Discussion
TRECs are thought to be the most
reliable tools for tracking recent thymic output. Indeed, such
measurements have been used in multiple clinical settings, including
diagnosis of SCID, T cell development-associated immune deficiencies, T
cell reconstitution after HSCT, aging, and autoimmune diseases.
In this study, we quantified TRECs levels in 475 children aged 0–18
years, including premature newborns and low BW newborns. To the best of
our knowledge, this is the largest study designed to determine TRECs
reference levels in a healthy Chinese pediatric population. We found
that TRECs levels tended to fall with age, mainly due to division of
peripheral cells and reduced thymic activity due to age-associated
thymic involution23. TRECs levels fell rapidly between
1 month and 1 year-of-age, reflecting increased rates of thymopoiesis. A
second large decrease occurred from 12–18 years-of-age, which might be
due to changes in hormone levels. We also found that TRECs levels were
lower in premature newborns and low BW newborns. TRECs levels tended to
increase with GA. These data suggest that during newborn screening, the
GA and its reciprocal BW should be included in the screening
strategies24. Ward et al found a 9.8% increase in
TRECs levels per week of gestation25; however, our
study identified no linear correlation between GA and TRECs levels,
although this may be due to an insufficient number of subjects in this
case.
Gender-related differences in TRECs levels were very
conflicting26,27. We found that TRECs levels in
females were similar to those in males. Sex hormones such as prolactin
may regulate development of CD4 T cells28. Indeed, the
number of mature CD4 T cells in estrogen-treated mice increases. Also,
testosterone may induce apoptosis of thymocytes29. In
addition, production of cytokines is affected by sex hormones such as
IL7 and IL15, which may drive development of T
cells30.
We found a weak correlation between TRECs levels and numbers of CD4
naïve T cells, which is inconsistent with the consensus that TRECs
levels are closely related to CD4 naïve T cell numbers. In the large
number of healthy young participants, we found a correlation between
TRECs levels and the absolute number of peripheral CD4 naïve T cells
only in the 1–4 y and 4–8 y age groups; no study has found this
before. This finding may be due to the fact that we examined CD4 naïve T
cell numbers in peripheral blood; thus the numbers may be affected by
factors other than thymic output (i.e., proliferation, death, and
redistribution of peripheral CD4 naïve T cells)31. The
CD4 naïve T cells have a short lifespan and soon undergo apoptosis,
thereby contributing to T cell homeostasis in which newly generated
thymic emigrants make up for the loss of peripheral
cells32. Activated of CD4 naïve T cells are regulated
by some nutritional factors33; therefore, measurement
of TRECs levels and naïve T cell numbers should be combined with an
assessment of thymus function.
WAS, characterized by eczema, thrombo-cytopenia, and immunodeficiency,
has three phenotypes: classical WAS, XLT, and X-linked neutropenia
(XLN)34. Treatment differs according to the phenotype
and associated complications. Those with classical WAS need urgent
curative treatment, such as a stem cell transplant or gene
therapy35. For those with XLT, symptomatic treatment
is the major therapy of choice, although splenectomy is sometimes
recommended as it effectively stops the tendency to
bleed36. XLT patients have a better prognosis than
those with WAS37. Thus, WAS may be a good model to
test whether TRECs are a good biomarker and predictor of disease
severity. Our data revealed that TRECs levels in patients with classical
WAS (n=14) were significantly suppressed, whereas those in patients with
XLT (n=8) were mildly suppressed or normal. There was a strong
correlation between TRECs levels and CD4 naïve cell numbers in those
with classic WAS, which is consistent with the findings of a previous
study36.
GOF mutations in the PI3K genes PIK3CD (p110δ) and PIK3R1 (p85α) cause a
combined immunodeficiency syndrome called APDS38.
TRECs levels and CD4 naïve T cell numbers were suppressed significantly
in those with APDS39. The PI3K-AKT pathway is crucial
for transition of intermediate single positive thymocytes to double
positive thymocytes40. T cell development in those
with PIK3CD GOF mutations is skewed, as evidenced by a reduction in CD4
and CD8 thymocyte numbers in an APDS mouse model41.
APDS mice show severe lymphopenia in the periphery, along with increased
senescence of effector T cells and total T cells42.
Signal transducer and activator of transcription 1 (STAT1) is a
transcription factor that mediates cellular responses to interferons
(IFNs), as well as other cytokines and growth factors. Mutation of STAT1
is associated with chronic mucocutaneous candidiasis or Mendelian
susceptibility to mycobacterial disease, with or without autoimmune
disease43. TRECs levels in patients with STAT1
mutation are normal in the publications before44,45; however, we found
that TRECs levels were consistent with the levels of CD4 naïve T cells,
which were normal or mildly decreased. A previous study shows that only
28% of patients with STAT1 mutations have low CD4 T cell
numbers46. We found a similar result
here47. Lower CD4 T cell counts are associated with a
higher mortality in STAT1 patients; therefore, patients with low TRECs
levels probably require HSCT as soon as possible. The proportion of
CD3/CD4 T cells and TRECs levels in NS patients were
normal48,49. Overall, the data suggest that TRECs
levels correlate with CD4 naïve T cell numbers; therefore, they may be a
good biomarker for multiple PIDs.
TRECs levels may also predict occurrence of GVHD and associated
complications. In the first few
months after HSCT, peripheral expansion of implanted cells is very
important50. We found that TRECs levels started to
rise within 4 weeks of HSCT, and achieved normal levels by 1 year
post-HSCT. Consistent with the findings of Weinberg et
al51, we found that GVHD might be associated with low
TRECs levels. P2 developed GVHD in multiple organs, and his TRECs levels
remained low. P5 and P6 had low TRECs levels after HSCT and developed
multiple organ disorders. The major factors that affect TRECs levels are
the number of HLA matches, the type of graft, TRECs levels before HSCT,
and the proposal conditioning regimen52. However, more
cases are needed to identify a correlation between TRECs levels and
complications after HSCT.