CD3ζ ITAMs enable ligand discrimination and antagonism by inhibiting TCR signaling in response to low-affinity peptides

The T cell antigen receptor (TCR) contains ten immunoreceptor tyrosine-based activation motif (ITAM) signaling sequences distributed within six CD3 subunits; however, the reason for such structural complexity and multiplicity is unclear. Here we evaluated the effect of inactivating the three CD3ζ chain ITAMs on TCR signaling and T cell effector responses using a conditional 'switch' mouse model. Unexpectedly, we found that T cells expressing TCRs containing inactivated (non-signaling) CD3ζ ITAMs (6F-CD3ζ) exhibited reduced ability to discriminate between low- and high-affinity ligands, resulting in enhanced signaling and cytokine responses to low-affinity ligands because of a previously undetected inhibitory function of CD3ζ ITAMs. Also, 6F-CD3ζ TCRs were refractory to antagonism, as predicted by a new in silico adaptive kinetic proofreading model that revises the role of ITAM multiplicity in TCR signaling. Finally, T cells expressing 6F-CD3ζ displayed enhanced cytolytic activity against solid tumors expressing low-affinity ligands, identifying a new counterintuitive approach to TCR-mediated cancer immunotherapy.

GRB2 promotes thymocyte positive selection by facilitating THEMIS-mediated inactivation of SHP1

The T-lineage restricted protein THEMIS has been shown to play a critical role in T cell development. THEMIS, via its distinctive CABIT domains, inhibits the catalytic activity of the tyrosine phosphatase SHP1 (PTPN6). SHP1 and THEMIS bind to the ubiquitous cytosolic adapter GRB2, and the purported formation of a tri-molecular THEMIS-GRB2-SHP1 complex facilitates inactivation of SHP1 by THEMIS. The importance of this function of GRB2 among its numerous documented activities is unclear as GRB2 binds to multiple proteins and participates in several signaling responses in thymocytes. Here, we show that similar to Themis-/- thymocytes, the primary molecular defect in GRB2-deficient thymocytes is increased catalytically active SHP1 and the developmental block in GRB2-deficient thymocytes is alleviated by deletion or inhibition of SHP1 and is exacerbated by SHP1 overexpression. Thus, the principal role of GRB2 during T cell development is to promote THEMIS-mediated inactivation of SHP1 thereby enhancing the sensitivity of TCR signaling in CD4+CD8+ thymocytes to low affinity positively selecting self-ligands.

THEMIS increases TCR signaling in CD4+CD8+ thymocytes by inhibiting the activity of the tyrosine phosphatase SHP1

The T cell lineage-restricted protein THEMIS plays a critical role in T cell development at the positive selection stage. In the SHP1 activation model, THEMIS is proposed to enhance the activity of the tyrosine phosphatase SHP1 (encoded by Ptpn6), thereby dampening T cell antigen receptor (TCR) signaling and preventing the inappropriate negative selection of CD4+CD8+ thymocytes by positively selecting ligands. In contrast, in the SHP1 inhibition model, THEMIS is proposed to suppress SHP1 activity, rendering CD4+CD8+ thymocytes more sensitive to TCR signaling initiated by low-affinity ligands to promote positive selection. We sought to resolve the controversy regarding the molecular function of THEMIS. We found that the defect in positive selection in Themis-/- thymocytes was ameliorated by pharmacologic inhibition of SHP1 or by deletion of Ptpn6 and was exacerbated by SHP1 overexpression. Moreover, overexpression of SHP1 phenocopied the Themis-/- developmental defect, whereas deletion of Ptpn6, Ptpn11 (encoding SHP2), or both did not result in a phenotype resembling that of Themis deficiency. Last, we found that thymocyte negative selection was not enhanced but was instead impaired in the absence of THEMIS. Together, these results provide evidence favoring the SHP1 inhibition model, supporting a mechanism whereby THEMIS functions to enhance the sensitivity of CD4+CD8+ thymocytes to TCR signaling, enabling positive selection by low-affinity, self-ligand-TCR interactions.

