Efforts of the human immune system to maintain the peripheral CD8+ T cell compartment after childhood thymectomy

Abstract

Background: Homeostatic mechanisms to maintain the T cell compartment diversity indicate an ongoing process of thymic activity and peripheral T cell renewal during human life. These processes are expected to be accelerated after childhood thymectomy and by the influence of cytomegalovirus (CMV) inducing a prematurely aged immune system. The study aimed to investigate proportional changes and replicative history of CD8+ T cells, of recent thymic emigrants (RTEs) and CD103+ T cells (mostly gut-experienced) and the role of Interleukin-(IL)-7 and IL-7 receptor (CD127)-expressing T cells in thymectomized patients compared to young and old healthy controls.

Results: Decreased proportions of naive and CD31 + CD8+ T cells were demonstrated after thymectomy, with higher proliferative activity of CD127-expressing T cells and significantly shorter relative telomere lengths (RTLs) and lower T cell receptor excision circles (TRECs). Increased circulating CD103+ T cells and a skewed T cell receptor (TCR) repertoire were found after thymectomy similar to elderly persons. Naive T cells were influenced by age at thymectomy and further decreased by CMV.

Conclusions: After childhood thymectomy, the immune system demonstrated constant efforts of the peripheral CD8+ T cell compartment to maintain homeostasis. Supposedly it tries to fill the void of RTEs by peripheral T cell proliferation, by at least partly IL-7-mediated mechanisms and by proportional increase of circulating CD103+ T cells, reminiscent of immune aging in elderly. Although other findings were less significant compared to healthy elderly, early thymectomy demonstrated immunological alterations of CD8+ T cells which mimic features of premature immunosenescence in humans.

Keywords: CD8; CMV; Naive T cells; TCR diversity; TRECs; Telomeres; Thymectomy.

Fig. 1

Analysis of CD127+, CD31+ and CD103+ CD8+ T cell proportions. Representative examples of flow cytometric analysis of CD8+ T cells of one YATEC patient, one YHC and one OHC are shown. Gating strategies were as follows: First, CD8+ T cells were analyzed within the CD3+ lymphocyte gate. Percentages of CD45RA+ and CD45RA- were analyzed in CD8+ within the CD3+ T cell gate (a). CD127+ was determined within CD45RA+ (b) and CD45RA- CD8+ T cells (c) together with CD31, percentages indicating positive events in the CD45RA+ or CD45RA- CD8+ T cell gate, respectively. Due to low percentages of circulating CD103 + CD8+, these cells were determined in the total lymphocyte gate (d). Percentages of Ki67+ were determined together with CD127+ in the CD45RA+ (e) and CD45RA- CD8+ gate (f), respectively

Fig. 2

Proportions of naive and CD31+ CD8+ T cells. YAT patients showed lower proportions of naive CD8+ T cells than YHC (a). YATEC patients demonstrated lower proportions of CD31 + CD8+ T cells than YHC. Lowest naive and CD31+ CD8+ T cells were found in OHC compared to YHC (b). Bars represent mean percentages ± standard deviation, numbers (n) of investigated individuals for each group. A p ≤ 0.05 indicates statistical significance (Mann–Whitney U test). YHC showed a negative correlation of CD31+ naive CD8+ T cells with chronological age (d), whereas YATEC did not (c). In YATEC patients, CD31+ naive CD8 + T cells negatively correlated with age at thymectomy (e). Spearman’s Rank correlation coefficient, R; p ≤ 0.05 indicates statistical significance

Fig. 3

Serum Interleukin-7 concentrations, proportions of CD127+ and Ki67+ CD8+ T cells. Serum IL-7 concentrations are shown in YATEC and YAT patients compared to YHC and OHC (a). Lower proportions of CD127+ naive CD8+ T cells were seen in YATEC patients compared to YHC, with lowest proportions in OHC (b). Ki67-expressing cells were demonstrated within the CD127+ naive (c) and memory CD8+ T cell subpopulations (d). Highest percentages of Ki67+ were demonstrated in OHC. Bars represent mean percentages ± standard deviation, numbers (n) of investigated individuals for each group. A p ≤ 0.05 indicates statistical significance (Mann–Whitney U test). Taken together, in YATEC and YAT (“thymectomized group”) serum Interleukin-7 (IL-7) concentrations showed a significant correlation with time post thymectomy (e). Lymphocyte counts significantly correlated with IL-7 concentrations in YHC (f), whereas YATEC did not (g). Spearman’s Rank correlation coefficient, R; p ≤ 0.05 indicates statistical significance

Fig. 4

Replicative history of naive T cells. Significantly lower TRECs per 1,000 CD8 + CD45RA+ T cells (a) and shorter relative telomere lengths (RTLs) (b) were found in YATEC patients compared to YHC. Bars represent mean percentages ± standard deviation, numbers (n) of investigated individuals for each group. No statistical analysis was performed with YAT, as only 2 of them had detectable TRECs and RTLs. Higher proportions of CD103-expressing lymphocytes in CD8 + CD103+ T cells were seen in YATEC patients compared to YHC (c). A p ≤ 0.05 indicates statistical significance (Mann–Whitney U test)

Fig. 5

Proportions of CD31+ and CD103+ CD8+ T cells, Interleukin-7 (IL-7) concentrations, TRECs and relative telomere lengths in YATEC aged >30 years (YATEC > 30a) and YHC aged >30 years (YHC > 30a). Proportions of CD31+ (a) and CD103+ cells in CD8+ T cells (b) are shown in YATEC aged >30 years (YATEC > 30a) who had thymectomy more than 25 years ago compared to YHC aged >30 years (YHC > 30a). Significantly higher proportions of CD103+ CD8+ T cells (b) and a trend for higher IL-7 concentrations (c) were found in YATEC > 30a compared to YHC > 30a. Negative TRECs were seen in five YATEC > 30a compared to YHC > 30a with only one individual with negative TRECs (d). A trend to lower relative telomere lengths (RTLs) was demonstrated in YATEC > 30a compared to YHC > 30a (e). Horizontal lines indicate the median. A p ≤ 0.05 indicates statistical significance (Mann–Whitney U test)

Fig. 6

Association between CMV-seropositivity, proportions of naive T cells and TCR Vβ repertoire diversity. No significant differences were seen between CMV positive (CMV+) or negative (CMV-) YATEC patients or YHC regarding proportions of naive CD4+ T cells (a). CMV positive YATEC patients had lower proportions of naive CD8+ T cells than CMV positive YHC and CMV negative YATEC (b). Horizontal line indicates the median. A p ≤ 0.05 indicates statistical significance (Mann–Whitney U test). Percentages of clonality score 1 (polyclonal) (c), score 2 (oligoclonal) (d) and score 3 (monoclonal) (e) are given for all evaluated Vβ families for YATEC patients, YHC and OHC. An arbitrary dotted line indicates 50 % reaching clonality score (C-E). Representative repertoire profiles for 5 out of 24 evaluated Vβ family primers are provided for one YATEC patient, one YHC and one OHC showing a skewed TCR repertoire in YATEC and OHC (f). More monoclonal (clonality score 3) and less polyclonal distributions (clonality score 1) were found in YATEC patients and OHC compared to YHC (g). The polyclonal/monoclonal distribution ratio (ratio clonality score 1/3) was lower in CMV positive YATEC and YHC compared to CMV negative individuals (h). Lower ratios were found in YATEC patients compared to YHC, with lowest ratios in OHC (h). Data represent a trend of scores, but differences between distributions (polyclonal/monoclonal) and CMV serostatus (positive/negative) did not reach statistical significance in any group (X ² test)