Modulation of T cell activation and function by PI3K-controlled transcriptional regulators Foxo1 and Id3.

Abstract

αβ T cells are a diverse population of cells which help to mediate both inflammation and tolerance. T cells can be broadly divided based on their localization, activation and expression of transcription factors into naïve, effector, memory and regulatory T cells. Conventional CD4+ and CD8+ T cells mediate inflammation and immune memory, helping to clear viral, bacterial and parasitic infections. Foxp3+ regulatory T cells (TR) are potent regulators of tolerance by preventing auto-reactive effector function and dampening immune responses after infection. Although diverse in their function and localization, all αβ T cells are regulated by the phosphoinositide 3-kinase (PI3K) signaling cascade. In T cells the PI3K pathway can be activated or inhibited by a variety of signals including T cell receptor (TCR) stimulation, cytokines, growth factors, co-stimulatory receptors and co-inhibitory receptors. In both CD4+ and CD8+ T cells varied strength and duration of PI3K signaling can instruct cells towards effector function, memory phenotypes, anergy or exhaustion. These observations demonstrate the essential role of PI3K signaling in regulating the complex pathways T cells must follow for appropriate responses to an immune threat. In this work we highlight two proteins, Foxo1 (Forkhead box O) and Id3 (inhibitor of DNA binding) which help regulate T cell function downstream of PI3K signaling. Herein, we demonstrate that Foxo1 regulates T cell homeostasis starting at thymic development and plays a vital role in regulating effector T cell differentiation and function. Using several transgenic mouse models, we prevented inactivation of Foxo1 in T cells, resulting in both CD4+ and CD8+ T cells which have constitutively active of Foxo1 (Foxo1CA). Foxo1 activation in the thymus prior to positive-selection resulted in decreased thymic output of mature CD4 single positive T cells and development of spontaneous autoimmunity associated with autoreactive B cells. Conversely, constitutive activation of Foxo1 after thymic maturation yielded healthy mice with a naïve-skewed T cell population, in which T cells were defective in effector function. RNA-seq analysis of stimulated control versus Foxo1CA CD8+ T cells highlighted several key pathways regulated by Foxo1. We show that during TCR stimulation inactivation of Foxo1 allows for coordinated changes in costimulatory receptors, inhibitory receptors and key transcription factors. Examining regulation of inhibitors of DNA binding (Id) proteins in TR, we show that dynamic downregulation of Id3 by PI3K signaling helps define three distinct TR populations, Id3+CD62LhiCD44lo central (c)TR, Id3+CD62LloCD44hi effector (e)TR and Id3- eTR. We further go on to show that Id3- eTR are highly enriched in non-lymphoid tissues and have a tissue resident TR molecular signature. Together our findings add to the known functions of PI3K regulation in αβ T cell homeostasis, differentiation and function.