Eomes-Dependent Loss of the Co-activating Receptor CD226 Restrains CD8+ T Cell Anti-tumor Functions and Limits the Efficacy of Cancer Immunotherapy

CD8⁺ T cells within the tumor microenvironment (TME) are exposed to various signals that ultimately determine functional outcomes. Here, we examined the role of the co-activating receptor CD226 (DNAM-1) in CD8⁺ T cell function. The absence of CD226 expression identified a subset of dysfunctional CD8⁺ T cells present in peripheral blood of healthy individuals. These cells exhibited reduced LFA-1 activation, altered TCR signaling, and a distinct transcriptomic program upon stimulation. CD226^neg CD8⁺ T cells accumulated in human and mouse tumors of diverse origin through an antigen-specific mechanism involving the transcriptional regulator Eomesodermin (Eomes). Despite similar expression of co-inhibitory receptors, CD8⁺ tumor-infiltrating lymphocyte failed to respond to anti-PD-1 in the absence of CD226. Immune checkpoint blockade efficacy was hampered in Cd226^-/- mice. Anti-CD137 (4-1BB) agonists also stimulated Eomes-dependent CD226 loss that limited the anti-tumor efficacy of this treatment. Thus, CD226 loss restrains CD8⁺ T cell function and limits the efficacy of cancer immunotherapy.

The costimulatory molecule CD226 signals through VAV1 to amplify TCR signals and promote IL-17 production by CD4+ T cells

The activation of T cells requires the guanine nucleotide exchange factor VAV1. Using mice in which a tag for affinity purification was attached to endogenous VAV1 molecules, we analyzed by quantitative mass spectrometry the signaling complex that assembles around activated VAV1. Fifty VAV1-binding partners were identified, most of which had not been previously reported to participate in VAV1 signaling. Among these was CD226, a costimulatory molecule of immune cells. Engagement of CD226 induced the tyrosine phosphorylation of VAV1 and synergized with T cell receptor (TCR) signals to specifically enhance the production of interleukin-17 (IL-17) by primary human CD4⁺ T cells. Moreover, co-engagement of the TCR and a risk variant of CD226 that is associated with autoimmunity (rs763361) further enhanced VAV1 activation and IL-17 production. Thus, our study reveals that a VAV1-based, synergistic cross-talk exists between the TCR and CD226 during both physiological and pathological T cell responses and provides a rational basis for targeting CD226 for the management of autoimmune diseases.

An Epistatic Interaction between Themis1 and Vav1 Modulates Regulatory T Cell Function and Inflammatory Bowel Disease Development

The development of inflammatory diseases depends on complex interactions between several genes and various environmental factors. Discovering new genetic risk factors and understanding the mechanisms whereby they influence disease development is of paramount importance. We previously reported that deficiency in Themis1, a new actor of TCR signaling, impairs regulatory T cell (Treg) function and predisposes Brown-Norway (BN) rats to spontaneous inflammatory bowel disease (IBD). In this study, we reveal that the epistasis between Themis1 and Vav1 controls the occurrence of these phenotypes. Indeed, by contrast with BN rats, Themis1 deficiency in Lewis rats neither impairs Treg suppressive functions nor induces pathological manifestations. By using congenic lines on the BN genomic background, we show that the impact of Themis1 deficiency on Treg suppressive functions depends on a 117-kb interval coding for a R63W polymorphism that impacts Vav1 expression and functions. Indeed, the introduction of a 117-kb interval containing the Lewis Vav1-R63 variant restores Treg function and protects Themis1-deficient BN rats from spontaneous IBD development. We further show that Themis1 binds more efficiently to the BN Vav1-W63 variant and is required to stabilize its recruitment to the transmembrane adaptor LAT and to fully promote the activation of Erk kinases. Together, these results highlight the importance of the signaling pathway involving epistasis between Themis1 and Vav1 in the control of Treg suppressive function and susceptibility to IBD development.

