Helios Regulates CD8 T Cell Activation in Response to TCR Affinity

Abstract Text: Helios (Ikzf2) belongs to a family of transcription factors that regulate lymphoid differentiation and function. In thymic-derived FoxP3 regulatory T cells, Helios promotes suppressive function. Previously, we observed that Helios is induced in CD8 T cells rendered anergic.
The aim of this work was to analyze Helios induction and function in CD8 T cells.
We quantified Helios expression in OT-I cells exposed to ovalbumin (OVA) presented in different contexts: (a) ubiquitous self-antigen; (b) tissue-restricted self-antigen; (c) bacterial antigen; (d) tumor-associated antigen. Helios was produced only by exposure to self- and tumor-associated antigens.
We mounted an in vitro assay to identify factors that modulate Helios expression. Productive CD8 activation, achieved by high affinity ligands and pre-activation of antigen presenting cells, inhibited Helios expression. In contrast, activation with low-affinity ligands led to robust Helios induction. Pharmacological dissection of signaling pathways indicated that activation of the IL-2-STAT5 pathway inhibited Helios induction. Accordingly, IL-2 blockade or STAT5 inhibition released Helios expression in CD8 T cells activated with high affinity ligands.
In vivo, Ikzf2-deficient OT-I cells exhibited altered proliferation when exposed to OVA as a self-antigen or associated with Listeria. scRNA sequencing revealed that Helios controls genes involved in cell cycle, cytokine response, and metabolic adaptation. Helios deficiency caused an abnormal expansion of effector cells.
In summary, Helios fine-tunes proliferation and effector differentiation during CD8 T cell activation. IL-2-STAT5 activation inhibits Helios expression during strong CD8 stimulation. This mechanism could differentially regulate the activation of lower affinity clones during immune responses.