TNF-Induced β2 Integrin Activation Involves Src Kinases and a Redox-Regulated Activation of p38 MAPK

Integration of an LPAR1 Antagonist into Liposomes Enhances Their Internalization and Tumor Accumulation in an Animal Model of Human Metastatic Breast Cancer

Lysophosphatidic acid receptor 1 (LPAR1) is elevated in breast cancer. The deregulation of LPAR1, including the function and level of expression, is linked to cancer initiation, progression, and metastasis. LPAR1 antagonists, AM095 or Ki16425, may be effective therapeutic molecules, yet their limited water solubility hinders in vivo delivery. In this study, we report on the synthesis of two liposomal formulations incorporating AM095 or Ki16425, embedded within the lipid bilayer, as targeted nanocarriers for metastatic breast cancer (MBC). The data show that the Ki16425 liposomal formulation exhibited a 50% increase in internalization by MBC mouse epithelial cells (4T1) and a 100% increase in tumor accumulation in a mouse model of MBC compared with that of a blank liposomal formulation (control). At the same time, normal mouse epithelial cells (EpH-4Ev) internalized the Ki16425 liposomal formulation 25% lesser than the control formulation. Molecular dynamics simulations show that the integration of AM095 or Ki16425 modified the physical and mechanical properties of the lipid bilayer, making it more flexible in these liposomal formulations compared with liposomes without drug. The incorporation of an LPAR1 antagonist within a liposomal drug delivery system represents a viable therapeutic approach for targeting the LPA-LPAR1 axis, which may hinder the progression of MBC.

A humanized β2 integrin knockin mouse reveals localized intra- and extravascular neutrophil integrin activation in vivo

β₂ integrins are leukocyte-specific adhesion molecules that are essential for leukocyte recruitment. The lack of tools for reporting β₂ integrin activation in mice hindered the study of β₂ integrin-related immune responses in vivo. Here, we generated a humanized β₂ integrin knockin mouse strain by targeting the human β₂ integrin coding sequence into the mouse Itgb2 locus to enable imaging of β₂ integrin activation using the KIM127 (extension) and mAb24 (high-affinity) reporter antibodies. Using a CXCL1-induced acute inflammation model, we show the local dynamics of β₂ integrin activation in arresting neutrophils in vivo in venules of the mouse cremaster muscle. Activated integrins are highly concentrated in a small area at the rear of arresting neutrophils in vivo. In a high-dose lipopolysaccharide model, we find that β₂ integrins are activated in association with elevated neutrophil adhesion in lung and liver. Thus, these mice enable studies of β₂ integrin activation in vivo.

CD44 engagement enhances acute myeloid leukemia cell adhesion to the bone marrow microenvironment by increasing VLA-4 avidity

Adhesive properties of leukemia cells shape the degree of organ infiltration and the extent of leukocytosis. CD44 and the integrin VLA-4, a CD49d/CD29 heterodimer, are important factors in progenitor cell adhesion in bone marrow. Here, we report their cooperation in acute myeloid leukemia (AML) by a novel non-classical CD44-mediated way of inside-out VLA-4 activation. In primary AML bone marrow samples from patients and the OCI-AML3 cell line, CD44 engagement by hyaluronan induced inside-out activation of VLA-4 resulting in enhanced leukemia cell adhesion on VCAM-1. This was independent of VLA-4 affinity regulation but based on ligand-induced integrin clustering on the cell surface. CD44-induced VLA-4 activation could be inhibited by the Src family kinase inhibitor PP2 and the multikinase inhibitor midostaurin. As a further consequence, the increased adhesion on VCAM-1 allowed AML cells to bind stromal cells strongly. Thereby, the VLA-4/VCAM-1 interaction promoted activation of Akt, MAPK, NF-kB and mTOR signaling and decreased AML cell apoptosis. Collectively, our investigations provide a mechanistic description of an unusual CD44 function in regulating VLA-4 avidity in AML, enhancing AML cell retention in the supportive bone marrow microenvironment.

Complement Receptor 3 Contributes to the Sexual Dimorphism in Neutrophil Killing of Staphylococcus aureus

We previously reported sex differences in innate susceptibility to Staphylococcus aureus skin infection and that bone marrow neutrophils (BMN) from female mice have an enhanced ability to kill S. aureus ex vivo compared with those of male mice. However, the mechanism(s) driving this sex bias in neutrophil killing have not been reported. Given the role of opsonins such as complement, as well as their receptors, in S. aureus recognition and clearance, we investigated their contribution to the enhanced bactericidal capacity of female BMN. We found that levels of C3 in the serum and CR3 (CD11b/CD18) on the surface of BMN were higher in female compared with male mice. Consistent with increased CR3 expression following TNF-α priming, production of reactive oxygen species (ROS), an important bactericidal effector, was also increased in female versus male BMN in response to serum-opsonized S. aureus Furthermore, blocking CD11b reduced both ROS levels and S. aureus killing by murine BMN from both sexes. However, at the same concentration of CD11b blocking Ab, S. aureus killing by female BMN was greatly reduced compared with those from male mice, suggesting CR3-dependent differences in bacterial killing between sexes. Overall, this work highlights the contributions of CR3, C3, and ROS to innate sex bias in the neutrophil response to S. aureus Given that neutrophils are crucial for S. aureus clearance, understanding the mechanism(s) driving the innate sex bias in neutrophil bactericidal capacity could identify novel host factors important for host defense against S. aureus.

Fungal Chitin Reduces Platelet Activation Mediated via TLR8 Stimulation

Platelets play an important role in the innate immune response. During candidaemia, circulating fungal polysaccharides, including chitin, are released into the bloodstream and can interact with platelets and induce modulation of platelet activities. However, the role of circulating chitin in platelet modulation has not been investigated. The aims of the present study were to assess the effect of fungal chitin on activation, adhesion, aggregation and receptor expression of platelets and their impact on the host defense against Candida albicans. Platelets pre-treated with different concentrations of chitin (10–400 μg/mL) extracted from C. albicans were analyzed in terms of activation, Toll-like receptor (TLR) expression, aggregation and adhesion to C. albicans. Chitin treatment reduced platelet adhesion to C. albicans and neutrophils. P-selectin expression was significantly decreased in platelets challenged with chitin. Aggregation and intracellular Ca²⁺ influx were also decreased in platelets. TLR8 mRNA and proteins were expressed in platelets pre-treated with chitin when compared to untreated platelets. Overall, chitin purified from C. albicans reduced the adhesion, activation and aggregation of platelets mediated via TLR8 stimulation by decreasing intracellular Ca²⁺ influx and P-selectin expression.

