Platelets mediate neutrophil-dependent immune complex nephritis in the rat

Association of neutrophil extracellular trap levels with Raynaud's phenomenon, glomerulonephritis and disease index score in SLE patients from Brazil

Neutrophil extracellular traps (NETs) are cell-extruded DNA strands coated with neutrophils' nuclear proteins and enzymes from cytotoxic granules, produced by NETosis, a cell death pathway. They perform an important defensive role in innate immunity, but their increased production and/or inefficient degradation expose new antigens, such as DNA or citrullinated histone peptides, triggering autoimmunity. This study aimed to access possible associations between serum NETs levels with epidemiological, clinical, and serological data from a well-characterized SLE Brazilian patients' cohort. NET levels were evaluated in one hundred seventy serum samples of patients with Systemic Lupus Erythematosus (SLE) using an Immunoassay. Univariate and multivariate binary logistic regression used clinical patients' data as independent variables. Parametric and non-parametric tests compared log10 base serum NET levels transformed between patients' groups. SLE patients were also dichotomized into "High serum NET levels" and "Low serum NET levels" groups. All analyses were performed in R language 4.1.2, and p < 0.05 were considered significant. Increased susceptibility for high serum NET levels was observed in SLE patients with Raynaud's phenomenon (OR = 2.30, 95 % CI = 1.06-5.21 and p = 0.039), independently of any other risk factor. Also, SLE patients with Raynaud's phenomenon presented higher mean NET serum levels (mean = -0.13 vs. -0.51, p = 0.01). In addition, higher mean NET serum levels were associated with glomerulonephritis (mean = -0.45 vs. -0.12, p = 0.03). Ultimately, the SLEDAI index scored higher in the high NETs serum levels group (median = 2.0 vs. 0.0, p = 6 × 10-3). The formation of NETs might be implicated in Raynaud's phenomenon, glomerulonephritis, and disease index score in SLE patients. Our results highlight the importance of serum NET levels as a possible therapeutical target to modulate the clinical course of SLE.

Neutrophils are key mediators in crescentic glomerulonephritis and targets for new therapeutic approaches

Crescentic glomerulonephritis (CGN) results from a diverse set of diseases associated with immune dysregulation and the breakdown of self-tolerance to a wide range of autoantigens, some known and some that remain unknown. Experimental data demonstrate that neutrophils have an important role in the pathogenesis of CGN. Upon activation, neutrophils generate reactive oxygen species, release serine proteases and form neutrophil extracellular traps (NETs), all of which can induce direct tissue damage. In addition, serine proteases such as myeloperoxidase and proteinase 3, presented on NETs, can be processed and recognized as autoantigens, leading to the generation and maintenance of autoimmune responses in susceptible individuals. The basis of the specificity of autoimmune responses in different patients to NET proteins is unclear, but relates at least in part to differences in human leucocyte antigen expression. Conditions associated with CGN are often characterized by aberrant neutrophil activation and NETosis and, in some, impaired NET degradation. Targeting neutrophil degranulation and NETosis is now possible using a variety of novel compounds and may provide a promising therapeutic alternative to glucocorticoid use, which has been a mainstay of management in CGN for decades and is associated with significant adverse effects. In this review, we discuss the evidence supporting the role of neutrophils in the development of CGN and the pathways identified in neutrophil degranulation and NETosis that may translate to novel therapeutic applications.

The non-haemostatic role of platelets in systemic lupus erythematosus

Dysregulation of lymphocyte function, accumulation of autoantibodies and defective clearance of circulating immune complexes and apoptotic cells are hallmarks of systemic lupus erythematosus (SLE). Moreover, it is now evident that an intricate interplay between the adaptive and innate immune systems contributes to the pathogenesis of SLE, ultimately resulting in chronic inflammation and organ damage. Platelets circulate in the blood and are chiefly recognized for their role in the prevention of bleeding and promotion of haemostasis; however, accumulating evidence points to a role for platelets in both adaptive and innate immunity. Through a broad repertoire of receptors, platelets respond promptly to immune complexes, complement and damage-associated molecular patterns, and represent a major reservoir of immunomodulatory molecules in the circulation. Furthermore, evidence suggests that platelets are activated in patients with SLE, and that they could contribute to the circulatory autoantigenic load through the release of microparticles and mitochondrial antigens. Herein, we highlight how platelets contribute to the immune response and review evidence implicating platelets in the pathogenesis of SLE.

Platelet retention in inflamed glomeruli occurs via selective prolongation of interactions with immune cells

Platelet-leukocyte interactions promote acute glomerulonephritis. However, neither the nature of the interactions between platelets and immune cells nor the capacity of platelets to promote leukocyte activation has been characterized in this condition. We used confocal intravital microscopy to define the interactions of platelets with neutrophils, monocytes, and endothelial cells in glomerular capillaries in mice. In the absence of inflammation, platelets underwent rapid on/off interactions with immune cells. During glomerulonephritis induced by in situ immune complex formation, platelets that interacted with neutrophils or monocytes, but not with other intraglomerular cells, were retained in the glomerulus for prolonged durations. Depletion of platelets inhibited both neutrophil recruitment and activation. Inhibition of platelet activating factor reduced neutrophil recruitment without impacting reactive oxygen species generation, while blocking CXC chemokine ligand 7 (CXCL7) reduced both responses. In contrast, inhibition of the adenosine diphosphate and thromboxane A2 pathways inhibited neutrophil reactive oxygen species generation without affecting neutrophil adhesion. Thus, platelet retention in glomerular capillaries following immune complex deposition stems from prolongation of platelet interactions with immune cells but not other substrates. Pro-inflammatory mediators play divergent roles in promoting neutrophil retention and activation in glomerular capillaries.

An inhibitor of spleen tyrosine kinase suppresses experimental crescentic glomerulonephritis

Non-selective inhibitors of spleen tyrosine kinase (SYK) efficiently suppress disease in T cell-dependent models of crescentic glomerulonephritis. However, the therapeutic potential of selective SYK inhibitors in this disease has not been established. In addition, we lack knowledge regarding SYK expression in non-myeloid cells in glomerulonephritis. We addressed these two issues in a rat model of nephrotoxic serum nephritis (NTN) using a SYK inhibitor, GS-492429. Disease was induced in Sprague-Dawley rats (Study 1) or Wistar-Kyoto (WKY) rats (Study 2) by immunization with sheep IgG and administration of sheep anti-rat nephrotoxic serum. Animals were untreated or received GS-492429 (30 mg/kg/bid) or vehicle treatment from 2 h before nephrotoxic serum injection until being killed 3 or 24 h later (Study 1) or 14 days later (Study 2). Two-colour confocal microscopy found that SYK expression in NTN kidney was restricted to myeloid cells and platelets, with no evidence of SYK expression by T cells, mesangial cells, podocytes or tubular epithelial cells. In Study 1, GS-492429 treatment significantly reduced glomerular neutrophil and macrophage infiltration, with protection from glomerular thrombosis and proteinuria. In Study 2, GS-492429 treatment reduced glomerular crescent formation by 70% on day 14 NTN in conjunction with reduced glomerular thrombosis, glomerulosclerosis and tubular damage. This was accompanied by a marked reduction in markers of inflammation (CCL2, TNF-α, NOS2, MMP-12). Importantly, the protective effects of GS-492429 were independent of T cell infiltration and activation and independent of JAK/STAT3 signalling. In conclusion, this study demonstrates that a SYK inhibitor can suppress the development of crescentic glomerulonephritis through effects upon myeloid cells and platelets.

