The unmet need in the elderly: how immunosenescence, CMV infection, co-morbidities and frailty are a challenge for the development of more effective influenza vaccines

Abstract

Influenza remains the single most important cause of excess disability and mortality during the winter months. In spite of widespread influenza vaccination programs leading to demonstrated cost-savings in the over 65 population, hospitalization and death rates for acute respiratory illnesses continue to rise. As a person ages, increased serum levels of inflammatory cytokines are commonly recorded (TNF-α, IL-1, IL-6). Termed "inflammaging", this has been linked to persistent cytomegalovirus (CMV) infection and immune senescence, while increased anti-inflammatory cytokines (IL-10, TGF-β) are possibly associated with more healthy aging. Paradoxically, a shift with aging toward an anti-inflammatory (IL-10) response and decline in the IFN-γ:IL-10 ratio in influenza-challenged peripheral blood mononuclear cells is associated with a decline in the cytolytic capacity of CD8+ T cells responsible for clearing influenza virus from infected lung tissue. Thus, it is seemingly counter intuitive that the immune phenotype of healthy aging predicts a poor cell-mediated immune response and more serious outcomes of influenza. Herein we postulate a mechanistic link between the accumulation of late-stage, potentially terminally differentiated T cells, many or most of which result from CMV infection, and the immunopathogenesis of influenza infection, mediated by granzyme B in older adults. Further, adjuvanted influenza vaccines that stimulate inflammatory cytokines and suppress the IL-10 response to influenza challenge, would be expected to enhance protection in the 65+ population.

Figure 1

With permission reproduced from Franceschi C et al. [33]. Health aging: the Balance.

Figure 2

The GrzB assay is based on the enzyme’s unique substrate specificity to cleave the four amino acid sequence (IEPD) at the aspartate (D) residue releasing the paranitroanalide, which undergoes a colorimetric change detected on a plate reader. In this case, “bGrzB” is measured in lysates of unstimulated CD3+ T cells purified by magnetic bead selection. bGrzB activity is significantly higher in CMV+ vs. CMV− older adults (p=0.029) and corresponds to the high proportion of GrzB+ CD8+ T cells found in unstimulated PBMC shown in Figure 3.

Figure 3

With permission, modified from McElhaney et al. [44]. Intracellular GrzB and the degranulation marker CD107a are shown for effector CD45RA+CD8+ T cells. Healthy young (HY) adults show low levels of GrzB+ cells (−virus) and degranulate (CD107a+) only in response to influenza stimulation (+virus). Healthy older adults (HO) vs. HY show a significantly increased proportion of GrzB+ cells (−virus) and higher proportions degranulate in response to influenza (+virus). Older adults with CHF (CHF) show a further increase in the proportion of GrzB+ cells, degranulate (CD107a+) in the resting state (in the absence of virus) and show no response to virus stimulation.

Figure 4

Granzyme B (GrzB) is released by CTL and NK cells to induce apoptosis of virus-infected cells and requires perforin for internalization into cells. Once inside the cell, GrzB induces apoptosis and this is referred to as the classical GrzB/apoptosis pathway. In the absence of perforin, GrzB is released into extracellular space (bGrzB is the biomarker) stimulating GrzB in other cells and causing cleavage of IL-1α and the extracellular matrix causing inflammation and loss of tissue integrity.

Figure 5

Summary of flow cytometry results from [46]. The response to influenza vaccination is shown as the change in memory T cell frequency and the generation of Effector T cells in response to ex vivo influenza challenge (*). Cytotoxicity of Effectors declines with age and with time after vaccination (lighter colour = less cytotoxicity). The Memory CD8+ T cell frequency is similar at 4 weeks post-vaccination but declines by 10 weeks post-vaccination in older compared to young adults. Thus, CD8+ T cell immunity through vaccination is lost by the time of the influenza season in older adults.