The Dynamics of T-Cell Receptor Repertoire Diversity Following Thymus Transplantation for DiGeorge Anomaly

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Abstract
T cell populations are regulated both by signals specific to the T-cell receptor (TCR) and by signals and resources, such as cytokines and space, that act independently of TCR specificity. Although it has been demonstrated that disruption of either of these pathways has a profound effect on T-cell development, we do not yet have an understanding of the dynamical interactions of these pathways in their joint shaping of the T cell repertoire. Complete DiGeorge Anomaly is a developmental abnormality that results in the failure of the thymus to develop, absence of T cells, and profound immune deficiency. After receiving thymic tissue grafts, patients suffering from DiGeorge anomaly develop T cells derived from their own precursors but matured in the donor tissue. We followed three DiGeorge patients after thymus transplantation to utilize the remarkable opportunity these subjects provide to elucidate human T-cell developmental regulation. Our goal is the determination of the respective roles of TCR-specific vs. TCR-nonspecific regulatory signals in the growth of these emerging T-cell populations. During the course of the study, we measured peripheral blood T-cell concentrations, TCRβ V gene-segment usage and CDR3-length spectratypes over two years or more for each of the subjects. We find, through statistical analysis based on a novel stochastic population-dynamic T-cell model, that the carrying capacity corresponding to TCR-specific resources is approximately 1000-fold larger than that of TCR-nonspecific resources, implying that the size of the peripheral T-cell pool at steady state is determined almost entirely by TCR-nonspecific mechanisms. Nevertheless, the diversity of the TCR repertoire depends crucially on TCR-specific regulation. The estimated strength of this TCR-specific regulation is sufficient to ensure rapid establishment of TCR repertoire diversity in the early phase of T cell population growth, and to maintain TCR repertoire diversity in the face of substantial clonal expansion-induced perturbation from the steady state. Protective adaptive immunity depends crucially on the enormous diversity of the T-cell receptor repertoire, the antigen receptors expressed collectively on T-cell populations. T cells develop from T-cell precursors that originate in the bone marrow and migrate to the thymus, where their T cell receptors are constructed stochastically, and tested for autoreactivity against a host of self antigens. Complete DiGeorge anomaly is a rare congenital disease in which the thymus fails to develop, blocking all T cell development and causing profound immunodeficiency. Thymus transplantation, performed in the first two post-natal years, allows the patient's own T cell precursors to develop in the engrafted thymus tissue into normal, functioning T cells. In addition to saving patients' lives, this procedure provides an extraordinary opportunity to study the de novo development of human T cell populations. We have developed a mathematical model to aid in the statistical analysis of the precious data from these patients. In addition to helping elucidate the means by which the size and diversity of T cell populations are jointly regulated, the insights gained from this study hold promise for the development of therapies to promote immune recovery after transplantation.