New Insights on Platelets and Platelet-Derived Microparticles in Systemic Lupus Erythematosus

PURPOSE OF REVIEW

Current knowledge on the role of platelets and platelet-derived microparticles (PMPs) on the immune system has been fast-growing. Systemic lupus erythematosus (SLE) is a systemic auto-immune disorder characterized by a loss of tolerance toward nuclear auto-antigens. Although recent studies allowed a better understanding of SLE pathogenesis, there is an urgent need for the development of new treatments and the identification of new biomarkers to assess the disease activity. We describe here the state-of-the-art knowledge linking platelets and PMPs to SLE.

RECENT FINDINGS

Platelet system activation is a key event in the pathogenesis of SLE. Circulating immune complexes, anti-phospholipid antibodies, and infectious agents such as virus are the main activators of platelets in SLE. Platelet activation can be monitored through different ways such as P-selectin expression, mean platelet volume, or circulating PMP levels, suggesting their potential use as biomarkers. Upon activation, platelets promote type I interferon production, NETosis, dendritic cell activation, and T and B lymphocyte activation, all essential events contributing to the development of SLE. Of interest, platelets also play a fundamental role in SLE organ disease such as the development of cardiovascular, thrombotic, and renal diseases. Finally, we review current knowledge on drugs targeting platelet activation and their potential impact on SLE pathogenesis. Platelets play a major role in SLE pathogenesis and organ disease and represent a great potential for novel biomarkers and drug development.

Platelet-derived sphingosine-1-phosphate and inflammation: from basic mechanisms to clinical implications

Beyond key functions in hemostasis and thrombosis, platelets are recognized as key players of inflammation, an underlying feature of a variety of diseases. In this regard, platelets act as a circulating source of several pro- and anti-inflammatory molecules, which are secreted from their intracellular stores upon activation. Among them, mounting evidence highlights a crucial role of sphingosine-1-phosphate (S1P), a multifunctional sphingoid mediator. S1P-induced pleiotropic effects include those crucial in inflammatory processes, such as the maintenance of the endothelial barrier integrity, and leukocyte activation and recruitment at the injured site. This review outlines the peculiar features and molecular mechanisms that allow platelets for acting as a unique factory that produces and stores S1P in large quantities. A particular emphasis is placed on the autocrine and paracrine roles of S1P derived from the "inflamed" platelets, highlighting the role of its cross-talk with endothelial and blood cells involved in inflammation, and the mechanisms of its contribution to the development and progression of inflammatory diseases. Finally, potential clinical implications of platelet-derived S1P as diagnostic tool of inflammatory severity, and as therapeutic target in inflammation are discussed.

Myeloid cell-mediated renal injury in rapidly progressive glomerulonephritis depends upon spleen tyrosine kinase

Antibody-dependent activation of myeloid cells within the glomerulus plays a central role in rapidly progressive forms of glomerulonephritis. The spleen tyrosine kinase (Syk) is expressed by all leukocytes, except mature T cells, and is required for signalling via the B-cell receptor, Fc receptors, and some integrins. Syk has been proposed as a therapeutic target in glomerulonephritis. However, little is known of Syk activation in human kidney disease, while studies in experimental glomerulonephritis using non-selective Syk inhibitors require validation via conditional gene deletion. The current study addressed both of these important points. Syk activation (Tyr(525/526) phosphorylation) was examined in a cohort of 96 patients with different glomerulonephritides. Syk activation was evident in infiltrating leukocytes, mainly neutrophils and macrophages, in 36/40 cases of rapidly progressive glomerulonephritis. In contrast, non-proliferative diseases showed little or no Syk activation. Glomerular and interstitial cells exhibiting Syk activation correlated with renal function and systemic inflammation. Next, we examined mice with conditional Syk gene deletion in myeloid cells (Syk(My) ) versus Syk(f/f) littermate controls in nephrotoxic serum nephritis - a model of rapidly progressive glomerulonephritis. Control Syk(f/f) mice featured a transient neutrophil influx at 3 h and severe disease on day 9 of nephrotoxic serum nephritis, with crescent formation, macrophage infiltration, inflammation, kidney fibrosis, and renal dysfunction. In contrast, Syk(My) mice had significantly reduced neutrophil and macrophage infiltration despite equivalent glomerular deposition of humoral reactants. Syk(My) mice exhibited reduced crescent formation, inflammation, and fibrosis, with improved renal function on day 9 of nephrotoxic serum nephritis. In conclusion, Syk activation is prominent in infiltrating myeloid cells in human rapidly progressive glomerulonephritis, and functional studies demonstrate that Syk deletion in myeloid cells is protective in mouse nephrotoxic serum nephritis.

Nicotine mediates hypochlorous acid-induced nuclear protein damage in mammalian cells

Activated neutrophils secrete hypochlorous acid (HOCl) into the extracellular space of inflamed tissues. Because of short diffusion distance in biological fluids, HOCl-damaging effect is restricted to the extracellular compartment. The current study aimed at investigating the ability of nicotine, a component of tobacco and electronic cigarettes, to mediate HOCl-induced intracellular damage. We report, for the first time, that HOCl reacts with nicotine to produce nicotine chloramine (Nic-Cl). Nic-Cl caused dose-dependent damage to proliferating cell nuclear antigen (PCNA), a nuclear protein, in cultured mammalian lung and kidney cells. Vitamin C, vitamin E analogue (Trolox), glutathione, and N-acetyl-L-cysteine inhibited the Nic-Cl-induced PCNA damage, implicating oxidation in PCNA damage. These findings point out the ability of nicotine to mediate HOCl-induced intracellular damage and suggest antioxidants as protective measures. The results also raise the possibility that Nic-Cl can be created in the inflamed tissues of tobacco and electronic cigarette smokers and may contribute to smoking-related diseases.

Platelets are relevant mediators of renal injury induced by primary endothelial lesions

Several studies have suggested a prominent (pro)inflammatory and harmful role of platelets in renal disease, and newer work has also demonstrated platelet release of proangiogenic factors. In the present study, we investigated the role of platelets in a mouse model of selective endothelial cell injury using either platelet depletion or the pharmacological P2Y12 receptor blocker clopidogrel as an interventional strategy. The concanavalin A/anti-concanavalin A model was induced in left kidneys of C57bl/6J wild-type mice after initial platelet depletion or platelet-inhibiting therapy using clopidogrel. FACS analysis of glycoprotein IIb/IIIa/P-selectin double-positive platelets and platelet-derived microparticles demonstrated relevant platelet activation after the induction of selective endothelial injury in mice. Enhanced platelet activation persisted for 5 days after disease induction and was accompanied by increased amounts of circulating platelet-derived microparticles as potential mediators of a prolonged procoagulant state. By immunohistochemistry, we detected significantly reduced glomerular injury in platelet-depleted mice compared with control mice. In parallel, we also saw reduced endothelial loss and a consequently reduced repair response as indicated by diminished proliferative activity. The P2Y12 receptor blocker clopidogrel demonstrated efficacy in limiting platelet activation and subsequent endothelial injury in this mouse model of renal microvascular injury. In conclusion, platelets are relevant mediators of renal injury induced by primary endothelial lesions early on, as demonstrated by platelet depletion as well as platelet inhibition via the P2Y12 receptor. While strategies to prevent platelet-endothelial interactions have shown protective effects, the contribution of platelets during renal regeneration remains unknown.

A Novel Mouse Model for MPO-ANCA-Associated Glomerulonephritis

We established a novel model mouse for myeloperoxidase anti-neutrophil cytoplasmic antibody (MPO-ANCA)-associated glomerulonephritis with crescentic formation, which was induced by administering bovine serum albumin (BSA). Neutrophil infiltration into the renal glomeruli began at 8 weeks and crescent formation was observed from 10 weeks after the first BSA injection. Platelet and neutrophil counts significantly increased, and proteinuria was observed from 5 weeks. MPO-ANCA increased slightly at 4 and markedly at 9 weeks, and the TNF-alpha level increased at 11 weeks. Glomerular neutrophil infiltration was correlated with MPO-ANCA levels. In addition, proteinuria also significantly correlated with MPO-ANCA levels. Finally, renal crescent formation was associated with an increase of MPO-ANCA levels and neutrophil infiltration into glomeruli. The glomerular immune deposition of IgG and C3 was observed. These findings indicate that BSA induces neutrophil activation of peripheral blood followed by the elevation of MPO-ANCA, resulting in the development of crescentic glomerulonephritis in mice.

