Sara van den Berg

138 Chapter 6 INTRODUCTION In an ongoing fight against new and emerging viruses and bacteria, the adaptive immune system provides a unique line of defence for its host. Adaptive immunity is not only highly specific against pathogens, but also enables protection for many years. This immunological memory requires long-term maintenance of memory T-cells. After an infection is cleared, the pool of generated effector memory (T EM ) and central memory (T CM ) T-cells typically contracts, and a small population of antigen-specific T-cells is maintained over time. Under steady state conditions, cell numbers are maintained by balancing the influx and efflux of cells; the production of new cells, either from a source or from proliferation, matches the death and differentiation of cells. Although it is challenging to measure the production rates of cell populations in vivo in humans, let alone to subsequently put this in context of the expression of proliferation, apoptosis, or senescence markers, it is important to obtain such quantitative insights in order to understand how T-cell memory is maintained. Moreover, features of the T-cell compartment can be severely impacted by chronic viral infections, specifically by human cytomegalovirus (CMV). Cytomegaloviruses are archaic double-stranded DNA viruses that have infected vertebrates for millennia [1]. CMV can cause pathology in severely immunocompromised states (e.g. after organ transplantation, or in advanced HIV infection), but in healthy individuals the primary infection typically remains asymptomatic. Nevertheless, CMV-seropositivity greatly influences the T-cell phenotype of healthy individuals — CD8 + T-cells in particular. It leads to a marked increase in the absolute number of CD8 + T EM [2] and effector memory re-expressing CD45RA (T EMRA ) T-cells [3, 4]. These cells also seem functionally different as they show a decreased expression of the costimulatory molecules CD27 and CD28 [3, 4], and upregulation of the late-differentiation markers CD57 and KLRG-1 [5]. Currently, it is still unclear to what extent the presence of CMV-specific T-cells themselves is responsible for these changes in the T-cell pool, and to what extent other antigen-specific memory T-cells are affected. The fact that T-cell changes in CMV infection resemble those observed in healthy ageing [2] has prompted the hypothesis that CMV infection contributes to the age-related decay of immune function, including diminished responses to infectious diseases and vaccination [6, 7]. The factor that is thought to be key in the unique T-cell response to CMV, is the ‘dynamic’ latency CMV establishes within its host [8] with frequent episodes of viral reactivation from latency [9, 10]. These reactivations are believed to play a significant role in the establishment of high numbers of CMV-specific T-cells. Especially in older adults, CMV-specific CD8 + T-cells can take up as much as 30%-90% of the total CD8 + T-cell pool [11, 12]. In mice, it has been shown that the number of CMV-specific memory T-cells gradually increases over time, a process termed ‘memory inflation’ [13-17]. There is also some evidence for memory inflation in humans [14, 18-20]. However, how a handful of T-cell clones can become and remain so numerous, or even increase over time, is not well understood. The underlying dynamics, i.e. the production and loss rates, of both CMV-specific T-cells and other cells in the CMV- reshaped T-cell pool, remain largely unknown. Based on in vivo deuterated glucose labelling