WAVE-ing T cell activation off
The WAVE regulatory complex (WRC) is a pentameric complex that regulates actin cytoskeleton dynamics. The precise role for the WRC in immunity has not been established, although recent work has implicated WRC components such as HEM1 in certain human immunodeficiencies. Liu et al. characterized mice with a conditional knockout of the WRC constituent WAVE2 (see the Perspective by Hambleton). These mice exhibited progressive severe autoimmune and inflammatory disease associated with the activation and accelerated differentiation of T cells. WAVE2’s suppression of T cell activation was not mediated by the T cell receptor but rather by its inhibition of mammalian target of rapamycin (mTOR) binding to RAPTOR and RICTOR. Accordingly, the autoimmune phenotype of these mice could be ameliorated by pharmacological inhibitors of mTOR signaling.
The essential role of actin cytoskeletal regulatory proteins in expression of effective immune responses is exemplified by the impaired immunity manifested by patients deficient for the Wiskott-Aldrich syndrome actin modulatory protein (WASp). Among other WASp-related drivers of actin rearrangement, the WASp family verprolin homologous protein 2 (WAVE2) is predominantly expressed in hematopoietic cells and has been implicated in the cytoskeletal remodeling required for T cell adhesion and organization of the immunological synapse. However, the precise roles of WAVE2 in modulating T cell functions that govern immunity remain unknown.
WAVE2 stability and subcellular localization depend on its constitutive association with the ABI1/2 (Abelson interactor 1/2), HEM1 (hematopoietic protein 1), HSPC300 (haematopoietic stem/progenitor cell protein 300), and CYFIP1 (cytoplasmic FMR1 interacting protein 1) proteins, which together constitute a pentameric WAVE regulatory complex (WRC) that supports WAVE2 activation by the Rac guanosine triphosphatase and other stimulatory signals that trigger WAVE2-driven actin-related protein-2/3 (Arp2/3)–dependent actin nucleation. Genetic data implicating several WAVE2-associated WRC proteins in autoimmune disease and the established link between cytoskeletal regulator dysfunction and immune deficiency raise the possibility that WAVE2 represents an important effector of T cell contributions to immune competence. Thus, we derived WAVE2 T cell conditional knockout (Wave2cKO) mice to enable investigation of WAVE2 influence on T cell homeostasis and function.
Characterization of Wave2cKO mice expressing WAVE2-deficient T cells revealed numbers of peripheral T cells to be reduced in young (<4 weeks old) mice but progressively increased and surpassed those of control mice by age 8 weeks. Although thymic T cell development appeared normal, Wave2cKO mice manifested progressive severe autoimmune and inflammatory disease characterized by splenomegaly, lymphadenopathy, multiorgan lymphocyte infiltration, and early death. Significant increases in proinflammatory cytokine and autoantibody expression were also detected in these mice and were accompanied by spontaneous activation and accelerated differentiation of peripheral T cells. Wave2cKO mice also exhibited markedly impaired antigen-specific immune responses associated with exhausted T cells, showing increases in T cell inhibitory receptor expression, dysregulated mitochondrial function, and increased activation-induced cell death. T cell antigen receptor (TCR)–driven actin polymerization and chemotaxis were also impaired in WAVE2-deficient T cells, with aberrant trafficking of these cells to nonlymphoid organs. WAVE2 deficiency had negligible effects on proximal TCR signaling but was associated with significantly increased activation of the mammalian target of rapamycin (mTOR) in response to antigen receptor or cytokine stimulation. Mechanistically, WAVE2 bound mTOR and constrained mTOR binding to RAPTOR (regulatory-associated protein of mTOR) and RICTOR (rapamycin-insensitive companion of mTOR), key components of the mTOR complexes 1 and 2, respectively. Consistent with WAVE2’s suppressive effect on mTOR activation, pharmacologic inhibition of mTOR ameliorated the T cell anomalies and the immunodysregulatory phenotypes manifested in Wave2cKO mice.
Our findings identify a critical role for WAVE2 in restraining T cell activation and effector differentiation, with T cell–selective WAVE2 ablation leading to combined immunodeficiency and autoimmune disease. The enhanced mTOR activation observed in WAVE2-deficient T cells and the suppression of disease in Wave2cKO mice treated with an mTOR inhibitor establish WAVE2 restriction of mTOR activity as a critical mechanistic pathway supporting T cell quiescence so as to maintain immune homeostasis and prevent inflammation and autoimmunity.
Mice expressing WAVE2-deficient T cells develop severe immunodysregulatory disease. In normal T cells, WAVE2 represses mTOR activation by restricting RAPTOR/RICTOR recruitment to mTOR. In the absence of WAVE2, enhanced mTOR signaling engenders spontaneous T cell activation, hyperproliferation, and accelerated differentiation, along with impaired antigen-evoked responses and an exhaustion phenotype. These T cell aberrancies and the concomitant autoimmune phenotype are ameliorated by pharmacological inhibition of mTOR.
Cytoskeletal regulatory protein dysfunction has been etiologically linked to inherited diseases associated with immunodeficiency and autoimmunity, but the mechanisms involved are incompletely understood. Here, we show that conditional Wave2 ablation in T cells causes severe autoimmunity associated with increased mammalian target of rapamycin (mTOR) activation and metabolic reprogramming that engender spontaneous activation and accelerated differentiation of peripheral T cells. These mice also manifest diminished antigen-specific T cell responses associated with increased inhibitory receptor expression, dysregulated mitochondrial function, and reduced cell survival upon activation. Mechanistically, WAVE2 directly bound mTOR and inhibited its activation by impeding mTOR interactions with RAPTOR (regulatory-associated protein of mTOR) and RICTOR (rapamycin-insensitive companion of mTOR). Both the T cell defects and immunodysregulatory disease were ameliorated by pharmacological mTOR inhibitors. Thus, WAVE2 restraint of mTOR activation is an absolute requirement for maintaining the T cell homeostasis supporting adaptive immune responses and preventing autoimmunity.