Rho-GTPases as key regulators of T lymphocyte biology

Rho-GTPases belong to the Ras superfamily and are crucial signal transducing proteins downstream of many receptors. In general, the Rho-GTPases function as molecular switches, cycling between inactive (GDP-bound) and active (GTP-bound) states. The activated GTP bound Rho-GTPases interact with a broad spectrum of effectors to regulate a plethora of biological pathways including cytoskeletal dynamics, motility, cytokinesis, cell growth, apoptosis, transcriptional activity and nuclear signaling. Recently, gene targeting in mice allowed the selective inactivation of different Rho-GTPases and has advanced our understanding of the physiological role of these proteins, particularly in the immune system. Particularly, these proteins are key signaling molecules in T lymphocytes, which are generated in the thymus and are major players in the immune system. The scope of this review is to discuss recent data obtained in Rho-GTPases deficient mice by focusing on the role-played by Rho-GTPases in T-lymphocyte development, migration, activation and differentiation.

VAV1 adaptor functions regulate both T cell polarization and susceptibility to immune mediated diseases (BA14P.211)

Vav1 is a guanine nucleotide exchange factor (GEF) that is essential for signal transduction of the T cell receptor (TCR) and plays an important role in the development and activation of T cells. Our previous results identified a polymorphism in the Vav1 gene that impacts on its adaptor functions without notable effect on its enzymatic activity. To analyze the impact of this polymorphism on the susceptibility to immune diseases, we generated a knock-in (KI) mouse bearing an arginine (R) to tryptophan (W) substitution in the Vav1 protein (Vav1R63W KI). Using this model, we confirmed that Vav1R63W mutation had no impact on Vav1 enzymatic activity in CD4 T cells but decreased its adaptor function, as revealed by the reduction of TCR induced calcium flux and ERK/AKT activation. Moreover, we showed that Vav1R63W favors Th2 cytokine production by CD4 T cells. We next analyzed the susceptibility of these mice to immune-mediated diseases and showed that KI mice were less susceptible to MOG-induced central nervous system inflammation. This finding likely resulted from a reduced production of effector cytokines (IFN-γ, IL-17 and GM-CSF) by autoreactive CD4 T cells. Finally, we found an increased susceptibility of the KI mice to asthma features, consistent with enhanced Th2 polarization. Together, these data highlight the importance of Vav1 adaptor functions in T cell polarization and susceptibility to immune mediated diseases, with opposite effects on autoimmune and allergic diseases.

Loss of the HPV-infection resistance EVER2 protein impairs NF-κB signaling pathways in keratinocytes

Homozygous mutations in EVER genes cause epidermodysplasia verruciformis (EV), characterized by an immune defect and the development of skin cancers associated with β-human papillomavirus (HPV) infections. The effects of EVER protein loss on the keratinocyte immune response remain unknown. We show here that EVER2 plays a critical role in the interplay between the NF-κB and JNK/AP-1 signaling pathways. EVER2-deficient cells overproduce IL-6 following the upregulation of JNK activation. They respond poorly to phorbol ester and TNF via the NF-κB pathway. They have lower levels of IKKα subunit, potentially accounting for impairments of p100 processing and the alternative NF-κB pathway. The loss of EVER2 is associated with an unusual TRAF protein profile. We demonstrate that EVER2 deficiency sustains TRAF2 ubiquitination and decreases the pool of TRAF2 available in the detergent-soluble fraction of the cell. Finally, we demonstrate that EVER2 loss induces constitutive PKCα-dependent c-jun phosphorylation and facilitates activation of the HPV5 long control region through a JNK-dependent pathway. These findings indicate that defects of the EVER2 gene may create an environment conducive to HPV replication and the persistence of lesions with the potential to develop into skin cancer.

