Age-dependent incidence, time course, and consequences of thymic renewal in adults

Abstract

Homeostatic regulation of T cells involves an ongoing balance of new T cell generation, peripheral expansion, and turnover. The recovery of T cells when this balance is disrupted provides insight into the mechanisms that govern homeostasis. In a long-term, single cohort study, we assessed the role of thymic function after autologous transplant in adults, correlating serial computed tomography imaging of thymic size with concurrent measurements of peripheral CD4(+) T cell populations. We established the age-dependent incidence, time course, and duration of thymic enlargement in adults and demonstrated that these changes were correlated with peripheral recovery of naive CD45RA(+)CD62L(+) and signal-joint TCR rearrangement excision circle-bearing CD4(+) populations with broad TCR diversity. Furthermore, we demonstrated that renewed thymopoiesis was critical for the restoration of peripheral CD4(+) T cell populations. This recovery encompassed the recovery of normal CD4(+) T cell numbers, a low ratio of effector to central memory cells, and a broad repertoire of TCR Vbeta diversity among these memory cells. These data define the timeline and consequences of renewal of adult thymopoietic activity at levels able to quantitatively restore peripheral T cell populations. They further suggest that structural thymic regrowth serves as a basis for the regeneration of peripheral T cell populations.

Figure 1

TI scale utilized to assess thymic enlargement in thoracic CT scans, based on McCune (28): 0, no soft tissue with thymus entirely replaced by fat (top left panel); 1, minimal soft tissue, barely recognizable (top right panel); 2, minimal soft tissue more obvious (typical thymus of a middle-aged adult; bottom left panel); 3, moderate soft tissue (bottom right panel). No cases of McCune’s level 4 (moderate soft tissue of greater extent, almost mass-like) or level 5 (mass-like appearance of concern for hyperplasia or thymoma) were observed in these patients.

Figure 2

Effect of age on recovery of thymic size after APBSCT. (A) Spearman nonparametric correlation demonstrates that the maximum TI achieved in the first 2 years after transplant (as assessed in Figure 1) is dependent on age at time of treatment. (B) Time course of thymic enlargement as stratified by age. Each line represents the individual time course during the first 2 years after transplant of changes in TI of a patient who achieved a maximum TI of 2, the size of a normal adult thymus. The number of patients in each age range and the incidence of those individuals with thymic enlargement (4 of 5, TI ≥ 2) is in parentheses above each graph. The gray bands represent the range of minimal thymic size observed after transplant in patients demonstrating no significant change, whose time courses would otherwise overlap.

Figure 3

Effect of age on recovery of naive CD4⁺ T cells after APBSCT. (A) Scatterplot of patient naive (CD45RA⁺CD62L⁺) CD4⁺ T cells after APBSCT. Naive CD4⁺ T cells were reduced substantially by the transplant conditioning regimen and recovered slowly over the next 2 years. Each filled circle represents a single patient value; the red line represents the median and the gray area represents the normal range in adults. Pre, pretreatment; 0, time of transplant. (B) Spearman nonparametric correlation demonstrates that number of naive (CD45RA⁺CD62L⁺) CD4⁺ T cells achieved after approximately 2 years (21–24 months) is dependent on age at time of treatment. (C) Line plots of recovery of naive CD4⁺ T cells in individual patients, stratified by age, during the first 2 years after transplant. The red lines represent patients with thymic enlargement (Figure 2B); the black lines represent those with no evident enlargement.

Figure 4

Effect of age on recovery of TREC-bearing CD4⁺ T cells. TREC frequency after approximately 2 years (21–24 months) is dependent on age at time of transplant.

Figure 5

Correlated recovery of thymic size, TREC-bearing and naive CD4⁺ T cells. (A) Spearman nonparametric correlation between the highest TI attained within the first 2 years and the number of TRECs per 10⁵ CD4⁺ T cells at 2 years. (B) Spearman nonparametric correlation between the highest TI attained within the first 2 years and the number of naive (CD45RA⁺CD62L⁺) CD4⁺ T cells per μl at 2 years. (C) Spearman nonparametric correlation between the frequency of TREC per 10⁵ CD4⁺ T cells and the number of CD45RA⁺CD62L⁺ CD4⁺ T cells per microliter of peripheral blood.

Figure 6

Naive CD4⁺ T cell repertoire diversity in an individual recovering more than 50% naive cells. At 6 months, 12 months, and 18 months, spectratypes were assessed by RT-PCR and CDR3 cDNA fragment length analysis for 23 Vβ families from 100,000 sorted CD45RA⁺ CD4⁺ T cells. Although significantly oligoclonal at 6 months after APBSCT, within the first year after APBSCT, naive CD4⁺ T cells recovered polyclonal spectratypes in most Vβ families.

Figure 7

Correlation of quantitative recovery of total CD4⁺ T cell numbers with renewed thymopoiesis. (A) Individual time courses of CD4⁺ T cell loss and recovery with APBSCT. Red lines indicate patients who attained frequencies of 3,000 TRECs or more per 10⁵ CD4⁺ cells at 2 years. These same individuals tended to have the highest total CD4 recovery. PRE TRT, pretreatment. (B–D) Spearman nonparametric correlations demonstrate that long-term recovery of total CD4 number is predicted by maximum TI (B), typically achieved at 1 year, and is correlated with the frequency of TREC-bearing CD4 cells in the peripheral blood (C) and with the number of naive phenotype cells at 2 years (D). (E) Spearman nonparametric correlation demonstrates that even after 4–5 years, the recovery of robust thymopoiesis is the main determinant of total CD4⁺ T cell number.

Figure 8

Effect of naive CD4⁺ T cells’ recovery on EM/CM ratio. (A) Time course of shifts in the EM/CM CD4⁺ T cell ratio in 6 representative patients. Red lines indicate patients who had a strong recovery of thymopoiesis (TRECs/10⁵ CD4⁺ T cells at least 3,000 at 2 years). (B) Spearman nonparametric correlation demonstrates that long-term recovery of the ratio of EM to CM CD4⁺ T cells correlates with the frequency of TREC-bearing cells in the CD4 population.

Figure 9

Effect of renewed thymopoiesis and age on TCR repertoire diversity. (A) Repertoire diversity in naive versus memory CD4⁺ T cells in 2 individuals, patient A (Pt. A) and Pt. B, with less than 15% naive CD4⁺ T cells after 2–4 years. Representative spectratypes of several Vβ families demonstrate that naive CD4⁺ populations have polyclonal spectratypes, whereas memory/activated CD45RA^– CD4⁺ T cells have more limited diversity. (B–D) Spearman correlation demonstrates that recovery of diverse CDR3 repertoire in memory/activated (CD45RA^–) CD4⁺ T cells is correlated with recovery of thymopoiesis, as assessed by TI (B) and number of naive CD45RA⁺CD4⁺ T cells (C). (D) Spearman correlation demonstrates that the capacity to recover a broad TCR repertoire after transplant is dependent upon patient age.