Ana Urzainqui

ITAM-Based Interaction of ERM Proteins with Syk Mediates Signaling by the Leukocyte Adhesion Receptor PSGL-1

P-selectin glycoprotein ligand 1 (PSGL-1) is a leukocyte adhesion molecule involved in cell tether and rolling on activated endothelium. Our work shows that PSGL-1 associates with Syk. This association is mediated by the actin-linking proteins moesin and ezrin, which directly interact with Syk in an ITAM-dependent manner. PSGL-1 engagement induces tyrosine phosphorylation of Syk and SRE-dependent transcriptional activity. Treatment of cells with the Syk inhibitor piceatannol and overexpression of either a Syk dead kinase mutant or an ITAM-mutated moesin abrogated PSGL-1-induced transcriptional activation. These data unveil a new functional role for the ERMs (ezrin/radixin/moesin) as adaptor molecules in the interactions of adhesion receptors and intracellular tyrosine kinases and show that PSGL-1 is a signaling molecule in leukocytes.

HDAC6 Deacetylase Activity Links the Tubulin Cytoskeleton with Immune Synapse Organization

We investigated the role of acetylated microtubules in the antigen-specific interaction of T helper and antigen-presenting cells (APCs). In T cells, acetylated microtubules concentrated at contact site with APCs, surrounding clusters of CD3 and LFA-1. TcR engagement induced a transient deacetylation of microtubules at early times and an enhanced acetylation at late times. Confocal videomicroscopy studies revealed that the HDAC6 tubulin deacetylase was translocated and concentrated at the contact site of T cells with APCs. Overexpression of HDAC6 but not a dead deacetylase mutant in T cells disorganized CD3 and LFA-1 at the immune synapse. This effect was reverted by treatment with the deacetylase inhibitor trichostatin A. The antigen-specific translocation of the microtubule organizing center (MTOC) and IL-2 production were also severely impaired by overexpression of HDAC6. Our results underscore the key role for HDAC6 in the organization of the immune synapse and the antigen-specific reorientation of the MTOC.

A juxta-membrane amino acid sequence of P-selectin glycoprotein ligand-1 is involved in moesin binding and ezrin/radixin/moesin-directed targeting at the trailing edge of migrating lymphocytes

P‐selectin glycoprotein ligand 1 (PSGL‐1) is an adhesion receptor localized on the tips of microvilli that is involved in the rolling of neutrophils on activated endothelium. We found that PSGL‐1 was concentrated at the uropod of chemokine‐stimulated lymphoid cells. Dynamic fluorescence videomicroscopy analyses of migrating lymphocytes demonstrated that PSGL‐1 and moesin redistributed towards the cellular uropod at the trailing edge of these cells, where activated ezrin/radixin/moesin (ERM) proteins were located. An eighteen amino acid sequence in the juxta‐membrane region of the PSGL‐1 cytoplasmic tail was found to be critical for uropod targeting and moesin binding. Substitution of S336, S348, and the basic cluster R337K338 by alanines within this region significantly impaired both moesin binding and PSGL‐1 polarization. These results underline the role of moesin in the subcellular redistribution of PSGL‐1 in lymphoid cells and make evident the importance of specific serine residues within the cytoplasmic tail of PSGL‐1 for this process.

Myosin IIA is involved in the endocytosis of CXCR4 induced by SDF-1α

Endocytosis of chemokine receptors regulates signal transduction initiated by chemokines, but the molecular mechanisms underlying this process are not fully defined. In this work, we assessed the involvement of the motor protein nonmuscle myosin heavy chain IIA (MIIA) in the endocytosis of CXCR4 induced by SDF-1α (also known as CXCL12) in T lymphocytes. Overexpression of the C-terminal half of MIIA inhibited the ligand-induced endocytosis of CXCR4, but not that of transferrin receptor. Targeting MIIA either by silencing its expression with small interfering RNA (siRNA) or by blebbistatin treatment also inhibited endocytosis of CXCR4. Inhibition of endocytosis of CXCR4 by targeting endogenous MIIA resulted in an increased migration of T cells induced by SDF-1α, and in the inhibition of the HIV-1-Env antifusogenic activity of this chemokine. Coimmunoprecipitation and protein-protein binding studies demonstrated that MIIA interacts with both the cytoplasmic tail of CXCR4 and β-arrestin. Moreover, SDF-1α promotes a rapid MIIA-β-arrestin dissociation. Our data reveal a novel role for MIIA in CXCR4 endocytosis, which involves its dynamic association with β-arrestin and highlights the role of endogenous MIIA as a regulator of CXCR4 internalization and, therefore, the onset of SDF-1α signaling.