TRPC channels: Regulation, dysregulation and contributions to chronic kidney disease

Mutations in the gene encoding canonical transient receptor potential-6 (TRPC6) channels result in severe nephrotic syndromes that typically lead to end-stage renal disease. Many but not all of these mutations result in a gain in the function of the resulting channel protein. Since those observations were first made, substantial work has supported the hypothesis that TRPC6 channels can also contribute to progression of acquired (non-genetic) glomerular diseases, including primary and secondary FSGS, glomerulosclerosis during autoimmune glomerulonephritis, and possibly in type-1 diabetes. Their regulation has been extensively studied, especially in podocytes, but also in mesangial cells and other cell types present in the kidney. More recent evidence has implicated TRPC6 in renal fibrosis and tubulointerstitial disease caused by urinary obstruction. Consequently TRPC6 is being extensively investigated as a target for drug discovery. Other TRPC family members are present in kidney. TRPC6 can form a functional heteromultimer with TRPC3, and it has been suggested that TRPC5 may also play a role in glomerular disease progression, although the evidence on this is contradictory. Here we review literature on the expression and regulation of TRPC6, TRPC3 and TRPC5 in various cell types of the vertebrate kidney, the evidence that these channels are dysregulated in disease models, and research showing that knock-out or pharmacological inhibition of these channels can reduce the severity of kidney disease. We also summarize several areas that remain controversial, and some of the large gaps of knowledge concerning the fundamental role of these proteins in regulation of renal function.

TNF/TNFR axis promotes pyrin inflammasome activation and distinctly modulates pyrin inflammasomopathy

Pyrin is an inflammasome sensor that promotes caspase-1-mediated pyroptotic cell death and maturation of proinflammatory cytokines IL-1β and IL-18. Familial Mediterranean fever (FMF), an autoinflammatory disorder, is associated with mutations in the gene encoding pyrin (MEFV). FMF-knockin (FMF-KI) mice that express chimeric pyrin protein with FMF mutation (MefvV726A/V726A) exhibit an autoinflammatory disorder mediated by autoactivation of the pyrin inflammasome. Increase in the levels of TNF are observed in FMF-KI mice, and many features of FMF overlap with the autoinflammatory disorder associated with TNF receptor signaling. In this study, we assessed the contribution of TNF signaling to pyrin inflammasome activation and its consequent role in distinct FMF pathologies. TNF signaling promoted the expression of pyrin in response to multiple stimuli and was required for inflammasome activation in response to canonical pyrin stimuli and in myeloid cells from FMF-KI mice. TNF signaling promoted systemic wasting, anemia, and neutrophilia in the FMF-KI mice. Further, TNF-induced pathology was induced specifically through the TNFR1 receptor, while TNFR2-mediated signaling was distinctly protective in colitis and ankle joint inflammation. Overall, our data show that TNF is a critical modulator of pyrin expression, inflammasome activation, and pyrin-inflammasomopathy. Further, specific blockade of TNFR1 or activation of TNFR2 could provide substantial protection against FMF pathologies.

Complement Factor H Inhibits Anti-Neutrophil Cytoplasmic Autoantibody-Induced Neutrophil Activation by Interacting With Neutrophils

Our previous study demonstrated that plasma levels of complement factor H (FH) were inversely associated with the disease activity of patients with anti-neutrophil cytoplasmic autoantibody (ANCA)-associated vasculitis (AAV). In addition to serving as an inhibitor of the alternative complement pathway, there is increasing evidence demonstrating direct regulatory roles of FH on several cell types. Here, we investigated the role of FH in the process of ANCA-mediated activation of neutrophils and neutrophil–endothelium interaction. We demonstrated that FH bound to neutrophils by immunostaining and flow cytometry. Interestingly, ANCA-induced activation of neutrophils, including respiratory burst and degranulation, was inhibited by FH. Although FH enhanced neutrophils adhesion and migration toward human glomerular endothelial cells (hGEnCs), it inhibited ANCA-induced activation of neutrophils in the coculture system of hGEnCs and neutrophils. Moreover, the activation and injury of hGEnCs, reflected by the level of endothelin-1 in the supernatant of cocultures, was markedly reduced by FH. However, we found that FH from patients with active AAV exhibited a deficient ability in binding neutrophils and inhibiting ANCA-induced neutrophil activation in fluid phase and on endothelial cells, as compared with that from healthy controls. Therefore, our findings indicate a novel role of FH in inhibiting ANCA-induced neutrophil activation and protecting against glomerular endothelial injury. However, FH from patients with active AAV are deficient in their ability to bind neutrophils and inhibit neutrophil activation by ANCA. It further extends the current understanding of the pathogenesis of AAV, thus providing potential clues for intervention strategies.

Changes in podocyte TRPC channels evoked by plasma and sera from patients with recurrent FSGS and by putative glomerular permeability factors

Primary forms of focal and segmental glomerulosclerosis (FSGS) are driven by circulating factors that cause dysfunction or loss podocytes. Rare genetic forms of FSGS can be caused by mutations in TRPC6, which encodes a Ca2+-permeable cationic channel expressed in mesangial cells and podocytes; and NPHS2, which encodes podocin, a TRPC6-binding protein expressed in podocyte slit diaphragm domains. Here we observed that exposing immortalized mouse podocytes to serum or plasma from recurrent FSGS patients for 24h increased the steady-state cell-surface abundance of TRPC6, accompanied by an increase in currents through endogenous TRPC6 channels evoked by a hypoosmotic stretch stimulus. These effects were mimicked by the soluble urokinase receptor (suPAR) and by tumor necrosis factor (TNF), circulating factors implicated in nephrotic syndromes. Most but not all of the recurrent FSGS plasma samples that we examined also caused a loss of podocin over a period of several hours. The loss of podocin was also seen following exposure to suPAR but not TNF. However, TNF increased the effects of suPAR on TRPC6 and podocin, and TNF and suPAR are required for the full effects of one of the recurrent FSGS plasma samples. The actions of FSGS plasma, suPAR and TNF on surface abundance of TRPC6 were blocked by cilengitide, an inhibitor of αvβ3-integrin signaling. These data suggest that primary FSGS is a heterogeneous condition mediated by multiple circulating factors, and support TRPC6 and αvβ3-integrin as potential therapeutic targets.

