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.