Cutting Edge: Association of the Motor Protein Nonmuscle Myosin Heavy Chain-IIA with the C Terminus of the Chemokine Receptor CXCR4 in T Lymphocytes

The binding of chemokines to their receptors guides lymphocyte migration. However, the precise mechanism that links the chemotactic signals with the energy and traction force generated by the actomyosin complex of the cell has not been elucidated. Using biochemical approaches and mass spectrometry analysis, we found an association between the C-termini of CXCR4 and CCR5 and the motor protein nonmuscle myosin H chain-IIA. Immunoprecipitation experiments revealed that this association also occurs between the endogenous molecules in T lymphocytes. As expected, myosin L chain was also associated with CXCR4. Confocal microscopy analysis showed that CXCR4 and motor protein nonmuscle myosin H chain-IIA colocalize at the leading edge of migrating T lymphocytes, together with filamentous actin and myosin L chain. These results provide the first evidence of a biochemical association between chemokine receptors and motor proteins, a mechanosignaling mechanism that may have a key role in lymphocyte migration.

Functional Role of P-Selectin Glycoprotein Ligand 1/P-Selectin Interaction in the Generation of Tolerogenic Dendritic Cells

Dendritic cells (DCs) have a key role in both the generation of the immune response and the induction of tolerance to self-Ags. In this work, the possible role of P-selectin glycoprotein ligand 1 (PSGL-1) on the tolerogenic activity of human DCs was explored. We found that the engagement of PSGL-1 by P-selectin on DCs induced the expression of c-Fos, IDO, IL-10, and TGF-β genes. Remarkably, stimulation of DCs through PSGL-1 with P-selectin enhanced their capability to generate CD4+CD25+Foxp3+ regulatory T cells, which expressed high levels of TGF-β1 mRNA, synthesized IL-10, and suppressed the proliferation of autologous CD4+CD25− T cells. Accordingly, we found that DCs from PSGL-1−/− mice expressed higher levels of MHC class II molecules, and exhibited an enhanced immunogenicity compared with wild-type mice. In addition, the percentage of CD4+CD25+Foxp3+ regulatory T cells in the thymus of PSGL-1-deficient animals was significantly reduced. Our data reveal an unexpected role of PSGL-1 on the tolerogenic function of DCs, and the regulation of the immune response.

Pleiotropic Effects of Cell Wall Amidase LytA on Streptococcus pneumoniae Sensitivity to the Host Immune Response

ABSTRACT The complement system is a key component of the host immune response for the recognition and clearance of Streptococcus pneumoniae. In this study, we demonstrate that the amidase LytA, the main pneumococcal autolysin, inhibits complement-mediated immunity independently of effects on pneumolysin by a complex process of impaired complement activation, increased binding of complement regulators, and direct degradation of complement C3. The use of human sera depleted of either C1q or factor B confirmed that LytA prevented activation of both the classical and alternative pathways, whereas pneumolysin inhibited only the classical pathway. LytA prevented binding of C1q and the acute-phase protein C-reactive protein to S. pneumoniae, thereby reducing activation of the classical pathway on the bacterial surface. In addition, LytA increased recruitment of the complement downregulators C4BP and factor H to the pneumococcal cell wall and directly cleaved C3b and iC3b to generate degradation products. As a consequence, C3b deposition and phagocytosis increased in the absence of LytA and were markedly enhanced for the lytA ply double mutant, confirming that a combination of LytA and Ply is essential for the establishment of pneumococcal pneumonia and sepsis in a murine model of infection. These data demonstrate that LytA has pleiotropic effects on complement activation, a finding which, in combination with the effects of pneumolysin on complement to assist with pneumococcal complement evasion, confirms a major role of both proteins for the full virulence of the microorganism during septicemia.

