Background
Loss of renal function is related to impaired function of the T cell-mediated immune system. Changes in T cell subsets and function may underlie this effect [
1,
2]. Clinical consequences of this T cell-mediated immune dysfunction are a reduced efficiency of vaccination [
3,
4], an enhanced susceptibility for infectious diseases [
5] and an enchanced risk for developing auto-immune diseases and tumors [
6].
T cells leave the thymus as naïve cells. Upon encountering of antigens presented by antigen presenting cells, naive T cells will differentiate into effector T cells and eventually only a fraction of these will develop into memory T cells. The expression of (chemokine C-C motif receptor 7) CCR7 and CD45RO can be used to distinguish between the different T cell subsets, i.e. naïve (CD45RO
-CCR7
+), central memory (CM, CD45RO
+ CCR7
+, able to home into lymphoid tissues), effector memory (EM, CD45RO
+CCR7
-, exerting direct effector functions) and the more terminally differentiated effector memory CD45RA
+ (EMRA, CD45RO
-CCR7
-, high in effector function) subset [
2,
7,
8]. In addition, the loss of cell surface CD28 expression identifies more differentiated T cells [
9].
During ageing in healthy individuals, the thymic output of new naïve T cells reduces due to the involution of the thymus. Absolute T cell numbers are largely conserved by homeostatic proliferation of both naïve and memory T cells but eventually this leads to a reduced population of naïve T cells and a relatively preserved population of memory T cells [
10]. Elderly individuals have a marked decrease in naïve T cells,, a decline in CD4/CD8 ratio and a relative increase in the number of differentiated memory T cells lacking CD28 [
10‐
12].
The thymic output of new naïve T cells can be determined by measuring the T cell receptor excision circles (TRECs)[
13]. These TRECs are small circular DNA episomes that are formed during rearrangement of the T cell receptor (TCR) genes in T cells that are present in the thymus. These TRECs are not replicated and therefore diluted with every cell division. Another hallmark of ageing is the reduction in telomere length [
14,
15]. Telomeres are small DNA sequences located at the end of a chromosome and with increasing age they become shorter due to the inability of telomerase to elongate these DNA sequences upon division. Together, TREC content and telomere length reflect the thymic output and replicative history of T cells and may provide a valuable tool to estimate the immunological age of the T cells within an individual.
Using these ageing parameters, the T cell system of a dialysis patient shows severe T cell ageing and resembles that of a 20–30 year older healthy individual [
1]. This premature ageing of the T cell system probably underlies the uremia-associated immune defect in dialysis patients. However, it is not known whether the T cell system of ESRD patients not on renal replacement therapy (RRT) shows a similar degree of immunological ageing and to what extent this is influenced by RRT.
In this cross-sectional study, we have analyzed the T cell ageing parameters in ESRD patients who are not on RRT and compared this to healthy individuals on one hand, and patients treated with RRT (i.e. hemodialysis or peritoneal dialysis) on the other hand.
Discussion
The results of this study show that based on several immunological parameters, ESRD without RRT is associated with premature immunological ageing of the T cell system. The added effect of RRT on this phenomenon is remarkably small and was predominantly limited to the CD8+ T cell compartment in young ESRD patients.
Most studies on the immune system of patients with renal failure have been performed in chronic ESRD patients treated with RRT, mainly hemodialysis. Lymphopenia and signs of T cell activation have been reported in a lot of the studies [
16,
17]. In-depth analysis of T cell subsets showed that lymphopenia is particularly prominent in the naïve T cell subset which showed a progressive decline in numbers as the stage of chronic kidney disease increased [
2,
18]. In a recent study, we showed that decreased numbers of naïve T cells in hemodialysis patients is related to decreased thymic output of naïve T cells with increased but insufficient homeostatic proliferation in the periphery [
19]. Memory T cells were in general more differentiated probably due to increased proliferation, given the decrease in relative telomere length. Similar findings were now observed for ESRD patients not on dialysis, indicating that loss of renal function is the most dominant factor for decreased thymic output of naïve T cells and increased differentiation/proliferation of memory T cells. In healthy individuals these changes are observed in the elderly and therefore considered as a physiological process of immunological ageing of the T cell system. In comparison to the CD8
+ T cells, the CD4
+ T cell system in healthy individuals remains relatively unaffected by age until the seventh or eighth decade [
10,
12,
20‐
23]. However, patients with ESRD not on RRT already showed all characteristics of immunological ageing (lower thymic output, shorter telomeres) of their T cell system, approximately 10–20 years ahead of their calendar age. The reasons for premature T cell ageing in patients with chronic renal failure are not known but a relative lack of the T cell growth factor IL-7 has been documented and may be important [
1,
2]. In animal models it was clear that sudden loss of renal function causes involution of the thymus and other lymphoid organs confirming a direct relationship between kidney function and lymphopoiesis [
24]. Lymphopenia may trigger increased homeostatic proliferative responses [
25], not only of the circulating naïve T cell compartment but also of memory T cells thereby inducing differentiation and loss of telomere length. However this concept is hypothetical and has not been tested yet in ESRD patients.
