Thymectomy in early childhood: Significant alterations of the CD4⁺CD45RA⁺CD62L⁺ T cell compartment in later life

Abstract

The study was aimed to assess indicators of immunosenescence, such as the total counts of peripheral blood CD4⁺CD45RA⁺CD62L⁺ (naive) T cells, the numbers of T cell receptor excision circles (TRECs), and Ki67-expression as marker of peripheral replication in thymectomized patients (TP) (n = 101) compared to age-matched healthy donors (HD) (n = 81).

In TP, there was an inverse correlation between naive T cells and chronological age (p < 0.001) or time post thymectomy (p < 0.001). TP demonstrated lower TREC numbers in naive T cells compared to HD (p < 0.001). TREC numbers negatively correlated with time post thymectomy (p < 0.001). Percentages of Ki67-expresssing naive T cells were higher in TP compared to HD (p < 0.05).

The findings of the presented long-term follow up cohort of thymectomized patients indicate that changes of the peripheral naive T cell subset in TP may resemble the findings of an aging immune system in elderly persons after thymic involution. Our data provide evidence that peripheral T cell homeostasis in TP is maintained at minimal levels mainly by extrathymic expansion of existing naive T cells in the periphery to compensate the diminished thymic output.

Introduction

The thymus, which is the primary site of T cell lymphopoiesis during fetal and early postnatal life, is almost fully developed at birth, but its involution continues throughout life [1], [2]. In the elderly, there is still ongoing T cell receptor gene rearrangement and T cell maturation, but a diminished thymic output of newly generated T cells and compensatory extrathymic expansion of existing naive T cells in the periphery [1], [2], [3]. Decrease of naive T cells which are characterized by the CD45RA isoform along with expression of the lymph node homing receptor CD62L [4] has been discussed to be responsible for the increased occurrence and severity of infections and autoimmune diseases in old age. Another most widely acknowledged phenotypic change of the aged individual is the loss of the major co-stimulatory molecule CD28 [3].

In open heart surgery for congenital heart diseases in children, the thymus is removed for better surgical access to the heart and great vessels. The thymus of children is highly active in the first months of life. Experiments in mice, discovering the importance of the thymus in early phases of life demonstrated that thymectomy resulted in partial immunodeficiency, mainly affecting cell mediated immune responses [5]. Thymectomy in children younger than 6 months has been shown to be tolerated without clinical consequences such as higher infection rates [6], [7], but revealed to cause alterations of the peripheral T cell compartment. Changes in T cell phenotype following thymectomy were also described by several other studies, which reported decreased CD4⁺ T cell numbers [8], [9], [10], [11], [12].

However, there are no phenotypic markers that distinguish recent thymic emigrants from naive T cells produced by peripheral clonal expansion. The evaluation of T cell receptor recombination excision circle (TREC) numbers in peripheral T cells can be a useful indicator of thymopoiesis [13], [14], [15], [16]. TRECs are not replicated during mitosis and, therefore, diluted out during cell divisions, which not only include priming of recent thymic emigrants to become memory T cells but also homeostatic cell division of naive T cells. Children with primary and secondary immune deficiency diseases including human immunodeficiency virus (HIV)-infected children and patients with absent thymus, such as in severe combined immune deficiency disease or in complete DiGeorge syndrome showed significant decreased TREC levels [17], [18], [19]. TRECs were markedly reduced in thymectomized patients concurrently with decreases in total T cells and phenotypic naive T cells [9], [20].

However, all these studies reported short-term surveys and did not consider long-term effects of thymectomy on the naive T cell subset which shows the most striking changes in the elderly after physiological thymic involution. Therefore, we examined thymic function and alterations to the peripheral naive T cell compartment in a cohort of patients who had undergone thymectomy in their first months of life for signs of premature immunosenescence in later life with the oldest patients having thymectomy more than 27 years ago. Premature immunosenescence after thymectomy may be of clinical relevance with advancing age of individuals regarding increased incidence of diseases of the elderly, such as infectious complications, artherosclerosis, neurodegenerative disorders and malignancies, and poor responses to new antigens, such as vaccine antigens [3]. To evaluate whether thymectomized patients demonstrate changes of the naive T cell pool with alterations reminiscent of the ones described in the elderly, three indicators of aging were measured: (1) the total counts of peripheral blood CD4⁺CD45RA⁺CD62L⁺ (naive) T cells; (2) the numbers of TRECs in CD4⁺CD45RA⁺CD62L⁺ T cells; (3) Ki67-expression as marker of replicative history and proliferation.