The metalloprotease ADAM8 is associated with and regulates the function of the adhesion receptor PSGL-1 through ERM proteins

The P‐selectin glycoprotein ligand‐1 (PSGL‐1) is involved in the initial contact of leukocytes with activated endothelium, and its adhesive function is regulated through its proteolytic processing. We have found that the metalloprotease ADAM8 is both associated with PSGL‐1 through the ezrin–radixin–moesin actin‐binding proteins and able to cause the proteolytic cleavage of this adhesion receptor. Accordingly, ADAM8 knockdown increases PSGL‐1 expression, and functional assays show that ADAM8 is able to reduce leukocyte rolling on P‐selectin and hence on activated endothelial cells. We conclude that ADAM8 modulates the expression and function of PSGL‐1.

Induction of Th17 Lymphocytes and Treg Cells by Monocyte-Derived Dendritic Cells in Patients with Rheumatoid Arthritis and Systemic Lupus Erythematosus

Dendritic cells (DCs) have a key role in the regulation of immune response. We herein explored, in patients with inflammatory diseases, the role of monocyte derived DCs (mo-DCs) on the generation of Th17 and T regulatory (Treg) lymphocytes. Peripheral blood was obtained from thirty-five patients with rheumatoid arthritis (RA), twelve with systemic lupus erythematosus (SLE), and twenty healthy subjects. Mo-DCs were generated under standard (IL-4/GM-CSF) or tolerogenic (IL-4/GM-CSF plus recombinant P-selectin or PD-1 or IL-10) conditions, and their ability to induce Th17 and Treg lymphocytes was tested. We detected that mo-DCs from patients with RA showed an enhanced release of IL-6 and IL-23 as well as an increased capability to induce Th17 cells. Although mo-DCs from SLE patients also released high levels of IL-6/IL-23, it did not show an increased ability to induce Th17 lymphocytes. In addition, mo-DCs, from patients with RA and SLE generated under the engagement of PSGL-1, showed a defective capability to induce Foxp3+ Treg cells. A similar phenomenon was observed in SLE, when DCs cells were generated under PDL-1 engagement. Our data indicate that DCs from patients with rheumatic inflammatory disease show an aberrant function that may have an important role in the pathogenesis of these conditions.

P-selectin glycoprotein ligand-1 modulates immune inflammatory responses in the enteric lamina propria.

P‐selectin glycoprotein ligand‐1 (PSGL‐1), a leukocyte adhesion receptor that interacts with selectins, induces a tolerogenic programme in bone marrow‐derived dendritic cells (DCs), which in turn promotes the generation of T regulatory (Treg) lymphocytes. In the present study, we have used a mouse model of dextran sulphate sodium (DSS)‐induced colitis and studied the characteristics of the inflammatory cell infiltrate in the lamina propria (LP), mesenteric lymph nodes (mLNs) and Peyers patches (PPs) to assess the possible role of PSGL‐1 in the modulation of the enteric immune response. We have found that untreated PSGL‐1‐deficient mice showed an altered proportion of innate and adaptive immune cells in mLNs and PPs as well as an activated phenotype of macrophages and DCs in the colonic LP that mainly produced pro‐inflammatory cytokines. Administration of an anti‐PSGL‐1 antibody also reduced the total numbers of macrophages, DCs and B cells in the colonic LP, and induced a lower expression of MHC‐II by DCs and macrophages. After DSS treatment, PSGL‐1−/− mice developed colitis earlier and with higher severity than wild‐type (WT) mice. Accordingly, the colonic LP of these animals showed an enhanced number of Th1 and Th17 lymphocytes, with enhanced synthesis of IL‐1α, IL‐6 and IL‐22, and increased activation of LP macrophages. Together, our data indicate that PSGL‐1 has a relevant homeostatic role in the gut‐associated lymphoid tissue under steady‐state conditions, and that this adhesion receptor is able to down‐regulate the inflammatory phenomenon in DSS‐induced colitis. Copyright