QRICH1 Mediates an Intracellular Checkpoint for CD8+ T Cell Activation via the CARD11 Signalosome

Antigen receptor signaling pathways that control lymphocyte activation depend upon signaling hubs and negative regulatory proteins to fine-tune signaling output to ensure host defense and avoid potentially pathogenic hyperresponses. CARD11 is a critical signaling scaffold that translates T Cell Receptor triggering into the activation of branching NF-kB, JNK, mTOR and Akt pathways. Here we identify QRICH1 as a regulator of CARD11 signaling that mediates a previously undiscovered intracellular checkpoint for CD8+ T cell activation. QRICH1 inducibly associates with CARD11 following TCR engagement and negatively regulates CARD11 signaling to the IKK complex and NF-kB. QRICH1 binding to CARD11 occurs after TCR-induced CARD11 opening and is controlled by an autoregulatory intramolecular interaction between QRICH1 protein domains of previously uncharacterized function. Using mice deficient in QRICH1 in the T cell lineage, we show that QRICH1 controls the antigen-induced activation, proliferation, and effector status of CD8+ T cells by regulating numerous gene targets critical for CD8+ T cell function. Our results define a previously unappreciated component of antigen receptor signaling circuitry that fine-tunes effector output in response to antigen recognition.