Staphylococcus aureus α-hemolysin promotes platelet-neutrophil aggregate formation

Community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) causes severe hemorrhagic necrotizing pneumonia associated with high mortality. Exotoxins have been implicated in the pathogenesis of this infection; however, the cellular mechanisms responsible remain largely undefined. Because platelet-neutrophil aggregates (PNAs) can dysregulate inflammatory responses and contribute to tissue destruction, we investigated whether exotoxins from MRSA could stimulate formation of PNAs in human whole blood. Strong PNA formation was stimulated by toxins from stationary phase but not log phase CA-MRSA, and α-hemolysin was singularly identified as the mediator of this activity. MRSA exotoxins also caused neutrophil (polymorphonuclear leukocyte) activation, as measured by increased CD11b expression, although platelet binding was not driven by this mechanism; rather, α-hemolysin-induced PNA formation was solely platelet P-selectin dependent. These findings suggest a role for S. aureus α-hemolysin-induced PNA formation in alveolar capillary destruction in hemorrhagic/necrotizing pneumonia caused by CA-MRSA and offer novel targets for intervention.

Bruton's Tyrosine Kinase mediates platelet receptor-induced generation of microparticles: a potential mechanism for amplification of inflammatory responses in rheumatoid arthritis synovial joints

Platelet microparticles (pMPs) are small membrane-coated vesicles that are released from the plasma membrane upon platelet activation. In the joint fluid of patients with rheumatoid arthritis, pMP can interact with and activate fibroblast-like synoviocytes (FLS), which are important effector cells that mediate both immune activation and joint destruction. The signaling process by which engagement of glycoprotein VI (GPVI), a surface glycoprotein receptor for collagen which is expressed on platelets, triggers pMP generation is poorly understood, but has been suggested to involve Spleen Tyrosine Kinase (SYK), best known as an upstream activator of Bruton's Tyrosine Kinase (BTK) in B cells. In this study, we showed that activation of human platelets triggered by convulxin or collagen, specific ligands for GPVI receptor, or alternatively by antibody-mediated cross-linking of another platelet receptor, C type lectin-like receptor 2 (CLEC2), resulted in phosphorylation of BTK and downstream effector, phospholipase Cγ2 (PLCγ2). A potent and selective BTK inhibitor, RN486, inhibited GPVI- or CLEC2-mediated PLCγ2 phosphorylation and pMP production in a dose-dependent manner. BTK is also an essential effector of B cell receptor (BCR)-induced B cell signaling. Consistent with the biology, the IC50s of BTK inhibitors with varying potencies in a BCR-dependent B cell activation marker assay correlated with those in the GPVI-mediated PLCγ2 phosphorylation. In a co-culture system consisting of human primary synovial FLS and activated human platelets, convulxin stimulation resulted in elevated production of pro-inflammatory cytokines, IL-6 and IL-8, an effect which was dose-dependently blocked by RN486. The effects are specific as RN486 abrogated platelet aggregation induced by GPVI ligands but not by other platelet surface receptor agonists. Taken together, our data further support the potential therapeutic utility of BTK inhibitors in RA therapy, by inhibiting GPVI-mediated platelet activation and thus subsequent amplification of inflammation driven by pMP-induced FLS cytokines production.

Platelets are not critical effector cells for the time course of murine passive crescentic glomerulonephritis

Although platelets are well-known effector cells of inflammatory renal disease, clinical studies were not able to establish platelet inhibition as an effective therapy. Our previous studies using Vasodilator stimulated Phosphoprotein- and P2Y1-deficient mice suggested some early, but no long-term effects of platelets in passive crescentic glomerulonephritis. To define the role of platelets for this disease model, passive crescentic glomerulonephritis was induced in 72 C57Bl/6 mice by intraperitoneal injection of sheep anti-rabbit glomerular basement membrane antibody on 2 consecutive days. Platelets were depleted using anti-glycoprotein Ibα antibodies (p0p3/p0p4) every 4th day. Mice treated with equal amounts of sterile Phosphate buffered solution or rat-IgG served as controls. Blood, urine, and tissues were harvested on days 3 and 28. Renal tissue sections were evaluated after immunostaining using (semi)quantitative and computer-assisted image analysis. Compared to controls, efficient depletion was achieved as indicated by a markedly prolonged bleeding time and a more than 90% reduction in platelet counts (800/nl vs. 42/nl; P < 0.001). Functional (creatinine-clearance and proteinuria) parameters demonstrated no significant differences between the groups. Neither parameters of renal injury (glomerulosclerosis and fibrosis) nor glomerular/tubulointerstitial matrix expansion (by collagen IV staining), glomerular capillary rarefaction (lectin staining), and the glomerular/tubulointerstitial proliferative response (proliferating cell nuclear antigen) demonstrated any differences between platelet-depleted mice and PBS- or rat-IgG-treated nephritic mice at any time point. Despite effective platelet inhibition/depletion, neither the short- nor long-term course of passive crescentic nephrotoxic nephritis was affected. These data indicate that platelets play a minor role during the time course of this disease model in the mouse.

Spleen tyrosine kinase promotes acute neutrophil-mediated glomerular injury via activation of JNK and p38 MAPK in rat nephrotoxic serum nephritis

Glomerular antibody deposition induces acute neutrophil-mediated glomerular injury via activation of c-Jun amino terminal kinase (JNK) and p38 mitogen-activated protein kinase (MAPK). However, the link between antibody deposition and activation of JNK/p38 MAPK signalling is unclear. This study tested the postulate that spleen tyrosine kinase (Syk), which is activated via Fcγ-receptor ligation, is required for activation of JNK and p38 signalling and acute neutrophil-mediated glomerular injury. We used a Syk inhibitor (SYKi) in rat nephrotoxic serum nephritis (NTN) in which neutrophil-mediated glomerular injury is dependent upon JNK and p38 signalling. SYKi or vehicle treatment of Sprague-Dawley rats began 30 min before administration of anti-GBM serum with rats killed 3 or 24 h later. Immunostaining identified de novo glomerular Syk activation (p-Tyr 525/526) in untreated NTN, being most prominent in neutrophils. Vehicle and untreated NTN exhibited heavy proteinuria and glomerular thrombosis at 24 h with P-selectin and fibrin immunostaining within capillaries, glomerular macrophage and T cell infiltration, activation of JNK and p38 MAPK signalling, and upregulation of glomerular mRNA levels of pro-inflammatory molecules (TNF-α, NOS2, MMP-12 and CCL2). In contrast, SYKi treatment provided complete protection from proteinuria, with a profound reduction in glomerular thrombosis and immunostaining for P-selectin and fibrin, and a substantial reduction in glomerular mRNA levels of pro-inflammatory molecules. SYKi treatment also reduced the acute glomerular neutrophil influx and pro-inflammatory response at 3 h in NTN. These protective effects were associated with a significant reduction in glomerular JNK and p38 MAPK activation. In addition, activation of Syk, JNK and p38 was identified in human biopsy samples of acute crescentic glomerulonephritis. In conclusion, this study demonstrates that Syk signalling is required for JNK and p38 MAPK signalling and acute neutrophil-dependent glomerular injury in rat NTN. These findings identify Syk as a potential therapeutic target in antibody-dependent kidney disease.

