Clinical impact of immunosenescence and the complex interaction with Cytomegalovirus

In this thesis we investigated the clinical impact of age-related changes to the immune system (cf. immunosenescence). Two large hallmarks were previously distinguished, namely ‘inflammaging’, a low-grade chronic systemic inflammatory status at old age. Second, a T-cell senescence leading to T-cell repertoire shrinkage with increasing age. We investigated the clinical impact of potential biomarkers of both concepts in very elderly individuals of the BELFRAIL study (a representative cohort study of persons aged 80 years and older). In the first part of my thesis we investigated the value of 13 serum ‘inflammaging’ markers for the identification (Chapter 2) and prediction (Chapter 3) of global functioning and global functional decline, respectively. Outside physical and mental functioning, we compared and identified the principal prognostic marker for mortality or hospitalization in a sample of 303 very elderly individuals (Chapter 4). We showed the prognostic superiority of IL-6, over 12 other markers, in all three chapters and reported on high to very high negative predictive values but low positive predictive values. Therefore, we suggested the simple measurement of serum levels of IL-6 in future gerontological research as the first-rated ‘not-at-risk’ marker to identify those persons in the community not in need of additional care. In the second part, we thoroughly investigated T-cell subset distribution by means of flow cytometry and identified the CD4:8 ratio as a potential important biomarker in a randomly selected subpopulation of 235 very elderly individuals of the BELFRAIL study and 25 younger individuals. We collected and processed all necessary peripheral blood mononuclear cells (PBMC) samples at a timepoint 1,5 years from baseline and measured 14 different T-cell subsets by means of CD-receptors (CD4, CD8, CD28, CD27, CD45RA, CCR7, CD57) with the help of our German colleagues at the university of Tübingen. We reported on a newly-identified large group (almost 33%) of individuals with an inflated CD4:8 ratio (R>5), which was absent in younger individuals. Therefore we divided the study population in three interesting groups, namely the R<1 (inverted ratio individuals), 15. The latter was a naïve-dominated phenotype independent of CMV seropositivity, with very low levels of late-stage effector memory T-cells. We also reported on a discondordant number of inverted ratio individuals (R<1), compared to the original OCTO/NONA study where these individuals where found to have the highest mortality rates. Our next step was then to investigate whether the CD4:8 ratio phenotypes can indeed be used as a marker for mortality or other adverse outcomes in a Belgian population. Counterintuitively, we showed a cross-sectional higher chance of physical impairment and higher mortality rates among the R>5, and lower chances or rates among the R<1, respectively. Even more, CMV seropositivity had an independent and protective effect on survival chances in these very elderly individuals of the BELFRAIL study. Mediation analyses showed the CD4;8 ratio not to mediate the strong independent relationship between CMV and mortality, suggesting independent concurrent pathways towards mortality. We interpreted this to imply that very old people living a sheltered life benefitted more from better control of persistent CMV infection than they did from possessing more naive T-cells specific for new pathogens that they never encountered. Following this hypothesis, the R<1 individuals exhibited a very efficient non-specific T-cell response and CMV-specific humoral immunosurveillance in the absence of chronic inflammation to combat superinfection or reactivation. These results stress the importance of changing associations with very high age and may indicate that we should consider a person’s immune adaptive capacity at very old age with regards to mortality. Due to the difficult nature of CMV history and infection, we suggested new measures of CMV reactivation to not merely distinguish between the infected vs non-infected, but between latent vs persistent infection. The latter could help to explain the heterogeneity seen in CMV-seropositive elderly persons. In the final part of my thesis, we explored one of the major consequences of immunosenescence, namely a reduced vaccination response at old age. We focused on influenza as one of the major infectious burden in Belgian very elderly. We set-up the new INCIVAR phase IV trial and recruited 203 elderly individuals aged 65 years or more through general practitioner centers in the region of Leuven and Brussels. As we would investigate the impact of CMV on the influenza vaccination effectively, we almost evenly distributed the number of CMV-seropositives (n=117) and CMV seronegatives (n=86). We completed the study population with 44 younger individuals between the age of 18 and 50 years. The INCIVAR study will not only assess the humoral response as is custom, but also the detailed cellular immune response to influenza vaccination to compare methods and effectively study vaccine efficacy. Additionally, the persistent CMV infection will be studied as a dynamic condition resulting from a complex host-pathogen interaction rather than just comparing vaccination response between CMV-infected and non-infected.