Rap1A regulates generation of T regulatory cells via LFA-1-dependent and LFA-1-independent mechanisms

Proteomics reveals a global phenotypic shift of NK cells in HCV patients treated with direct-acting antivirals

Chronic hepatitis C virus (HCV) infections compromise natural killer (NK)-cell immunity. Direct-acting antivirals (DAA) effectively eliminate HCV, but the long-term effects on NK cells in cured patients are debated. We conducted a proteomic study on CD56+ NK cells of chronic HCV-infected patients before and 1 year after DAA therapy. Donor-variation was observed in NK-cell proteomes of HCV-infected patients, with 46 dysregulated proteins restored after DAA therapy. However, 30% of the CD56+ NK-cell proteome remained altered 1 year post-therapy, indicating a phenotypic shift with low donor-variation. NK cells from virus-negative cured patients exhibited global regulation of RNA-processing and pathways related to "stimuli response", "chemokine signaling", and "cytotoxicity regulation". Proteomics identified downregulation of vesicle transport components (CD107a, COPI/II complexes) and altered receptor expression profiles, indicating an inhibited NK-cell phenotype. Yet, activated NK cells from HCV patients before and after therapy effectively upregulated IFN-γ and recruited CD107a. Conversely, reduced surface expression levels of Tim-3 and 2B4 were observed before and after therapy. In conclusion, this study reveals long-term effects on the CD56+ NK-cell compartment in convalescent HCV patients 1 year after therapy, with limited abundance of vesicle transport complexes and surface receptors, associated with a responsive NK-cell phenotype.

Rap1 prevents colitogenic Th17 cell expansion and facilitates Treg cell differentiation and distal TCR signaling

T-cell-specific Rap1 deletion causes spontaneous colitis in mice. In the present study, we revealed that Rap1 deficiency in T cells impaired the preceding induction of intestinal RORγt+ Treg cells. In the large intestinal lamina propria (LILP) of T-cell-specific Rap1-knockout mice (Rap1KO mice), Th17 cells were found to increase in a microbiota-dependent manner, and the inhibition of IL-17A production prevented the development of colitis. In the LILP of Rap1KO mice, RORγt+ Treg cells were scarcely induced by 4 weeks of age. The expression of CTLA-4 on Rap1-deficient Treg cells was reduced and the expression of CD80 and CD86 on dendritic cells was consequently elevated in Rap1KO mice. When cultured under each polarizing condition, Rap1-deficient naïve CD4+ T cells did not show biased differentiation into Th17 cells; their differentiation into Treg cells as well as Th1 and Th2 cells was lesser than that of wild-type cells. Rap1-deficient naïve CD4+ T cells were found to exhibit the defective nuclear translocation of NFAT and formation of actin foci in response to TCR engagement. These data suggest that Rap1 amplifies the TCR signaling required for Treg-mediated control of intestinal colitogenic Th17 responses.

Structural, biochemical, and functional properties of the Rap1-Interacting Adaptor Molecule (RIAM)

Leukocytes, the leading players of immune system, are involved in innate and adaptive immune responses. Leukocyte adhesion to endothelial cells during transmigration or to antigen presenting cells during T cell activation, requires integrin activation through a process termed inside-out integrin signaling. In hematopoietic cells, Rap1 and its downstream effector RIAM (Rap1-interacting adaptor molecule) form a cornerstone for inside-out integrin activation. The Rap1/RIAM pathway is involved in signal integration for activation, actin remodeling and cytoskeletal reorganization in T cells, as well as in myeloid cell differentiation and function. RIAM is instrumental for phagocytosis, a process requiring particle recognition, cytoskeletal remodeling and membrane protrusion for engulfment and digestion. In the present review, we discuss the structural and molecular properties of RIAM and the recent discoveries regarding the functional role of the Rap1/RIAM module in hematopoietic cells.

Pregnancy Induces an Immunological Memory Characterized by Maternal Immune Alterations Through Specific Genes Methylation

During pregnancy, the maternal immune system undergoes major adaptive modifications that are necessary for the acceptance and protection of the fetus. It has been postulated that these modifications are temporary and limited to the time of pregnancy. Growing evidence suggests that pregnancy has a long-term impact on maternal health, especially among women with pregnancy complications, such as preeclampsia (PE). In addition, the presence of multiple immunological-associated changes in women that remain long after delivery has been reported. To explain these long-term modifications, we hypothesized that pregnancy induces long-term immunological memory with effects on maternal well-being. To test this hypothesis, we evaluated the immunological phenotype of circulating immune cells in women at least 1 year after a normal pregnancy and after pregnancy complicated by PE. Using multiparameter flow cytometry (FCM) and whole-genome bisulfite sequencing (WGBS), we demonstrate that pregnancy has a long-term effect on the maternal immune cell populations and that this effect differs between normal pregnancy and pregnancy complicated by PE; furthermore, these modifications are due to changes in the maternal methylation status of genes that are associated with T cell and NK cell differentiation and function. We propose the existence of an "immunological memory of pregnancy (IMOP)" as an evolutionary advantage for the success of future pregnancies and the proper adaptation to the microchimeric status established during pregnancy. Our findings demonstrate that the type of immune cell populations modified during pregnancy may have an impact on subsequent pregnancy and future maternal health.