Activation of human liver sinusoidal endothelial cell by human platelets induces hepatocyte proliferation

BACKGROUND & AIMS

We previously reported that platelets promote hepatocyte proliferation. In this study, we focused on the role of platelets in liver sinusoidal endothelial cells (LSECs) in addition to their role in hepatocyte in liver regeneration.

METHODS

Immortalized human LSECs (TMNK-1) were used. The LSECs were co-cultured with human platelets, and the proliferation of LSECs and the excretion of growth factors and interleukin-6 (IL-6) were subsequently measured. The main factor from platelets which induced the excretion of IL-6 from LSECs was determined using inhibitors of each component contained in the platelets. The need for direct contact between platelets and LSECs was investigated using cell culture inserts. The proliferation of human primary hepatocytes was measured after the addition of the supernatant of LSECs cultured with or without platelets.

RESULTS

The number of LSECs cocultured with platelets significantly increased. Excretion of IL-6 and vascular endothelial growth factor (VEGF) increased in LSECs with platelets. JTE-013, a specific antagonist for sphingosine 1-phosphate (S1P) 2 receptors, inhibited the excretion of IL-6 from LSECs after the addition of platelets. When the platelets and LSECs were separated by the cell culture insert, the excretion of IL-6 from LSECs was decreased. DNA synthesis was significantly increased in human primary hepatocytes cultured with the supernatant of LSECs with platelets.

CONCLUSIONS

Platelets promote LSEC proliferation and induce IL-6 and VEGF production. Direct contact between the platelets and LSECs and S1P, that are contained in platelets, were involved in the excretion of IL-6 from LSECs. IL-6 from LSECs induced proliferation of parenchymal hepatocytes.

Platelet recruitment to the inflamed glomerulus occurs via an alphaIIbbeta3/GPVI-dependent pathway

Recruitment of leukocytes to glomeruli is fundamental to the pathogenesis of many forms of glomerulonephritis. In a model of glomerulonephritis induced by in situ immune complex deposition, we previously observed that, in addition to leukocytes, platelets accumulate in glomerular capillaries, where they contribute to leukocyte recruitment. However, the mechanisms of platelet recruitment and the role of platelet-expressed P-selectin in leukocyte recruitment require further investigation. We used intravital microscopy to examine the mechanisms of platelet and leukocyte recruitment to glomeruli of mice following administration of an antibody against the glomerular basement membrane (anti-GBM antibody). Platelet recruitment was initiated within five minutes of administration of anti-GBM antibody. This was unaltered by inhibition of platelet GPIbalpha but was prevented by the absence of platelet GPVI. Fibrinogen was deposited in glomerular capillaries via a partially intercellular adhesion molecule 1 (ICAM-1)-dependent mechanism, and inhibition of alpha(IIb)beta(3), fibrinogen and ICAM-1 inhibited platelet recruitment. Notably, neutrophil depletion also reduced platelet accumulation, indicating a cooperative interaction underlying recruitment of platelets and neutrophils. Finally, using bone marrow chimeras to restrict expression of P-selectin to platelets or endothelial cells, platelet but not endothelial P-selectin was required for glomerular leukocyte recruitment. Together these data indicate that platelet recruitment in this model is dependent on the combined actions of GPVI and the alpha(IIb)beta(3)/fibrinogen/ICAM-1 pathway and that platelet P-selectin is crucial for subsequent leukocyte recruitment.

Platelet dynamics in the early phase of postischemic liver in vivo

BACKGROUND

In liver surgery, ischemia/reperfusion injury occasionally leads to liver failure by activating Kupffer cells (KCs) and leukocytes. However, few reports have demonstrated a relationship between KCs and platelets in vivo. This study investigated the relationship between these cells using intravital microscopy.

MATERIALS AND METHODS

Male Wistar rats were divided into two groups: (1) KC+ group, receiving 1 mL saline; and (2) KC- group, intravenously injected with liposome-encapsulated dichloromethylene disphosphonate for elimination of KCs. At 48 h after administration, 20 min of total normothermic hepatic ischemia was induced. Rhodamine-6G-labeled platelets and sinusoidal alterations were monitored using intravital microscopy up to 120 min after reperfusion. P-selectin, accumulated leukocytes and morphological damage, and alanine aminotransferase were evaluated.

RESULTS

In the KC+ group, numbers of adherent platelets increased significantly within 30 min after reperfusion. Endothelial cells of sinusoids in which KCs were mainly located were destroyed and the sinusoids were significantly constricted after reperfusion. Conversely, in the KC- group, adherent platelets in sinusoids were suppressed, and sinusoidal perfusion, endothelial cell damage and serum alanine aminotransferase levels were significantly improved. P-selectin on sinusoidal endothelial cells was not observed up to 120 min after reperfusion in either group.

CONCLUSIONS

Adherent platelets appear to reflect activation of KCs and lead to leukocyte accumulation, resulting in sinusoidal perfusion disturbance and liver failure. Evaluation of adherent platelets in the microcirculation offers an important marker of hepatic injury.

The simple design of complement factor H: Looks can be deceiving

The complement system is a powerful component of innate immunity which recognizes and facilitates the elimination of pathogens and unwanted host material. Since complement can also lead to host tissue injury and inflammation, strict regulation of its activation is important. One of the key regulators is complement factor H (CFH), a protein with an ever-expanding list of relevant functions. Inherited mutations in CFH can account for membranoproliferative glomerulonephritis (MPGN) type II, atypical hemolytic uremic syndrome, and age-related macular degeneration. The former can be associated with excessive systemic complement activation from dysfunctional CFH, while the latter two are associated with mutations affecting the ability of CFH to bind to anionic surfaces such as on endothelial cells and glomerular and retinal capillary walls. Mice with targeted deletion of CFH can spontaneously develop MPGN and have increased susceptibility to models of GN. In the rodent, CFH on platelets functions as the immune adherence receptor, analogous to CR1 on primate erythrocytes. In mice, platelets lacking CFH are unable to effectively clear immune complexes which results in their accumulation in glomeruli. The same switch also appears to be true in the rodent podocyte where CFH is present in place of CR1 in human podocytes. Thus, CFH has a variety of functions which can affect the diverse roles the complement system plays in health and disease.

Leukocyte recruitment to the inflamed glomerulus: a critical role for platelet-derived P-selectin in the absence of rolling

The renal glomerulus is one of the few sites within the microvasculature in which leukocyte recruitment occurs in capillaries. However, due to the difficulty of directly visualizing the glomerulus, the mechanisms of leukocyte recruitment to glomerular capillaries are poorly understood. To overcome this, we rendered murine kidneys hydronephrotic to allow the visualization of the functional glomerular microvasculature during an inflammatory response. These experiments demonstrated that following infusion of anti-glomerular basement membrane (GBM) Ab, leukocytes became adherent in glomerular capillaries via a process of immediate arrest, without undergoing prior detectable rolling. However, despite the absence of rolling, this recruitment involved nonredundant roles for the P-selectin/P-selectin glycoprotein ligand-1 and beta2 integrin/ICAM-1 pathways, suggesting that a novel form of the multistep leukocyte adhesion cascade occurs in these vessels. Anti-GBM Ab also increased glomerular P-selectin expression and induced a P-selectin-independent increase in platelet accumulation. Moreover, platelet depletion prevented both the increase in glomerular P-selectin, and the leukocyte recruitment induced by anti-GBM Ab. Furthermore, depletion of neutrophils and platelets also prevented the increase in urinary protein excretion induced by anti-GBM Ab, indicating that their accumulation in glomeruli contributed to the development of renal injury. Finally, infusion of wild-type platelets into P-selectin-deficient mice restored the ability of glomeruli in these mice to support leukocyte adhesion. Together, these data indicate that anti-GBM Ab-induced leukocyte adhesion in glomeruli occurs via a novel pathway involving a nonrolling interaction mediated by platelet-derived P-selectin.