Multiple Phenotypic Changes Define Neutrophil Priming

Exposure to pro-inflammatory cytokines, chemokines, mitochondrial contents, and bacterial and viral products induces neutrophils to transition from a basal state into a primed one, which is currently defined as an enhanced response to activating stimuli. Although, typically associated with enhanced generation of reactive oxygen species (ROS) by the NADPH oxidase, primed neutrophils show enhanced responsiveness of exocytosis, NET formation, and chemotaxis. Phenotypic changes associated with priming also include activation of a subset of functions, including adhesion, transcription, metabolism, and rate of apoptosis. This review summarizes the breadth of phenotypic changes associated with priming and reviews current knowledge of the molecular mechanisms behind those changes. We conclude that the current definition of priming is too restrictive. Priming represents a combination of enhanced responsiveness and activated functions that regulate both adaptive and innate immune responses.

Rac GTPase Activating Protein ARHGAP25 Regulates Leukocyte Transendothelial Migration in Mice

ARHGAP25 is a Rac-specific GTPase-activating protein that is expressed primarily in hematopoietic cells. The involvement of ARHGAP25 in regulating the recruitment of leukocytes to inflammatory sites was investigated in genetically modified mice. Using intravital microscopy, we show that Arhgap25 deficiency affects all steps of leukocyte recruitment with a predominant enhancement of transendothelial migration of neutrophilic granulocytes. Increased transmigration of Arhgap25-deficient leukocytes is demonstrated in inflamed cremaster muscle venules, in a peritonitis model, and in an in vitro chemotaxis assay. Using bone marrow chimeric mice lacking ARHGAP25 in the hematopoietic compartment, we show that enhanced migration in the absence of ARHGAP25 is due to defective leukocyte function. In search for potential mechanisms of ARHGAP25-regulated migration of neutrophils, we detected an increase in the amount of active, GTP-bound Rac and Rac-dependent cytoskeletal changes in the absence of ARHGAP25, suggesting a critical role of ARHGAP25 in counterbalancing the Rac-activating effect of nucleotide exchange factors. Taken together, using Arhgap25-deficient mice, we identified ARHGAP25 as a relevant negative regulator of leukocyte transendothelial migration.

Reinforcement of integrin-mediated T-Lymphocyte adhesion by TNF-induced Inside-out Signaling

Integrin-mediated leukocyte adhesion to endothelial cells is a crucial step in immunity against pathogens. Whereas the outside-in signaling pathway in response to the pro-inflammatory cytokine tumour necrosis factor (TNF) has already been studied in detail, little knowledge exists about a supposed TNF-mediated inside-out signaling pathway. In contrast to the outside-in signaling pathway, which relies on the TNF-induced upregulation of surface molecules on endothelium, inside-out signaling should also be present in an endothelium-free environment. Using single-cell force spectroscopy, we show here that stimulating Jurkat cells with TNF significantly reinforces their adhesion to fibronectin in a biomimetic in vitro assay for cell-surface contact times of about 1.5 seconds, whereas for larger contact times the effect disappears. Analysis of single-molecule ruptures further demonstrates that TNF strengthens sub-cellular single rupture events at short cell-surface contact times. Hence, our results provide quantitative evidence for the significant impact of TNF-induced inside-out signaling in the T-lymphocyte initial adhesion machinery.

Short fungal fractions of β-1,3 glucans affect platelet activation

Platelets are capable of binding, aggregating, and internalizing microorganisms, which enhances the elimination of pathogens from the blood. The yeast Candida albicans is a pathobiont causing life-threatening invasive infections. Its cell wall contains β-1,3 glucans that are known to trigger a wide range of host cell activities and to circulate during infection. We studied the effect of β-1,3 glucan fractions (BGFs) consisting of diglucosides (Glc2), tetraglucosides (Glc4), and pentaglucosides (Glc5) on human platelets, their mechanisms of action, and their possible impact on host defenses. The effect of BGFs on the coagulation process was determined by measuring thrombin generation. Platelets pretreated with BGFs were analyzed in terms of activation, receptor expression, aggregation, and adhesion to neutrophils and to C. albicans The results show that BGFs affected the endogenous thrombin potential in a concentration-dependent manner. For platelet activation, BGFs at a low concentration (2 μmol/l) reduced ATP release and prevented the phosphorylation of protein kinase C. BGFs diminished the expression of P-selectin and the activation of αIIbβ3 BGFs decreased platelet aggregation and the interaction between thrombin-stimulated platelets and neutrophils, fibrinogen, and C. albicans GLc5 decreased ATP release and TGF-β1 production in response to TLR4 upregulation in thrombin-stimulated platelets, but TLR4 blockage abolished the effect of BGFs on platelets. This study provides evidence that fungal pentaglucosides modulate platelet activity mediated via TLR4 stimulation and reduce platelet-neutrophil interaction.

Chemerin15 inhibits neutrophil-mediated vascular inflammation and myocardial ischemia-reperfusion injury through ChemR23

Neutrophils are shown to express ChemR23 and respond to C15, which inhibits integrin activation and clustering, reducing neutrophil adhesion and chemotaxis in vitro, and neutrophil recruitment and heart damage in a murine myocardial infarction model.

Neutrophil activation and adhesion must be tightly controlled to prevent complications associated with excessive inflammatory responses. The role of the anti-inflammatory peptide chemerin15 (C15) and the receptor ChemR23 in neutrophil physiology is unknown. Here, we report that ChemR23 is expressed in neutrophil granules and rapidly upregulated upon neutrophil activation. C15 inhibits integrin activation and clustering, reducing neutrophil adhesion and chemotaxis in vitro. In the inflamed microvasculature, C15 rapidly modulates neutrophil physiology inducing adherent cell detachment from the inflamed endothelium, while reducing neutrophil recruitment and heart damage in a murine myocardial infarction model. These effects are mediated through ChemR23. We identify the C15/ChemR23 pathway as a new regulator and thus therapeutic target in neutrophil-driven pathologies.