MST1/2 Balance Immune Activation and Tolerance by Orchestrating Adhesion, Transcription, and Organelle Dynamics in Lymphocytes

The STE20-like serine/threonine kinases MST1 and MST2 (MST1/2) are mammalian homologs of Hippo in flies. MST1/2 regulate organ size by suppressing the transcription factor YAP, which promotes proliferation. MST1 is predominantly expressed in immune cells, where it plays distinct roles. Here, we review the functions of MST1/2 in immune cells, uncovered by a series of recent studies, and discuss the connection between MST1/2 function and immune responses. MST1/2 regulate lymphocyte development, trafficking, survival, and antigen recognition by naive T cells. MST1/2 also regulate the function of regulatory T cells and effector T cell differentiation, thus acting to balance immune activation and tolerance. Interestingly, MST1/2 elicit these functions not by the “canonical” Hippo pathway, but by the non-canonical Hippo pathway or alternative pathways. In these pathways, MST1/2 regulates cellular processes relating to immune response, such as chemotaxis, cell adhesion, immunological synapse, gene transcriptions. Recent advances in our understanding of the molecular mechanisms of these processes have revealed important roles of MST1/2 in regulating cytoskeleton remodeling, integrin activation, and vesicular transport in lymphocytes. We discuss the significance of the MST1/2 signaling in lymphocytes in the regulation of organelle dynamics.

Yersinia Pseudotuberculosis Modulates Regulatory T Cell Stability via Injection of Yersinia Outer Proteins in a Type III Secretion System-Dependent Manner

Adaptive immunity is essentially required to control acute infection with enteropathogenic Yersinia pseudotuberculosis (Yptb). We have recently demonstrated that Yptb can directly modulate naïve CD4⁺ T cell differentiation. However, whether fully differentiated forkhead box protein P3 (Foxp3⁺) regulatory T cells (Tregs), fundamental key players to maintain immune homeostasis, are targeted by Yptb remains elusive. Here, we demonstrate that within the CD4⁺ T cell compartment Yptb preferentially targets Tregs and injects Yersinia outer proteins (Yops) in a process that depends on the type III secretion system and invasins. Remarkably, Yop-translocation into ex vivo isolated Foxp3⁺ Tregs resulted in a substantial downregulation of Foxp3 expression and a decreased capacity to express the immunosuppressive cytokine interleukin-10 (IL-10). Together, these findings highlight that invasins are critically required to mediate Yptb attachment to Foxp3⁺ Tregs, which allows efficient Yop-translocation and finally enables the modulation of the Foxp3⁺ Tregs' suppressive phenotype.

Not Just an Adhesion Molecule: LFA-1 Contact Tunes the T Lymphocyte Program

The α_Lβ₂ integrin LFA-1 is known to play a key role in T lymphocyte migration, which is necessary to mount a local immune response, and is also the main driver of autoimmune diseases. This migration-triggering signaling process in T cells is tightly regulated to permit an immune response that is appropriate to the local trigger, as well as to prevent deleterious tissue-damaging bystander effects. Emerging evidence shows that, in addition to prompting a diverse range of downstream signaling cascades, LFA-1 stimulation in T lymphocytes modulates gene-transcription programs, including genetic signatures of TGF-β and Notch pathways, with multifactorial biological outcomes. This review highlights recent findings and discusses molecular mechanisms by which LFA-1 signaling influence T lymphocyte differentiation into the effector subsets Th1, Th17, and induced regulatory T cells. We argue that LFA-1 contact with a cognate ligand, such as ICAM-1, independent of the immune synapse activates a late divergence in T cells' effector phenotypes, hence fine-tuning their functioning.