Activation of mesangial cells by platelets in systemic lupus erythematosus via a CD154-dependent induction of CD40

BACKGROUND

Platelets are potential contributors to glomerular injury via the release of chemotactic and/or mitogenic mediators upon activation or through direct CD154/CD40-dependent interaction with cell components of the glomerulus. We examined whether platelets could activate mesangial cells and the potential role of the platelet-associated CD154.

METHODS

Thrombin-activated platelets from systemic lupus erythematosus (SLE) patients or from disease or healthy controls were grown with human mesangial cells in the presence or not of a neutralizing anti-CD154 antibody either in contact or in a noncontact setting, the platelets and mesangial cells being separated by a pore size semipermeable membrane. The induction of mesangial cell surface antigens was assayed by flow cytometry. The quantification of mesangial cell proliferation was performed by 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT) assay and the production of transforming growth factor-beta1 (TGF-beta1), platelet-derived growth factor (PDGF) and soluble CD40 were measured by enzyme-linked immunosorbent assay (ELISA).

RESULTS

Activated platelets from patients with SLE could induce an up-regulation of the expression of CD40 on mesangial cells with a concomitant release of soluble CD40. This induction required a direct contact between platelets and mesangial cells and was dependent upon the platelet-associated CD154. Pathologic consequences of the up-regulation of CD40 were a CD40-dependent stimulation of the proliferation of mesangial cells and a CD40-dependent increased production of TGF-beta1 by these cells.

CONCLUSION

Platelets from patients with SLE can activate mesangial cells through CD40/CD154 interactions, leading to an induction of proliferation of the mesangial cells and an enhanced production of TGF-beta1, a profibrotic cytokine.

Platelet activation leads to activation and propagation of the complement system

Inflammation and thrombosis are two responses that are linked through a number of mechanisms, one of them being the complement system. Various proteins of the complement system interact specifically with platelets, which, in turn, activates them and promotes thrombosis. In this paper, we show that the converse is also true: activated platelets can activate the complement system. As assessed by flow cytometry and immunoblotting, C3 deposition increased on the platelet surface upon cell activation with different agonists. Activation of the complement system proceeded to its final stages, which was marked by the increased generation of the anaphylotoxin C3a and the C5b-9 complex. We identified P-selectin as a C3b-binding protein, and confirmed by surface plasmon resonance binding that these two proteins interact specifically with a dissociation constant of 1 μM. Using heterologous cells expressing P-selectin, we found that P-selectin alone is sufficient to activate the complement system, marked by increases in C3b deposition, C3a generation, and C5b-9 formation. In summary, we have found that platelets are capable of activating the complement system, and have identified P-selectin as a receptor for C3b capable of initiating complement activation. These findings point out an additional mechanism by which inflammation may localize to sites of vascular injury and thrombosis.

Effects of leukocytosis and macrophage activation on anti-Thy 1.1 glomerulonephritis in the rat

OBJECTIVE

In anti-Thy 1.1 proliferative glomerulonephritis (GN), glomerular infiltration of polymorphonuclear leukocytes (PMNs) reaches a peak level after 1 h and that of macrophages does so 24 h after induction. However, the roles of PMNs and macrophages in the pathogenesis of anti-Thy 1.1 GN remain unclear. We examined the effects on this model of leukocytosis induced by granulocyte colony-stimulating factor (G-CSF) and of macrophages stimulated by macrophage colony-stimulating factor (M-CSF).

MATERIAL AND METHODS

Anti-Thy 1.1 GN was induced in male Wistar rats by intravenous injection of OX-7, a monoclonal antibody to the Thy 1 antigen. G-CSF (10 microg/kg/day), M-CSF (20 microg/kg/day) or vehicle was administered intraperitoneally for 7 days starting 24 h before the injection of OX-7. Histological examination of renal biopsy specimens was performed on Days 1, 5 and 14 after induction.

RESULTS

Circulating and glomerular-infiltrating PMNs (RP-3-positive cells) were increased at Day 5 in G-CSF-treated rats compared with controls receiving vehicle, and glomerular mesangiolysis continued at Day 5. The number of proliferating cells positive for proliferating cell nuclear antigen at Day 5 and matrix scores at Day 14 were smaller in G-CSF-treated rats than in control rats. The mesangiolysis score was significantly higher in the G-CSF group than in the control group at Days 5 and 14, but not at Day 1. ED-1-positive cells were increased in number at Day 5 and matrix accumulation decreased at Day 14 in M-CSF-treated rats compared with controls. Serum creatinine level at Day 14 was lower in the M-CSF group, but not in the G-CSF group, compared with the control group.

CONCLUSIONS

Activated macrophages may inhibit excess matrix accumulation and ameliorate the recovery of renal function, whereas leukocytosis inhibits the repair of mesangial cell injury in this model.

P2 receptor antagonist PPADS inhibits mesangial cell proliferation in experimental mesangial proliferative glomerulonephritis

BACKGROUND

Although extracellular nucleotides have been shown to confer mitogenic effects in cultured rat mesangial cells through activation of purinergic P2 receptors (P2Y receptors), thus far the in vivo relevance of these findings is unclear. Virtually all cells and in particular the dense granules of platelets contain high levels of nucleotides that are released upon cell injury or platelet aggregation. In experimental mesangial proliferative glomerulonephritis in the rat (anti-Thy1 model), mesangiolysis and glomerular platelet aggregation are followed by a pronounced mesangial cell (MC) proliferative response leading to glomerular hypercellularity. Therefore, we examined the role of extracellular nucleotides and their corresponding receptors in nucleotide-stimulated cultured mesangial cells and in inflammatory glomerular disease using the P2 receptor antagonist PPADS.

METHODS

The effects of PPADS on nucleotide- or fetal calf serum (FCS)-stimulated proliferation of cultured MC were measured by cell counting and [3H]thymidine incorporation assay. After induction of the anti-Thy1 model, rats received injections of the P2-receptor antagonist PPADS at different doses (15, 30, 60 mg/kg BW). Proliferating mesangial and non-mesangial cells, mesangial cell activation, matrix accumulation, influx of inflammatory cells, mesangiolysis, microaneurysm formation, and renal functional parameters were assessed during anti-Thy1 disease. P2Y-mRNA and protein expression was assessed using RT-PCR and real time PCR, Northern blot analysis, in situ hybridization, and immunohistochemistry.

RESULTS

In cultured mesangial cells, PPADS inhibited nucleotide, but not FCS-stimulated proliferation in a dose-dependent manner. In the anti-Thy1 model, PPADS specifically and dose-dependently reduced early (day 3), but not late (day 8), glomerular mesangial cell proliferation as well as phenotypic activation of the mesangium and slightly matrix expansion. While no consistent effect was obtained in regard to the degree of mesangiolysis, influx of inflammatory cells, proteinuria or blood pressure, PPADS treatment increased serum creatinine and urea in anti-Thy1 rats. P2Y receptor expression (P2Y2 and P2Y6) was detected in cultured MC and isolated glomeruli, and demonstrated a transient marked increase during anti-Thy1 disease.

CONCLUSION

These data strongly suggest an in vivo role for extracellular nucleotides in mediating early MC proliferation after MC injury.