Restraint of spreading-dependent activation of polymorphonuclear leukocyte NADPH oxidase in an acidified environment

Elucidation of the mechanisms by which environmental pH affects or regulates the functions of polymorphonuclear leukocytes (PMNs) is important because severe acidification of the microenvironment often prevails at sites of inflammation where they act in host defense. In the present study, we investigated the effect of an acidic environment on spreading-dependent activation of O2- -producing NADPH oxidase in PMNs. We found that PMNs underwent spreading spontaneously over type I collagen and plastic surfaces at both neutral and acidic pH, although spreading over fibrinogen surfaces, for which cellular stimulation with H2O2 is required, was inhibited by acidic pH. At acidic pH, however, PMNs were unable to undergo spreading-dependent production of O2-. Pharmacological experiments showed that p38 mitogen-activated protein kinase (MAPK) was involved in the signaling pathways mediating the spreading-dependent activation of NADPH oxidase, and that its spreading-dependent phosphorylation of Thr-180 and Tyr-182, a hallmark of activation, was impaired at acidic pH. Furthermore, the inhibition by acidic pH of O2- production as well as p38 MAPK phosphorylation subsequent to spreading induction was reversible; environmental neutralization and acidification after induction of spreading at acidic and neutral pH, respectively, up- and down-regulated the two phenomena. Acidic pH did not affect the O2- production activity of NADPH oxidase pre-activated by phorbol 12-myristate 13-acetate (PMA). These results suggest that, in PMNs, the p38 MAPK-mediated signaling pathway functions as a pH-sensing regulator of spreading-dependent NADPH oxidase activation.

Talin and signaling through integrins

Integrin adhesion receptors are essential for the development and functioning of multicellular animals. Integrins mediate cell adhesion to the extracellular matrix and to counter-receptors on adjacent cells, and the ability of integrins to bind extracellular ligands is regulated in response to intracellular signals that act on the short cytoplasmic tails of integrin subunits. Integrin activation, the rapid conversion of integrin receptors from low to high affinity, requires binding of talin to integrin β tails and, once bound, talin provides a connection from activated integrins to the actin cytoskeleton. A wide range of experimental approaches have contributed to the current understanding of the importance of talin in integrin signaling. Here, we describe two methods that have been central to our investigations of talin; a biochemical assay that has allowed characterization of interactions between integrin cytoplasmic tails and talin, and a fluorescent-activated cell-sorting procedure to assess integrin activation in cultured cells expressing talin domains, mutants, dominant negative constructs, or shRNA.

Priming of neutrophils and differentiated PLB-985 cells by pathophysiological concentrations of TNF-α is partially oxygen dependent

Activation of polymorphonuclear leukocytes (PMN) can be modulated to intermediate 'primed' states characterized by enhanced responsiveness to subsequent stimuli. We studied priming in response to TNF-α in human PMN and PLB-985 cells, a myeloid cell line differentiated to a neutrophilic phenotype (PLB-D). PMN generated reactive oxygen species (ROS) in response to TNF-α alone, and NADPH oxidase activity increased in response to stimulation with formyl-Met-Leu-Phe after priming. PLB-D cells also demonstrated priming of NADPH oxidase activity. Similar to priming by endotoxin, priming of the respiratory burst by TNF-α was predominantly oxygen dependent, with marked attenuation of ROS generation if primed anaerobically. Both PMN and PLB-D cells displayed significant increases in cell surface CD11b and gp91(phox) expression after TNF-α priming and PMN displayed activation of MAPK. In response to TNF-α priming, neither mobilization of intracellular proteins nor activation of MAPK pathways was NADPH oxidase dependent. Priming of PMN and PLB-D cells by low TNF-α concentrations enhanced chemotaxis. These data demonstrate that pathophysiological concentrations of TNF-α elicit NADPH oxidase-derived ROS and prime cells for enhanced surface protein expression, activation of p38 and ERK1/2 MAPK pathways, and increased chemotaxis. Furthermore, PLB-D cells undergo TNF-α priming and provide a genetically modifiable model to study priming mechanisms.

The Polycomb group protein EED couples TNF receptor 1 to neutral sphingomyelinase

The phospholipase neutral sphingomyelinase (N-SMase) has been recognized as a major mediator of processes such as inflammation, development and growth, differentiation and death of cells, as well as in diseases such as Alzheimer's, atherosclerosis, heart failure, ischemia/reperfusion damage, or combined pituitary hormone deficiency. Although activation of N-SMase by the proinflammatory cytokine TNF was described almost two decades ago, the underlying signaling pathway is unresolved. Here, we identify the Polycomb group protein EED (embryonic ectodermal development) as an interaction partner of nSMase2. In yeast, the N terminus of EED binds to the catalytic domain of nSMase2 as well as to RACK1, a protein that modulates the activation of nSMase2 by TNF in concert with the TNF receptor 1 (TNF-R1)-associated protein FAN. In mammalian cells, TNF causes endogenous EED to translocate from the nucleus and to colocalize and physically interact with both endogenous nSMase2 and RACK1. As a consequence, EED and nSMase2 are recruited to the TNF-R1.FAN.RACK1-complex in a timeframe concurrent with activation of nSMase2. After knockdown of EED by RNA interference, the TNF-dependent activation of nSMase2 is completely abrogated, identifying EED as a protein that both physically and functionally couples TNF-R1 to nSMase2, and which therefore represents the "missing link" that completes one of the last unresolved signaling pathways of TNF-R1.

Looking beyond death: a morphogenetic role for the TNF signalling pathway

Tumour necrosis factor alpha (TNFalpha) is a pro-inflammatory mediator with the capacity to induce apoptosis. An integral part of its apoptotic and inflammatory programmes is the control of cell shape through modulation of the cytoskeleton, but it is now becoming apparent that this morphogenetic function of TNF signalling is also employed outside inflammatory responses and is shared by the signalling pathways of other members of the TNF-receptor superfamily. Some proteins that are homologous to the components of the TNF signalling pathway, such as the adaptor TNF-receptor-associated factor 4 and the ectodysplasin A receptor (and its ligand and adaptors), have dedicated morphogenetic roles. The mechanism by which TNF signalling affects cell shape is not yet fully understood, but Rho-family GTPases have a central role. The fact that the components of the TNF signalling pathway are evolutionarily old suggests that an ancestral cassette from unicellular organisms has diversified its functions into partly overlapping morphogenetic, inflammatory and apoptotic roles in multicellular higher organisms.