The PD1:PD-L1/2 Pathway from Discovery to Clinical Implementation

The immune system maintains a critically organized network to defend against foreign particles, while evading self-reactivity simultaneously. T lymphocytes function as effectors and play an important regulatory role to orchestrate the immune signals. Although central tolerance mechanism results in the removal of the most of the autoreactive T cells during thymic selection, a fraction of self-reactive lymphocytes escapes to the periphery and pose a threat to cause autoimmunity. The immune system evolved various mechanisms to constrain such autoreactive T cells and maintain peripheral tolerance, including T cell anergy, deletion, and suppression by regulatory T cells (TRegs). These effects are regulated by a complex network of stimulatory and inhibitory receptors expressed on T cells and their ligands, which deliver cell-to-cell signals that dictate the outcome of T cell encountering with cognate antigens. Among the inhibitory immune mediators, the pathway consisting of the programed cell death 1 (PD-1) receptor (CD279) and its ligands PD-L1 (B7-H1, CD274) and PD-L2 (B7-DC, CD273) plays an important role in the induction and maintenance of peripheral tolerance and for the maintenance of the stability and the integrity of T cells. However, the PD-1:PD-L1/L2 pathway also mediates potent inhibitory signals to hinder the proliferation and function of T effector cells and have inimical effects on antiviral and antitumor immunity. Therapeutic targeting of this pathway has resulted in successful enhancement of T cell immunity against viral pathogens and tumors. Here, we will provide a brief overview on the properties of the components of the PD-1 pathway, the signaling events regulated by PD-1 engagement, and their consequences on the function of T effector cells.

Regulatory T-Cell-Mediated Suppression of Conventional T-Cells and Dendritic Cells by Different cAMP Intracellular Pathways

Regulatory T-cells (Tregs) mediate their suppressive action by acting directly on conventional T-cells (Tcons) or dendritic cells (DCs). One mechanism of Treg suppression is the increase of cyclic adenosine 3′,5′-monophosphate (cAMP) levels in target cells. Tregs utilize cAMP to control Tcon responses, such as proliferation and cytokine production. Tregs also exert their suppression on DCs, diminishing DC immunogenicity by downmodulating the expression of costimulatory molecules and actin polymerization at the immunological synapse. The Treg-mediated usage of cAMP occurs through two major mechanisms. The first involves the Treg-mediated influx of cAMP in target cells through gap junctions. The second is the conversion of adenosine triphosphate into adenosine by the ectonucleases CD39 and CD73 present on the surface of Tregs. Adenosine then binds to receptors on the surface of target cells, leading to increased intracellular cAMP levels in these targets. Downstream, cAMP can activate the canonical protein kinase A (PKA) pathway and the exchange protein activated by cyclic AMP (EPAC) non-canonical pathway. In this review, we discuss the most recent findings related to cAMP activation of PKA and EPAC, which are implicated in Treg homeostasis as well as the functional alterations induced by cAMP in cellular targets of Treg suppression.

Sema3e/Plexin D1 Modulates Immunological Synapse and Migration of Thymocytes by Rap1 Inhibition

Regulation of thymocyte trafficking plays an important role during thymic selection, but our understanding of the molecular mechanisms underlying these processes is limited. In this study, we demonstrated that class III semaphorin E (sema3e), a guidance molecule during neural and vascular development, directly inhibited Rap1 activation and LFA-1-dependent adhesion through the GTPase-activating protein activity of plexin D1. Sema3e inhibited Rap1 activation of thymocytes in response to chemokines and TCR stimulation, LFA-mediated adhesion, and T cell-APC interactions. Immunological synapse (IS) formation in mature thymocytes on supported lipid bilayers was also attenuated by sema3e. Impaired IS formation was associated with reduced Rap1 activation on the contact surface and cell periphery. Moreover, a significant increase of CD4(+) thymocytes was detected in the medulla of mice with T cell lineage-specific deletion of plexin D1. Two-photon live imaging of thymic explants and slices revealed enhanced Rap1 activation and migration of CD69(+) double-positive and single-positive cells with plexin D1 deficiency. Our results demonstrate that sema3e/plexin D1 modulates IS formation and Ag-scanning activities of thymocytes within thymic tissues.

Biochemical signaling of PD-1 on T cells and its functional implications

Maintenance of peripheral tolerance is essential for homeostasis of the immune system. While central tolerance mechanisms result in deletion of the majority of self-reactive T cells, T lymphocytes specific for self-antigens also escape this process and circulate in the periphery. To control the development of autoimmunity, multiple mechanisms of peripheral tolerance have evolved, including T cell anergy, deletion, and suppression by regulatory T (Treg) cells. The pathway consisting of the programmed cell death 1 (PD-1) receptor (CD279) and its ligands PD-L1 (B7-H1, CD274) and PD-L2 (B7-DC; CD273) plays a vital role in the induction and maintenance of peripheral tolerance. This pathway also regulates the balance between stimulatory and inhibitory signals needed for effective immunity and maintenance of T cell homeostasis. In contrast to this important beneficial role in maintaining T cell homeostasis, PD-1 mediates potent inhibitory signals that prevent the expansion and function of T effector cells and have detrimental effects on antiviral and antitumor immunity. Despite the compelling studies on the significant functional role of PD-1 in mediating inhibition of activated T cells, little is known about how PD-1 blocks T cell activation. Here, we will provide a brief overview of the signaling events that are regulated by PD-1 triggering, and we will discuss their implications on cell intrinsic and extrinsic mechanisms that determine the fate and function of T effector cells.

