Thymic crosstalk, a set of reciprocal regulations between thymocytes and thymic environment, is relevant for orchestrating the appropriate development of thymocytes as well as the recovery of the thymus from various exogenous insults. Nevertheless, the dynamic and regulatory aspects of the thymic crosstalk have not yet been clarified. In this work, we inferred the interactions shaping the thymic crosstalk and its resultant dynamics between the thymocytes and the thymic epithelial cells (TECs) by quantitative analysis and modelling of the recovery dynamics induced by irradiation. The analysis identified regulatory interactions consistent with the known molecular evidence and revealed their dynamic roles in the recovery process. Moreover, the analysis also predicted, and a subsequent experiment verified a new regulation of CD4+CD8+ double positive (DP) thymocytes, which temporarily increases their proliferation rate upon the decrease in their population size. Our model established the pivotal step towards a dynamic understanding of the thymic crosstalk as a regulatory network system.
Main textThe thymus is an organ responsible for producing a large part of T cells with appropriate repertoires [1]. Nevertheless, it is relatively sensitive to insults by stress, virus infection, radiation, and other extra stimuli [2,3]. While a thymus in a healthy animal can be normally recovered from these damages, a relatively prolonged process of the thymic recovery may impair T cell-mediated immunity due to a reduced replenishment of naïve T cell repertoire during the recovery period [3,4].Sub-lethal dose radiation on mice has been utilized as an experimental model of the thymic regeneration after insults [5,6]. Ionizing irradiation is also broadly used for hematopoietic transplantation and cancer therapy [7,8], and total body irradiation causes an acute thymic injury and slow recovery of thymopoiesis. Several studies showed that irradiation reduces cellularity not only of thymocytes but also of thymic epithelial cells (TECs), which are major constituents of the thymic environment [5,9,10]. Because the thymopoiesis is supported by interactions between the thymocytes and the TECs [11], understanding thymic recovery requires a characterization of the reciprocal regulations between the thymocytes and the TECs.Concomitantly, various techniques to trace, perturb, and quantify the cells involved in the events have enabled us to characterize their kinetics and dynamics quantitatively [12][13][14][15]. By combining mathematical models with such quantitative data, dynamic aspects of thymopoiesis have been distilled in the forms of more detailed kinetic information, e.g., rates of proliferation, death, and differentiation [12,16]. Mehr et al.[17] had developed the first kinetic model of the thymocyte development using ordinary differential equations (ODEs) [18]. Since this pioneering work, kinetic models of the thymopoiesis have been progressively refined by taking into account of the detailed cellularity and developmental states of the thy...