Reactive oxygen intermediate-induced pathomechanisms contribute to immunosenescence, chronic inflammation and autoimmunity

Deregulation of reactive oxygen intermediates (ROI) resulting in either too high or too low concentrations are commonly recognized to be at least in part responsible for many changes associated with aging. This article reviews ROI-dependent mechanisms critically contributing to the decline of immune function during physiologic - or premature - aging. While ROI serve important effector functions in cellular metabolism, signalling and host defence, their fine-tuned generation declines over time, and ROI-mediated damage to several cellular components and/or signalling deviations become increasingly prevalent. Although distinct ROI-associated pathomechanisms contribute to immunosenescence of the innate and adaptive immune system, mutual amplification of dysfunctions may often result in hyporesponsiveness and immunodeficiency, or in chronic inflammation with hyperresponsiveness/deregulation, or both. In this context, we point out how imbalanced ROI contribute ambiguously to driving immunosenescence, chronic inflammation and autoimmunity. Although ROI may offer a distinct potential for therapeutic targeting along with the charming opportunity to rescue from deleterious processes of aging and chronic inflammatory diseases, such modifications, owing to the complexity of metabolic interactions, may carry a marked risk of unforeseen side effects.

Receptor for advanced glycation end-products (RAGE) modulates neutrophil adhesion and migration on glycoxidated extracellular matrix

AGEs (advanced glycation end-products) accumulate in collagen molecules during uraemia and diabetes, two diseases associated with high susceptibility to bacterial infection. Because neutrophils bind to collagen during their locomotion in extravascular tissue towards the infected area we investigated whether glycoxidation of collagen (AGE-collagen) alters neutrophil migration. Type I collagen extracted from rat tail tendons was used for in vitro glycoxidation (AGE-collagen). Neutrophils were obtained from peripheral blood of healthy adult volunteers and were used for the in vitro study of adhesion and migration on AGE- or control collagen. Glycoxidation of collagen increased adhesion of neutrophils to collagen surfaces. Neutrophil adhesion to AGE-collagen was inhibited by a rabbit anti-RAGE (receptor for AGEs) antibody and by PI3K (phosphoinositide 3-kinase) inhibitors. No effect was observed with ERK (extracellular-signal-regulated kinase) or p38 MAPK (mitogen-activated protein kinase) inhibitors. AGE-collagen was able to: (i) induce PI3K activation in neutrophils, and (ii) inhibit chemotaxis and chemokinesis of chemoattractant-stimulated neutrophils. Finally, we found that blocking RAGE with anti-RAGE antibodies or inhibiting PI3K with PI3K inhibitors restored fMLP (N-formylmethionyl-leucyl-phenylalanine)-induced neutrophil migration on AGE-collagen. These results show that RAGE and PI3K modulate adhesion and migration rate of neutrophils on AGE-collagen. Modulation of adhesiveness may account for the change in neutrophil migration rate on AGE-collagen. As neutrophils rely on their ability to move to perform their function as the first line of defence against bacterial invasion, glycoxidation of collagen may participate in the suppression of normal host defence in patients with diabetes and uraemia.

Redox-regulation of Erk1/2-directed phosphatase by reactive oxygen species: role in signaling TPA-induced growth arrest in ML-1 cells

Extracellular signal-regulated kinase (Erk)1/2 activity signals myeloid cell differentiation induced by 12-O-tetradecanoyl-phorbol-13-acetate (TPA). Previously, we reported that Erk1/2 activation (phosphorylation) induced by TPA required reactive oxygen species (ROS) as a second messenger. Here, we hypothesized that ROS generated in response to TPA inhibit Erk1/2-directed phosphatase activity, which leads to an increase phosphorylation of Erk1/2 to signal p21(WAF1/Cip1)-mediated growth arrest in ML-1 cells. Incubation of ML-1 cells with TPA resulted in a marked accumulation of phosphorylated Erk1/2, and is subsequent to H2O2 generation. Interestingly, post-TPA-treatment with N-acetylcysteine (NAC) stimulated a marked and a rapid dephosphorylation of Erk1/2, suggesting a regeneration of Erk1/2-directed phospahatase activity by NAC. ROS generation in ML-1 cells induced by TPA was suggested to occur in the mitochondrial electron transport chain (METC) based on the following observations: (i) undifferentiated ML-1 cells not only lack p67-phox and but also express a low level of p47-phox key components required for NADPH oxidase enzymatic activity, (ii) pretreatment with DPI, an inhibitor of NADH- and NADPH-dependent enzymes, or rhein, an inhibitor of complex I, blocked the ROS generation, and (iii) examination of the microarray analysis data and Western blot analysis data revealed an induction of MnSOD expression at both mRNA and protein levels in response to TPA. MnSOD is a key member of the mitochondrial defense system against mitochondrial-derived superoxide. Together, this study suggested that TPA stimulated ROS generation as a second messenger to activate Erk1/2 via a redox-mediated inhibition of Erk1/2-directed phosphatase in ML-1 cells.

Inhibition of Src and p38 MAP kinases suppresses the change of claudin expression induced on dedifferentiation of primary cultured parotid acinar cells

Sjögren's syndrome and therapeutic radiation for head and neck cancers result in irreversible changes in the parenchyma of salivary glands, loss of acinar cells, prominence of duct cells, and fibrosis. To clarify mechanisms of salivary gland dysfunction, we identified a signaling pathway involved in the dedifferentiation of primary cultures of parotid acinar cells. We reported previously that the expression pattern of claudins changes during culture, is related to the three-dimensional organization of the cells, and reflects their ability to function as acinar cells. In this study, we found that this change of claudin expression is a process of dedifferentiation, because expression of other differentiation markers also changes during culture. The expression levels of claudins-4 and -6, cytokeratin 14, and vimentin are increased, and those of claudin-10, aquaporin 5, and amylase are decreased. Inhibitors of Src and p38 MAP kinases suppress these changes and increase the expression of acinar marker proteins. Differences in extracellular matrix components have no effect. Activation of p38 MAP kinase occurs during cell isolation from the parotid glands and is retained up to 6 h after the isolation. In contrast, activation of Src kinases does not increase during the cell isolation. The Src inhibitor PP1 suppresses the activation of p38 MAP kinase. Therefore, cellular stresses induced during cell isolation cause dedifferentiation and transition to duct-like cells through activation of p38 MAP kinase and constitutively active Src kinases.

