Mechanisms and consequences of robust PD1 expression by neonatal regulatory T cells in the diabetes-prone NOD mouse – TregNOD

Regulatory T cells (Treg), most notably those developing very early on (in the mouse during the neonatal period), play a central role in the control of immune-responses and thus prevent immunopathologies such as autoimmune diseases and chronic inflammation. Our recent data show that in NOD mice, the main experimental type I diabetes (T1D)-model, the T1D susceptibility locus Idd5.1 contributes to an exceptionally high frequency of PD1-expressing cells among Treg developing during the neonatal period. We showed that these neonatal PD1(pos) Treg are very autoreactive and strongly recirculate from the periphery back to the thymus. Based on these observations, we postulate that 1) PD1 expression by neonatal Treg impairs their suppressive activity and/or 2) that these very autoreactive Treg, upon recirculation back to the thymus, disturb induction of T cell-tolerance in the thymus, thus contributing to the development of T1D.

WP1: We will first investigate how particularly high levels of PD1-expressing Treg develop in the neonatal NOD thymus. We will identify the gene (among the four located in Idd5.1) that controls PD1 expression and decipher the molecular mechanisms involved. Our recent data show that PI3Kdelta controls PD1-expression by neonatal Treg and we will study the upstream and downstream mechanisms involved, including identification of transcription factors.

WP2 & 3: We will then extensively characterise the in vivo (clonal) dynamics of neonatal PD1(pos) Treg in pre-diabetic and diabetic NOD mice and compare them to those of neonatal PD1(neg) Treg. Based on the to-be-acquired multi-omics data, we will construct mathematical models to better describe and understand neonatal PD1(pos) Treg-behaviour in vivo. To assess the potential involvement of neonatal PD1(pos) Treg in susceptibility to T1D, we will construct NOD mice in which neonatal PD1+ Treg are absent, analyse immune-activation and monitor T1D development. Since we found that neonatal PD1+ Treg strongly recirculate back to the thymus, we will also analyse their capacity to obstruct T cell-tolerance induction in the thymus, potentially involved in T1D-development. To assess a potential effect of PD1 on neonatal Treg function (e.g. inhibition, modulation of metabolism), we will ablate its expression exclusively on neonatal Treg and study homing, activation, differentiation, and metabolism of Treg, and T1D development of the mice.

WP4: Since the locus controlling part of the PD1-phenotype of neonatal Treg in NOD mice (Idd5.1) has an orthologue in humans also controlling T1D-susceptibility (IDDM12), we feel that abnormalities of the earliest Treg developing in humans may be involved in human T1D. In preparation for a clinical research program on early Treg development in T1D-patients, we will set up in vitro mouse foetal thymus organ cultures in which human Treg develop from (non-mobilised) peripheral blood stem-cells from young adult donors.

Our project, which is based on a large body of solid but still unpublished data, will thus provide a better understanding of the complex role of inherited functional defects of Tregs in the aetiology of T1D in NOD mice. It may thus have repercussions for development of Treg-based or Treg-targeting immunotherapies against T1D in humans.