A complement-dependent model of thrombotic thrombocytopenic purpura induced by antibodies reactive with endothelial cells

Thrombotic thrombocytopenic purpura (TTP) is an immunologically mediated disease characterized by thrombocytopenia, hemolytic anemia, and pathologic changes in various organs, including the kidney, which are secondary to widespread thromboses. Central to TTP is platelet activation, which may occur from a variety of mechanisms, including endothelial cell activation or injury. In this study, injection of K6/1, a monoclonal antibody with widespread reactivity toward endothelia, led to dose-dependent thrombocytopenia in rats. This was magnified if animals were preimmunized with mouse IgG, thereby resulting in an accelerated autologous phase of injury. In this setting, significant anemia also resulted. Rats injected with K6/1 developed renal injury, consisting of tubular damage and glomerular thrombi. Thrombocytopenia and renal morphological abnormalities were eliminated if animals were complement depleted with cobra venom factor prior to K6/1 injection and worsened when the activity of the ubiquitous complement regulator Crry was inhibited with function-neutralizing antibodies. Therefore, we have developed a complement-dependent model of TTP in rats by injecting monoclonal antibodies reactive with endothelial cells. Antibody-directed complement activation leads to stimulation of platelets, through direct interactions with complement fragments and/or indirectly through endothelial cell activation or injury, with the subsequent development of TTP.

Sphingosine 1-phosphate stimulates rat mesangial cell proliferation from outside the cells

BACKGROUND

Proliferation of mesangial cells (MCs) is the initial step in glomerulonephritis, and platelet-derived mediators have been shown to play a significant role in this proliferation. Sphingosine 1-phosphate (S1P), one of the sphingolipids, is abundantly stored in platelets and is released upon stimulation. We examined the effects of S1P and related sphingolipids on the cell fate of cultured MCs in order to elucidate potential roles of these lipid mediators in glomerulonephritis.

METHODS

Cell proliferation was evaluated by bromodeoxy uridine (BrdU) incorporation together with MTS assay. Apoptosis of MCs was evaluated by examining annexin V staining and typical morphological changes in nuclei. We also examined the metabolism of [(3)H]sphingosine in MCs in either the presence or absence of platelet-derived growth factor (PDGF). The expression of endothelial differentiation genes (edg), which are the cell surface receptors for S1P in MCs, was examined by RT-PCR.

RESULTS

S1P, but not the other sphingolipids, stimulated MC proliferation. In contrast, dimethylsphingosine (DMS) induced apoptosis in the MCs. The amount of sphingosine (Sph) converted into S1P was small and was not affected by PDGF. This observation suggested that Sph kinase activity producing S1P from Sph was low in the MCs. Furthermore, expression of edg-1, -2 and -5 in MCs was confirmed by RT-PCR.

CONCLUSIONS

Our observations suggest that S1P stimulates MC proliferation from outside the cells, and not as a second messenger for PDGF. The modulation of MC fate with sphingolipids may provide possible strategies for the treatment of glomerulonephritis.

Neutrophil-platelet interactions and their relevance to bovine respiratory disease

Respiratory disease is a serious and significant health problem for the bovine industry. Classically, the clinical and research focus has been on the putative causative agents and conditions, and their interactions with host inflammatory cells, particularly alveolar macrophages and blood neutrophils. There is, currently, growing acceptance of the concept that blood platelets play a primary role in the inflammatory process. This review explores the implications of such pro-inflammatory activity, especially in the context of neutrophil-platelet interactions, and the species specificity of cellular responses. The relevance of these issues for the treatment and prevention of bovine respiratory disease is also discussed.

C5b-9 membrane attack complex mediates endothelial cell apoptosis in experimental glomerulonephritis

We studied the role of the C5b-9 membrane attack complex in two models of inflammatory glomerulonephritis (GN) initiated by acute glomerular endothelial injury in Piebold-viral-Glaxo (PVG) complement-sufficient rats (C+), C6-deficient rats (C6-), and rats systematically depleted of complement with cobra venom factor (CVF). GN was induced by performing a left nephrectomy and selectively perfusing the right kidney with either 1) the lectin concanavalin A (Con A) followed by complement-fixing anti-Con A (Con A GN) or 2) purified complement-fixing goat anti-rat glomerular endothelial cell (GEN) antibody [immune-mediated thrombotic microangiopathy (ITM)]. Comparable levels of GEN apoptosis were detected in C+ animals in both models. CVF administration reduced GEN apoptosis by 10- to 12-fold. GEN apoptosis was C5b-9 dependent because PVG C6- rats were protected from GEN loss. Furthermore, functional inhibition of the cell surface complement regulatory protein CD59 by renal perfusion with anti-CD59 antibody in ITM resulted in a 3.5-fold increase in GEN apoptosis. Last, in Con A GN, abrogation of GEN apoptosis preserved endothelial integrity and renal function. This study demonstrates the specific role of C5b-9 in the induction of GEN apoptosis in experimental inflammatory GN, a finding with implications for diseases associated with the presence of antiendothelial cell antibodies.

Role of ticlopidine on adriamycin-induced nephropathy

The effect of ticlopidine on rats with adriamycin nephropathy was observed during 26 weeks. In the ticlopidine-treated nephrotic animals (TNG), proteinuria was less than in the untreated nephrotic animals (NG), but this difference was significant only at week 6 (TNG = 47.27 +/- 16.52 versus NG = 100.08 +/- 13.83 mg/24 h, p < 0.01) and week 26 (TNG = 157.00 +/- 28.73 versus NG = 217.00 +/- 21.73 mg/24 h, p < 0.01) after ADR injection. NG presented severe tubulointerstitial abnormalities with a tubulointerstitial lesion index of 3+. No difference in glomerular lesions was observed among the groups (NG median = 6%, TNG median = 4% and TCG median = 2%). The tubulointerstitial lesion index of TNG was less intense (median = 2+) but not different from those of the control groups (CG median = 1+; TCG median = 0+) nor NG (median = 3+). We concluded that the treatment with ticlopidine produced some partially beneficial effects but did not prevent the development of adriamycin-induced nephropathy.

Effects of immune complex formation and complement activation on circulating platelets in the primate

Primate platelets are different from rodent and rabbit platelets in that they do not express receptors for C3a or C5a or immune adherence receptors. This study assessed the effects of immune complex (IC)-induced complement activation on primate platelets in the circulation. Cynomolgus monkeys (CYN, N = 4) immunized to bovine gamma globulin (BGG) were infused with BGG over 5 min to induce acute intravascular IC formation and complement activation. The studies were carried out under normal complement conditions (N = 12), partial complement inhibition (CAB-2 treated, N = 3), or total complement inhibition (CVF treated, N = 1). Under normal complement conditions, BGG infusion increased C3a levels from undetectable to an average of 11.9 +/- 2.6 micrograms/ml. At this time, decreases occurring in both circulating neutrophils (85 +/- 6%) and monocytes (78 +/- 6%) were significantly greater than decreases in circulating platelets (13 +/- 3%, p < 0.001). Partial complement inhibition had an equivocal effect on the BGG-induced changes in circulating leukocytes, while total complement inhibition abrogated these changes. In contrast, platelet changes were unaffected by complement inhibition. We conclude that, compared to circulating leukocytes, circulating platelets are insensitive to intravascular complement activation induced by IC in the nonhuman primate. These results contrast with previous studies in rodents which demonstrate strong effects of IC-induced intravascular complement activation on both circulating neutrophils and platelets.

Renal microvascular injury induced by antibody to glomerular endothelial cells is mediated by C5b-9

We have recently developed a model of thrombotic microangiopathy with injury to the glomerular endothelial cell (GEN) induced by heterologous antibody to rat GEN. In addition to GEN injury rats developed glomerular platelet aggregation and fibrin deposition, acute renal failure, and acute tubular necrosis with interstitial inflammation. To study the role of complement in mediating this lesion, we induced the disease in normal complement PVG rats and measured the effects of generalized complement depletion with cobra venom factor (CVF) and of selective C6 deficiency using genetically C6 deficient PVG animals. Complement sufficient rats developed severe endothelial injury accompanied by platelet aggregation, fibrin deposition, decrease in endothelial cells assessed by antibody staining in the glomerulus, and macrophage infiltration. These changes were associated with marked reduction in renal function. These features were either absent or markedly diminished in complement depleted or C6 deficient rats. This demonstrates that C5b-9, the terminal product of activation of the complement cascade, plays an important role in the pathogenesis of this immune renal microvascular endothelial injury model. Thus, the complement system may play a pathogenic role in renal microvascular diseases such as thrombotic microangiopathy.