Src tyrosine kinases mediate activations of NF-kappaB and integrin signal during lipopolysaccharide-induced acute lung injury

Src tyrosine kinases (TKs) are signaling proteins involved in cell signaling pathways toward cytoskeletal, membrane and nuclear targets. In the present study, using a selective Src TK inhibitor, PP1, we investigated the roles of Src TKs in the key pulmonary responses, NF-kappaB activation, and integrin signaling during acute lung injury in BALB/C mice intratracheally treated with LPS. LPS resulted in c-Src phosphorylation in lung tissue and the phospho-c-Src was predominantly localized in recruited neutrophils and alveolar macrophages. PP1 inhibited LPS-induced increases in total protein content in bronchoalveolar lavage fluid, neutrophil recruitment, and increases in the production or activity of TNF-alpha and matrix metalloproteinase-9. PP1 also blocked LPS-induced NF-kappaB activation, and phosphorylation and degradation of IkappaB-alpha. The inhibition of NF-kappaB activation by PP1 correlated with a depression of LPS-induced integrin signaling, which included increases in the phosphorylations of integrin beta(3), and of the focal adhesion kinase (FAK) family members, FAK and Pyk2, in lung tissue, and reductions in the fibrinogen-binding activity of alveolar macrophages. Moreover, treatment with anti-alpha(v), anti-beta(3), or Arg-Gly-Asp-Ser (RGDS), inhibited LPS-induced NF-kappaB activation. Taken together, our findings suggest that Src TKs play a critical role in LPS-induced activations of NF-kappaB and integrin (alpha(v)beta(3)) signaling during acute lung injury. Therefore, Src TK inhibition may provide a potential means of ameliorating inflammatory cascade-associated lung injury.

Endotoxin priming of neutrophils requires NADPH oxidase-generated oxidants and is regulated by the anion transporter ClC-3

Several soluble mediators, including endotoxin, prime neutrophils for an enhanced respiratory burst in response to subsequent stimulation. Priming of neutrophils occurs in vitro, and primed neutrophils are found in vivo. We previously localized the anion transporter ClC-3 to polymorphonuclear leukocytes (PMN) secretory vesicles and demonstrated that it is required for normal NADPH oxidase activation in response to both particulate and soluble stimuli. We now explore the contribution of the NADPH oxidase and ClC-3 to endotoxin-mediated priming. Lipooligosaccharide (LOS) from Neisseria meningitidis enhances the respiratory burst in response to formyl-Met-Leu-Phe, an effect that was impaired in PMNs lacking functional ClC-3 and under anaerobic conditions. Mobilization of receptors to the cell surface and phosphorylation of p38 MAPK by LOS were both impaired in PMN with the NADPH oxidase chemically inhibited or genetically absent and in cells lacking functional ClC-3. Furthermore, inhibition of the NADPH oxidase or ClC-3 in otherwise unstimulated cells elicited a phenotype similar to that seen after endotoxin priming, suggesting that basal oxidant production helps to maintain cellular quiescence. In summary, NADPH oxidase activation was required for LOS-mediated priming, but basal oxidants kept unstimulated cells from becoming primed. ClC-3 contributes to both of these processes.

Increased membrane expression of proteinase 3 during neutrophil adhesion in the presence of anti proteinase 3 antibodies

We investigated membrane proteinase 3 (mPR3) expression during TNF-alpha-induced adhesion of neutrophils in the presence of anti-PR3 antibodies, a situation occurring during anti-neutrophil cytoplasmic autoantibodies (ANCA)-associated vasculitis. Three increasing levels of mPR3 expression were observed on the mPR3(+) neutrophil subset after stepwise cell activation. TNF-alpha activation without adhesion, TNF-alpha-induced adhesion, and adhesion in the presence of anti-PR3 mAb or human anti-PR3 ANCA resulted, respectively, in a two-, seven-, and 24-fold increase of mPR3 levels. In plasma, anti-PR3 antibodies poorly recognized suspended neutrophils, whereas they bound to mPR3 on adherent cells. mPR3 upregulation was also triggered by IL-8, formyl-methionyl-leucyl-phenylalanine (fMLP), and neutrophil adhesion to activated human umbilical vein endothelial cells. It involved beta2 integrins and Fcgamma receptor, because it was prevented by anti-CD18 antibodies and was not observed with anti-PR3 F(ab')(2). Furthermore, it was specific to anti-PR3 mAb, and no mPR3 upregulation was observed with anti-myeloperoxidase or anti-HLA-ABC mAb. Newly expressed mPR3 molecules, after TNF-induced adhesion, were mobilized from secretory vesicles (CD35(+)) and secondary granules (CD11b(+)). The adhesion- and antibody-dependent upregulations of mPR3 expression occurred with little azurophilic granule degranulation, no sign of apoptosis, and no further CD177 upregulation. In conclusion, this study describes an amplifying loop in polymorphonuclear neutrophil activation process, whereby ANCA are involved in the membrane expression of their own antigen during cell adhesion. This could explain the restriction of ANCA-associated vasculitis to small vessels, the main site of neutrophil adhesion.

Leucocyte-endothelial interactions in health and disease

The emigration of leucocytes into the tissue as a crucial step in the response to inflammatory signals has been acknowledged for more than 100 years. The endothelium does not only represent a mechanical barrier between blood and tissue, the circulatory system also connects different organ systems with each other, thus allowing the communication between remote systems. Leukocytes can function as messengers and messages at the same time. Failure or dysregulation of leucocyte-endothelial communication can severely affect the integrity of the organism. The interaction between leucocytes and the vascular endothelium has been recognised as an attractive target for the therapy of numerous disorders and diseases, including excessive inflammatory responses and autoimmune diseases, both associated with enormous consequences for patients and the health care system. There is promising evidence that the success rate of such treatments will increase as the understanding of the molecular mechanisms keeps improving. This chapter reviews the current knowledge about leucocyte-endothelial interaction. It will also display examples of both physiological and dysregulated leucocyte-endothelial interactions and identify potential therapeutical approaches.

