Inside-out regulation of Fc alpha RI (CD89) depends on PP2A.

To achieve a correct cellular immune response toward pathogens, interaction between FcR and their ligands must be regulated. The Fc receptor for IgA, FcalphaRI, is pivotal for the inflammatory responses against IgA-opsonized pathogens. Cytokine-induced inside-out signaling through the intracellular FcalphaRI tail is important for FcalphaRI-IgA binding. However, the underlying molecular mechanism governing this process is not well understood. In this study, we report that PP2A can act as a molecular switch in FcalphaRI activation. PP2A binds to the intracellular tail of FcalphaRI and, upon cytokine stimulation, PP2A becomes activated. Subsequently, FcalphaRI is dephosphorylated on intracellular Serine 263, which we could link to receptor activation. PP2A inhibition, in contrast, decreased FcalphaRI ligand binding capacity in transfected cells but also in eosinophils and monocytes. Interestingly, PP2A activity was found crucial for IgA-mediated binding and phagocytosis of Neisseria meningitidis. The present findings demonstrate PP2A involvement as a molecular mechanism for FcalphaRI ligand binding regulation, a key step in initiating an immune response.

Signal regulatory protein alpha ligation induces macrophage nitric oxide production through JAK/STAT- and phosphatidylinositol 3-kinase/Rac1/NAPDH oxidase/H2O2-dependent pathways.

Signal regulatory protein alpha (SIRPalpha) is a glycoprotein receptor that recruits and signals via the tyrosine phosphatases SHP-1 and SHP-2. In macrophages SIRPalpha can negatively regulate the phagocytosis of host cells and the production of tumor necrosis factor alpha. Here we provide evidence that SIRPalpha can also stimulate macrophage activities, in particular the production of nitric oxide (NO) and reactive oxygen species. Ligation of SIRPalpha by antibodies or soluble CD47 triggers inducible nitric oxide synthase expression and production of NO. This was not caused by blocking negative-regulatory SIRPalpha-CD47 interactions. SIRPalpha-induced NO production was prevented by inhibition of the tyrosine kinase JAK2. JAK2 was found to associate with SIRPalpha in macrophages, particularly after SIRPalpha ligation, and SIRPalpha stimulation resulted in JAK2 and STAT1 tyrosine phosphorylation. Furthermore, SIRPalpha-induced NO production required the generation of hydrogen peroxide (H(2)O(2)) by a NADPH oxidase (NOX) and the phosphatidylinositol 3-kinase (PI3-K)-dependent activation of Rac1, an intrinsic NOX component. Finally, SIRPalpha ligation promoted SHP-1 and SHP-2 recruitment, which was both JAK2 and PI3-K dependent. These findings demonstrate that SIRPalpha ligation induces macrophage NO production through the cooperative action of JAK/STAT and PI3-K/Rac1/NOX/H(2)O(2) signaling pathways. Therefore, we propose that SIRPalpha is able to function as an activating receptor.

FcαRI Dynamics Are Regulated by GSK-3 and PKCζ During Cytokine Mediated Inside-Out Signaling.

IgA binding to FcαRI (CD89) is rapidly enhanced by cytokine induced inside-out signaling. Dephosphorylation of serine 263 in the intracellular tail of FcαRI by PP2A and PI3K activation are instrumental in this process. To further investigate these signaling pathways, we targeted downstream kinases of PI3K. Our experiments revealed that PI3K activates PKCζ, which subsequently inhibits GSK-3, a constitutively active kinase in resting cells and found here to be associated with FcαRI. We propose that GSK-3 maintains FcαRI in an inactive state at homeostatic conditions. Upon cytokine stimulation, GSK-3 is inactivated through a PI3K-PKCζ pathway, preventing the maintenance of phosphorylated inactive FcαRI. The concomitantly activated PP2A is then able to dephosphorylate and activate FcαRI. Moreover, FRAP and FLIP studies showed that FcαRI activation coincides with an increased mobile fraction of the receptor. This can enhance FcαRI valency and contribute to stronger avidity for IgA immune complexes. This tightly regulated inside-out signaling pathway allows leukocytes to respond rapidly and efficiently to their environment and could be exploited to enhance the efficacy of future IgA therapeutics.

RhoA activation promotes transendothelial migration of monocytes via ROCK.

Monocyte infiltration into inflamed tissue requires the initial arrest of the cells on the endothelium followed by firm adhesion and their subsequent migration. Migration of monocytes and other leukocytes is believed to involve a coordinated remodeling of the actin cytoskeleton. The small GTPases RhoA, Rac1, and Cdc42 are critical regulators of actin reorganization. In this study, we have investigated the role of Rho-like GTPases RhoA, Rac1, and Cdc42 in the adhesion and migration of monocytes across brain endothelial cells by expressing their constitutively active or dominant-negative constructs in NR8383 rat monocytic cells. Monocytes expressing the active form of Cdc42 show a reduced migration, whereas Rac1 expression did not affect adhesion or migration. In contrast, expression of the active form of RhoA in monocytes leads to a dramatic increase in their adhesion and migration across endothelial cells. The effect of RhoA was found to be mediated by its down-stream effector Rho kinase (ROCK), as pretreatment with the selective ROCK inhibitor Y-27632 prevented this enhanced adhesion and migration. These results demonstrate that RhoA activation in monocytes is sufficient to enhance adhesion and migration across monolayers of endothelial cells.

Integrins on neutrophils are dispensable for migration into three-dimensional fibrin gels.