A new model of renal microvascular endothelial injury

Although the importance of injury with consequent activation of endothelium is well-recognized in diseases affecting the glomerular endothelial cell (GEN), research on GEN injury in vivo has been hampered by the lack of adequate animal models. Here we report the establishment and characterization of a new GEN injury model in rats. This model was induced by selective renal artery perfusion with anti-GEN IgG and resulted in the severe acute renal failure with marked platelet deposition and development of a thrombotic microangiopathy involving glomeruli. Peritubular capillary endothelial cells were also damaged that was associated with severe tubular necrosis. Although the glomerular changes were severe, half of the glomeruli recovered by day 10, while interstitial changes remained throughout our observation time course. Proliferation of GEN was observed during the recovery phase. An increased expression of endothelial nitric oxide synthase in GEN was also observed, and may be an adaptive mechanism to counteract the thrombosis and ischemia. This model should be useful to investigate the pathophysiology of renal microvascular diseases and the mechanisms of GEN injury, activation and recovery in vivo.

The role of the kidney biopsy in the treatment of lupus nephritis

The renal biopsy plays an important role in the clinical evaluation of patients with lupus erythematosus. From numerous studies of renal biopsies in lupus nephritis, it has become clear that although the renal lesions are quite varied, the pattern of renal involvement correlates with the clinical outcome. The broad spectrum of lesions seen in lupus nephritis has been attributed to individual differences in the immune response in different patients or in the same patient during the course of disease. Classification of lupus nephritis authorized by the World Health Organization has provided a standardized approach to the findings on renal biopsy. This classification combines all of the morphologic modalities of biopsy interpretation including light, immuno-fluorescence, and electron microscopy as well as providing a semiquantitative assessment of activity and chronicity. The clinical correlations utilizing this histologic classifications have demonstrated that the specific nature of the renal histopathology can predict both the acute and long-term outcome of the renal disease in patients with lupus erythematosus and can be useful in determining the management of individual patients.

Expression of a nonmuscle myosin heavy chain in glomerular cells differentiates various types of glomerular disease in rats

To characterize the phenotypic modulation of mesangial and glomerular epithelial cells, we investigated the expression of a nonmuscle type myosin heavy chain, SMemb, and alpha-smooth muscle actin (alpha-SM actin) in rat experimental glomerular diseases, which included anti-Thy 1 nephritis, 5/6 nephrectomy, diabetes, and anti-glomerular basement membrane nephritis. SMemb was only slightly expressed in normal glomerular epithelial cells but not in mesangial cells. In the anti-Thy 1 nephritis rats, both SMemb and alpha-SM actin were most conspicuously induced in mesangial cells. However, the expression profile was shifted from alpha-SM actin to SMemb dominant pattern over the course of glomerulonephritis. The expression of SMemb was also increased in epithelial cells in this model. In the other three models, glomerular cells did not express alpha-SM actin, but did so for SMemb. In the nephrectomized and the diabetic rats SMemb was newly expressed in mesangial cells at earlier stages, but at later stages was remarkably enhanced in epithelial cells when severe glomerular hypertrophy developed. In the anti-GBM nephritis rats, SMemb expression was increased in epithelial cells. In all models examined, mesangial and epithelial expression of SMemb was confirmed by immunoelectron microscopy, and enhanced expression of SMemb mRNA in glomeruli was verified by RNase protection assay. We conclude from these results that glomerular cells change their phenotypes differently depending on various types of glomerular diseases. These phenotypic changes in glomerular cells can be revealed by the combined immunostaining for SMemb and alpha-SM actin. SMemb is especially useful to detect both mesangial and glomerular epithelial cell activation in these glomerular disease models. Understanding the functional difference and regulatory mechanisms of these cytoskeletal proteins will provide insight into the pathogenesis and progression of glomerular diseases.

Acute passive anti-glomerular basement membrane nephritis in P-selectin-deficient mice

P-selectin present on surfaces of activated endothelium and platelets mediates neutrophil-endothelial and neutrophilplatelet interactions. The role of P-selectin in vivo was examined in a model of acute passive anti-GBM nephritis in P-selectin-deficient and wild-type mice which was induced by intravenous injection of anti-GBM serum. There were two major differences between P-selectin-deficient and wild-type mice. Firstly, mutant mice had approximately two fold more glomerular PMNs and albuminuria than wild-type animals at the peak of neutrophil influx and proteinuria. Secondly, Lipoxin A4 (LXA4), an eicosanoid which inhibits leukocyte-endothelial adhesion in vitro, and is generated primarily by transcellular biosynthetic routes during P-selectin-mediated platelet-PMN interaction [1], was approximately 60% of wild type levels in nephritic kidneys of P-selectin-deficient mice. Injection of wild-type platelets into P-selectin-null mice restored LXA4 to wild-type levels. The corresponding PMN influx approximated PMN levels in wild-type mice receiving platelets but urine albuminuria remained higher. Although these two P-selectin-dependent events cannot be directly linked, our results point to the importance of considering both platelet and endothelial P-selectin in determining the cellular events in inflammation.

Platelet association with gingival tissue inflammation

Platelets (PL) may be involved in the inflammatory process through the release of a variety of factors which could contribute to gingival tissue injury. Thus, conditions which result in the localized discharge of PL constitutents could lead to amplification of the inflammatory process at these sites. The purpose of this study was to determine if there was evidence of PL activation in gingival crevicular fluid and whether the degree of gingival inflammation, as measured by the gingival index (GI), was associated with the degree of platelet activation. This was monitored by assaying for beta-thromboglobulin (beta-TG), a platelet specific protein released from alpha granules of PL when activated. One uL samples of the fluids were obtained from human subjects from gingival sites with various GI scores. Fluid samples were also obtained at probe-induced bleeding gingival crevicular sites. beta-TG levels in the various fluids obtained from the crevice were determined by radioimmunoassay (RIA). The RIA data indicated that detectable beta-TG levels were observed in all samples, the means ranging from 5.5 ng/ml to 45.2 ng/ml. Additionally, a positive association between the GI scores of 0 and 1 and the beta-TG levels where observed. For GI scores of 2 and above the beta-TG concentrations appeared to approach a maximum value. These findings provide evidence for PL activation and suggest a relationship with gingival inflammation.

Transcellular biosynthesis of lipoxin A4 during adhesion of platelets and neutrophils in experimental immune complex glomerulonephritis

Polymorphonuclear neutrophils are important effectors of injury in host defense and inflammation. Many inflammatory diseases are self-limiting, raising the possibility that compounds are generated in vivo during the course of inflammation that inhibit neutrophil recruitment and tissue destruction. Lipoxins, a more recent addition to the families of bioactive eicosanoids, are potential candidates in this regard. Lipoxins are generated via pathways that initially involve the dual lipoxygenation of arachidonic acid and are potent inhibitors of several neutrophil trafficking events in vitro. Here, we present evidence that lipoxin A4 is generated in rat kidneys during experimental immune complex-mediated glomerulonephritis in vivo. Renal lipoxin A4 levels were markedly reduced by prior depletion of animals of either neutrophils or platelets, suggesting that most lipoxin A4 generated in vivo was derived from transcellular biosynthetic pathways during platelet-neutrophil interactions. Electron microscopic examination of glomerulonephritic kidneys revealed areas of intimate contact between neutrophils and platelets within the lumen of glomerular capillaries. P-selectin on platelets is an important mediator of platelet-neutrophil adhesion in vitro and in vivo. Prior treatment of animals with a blocking monoclonal antibody (mAb) against P-selectin (mAb CY1747), but not an isotype-matched non-blocking control mAb (mAb PNB1.6), caused striking inhibition of lipoxin A4 generation without attenuating neutrophil recruitment. Anti-P-selectin mAb also blunted transcellular lipoxin A4 generation during coincubations of activated neutrophils and platelets in vitro.(ABSTRACT TRUNCATED AT 250 WORDS)