Osteopontin promotes integrin activation through outside-in and inside-out mechanisms: OPN-CD44V interaction enhances survival in gastrointestinal cancer cells

Osteopontin (OPN) and splice variants of CD44 (CD44(V)) have independently been identified as markers for tumor progression. In this study, we show that both OPN and CD44(V) are frequently overexpressed in human gastric cancer and that OPN-engaged CD44(V) ligation confers cells an increased survival mediated through integrin activation. First, we show that OPN treatment confers cells an increased resistance to UV-induced apoptosis. The OPN-mediated antiapoptosis is dependent on the expression of the variant exon 6 (V6)- or V7-containing CD44 as shown by overexpression of individual CD44(V) in gastric AZ521 cells that express no or very low level of endogenous CD44 and by knockdown of the constitutively expressed V6-containing CD44 isoforms in colon HT29 cells. Although OPN also interacts with RGD integrins, OPN-RGD sequence is dispensable for OPN-mediated antiapoptosis. OPN-induced antiapoptosis is mainly attributed to the engagement of CD44(V) isoforms and the relay of an inside-out signaling via Src activity, leading to robust integrin activation. Furthermore, OPN-elicited antiapoptosis was observed when cells were plated on fibronectin but not on poly-D-lysin, and preincubation of cells with anti-integrin beta(1) antibody to block integrin-extracellular matrix (ECM) interaction or ectopic expression of the dominant-negative forms of focal adhesion kinase to block ECM-derived signal abolished OPN-induced survival, suggesting that OPN-elicited antiapoptotic function is propagated from matrix transduced by integrin. Taken together, we showed that OPN-CD44(V) interaction promotes ECM-derived survival signal mediated through integrin activation, which may play an important role in the pathogenic development and progression of gastric cancer.

The neutrophil-activating protein of Helicobacter pylori crosses endothelia to promote neutrophil adhesion in vivo

Helicobacter pylori induces an acute inflammatory response followed by a chronic infection of the human gastric mucosa characterized by infiltration of neutrophils/polymorphonuclear cells (PMNs) and mononuclear cells. The H. pylori neutrophil-activating protein (HP-NAP) activates PMNs, monocytes, and mast cells, and promotes PMN adherence to the endothelium in vitro. By using intravital microscopy analysis of rat mesenteric venules exposed to HP-NAP, we demonstrated, for the first time in vivo, that HP-NAP efficiently crosses the endothelium and promotes a rapid PMN adhesion. This HP-NAP-induced adhesion depends on the acquisition of a high affinity state of beta(2) integrin on the plasma membrane of PMNs, and this conformational change requires a functional p38 MAPK. We also show that HP-NAP stimulates human PMNs to synthesize and release a number of chemokines, including CXCL8, CCL3, and CCL4. Collectively, these data strongly support a central role for HP-NAP in the inflammation process in vivo: indeed, HP-NAP not only recruits leukocytes from the vascular lumen, but also stimulates them to produce messengers that may contribute to the maintenance of the flogosis associated with the H. pylori infection.

Regulation of TNF mediated antiapoptotic signaling in human neutrophils: role of delta-PKC and ERK1/2

TNF is implicated in the suppression of neutrophil apoptosis during sepsis. Multiple signaling pathways are involved in TNF-mediated antiapoptotic signaling; a role for the MAP kinases (MAPK), ERK1/2, and p38 MAPK has been suggested. Antiapoptotic signaling is mediated principally through TNF receptor-1 (TNFR-1), and the PKC isotype-delta (delta-PKC) is a critical regulator of TNFR-1 signaling. delta-PKC associates with TNFR-1 in response to TNF and is required for NFkappaB activation and inhibition of caspase 3. The role of delta-PKC in TNF-mediated activation of MAPK is not known. The purpose of this study was to determine whether the MAPK, ERK1/2, and p38 MAPK are involved in TNF antiapoptotic signaling and whether delta-PKC is a key regulator of MAPK activation by TNF. In human neutrophils, TNF activated both p38 MAPK and ERK1/2 principally via TNFR-1. The MEK1/2 inhibitors PD098059 and U0126, but not the p38 MAPK inhibitor SB203580, decreased TNF antiapoptotic signaling as measured by caspase 3 activity. A specific delta-PKC antagonist, V1.1delta-PKC-Tat peptide, inhibited TNF-mediated ERK1/2 activation, but not p38 MAPK. ERK1/2 inhibition did not alter recruitment of delta-PKC to TNFR-1, indicating delta-PKC is acting upstream of ERK1/2. In HL-60 cells differentiated to a neutrophilic phenotype, delta-PKC depletion by delta-PKC siRNA resulted in inhibition of TNF mediated ERK1/2 activation but not p38 MAPK. Thus, ERK1/2, but not p38 MAPK, is an essential component of TNF-mediated antiapoptotic signaling. In human neutrophils, delta-PKC is a positive regulator of ERK1/2 activation via TNFR-1 but has no role in p38 MAPK activation.

Lipopolysaccharide Transiently Activates THP-1 Cell Adhesion

Lipopolysaccharide stimulation of adherent THP-1 cells induces morphological changes that are associated with the reorganization of the actin cytoskeleton. We hypothesized that LPS would also increase THP-1 cell adhesion and sought to determine the signaling mechanisms regulating this response. We show that LPS significantly increases THP-1 cell attachment after 1 h, supporting the idea that LPS can stimulate integrin function. By 4 h however, the number of adherent cells returned to control levels. Importantly, detached cells were determined to be viable by propidium iodide staining, indicating that the increase in cell adhesion was transient. LPS-induced adhesion to fibrinogen- but not fibronectin-coated wells was also transient, suggesting that adhesion reflected beta2 integrin activation. This idea was supported by the fact that LPS-induced adhesion could be blocked by a function-blocking anti-beta2 integrin antibody. Interestingly, the protein tyrosine phosphatase (PTP) inhibitor, phenylarsine oxide, prevented cell detachment. Taken together, these data suggest that LPS-mediated integrin activation is transient and can be regulated by PTP-mediated signaling events.

Albumin attenuates neutrophil activation induced by stimulators including antibodies against neutrophil-specific antigens

As neutrophil activation is related to several adverse transfusion reactions, we studied about the activation induced by anti-neutrophil antibodies and the stabilizing effects of albumin pretreatment by means of flow cytometry. Anti-neutrophil monoclonal antibody (anti-HNA-1a, 1b, 2a) alone induced CD11b expression and shedding of L-selectin, and anti-HNA-1a reinforced reactive oxygen species (ROS) production. Albumin pretreatment significantly reduced CD11b expression and L-selectin shedding induced by fMLP and ROS production induced by PMA, G-CSF combined with PMA or LPS-fMLP, or anti-HNA-1a combined with PMA. These findings suggest that anti-HNA-1a is related to adverse reactions and albumin has a regulating effect on neutrophil activation.

