Genetic control of TCR diversity within the natural regulatory T cell lineage

T cells are an essential component of the immune system crucial for fighting infection and to maintain a healthy organism. Their function is linked to the T cell receptor (TCR), a molecule expressed at the cell surface and through which T cells recognise pathogens. Although each T cell usually expresses only one type of TCR, the T cell population as a whole comprises a wide repertoire facilitating the detection of many different pathogens. T cells recognise fragments of pathogen-derived proteins when they are presented on the surface of infected cells in combination with specialised proteins called Major Histocompatibility Complex (MHC) molecules. It is the interaction between the TCR and these complexes that triggers the response. T cells are generated and their repertoire of TCRs determined in the thymus. Repertoires are selected which eliminate both 'useless' TCRs that do not interact with MHC and autoreactive TCRs that may cause autoimmunity. Thus, most T cells leaving the thymus express TCRs of intermediate affinity. However, some high-affinity TCRs escape the thymic elimination mechanism. To keep these potentially autoreactive T cells in check, the organism has a regulatory T cell subset that has suppressive activity (Treg). Like conventional T cells (Tconv), Tregs also depend on the TCR for their function. Thus Treg efficiency is very much dependent on their TCR repertoire. We have observed that in a mouse model of autoimmunity resembling human type I diabetes, the nTreg TCR repertoire has extremely low diversity. This will lead to a defective regulatory population which is likely to be related to development of disease. As reduced repertoire diversity is not observed in the conventional T cells, the defect appears to be Treg specific. The aim of this project is to identify the defective gene or genes in order to understand the genetic basis of TCR diversity in the Treg population. Several different approaches will be used and although part of the research rests on the study of a diabetes prone mouse strain, this will be used only as a model to unravel the biological mechanism underlying the phenotype. Identification of the genetic factors controlling TCR diversity in T regulatory cells would be an essential link in understanding how this population works and protects from autoimmune disease. This information could be applied to manipulate the TCR repertoire and will be potentially useful for the development of novel therapeutic approaches for autoimmunity, cancer and infectious disease.

This proposal will dissect the genetic control of TCR diversity. T cells are a major component of adaptive immunity with both effector and regulatory functions. TCR gene rearrangement produces a highly diverse pre-selection receptor repertoire which is shaped by thresholds which remove both excessively self-reactive receptors (negative selection) and those which do not interact sufficiently. The effector functions of T cells are tightly controlled by specialised 'regulatory' T cells. Natural regulatory T cells, also selected in the thymus, are considered to have distinct receptor repertoires with enhanced self-reactivity. Control of autoimmunity is absolutely dependent on this natural regulatory repertoire. Selection of T cell repertoires is intimately linked with signalling events downstream of the TCR which ultimately control thymocyte survival. Integrity of signalling pathways is thus crucial to set appropriate thresholds for negative and positive selection. The natural regulatory T cell population is a distinct lineage within which many components of the downstream signalling pathways are differentially regulated. In this context, our hypothesis and specific objectives arise from detailed analysis of the regulatory and conventional repertoires of autoimmune prone mice in which TCR diversity is markedly reduced in the regulatory arm suggesting a higher stringency of selection. Using SNP based mapping of backcross mice we will determine the genetic basis of repertoire restriction in the autoimmune NOD strain. To test whether T cell signal transduction is perturbed in the NOD strain, we will determine whether thymic selection and regulatory activity of NOD nTregs is defective. By using mouse strains with defined defects in T cell signal transduction we will directly assess whether reduced signalling generates T cell repertoires with altered diversity. Finally, we will determine whether the diversity of the thymic nTreg reperoire is modified in the periphery.