The innate immune system and the blood haemostasis system function cooperatively in many pathological conditions such as acute respiratory distress syndrome, deep venous thrombosis, ischaemia/reperfusion injury and cardiovascular disease. Infiltration of neutrophils into thrombotic substrates such as fibrin clots supports fibrinolysis, tissue damage and inflammation. Despite the importance of integrins in neutrophil attachment to fibrin-coated surfaces under flow conditions, little is known about their role in migration processes in shear free two-dimensional (2D) and three-dimensional (3D) fibrin(ogen) environments. Therefore, the present study was designed to study the role of functional integrins in mediating neutrophil migration on and in fibrin matrices. Time lapse video sequences of neutrophil chemokinesis and chemotaxis were made under conditions of active- or non-active integrins. Interestingly, migration of neutrophils on 2D fibrinogen coated surfaces and 3D fibrin matrices is independent of integrins as the response is not sensitive to alphaM-(CD11b) and beta2-(CD18) blocking antibodies and/or chelation of Ca2+ and Mg2+ by EDTA in bivalent ion-free buffers. The blocking integrin antibodies were shown to be functionally active in regular adhesion assays. Our study shows that integrins are dispensable for migration on 2D and in 3D fibrin matrices, both when neutrophils enter into the fibrin matrix and when captured in the matrix.

Characterization of the role of CaMKI-like kinase (CKLiK) in human granulocyte function.

Activation of granulocyte effector functions, such as induction of the respiratory burst and migration, are regulated by a variety of relatively ill-defined signaling pathways. Recently, we identified a novel Ca2+/calmodulin-dependent kinase I-like kinase, CKLiK, which exhibits restricted mRNA expression to human granulocytes. Using a novel antibody generated against the C-terminus of CKLiK, CKLiK was detected in CD34+-derived neutrophils and eosinophils, as well as in mature peripheral blood granulocytes. Activation of human granulocytes by N-formyl-methionyl-leucyl-phenylalanine (fMLP) and platelet-activating factor (PAF), but not the phorbol ester PMA (phorbol 12-myristate-13-acetate), resulted in induction of CKLiK activity, in parallel with a rise of intracellular Ca2+ [Ca2+]i. To study the functionality of CKLiK in human granulocytes, a cell-permeable CKLiK peptide inhibitor (CKLiK297-321) was generated which was able to inhibit kinase activity in a dose-dependent manner. The effect of this peptide was studied on specific granulocyte effector functions such as phagocytosis, respiratory burst, migration, and adhesion. Phagocytosis of Aspergillus fumigatus particles was reduced in the presence of CKLiK297-321 and fMLP-induced reactive oxygen species (ROS) production was potently inhibited by CKLiK297-321 in a dose-dependent manner. Furthermore, fMLP-induced neutrophil migration on albumin-coated surfaces was perturbed, as well as beta2-integrin-mediated adhesion. These findings suggest a critical role for CKLiK in modulating chemoattractant-induced functional responses in human granulocytes.

Signal-regulatory protein alpha-CD47 interactions are required for the transmigration of monocytes across cerebral endothelium.

Monocyte infiltration into inflamed tissue requires their initial arrest onto the endothelial cells (ECs), followed by firm adhesion and subsequent transmigration. Although several pairs of adhesion molecules have been shown to play a role in the initial adhesion of monocytes to ECs, the mechanism of transendothelial migration is poorly defined. In this study, we have investigated the role of signal-regulatory protein (SIRP)alpha-CD47 interactions in monocyte transmigration across brain ECs. CD47 expression was observed in vivo on cerebral endothelium of both control animals and animals suffering from experimental allergic encephalomyelitis. To investigate whether SIRPalpha-CD47 interactions are instrumental in the trafficking of monocytes across cerebral EC monolayers, in vitro assays were conducted in which the migration of monocytes, but not adhesion, was found to be effectively diminished by blocking SIRPalpha and CD47 on monocytes and ECs, respectively. In this process, SIRPalpha was found to interact solely with its counterligand CD47 on ECs. Overexpression of the CD47 molecule on brain ECs significantly enhanced monocytic transmigration, but did not affect adhesion. SIRPalpha-CD47-mediated transendothelial migration involved Gi protein activity, a known signaling component of CD47. Finally, cross-linking of CD47 on brain ECs induced cytoskeletal reorganization of the endothelium, a process that was Gi protein independent. These data provide the first evidence that the interaction of CD47 with its monocytic counterligand SIRPalpha is of importance in the final step of monocyte trafficking into the brain, a critical event in the development of neuroinflammatory diseases.

LacdiNAc-glycans constitute a parasite pattern for galectin-3-mediated immune recognition.

Although Gal beta 1-4GlcNAc (LacNAc) moieties are the most common constituents of N-linked glycans on vertebrate proteins, GalNAc beta 1-4GlcNAc (LacdiNAc, LDN)-containing glycans are widespread in invertebrates, such as helminths. We postulated that LDN might be a molecular pattern for recognition of helminth parasites by the immune system. Using LDN-based affinity chromatography and mass spectrometry, we have identified galectin-3 as the major LDN-binding protein in macrophages. By contrast, LDN binding was not observed with galectin-1. Surface plasmon resonance (SPR) analysis and a solid phase binding assay demonstrated that galectin-3 binds directly to neoglycoconjugates carrying LDN glycans. In addition, galectin-3 bound to Schistosoma mansoni soluble egg Ags and a mAb against the LDN glycan inhibited this binding, suggesting that LDN glycans within S. mansoni soluble egg Ags contribute to galectin-3 binding. Immunocytochemistry demonstrated high levels of galectin-3 in liver granulomas of S. mansoni-infected hamsters, and a colocalization of galectin-3 and LDN glycans was observed on the parasite eggshells. Finally, we demonstrate that galectin-3 can mediate recognition and phagocytosis of LDN-coated particles by macrophages. These findings provide evidence that LDN-glycans constitute a parasite pattern for galectin-3-mediated immune recognition.