Ageing Affects Thymopoiesis and Experimental Autoimmune Encephalomyelitis Development in a Strain-Dependent Manner.

INTRODUCTION: Considering significance of mechanisms of central tolerance for development of autoimmune diseases, including experimental autoimmune encephalomyelitis (EAE), and suppressive influence of circulating proinflammatory cytokines and alterations in brain-thymus communication, characteristic for the central nervous system (CNS) autoimmune diseases, on thymopoiesis, the study interogated putative strain-based thymus-related specificities relevant for the opposite effects of ageing on susceptibility of Dark Agouti (DA) and Albino Oxford (AO) rats to EAE. METHODS: Quantitative and qualitative changes in thymopoiesis including underlying mechanisms were examined using flow cytometry and RT-qPCR quantification of mRNAs for molecules relevant for integrity of stroma and T-cell development, respectively. RESULTS: With ageing, differently from DA rats, in AO rats the surface density of CD90, a negative regulator of selection threshold, on thymocytes undergoing lineage commitment was upregulated (consistent with TGF-ß expression downregulation), whereas the generation of natural CD4+CD25+Foxp3+ regulatory T cells (nTregs) was impaired reflecting differences in thymic expression of cytokines supporting their development. Additionally, specifically in old AO rats, in whom EAE development depends on IL-17-producing CD8+ T cells, their thymic differentiation was augmented, reflecting augmented thymic IL-4 expression. In turn, differently from old DA rats developing self-limiting EAE, in age-matched AO rats developing EAE of prolonged duration, EAE development led to impaired generation of nTregs and accumulation of proinflammatory, cytotoxic CD28-CD4+ T cells in the periphery. DISCUSSION: The study indicates that strain differences in age-related changes in the efficacy of central tolerance, in addition to enhanced thymic generation of CD8+ T cells prone to differentiate into IL-17-producing cells, could partly explain the opposite effect of ageing on DA and AO rat susceptibility to EAE induction. Additionally, it suggested that EAE development leading to a less efficient thymic output of CD4+ cells and nTregs in old AO rats than their DA counterparts could contribute to prolonged EAE duration in AO compared with DA rats. CONCLUSION: The study warns to caution when designing therapeutic interventions to enhance thymic activity in genetically diverse populations, e.g., humans, and interpreting their outcomes. Furthermore, it indicates that CNS autoimmune pathology may additionally worsen thymic involution and age-related immune changes.

Thymic changes as a contributing factor in the increased susceptibility of old Albino Oxford rats to EAE development.

The study was aimed to examine putative contribution of thymic involution to ageing-associated increase in susceptibility of Albino Oxford (AO) rats to the development of clinical EAE, and vice versa influence of the disease on the progression of thymic involution. To this end we examined (i) the parameters of thymocyte negative selection efficacy, the thymic generation of CD4+CD25+Foxp3+ T regulatory cells (Tregs) and thymic capacity to instruct/predetermine IL-17-producing T-cell differentiation, and thymopietic efficacy-associated accumulation of "inflammescent" cytotoxic CD28- T cells in the periphery, and (ii) the key underlying mechanisms in young and old non-immunised AO rats and their counterparts immunised for EAE (on the 16th day post-immunisation when the disease in old rats reached the plateau) using flow cytometry analysis and/or RT-qPCR. It was found that thymic involution impairs: (i) the efficacy of negative selection (by affecting thymocyte expression of CD90, negative regulator of selection threshold and the expression of thymic stromal cell integrity factors) and (ii) Treg generation (by diminishing expression of cytokines supporting their differentiation/maturation). Additionally, the results suggest that thymic involution facilitates CD8+ T-cell differentiation into IL-17-producing cells (previously linked to the development of clinical EAE in old AO rats). Furthermore, they confirmed that ageing-related decrease in thymic T-cell output (as indicated by diminished frequency of recent thymic emigrants in peripheral blood) resulted in the accumulation of CD28- T cells in peripheral blood and, upon immunisation, in the target organ. On the other hand, the development of EAE (most likely by increasing circulatory levels of proinflammatory cytokines) contributed to the decline in thymic output of T cells, including Tregs, and thereby to the progression/maintenance of clinical EAE. Thus, in AO rats thymic involution via multi-layered mechanisms may favour the development of clinically manifested autoimmunity, which, in turn, precipitates the thymus atrophy.