Induction of phospholipid hydroperoxide glutathione peroxidase in human polymorphonuclear neutrophils and HL60 cells stimulated with TNF-alpha

Phospholipid hydroperoxide glutathione peroxidase (PHGPx) is characterized as an important enzyme for protecting cells from oxidative stress-induced apoptosis and regulating the production of leukotrienes and prostanoids in cells overexpressing PHGPx. We studied whether the expression level of PHGPx fluctuates in polymorphonuclear leukocytes (PMNs) which were exposed to reactive oxygen species (ROS) and inflammatory cytokines at an inflammation site. Human peripheral PMNs up-regulated the expression level of PHGPx following culture with TNF-alpha, but not with IL-1beta, IL-8, and GRO. The up-regulated PHGPx expression was also observed in neutrophil-like cells that differentiated from the human leukemia cell line HL60 only after stimulation with TNF-alpha. However, macrophage-like differentiated HL60 cells and other cell lines, A498, ECV304, HeLa, U937, and HEK293, showed no increase in the PHGPx expression. This up-regulation of PHGPx was inhibited by treatment with the anti-oxidants, pyrrolidine dithiocarbamate, and N-acetyl-L-cysteine, and by inhibitors of NFkappaB and Src kinases. The stimulation of neutrophil-like differentiated HL60 cells with TNF-alpha induced activation of NFkappaB and c-Src kinase, and the activation was attenuated by treatment with the anti-oxidants. Up-regulation in neutrophil-like HL60 cells was also observed following exposure to H(2)O(2). These results indicate that activation of NFkappaB and/or Src kinases through ROS signaling may be involved in the up-regulation of the PHGPx in human PMNs stimulated by TNF-alpha.

Involvement of SHIP in TLR2-induced neutrophil activation and acute lung injury

The SHIP converts phosphatidylinositol 3,4,5 triphosphate to phosphatidyl 3,4 biphosphate. SHIP has negative regulatory functions on PI3K-dependent signaling pathways, which occupy important roles in modulating neutrophil functions. We used neutrophils from transgenic SHIP(-/-) and SHIP(+/+) mice that were stimulated with peptidoglycan (PGN) to examine the role of SHIP in TLR2-induced neutrophil activation. SHIP(-/-) neutrophils demonstrated significantly increased activation of the PI3K-dependent kinase Akt after exposure to PGN. Release of cytokines and chemokines, including TNF-alpha, IL-1beta, IL-6, IL-10, and MIP-2, was also increased in SHIP(-/-) compared with SHIP(+/+) neutrophils. There was no difference in the nuclear translocation of the transcriptional factor NF-kappaB between PGN-stimulated SHIP(-/-) and SHIP(+/+) neutrophils. However, phosphorylation of the p65 subunit of NF-kappaB, an event essential for optimal transcriptional activity of NF-kappaB, was increased in TLR2-activated SHIP(-/-) neutrophils. SHIP(-/-) neutrophils demonstrated greater activation of ERK1/2 and p38 MAPKs than did SHIP(+/+) neutrophils after exposure to PGN. The severity of acute lung injury induced by PGN was greater in SHIP(-/-) as compared with SHIP(+/+) mice. These results demonstrate that SHIP has a negative regulatory role in TLR2-induced neutrophil activation and in the development of related in vivo neutrophil-dependent inflammatory processes, such as acute lung injury.

Critical role of proline-rich tyrosine kinase 2 in reversion of the adhesion-mediated suppression of reactive oxygen species generation by human neutrophils

Neutrophils act as the first line of innate immune defense against invading microorganisms during infection and inflammation. The tightly regulated production of reactive oxygen species (ROS) through activation of NADPH oxidase is a major weapon used by neutrophils and other phagocytic leukocytes to combat such pathogens. Cellular adhesion signals play important physiological roles in regulating the activation of NADPH oxidase and subsequent ROS formation. We previously showed that the initial suppression of the oxidase response of chemoattractant-stimulated adherent neutrophils is mediated via inhibition of Vav1-induced activation of the NADPH oxidase regulatory GTPase Rac2 by adhesion signals. In this study we show that prior exposure of neutrophils to a number of cytokines and inflammatory mediators, including TNF-alpha, GM-CSF, and platelet-activating factor, overcomes the adhesion-mediated suppression of ROS formation. Proline-rich tyrosine kinase 2 (pyk2) activity is enhanced under these conditions, correlating with the restoration of Vav1 and Rac2 activities. Both dominant negative pyk2 and a pyk2-selective inhibitor prevented restoration of ROS production induced by TNF-alpha, GM-CSF, and platelet-activating factor, and this loss of pyk2 activity resulted in decreased Vav1 tyrosine phosphorylation and subsequent Rac2 activation. Our studies identify pyk2 as a critical regulatory component and a molecular switch to overcome the suppression of leukocyte oxidant generation by cell adhesion. This activity constitutes a mechanism by which cytokines might lead to rapid elimination of invading pathogens by adherent neutrophils under normal conditions or enhance tissue damage in pathological states.

Cross-Talk between CD14 and Complement Receptor 3 Promotes Phagocytosis of Mycobacteria: Regulation by Phosphatidylinositol 3-Kinase and Cytohesin-1

The glycosylphosphatidyl anchored molecule CD14 to the monocyte membrane plays a prominent role in innate immunity, and the paradigms for CD14 selective signaling are beginning to be elucidated. In this study, transfected human monocytic cell line THP-1 and Chinese hamster ovary (CHO) fibroblastic cells were used to examine phagocytosis of Mycobacterium bovis bacillus Calmette-Guerin (BCG). Flow cytometry was combined with molecular and biochemical approaches to demonstrate a dual mechanism for BCG internalization involving either CD14 alone or a CD14-regulated complement receptor (CR)3-dependent pathway. Phagocytosis by CD14-positive THP-1 cells was attenuated by phosphatidylinositol-3 inhibitors LY294002 and wortmannin and experiments using transfected CHO cells showed substantial accumulation of phosphatidylinositol-3,4,5-trisphosphate at the BCG attachment site in CHO cells expressing CD14 and TLR2 suggesting that bacteria bind to CD14 and use TLR2 to initiate a PI3K signaling pathway. Additional experiments using blocking Abs showed that anti-TLR2 Abs inhibit phagocytosis of BCG by THP-1 cells. Furthermore, knockdown of cytohesin-1, a PI3K-regulated adaptor molecule for beta(2) integrin activation, specifically abrogated CD14-regulated CR3 ingestion of BCG consistent with the observation of physical association between CR3 and cytohesin-1 in cells stimulated with mycobacterial surface components. These findings reveal that mycobacteria promote their uptake through a process of "inside-out" signaling involving CD14, TLR2, PI3K, and cytohesin-1. This converts low avidity CR3 into an active receptor leading to increased bacterial internalization.