inflated CMV-specific memory T cell populations retain robust effector function for the life of the individual, and have been described as “the last man standing” in the sea of reduced immune responses to other, acute pathogens [41,42]. …”

A constant companion: immune recognition and response to cytomegalovirus with aging and implications for immune fitness

“…This imprint of CMV infection on the memory T cell population is so robust that CMV infection status can be determined for human samples by sequencing the total T cell repertoire from peripheral blood-the frequency of common CMVspecific T cell receptors is so frequent and shared across HLA-disparate individuals (Emerson et al 2017). In contrast to chronic infections where antigen load is high and T cells are driven to functional exhaustion (hepatitis B/C viruses, HBV/HCV; human immunodeficiency virus, HIV), the inflated CMV-specific memory T cell populations retain robust effector function for the life of the individual (Cicin-Sain et al 2011;Riddell et al 2015;Wherry and Kurachi 2015). However, previous studies of late-differentiated memory CMV-specific T cells in older adults have shown impaired immune competence, such as limited replicative capacity (Fletcher et al 2005;Hadrup et al 2006).…”