In general, immunological ageing of T cells (e.g. increased numbers of CD28null T cells) has been associated with decreased T cell immunity. Maintenance and generation of a number of antibody responses seems critically dependent on the presence of antigen-specific CD4
+ T cells [
3,
26]. Any major disturbances of the T cell system will therefore affect the humoral immune response as well. ESRD-related premature immunological T cell ageing may therefore underlie the well-established uremia-associated cellular and humoral immune deficiency in ESRD patients [
1].
In a previous study, it was shown that loss of naïve T cells and increased memory T cell differentiation progresses with increasing stage of chronic kidney disease but with little difference between ESRD patients with or without RRT [
2,
18]. Also on the level of T cell chemokine receptor expression, which is indicative for functional capacities of T cells few differences were observed between T cells of ESRD patient with or without RRT. These findings are remarkable as most patients on dialysis have little to no residual renal function and as such are metabolically more affected than ESRD patient not on dialysis. The results in this study are largely in accordance with these data and show that patients with RRT do not have an altered thymic output of naïve T cells and total numbers of naïve T cells. Summarizing the present data, it appears that the maximum effect of loss of renal function on the T cell immune system is reached at the level of ESRD. Whether RRT prevents further immunological ageing or not is difficult to assess and cannot be inferred from our data.
However, the CD4+ and CD8+ memory T cells in young dialysis patients showed more differentiation and in the CD8+ T cells this was accompanied by a decrease in RTL. Thus, particularly memory CD8+ T cells in younger patients show a history of more proliferation without the presence of decreased thymic output. This finding indicates that in younger dialysis patients, on top of ESRD-related immunological ageing, other factors drive the proliferation of memory T cells. The dialysis vintage of younger patients was on average higher than the old group suggesting a role for duration of RRT and loss of telomere length. However, we could not find an independent statistically significant relation between duration of RRT and any of the immunological parameters measured. In addition, the type of underlying kidney diseases was not related to any parameter of immunological ageing. Another possible scenario may be that RRT actually improves homeostatic proliferation of memory T cells in the young but not in the elderly patients. However, this should result in a relative increase in memory T cell numbers in the young RRT patients compared to ESRD patients without RRT which was not observed. The lack of such a finding would argue against this explanation.
A limitation of the present study is the cross-sectional design which may obscure subtle changes in immunological T cell ageing after patients with ESRD have started RRT. However, the large number of patients included in this study adds to the reliability of the results.
In conclusion, severe loss of renal function leading to ESRD is a very potent inducer of premature immunological T cell ageing of both the CD4
+ and CD8
+ T cells. Renal replacement therapy is associated with a small increase of memory T cell ageing in patients <50 years of age, particularly in the CD8
+ T cell subset. Further research is needed to establish the pathophysiology of ESRD-related T cell ageing and whether this can be reversed by e.g. interleukin-7 therapy [
27,
28] or kidney transplantation.
Competing interests
All the authors declared no competing interests. This study was funded by the Dutch Kidney Foundation (KSPB.10.12).
Authors’ contributions
RM: performed the experiments, statistical analysis and drafted the manuscript. NL: designed the study and drafted the manuscript. EdW: performed the experiments. AL: contributed in writing the manuscript. AvdS: performed some of the experiments. CB: contributed in writing the manuscript. WW: contributed in writing the manuscript and provided patient data. MB: designed the study and drafted the manuscript. All authors read and approved the final manuscript.