A novel Zap70 mutation with reduced protein stability demonstrates the rate-limiting threshold for Zap70 in T-cell receptor signalling

Loss of ζ-associated protein 70 (Zap70) results in severe immunodeficiency in humans and mice because of the critical role of Zap70 in T-cell receptor (TCR) signalling. Here we describe a novel mouse strain generated by N-ethyl-N-nitrosourea mutagenesis, with the reduced protein stability (rps) mutation in Zap70. The A243V rps mutation resulted in decreased Zap70 protein and a reduced duration of TCR-induced calcium responses, equivalent to that induced by a 50% decrease in catalytically active Zap70. The reduction of signalling through Zap70 was insufficient to substantially perturb thymic differentiation of conventional CD4 and CD8 T cells, although Foxp3(+) regulatory T cells demonstrated altered thymic production and peripheral homeostasis. Despite the mild phenotype, the Zap70(A243V) variant lies just above the functional threshold for TCR signalling competence, as T cells relying on only a single copy of the Zap70(rps) allele for TCR signalling demonstrated no intracellular calcium response to TCR stimulation. This addition to the Zap70 allelic series indicates that a rate-limiting threshold for Zap70 protein levels exists at which signalling capacity switches from nearly intact to effectively null.

The dynamic of the apical ectoplasmic specialization between spermatids and Sertoli cells: the case of the small GTPase Rap1

Despite advances in assisted reproductive technologies, infertility remains a consistent health problem worldwide. Spermiation is the process through which mature spermatids detach from the supporting Sertoli cells and are released into the tubule lumen. Spermiation failure leads to lack of mature spermatozoa and, if not occasional, could result into azoospermia, major cause of male infertility in human population. Spermatids are led through their differentiation into spermatozoa by the apical ectoplasmic specialization (aES), a testis-specific, actin-based anchoring junction restricted to the Sertoli-spermatid interface. The aES helps spermatid movement across the seminiferous epithelium, promotes spermatid positioning, and prevents the release of immature spermatozoa. To accomplish its functions, aES needs to undergo tightly and timely regulated restructuring. Even if components of aES are partly known, the mechanism/s through which aES is regulated remains still elusive. In this review, we propose a model by which the small GTPase Rap1 could regulate aES assembly/remodelling. The characterization of key players in the dynamic of aES, such as Rap1, could open new possibility to develop prognostic, diagnostic, and therapeutic approaches for male patients under treatment for infertility as well as it could lead to the identification of new target for male contraception.

Signal transduction pathways in chronic inflammatory autoimmune disease: small GTPases

Ras superfamily small GTPases represent a wide and diverse class of intracellular signaling proteins that are highly conserved during evolution. These enzymes serve as key checkpoints in coupling antigen receptor, growth factor, cytokine and chemokine stimulation to cellular responses. Once activated, via their ability to regulate multiple downstream signaling pathways, small GTPases amplify and diversify signaling cascades which regulate cellular proliferation, survival, cytokine expression, trafficking and retention. Small GTPases, particularly members of the Ras, Rap, and Rho family, critically coordinate the function and interplay of immune and stromal cells during inflammatory respones, and increasing evidence indicates that alterations in small GTPase signaling contribute to the pathological behavior of these cell populations in human chronic inflammatory diseases such as rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE). Here, we review how Ras, Rap, and Rho family GTPases contribute to the biology of cell populations relevant to human chronic inflammatory disease, highlight recent advances in understanding how alterations in these pathways contribute to pathology in RA and SLE, and discuss new therapeutic strategies that may allow specific targeting of small GTPases in the clinic.

Integrin signalling and function in immune cells

Integrins not only mediate cell-cell and cell-extracellular matrix adhesion, but also affect the multitude of signal transduction cascades in control of cell survival, proliferation, differentiation and organ development. Mutations in integrins or the major effectors of integrin signalling pathways cause defective organ development, immunodeficiency, cancer or autoimmune disease. Understanding of the signalling events that drive integrin activation and signalling is therefore crucial to uncover the molecular mechanisms of these diseases. This review discusses the key signalling complexes regulating integrin activation and function in both 'inside-out' and 'outside-in' pathways in T lymphocytes, including kinases, SLP-76, VAV1, ADAP, SKAP-55, RapL, RIAM, Rap1, Talin and Kindlin.