EVER2 protein binds TRADD to promote TNF-α-induced apoptosis

EVER1 and 2 confer resistance to cutaneous oncogenic human papillomavirus infections by downregulating the activating protein 1 (AP-1) signaling pathway. Defects in their expression are associated with susceptibility to epidermodysplasia verruciformis, which is characterized by persistent β-HPV infection, tumor necrosis factor alpha (TNF-α) overproduction in keratinocytes and the development of skin cancers. TNF-α-induced apoptosis is a key defense strategy, preventing the persistence of the virus within cells, but the role of EVER proteins in this cell death mechanism triggered by extrinsic stimuli is unknown. We show here that EVER2 induces TNF-α- and TRAIL-dependant apoptosis. It interacts with the N-terminal domain of TRADD, impairs the recruitment of TRAF2 and RIPK1 and promotes apoptosis. The skin cancer-associated EVER2 I306 allele results in an impaired TRADD-EVER2 interaction, with lower levels of cell death following treatment with TNF-α. These data highlight a new, critical function of EVER2 in controlling cell survival in response to death stimuli.

TFPI-2 silencing increases tumour progression and promotes metalloproteinase 1 and 3 induction through tumour-stromal cell interactions

Tissue factor pathway inhibitor-2 (TFPI-2) is a potent inhibitor of plasmin which activates matrix metalloproteinases (MMPs) involved in degradation of the extracellular matrix. Its secretion in the tumour microenvironment makes TFPI-2 a potential inhibitor of tumour invasion and metastasis. As demonstrated in aggressive cancers, TFPI-2 is frequently down-regulated in cancer cells, but the mechanisms involved in the inhibition of tumour progression remained unclear. We showed in this study that stable TFPI-2 down-regulation in the National Cancer Institute (NCI)-H460 non-small cell lung cancer cell line using specific micro interfering micro-interfering RNA promoted tumour progression in a nude mice orthotopic model that resulted in an increase in cell invasion. Moreover, TFPI-2 down-regulation enhanced cell adhesion to collagen IV and laminin via an increase in α₁ integrin on cell surface, and increased MMP expression (mainly MMP-1 and -3) contributing to cancer cell invasion through basement membrane components. This study also reveals for the first time that pulmonary fibroblasts incubated with conditioned media from TFPI-2 silencing cancer cells exhibited increased expression of MMPs, particularly MMP-1, -3 and -7, that are likely involved in lung cancer cell invasion through the surrounding stromal tissue, thus enhancing formation of metastases.

Inhibition of cervical cancer cell growth by human papillomavirus virus-like particles packaged with human papillomavirus oncoprotein short hairpin RNAs

Overexpression of human papillomavirus (HPV E6 and HPV E7) oncogenes in human cervical cells results in the development of cancer, and E6 and E7 proteins are therefore targets for preventing cervical cancer progression. Here, we describe the silencing of E6 and E7 expression in cervical carcinoma cells by RNA interference. In order to increase the efficacy of the RNA interference, HPV pseudovirions coding for a short hairpin RNA (shRNA) sequence were produced. The results indicated the degradation of E6 and E7 mRNAs when shRNA against E6 or E7 were delivered by pseudovirions in HPV-positive cells (CaSki and TC1 cells). E6 silencing resulted in the accumulation of cellular p53 and reduced cell viability. More significant cell death was observed when E7 expression was suppressed. Silencing E6 and E7 and the consequences for cancer cell growth were also investigated in vivo in mice using the capacity of murine TC1 cells expressing HPV-16 E6 and E7 oncogenes to induce fast-growing tumors. Treatment with lentiviruses and HPV virus-like particle vectors coding for an E7 shRNA sequence both resulted in dramatic inhibition of tumor growth. These results show the ability of pseudovirion-delivered shRNA to produce specific gene suppression and provide an effective means of reducing HPV-positive tumor growth.