Cellular events in the evolution of experimental diabetic nephropathy

In several models of progressive glomerular disease, mesangial cell proliferation, phenotypic change and increased growth factor expression precede up-regulation of genes for extracellular matrix components (ECM) and mesangial expansion. To examine these events in diabetic nephropathy (DN) we conducted sequential studies of glomeruli in rats with streptozotocin induced DN. We found prominent mesangial cell proliferation at three days (4.34 +/- 2.24 PCNA + cells/glom vs. 1.6 +/- 0.74 in controls, P < 0.001) associated with increased alpha-actin expression. PDGF B-chain mRNA was slightly increased at day one, and PDGF B-chain immunostaining was slightly increased at days one and six. Staining for bFGF was significantly increased at three days (2.2 +/- 0.6 vs. 1.2 +/- 0.1 in controls, P < 0.01). There was also an early increase in platelets in glomeruli of diabetic animals, and platelet depletion significantly inhibited the early phase of proliferation. In addition to mesangial cell proliferation, a prominent glomerular macrophage infiltration began at day three and peaked at day 30 (3.94 +/- 1.47 vs. 2.08 +/- 1.13 in controls, P < 0.01). TGF-beta mRNA increased at days 14 and 30. Insulin treatment prevented mesangial cell proliferation, actin expression, and macrophage infiltration, and normalized TGF-beta expression at 14 and 30 days. These multiple cellular events preceded any detectable increases in glomerular gene expression or deposition of collagen I, IV or laminin.

Relative roles of elastase and reactive oxygen species in the degradation of human glomerular basement membrane by intact human neutrophils

Glomerular basement membrane (GBM) damage and proteinuria occurring during the early phase of acute glomerulonephritis are often neutrophil (PMN) dependent. The present study sought to identify the potential roles of PMN derived elastase and reactive oxygen species (ROS) in the pathogenesis of glomerular basement damage in an homologous in vitro model of anti-GBM nephritis using intact PMN. Human PMN (5 x 10(6)), incubated with human GBM (0.5 mg) pretreated with human anti-GBM IgG, degraded 10.3 +/- 1.1% of the GBM type IV collagen in six hours (8 micrograms/hr), and underwent a two-hour respiratory burst. The same number of sonically disrupted PMN solubulized 22.4 +/- 5.1% of GBM under the same incubation conditions. The inclusion of the elastase inhibitors alpha 1 proteinase inhibitor (alpha 1Pi), and a smaller highly-specific synthetic compound (L658,758), reduced degradation by PMN homogenates by 84.8% and 85.7%, respectively, whereas they were only able to inhibit intact PMN mediated degradation by a maximum of 49.2% and 50.9%, respectively. The inclusion of EDTA (10 mM), an inhibitor of metalloproteinases, reduced GBM degradation by APMA activated and disrupted PMN by only 7.5%. Incubation of PMN with diphenylene iodonium (DPI) abolished PMN reactive oxygen species generation by > 95% but preserved elastase release. This compound did not directly affect GBM degradation. It did, however, abolish the inhibitory effect of ROS on alpha 1Pi activity and thus indirectly reduced GBM damage by up to 20%.

New insights into mechanisms of immune glomerular injury

Although glomerular disease remains the most common cause of end-stage renal disease worldwide, major advances have been made recently in understanding the cellular and molecular mechanisms that mediate these disorders. The nephrotic syndrome in noninflammatory lesions such as minimal change or focal sclerosis and membranous nephropathy results from disorders of the glomerular epithelial cell that can be simulated in animal models by antibodies to various epithelial cell membrane epitopes. Clarification of how these antibodies affect epithelial cells to induce a loss of glomerular barrier function should substantially improve understanding of the pathogenesis of minimal change or focal sclerosis. In membranous nephropathy, proteinuria is mediated primarily by the C5b-9 complex through similar mechanisms that also involve glomerular epithelial cells as targets. Inflammatory glomerular lesions are induced by circulating inflammatory cells or proliferating resident glomerular cells. Understanding of how these cells induce tissue injury has also evolved considerably over the past decade. Neutrophil-induced disease involves leukocyte adhesion molecules in regulating neutrophil localization; proteases, oxidants, and myeloperoxidase in mediating injury; and platelets in augmenting these processes. The activated mesangial cell exhibits altered phenotype and proliferation with the release of oxidants and proteases. Mesangial cell proliferation may be initiated by basic fibroblast growth factor and is maintained by an autocrine mechanism involving platelet-derived growth factor. Transforming growth factor beta is important in the subsequent development of sclerosis.

Effect of PAF on rat lung vascular permeability: role of platelets and polymorphonuclear leucocytes

1. The objectives of the present experiments were to assess the contribution of polymorphonuclear leucocytes (PMNLs), platelets and their products such as thromboxane A2 (TxA2), histamine and 5-hydroxytryptamine to platelet activating factor (PAF)-mediated protein extravasation in rat lungs. 2. Intravenous injection of PAF (1.0 and 5.0 micrograms kg-1) increased dose-dependently (up to 7.5 fold) the vascular permeability of the trachea, upper and lower bronchi to Evans blue dye (EB), a marker of albumin extravasation. The permeability of the pulmonary parenchyma was not affected significantly by PAF. 3. Thrombocytopenia induced by administration of the IgG fraction of goat anti-rat platelet serum (APS; 15 mg 100 g-1, i.p., 16-18 h) reduced by 55, 58 and 40% the effects of the lower dose of PAF (1.0 microgram kg-1) and by 31, 23 and 15% the effects of the higher dose of PAF (5.0 micrograms kg-1) on the permeability of the trachea, upper and lower bronchi respectively to albumin. 4. PMNL depletion induced by administration of rabbit anti-rat polymorphonuclear serum (ANS; 2 mg kg-1, i.v., 24 h) did not reduce significantly the effects of the lower dose of PAF (1.0 microgram kg-1) on the airways, however the effects of the higher dose of PAF (5.0 micrograms kg-1) on the permeability of the trachea, upper and lower bronchi to albumin were reduced by 43, 25 and 23% respectively. 5. The injection of both the anti-platelet and the anti-PMNL sera reduced by 61, 62 and 96% the effects of the lower dose of PAF (1.0 microg kg-1) and by 44, 39 and 47% the effects of the higher dose of PAF (5.0 microg kg-1) on the permeability of the trachea, upper and lower bronchi respectively.6. The combined injection of the TxA2-mimetic (U-44069; 5.0 microg kg-1) and PAF (1.0 and 5.0 microg kg-1)in thrombocytopenic rats overcame the vascular permeability decrease induced by APS treatment.7. Pretreatment of the animals with a combination of antagonists to histamine (mepyramine;3.0 mg kg-1) and 5-hydroxytryptamine (methysergide; 2.5 mg kg-1) did not cause a significant inhibition of the effect of PAF (1.0 and 5.0 microg kg-1) on EB extravasation in the airways.8. These data show that the effect of intravenous PAF on rat vascular permeability is partly modulated by polymorphonuclear leucocyte and platelet activation. Our results suggest that following its release,TxA2 could increase postcapillary hydrostatic pressure by inducing a venoconstriction and potentiate the extravasation elicited by PAF. These results do not suggest a major role for histamine and/or 5-hydroxytryptamine on PAF-induced albumin extravasation.