Induction of high-affinity paf receptor expression during T cell activation

Is there an interplay between the SARS-CoV-2 spike protein and Platelet-Activating factor?

Previous publications have reported a potent effect of COVID-19 on platelet function and that the Spike protein enhances washed human platelet aggregation induced by various agonists. This study aims to evaluate whether mRNA vaccination for COVID-19 affects human platelet-rich plasma (hPRP) aggregation response, whether a recombinant Spike protein modulates PAF-induced aggregation in hPRP and in washed rabbit platelets (WRP), and to investigate the effect of recombinant Spike protein on the PAF production in the U-937 cell line. Our results showed that PRP from vaccinated individuals exhibited ex vivo lower EC₅₀ values in response to PAF, ADP, and collagen. Platelet incubation with the Spike protein alone did not induce aggregation either in hPRP or in WRP, but resulted in augmentation of in vitro PAF-induced aggregation in hPRP from non-vaccinated individuals and in WRP. When PRP from vaccinated individuals was incubated with the Spike protein and PAF was subsequently added, elimination of the secondary wave of the biphasic aggregation curve was recorded compared with the aggregation induced by PAF alone. Collagen-induced in vitro aggregation was dose-dependently reduced when platelets were pre-incubated with the Spike protein in all tested aggregation experiments. Stimulation of U-937 by the Spike protein induced an increase in intracellular PAF production accompanied by elevation of the activities of all three PAF biosynthetic enzymes. In conclusion, since the Spike protein appears to modulate PAF production and activity, the use of compounds that act as PAF inhibitors, could be considered at least in mild cases of patients infected with SARS-CoV-2.

Boosting Akt Pathway by Rupatadine Modulates Th17/Tregs Balance for Attenuation of Isoproterenol-Induced Heart Failure in Rats

Disruption of Th17/Tregs homeostasis plays a crucial role in governing the immune response during myocardial fibrosis and its progression to heart failure. The present study aimed to assess for the first time the possible protection afforded by rupatadine against isoproterenol-induced heart failure in rats. It also explored the role of PI3k/Akt as a possible mechanistic pathway, through which rupatadine could modulate Th17/Tregs balance to display its effect. Isoproterenol (85 and 170 mg/kg/day) was injected subcutaneously for 2 successive days, respectively and rupatadine (4 mg/kg/day) was then given orally for 14 days with or without wortmannin (PI3K/Akt inhibitor). Rupatadine succeeded to completely ameliorate isoproterenol-induced cardiac dysfunction as demonstrated by improvements of electrocardiographic and echocardiographic measurements. Moreover, rupatadine prevented the marked elevation of PAF and oxidative stress in addition to Th17 promoting cytokines (IL-6, IL-23, and TGF-β). Accordingly, rupatadine prevented Th17 stimulation or expansion as indicated by increased Foxp3/RORγt ratio and decreased production of its pro-inflammatory cytokine (IL-17). Rupatadine treatment mitigated isoproterenol-induced activation of STAT-3 signaling and the imbalance in p-Akt/total Akt ratio affording marked decrease in atrogin-1 and apoptotic biomarkers. Finally, this therapy was effective in averting cardiac troponin loss and reverting the histological alterations as assessed by myocardial fibrosis and hypertrophy grading. Contrariwise, co-administration of wortmannin mostly attenuated the protective effects of rupatadine affording more or less similar results to that of isoproterenol-untreated rats. In conclusion, rupatadine could be an effective therapy against the development of isoproterenol-induced heart failure where PI3K/Akt pathway seems to play a crucial role in its protective effect.

PAF-R on activated T cells: Role in the IL-23/Th17 pathway and relevance to multiple sclerosis

IL-23 is a potent stimulus for Th17 cells. These cells have a distinct developmental pathway from Th1 cells induced by IL-12 and are implicated in autoimmune and inflammatory disorders including multiple sclerosis (MS). TGF-β, IL-6, and IL-1, the transcriptional regulator RORγt (RORC) and IL-23 are implicated in Th17 development and maintenance. In human polyclonally activated T cells, IL-23 enhances IL-17 production. The aims of our study were: 1). To validate microarray results showing preferential expression of platelet activating factor receptor (PAF-R) on IL-23 stimulated T cells. 2). To determine whether PAF-R on activated T cells is functional, whether it is co-regulated with Th17-associated molecules, and whether it is implicated in Th17 function. 3). To determine PAF-R expression in MS. We show that PAF-R is expressed on activated T cells, and is inducible by IL-23 and IL-17, which in turn are induced by PAF binding to PAF-R. PAF-R is co-expressed with IL-17 and regulated similarly with Th17 markers IL-17A, IL-17F, IL-22 and RORC. PAF-R is upregulated on PBMC and T cells of MS patients, and levels correlate with IL-17 and with MS disability scores. Our results show that PAF-R on T cells is associated with the Th17 phenotype and function. Clinical Implications Targeting PAF-R may interfere with Th17 function and offer therapeutic intervention in Th17-associated conditions, including MS.

CD4+ T cells in atherosclerosis: Regulation by platelets

Atherosclerosis is an inflammatory and thrombotic disease, in which both CD4+ T cells and platelets play important roles throughout all stages of atherogenesis. CD4+ T cells are the most abundant T cells present in atherosclerotic lesions. They are primarily seen as type 1 T helper (Th1) cells, while the other CD4+ T cell subsets Th2, Th17, and regulatory T (Treg) cells are also found in the lesions with lower frequencies. CD4+ T effector cells release various cytokines, which exert paracrine or autocrine effects among different CD4+ T cell subsets and other lesional cells and subsequently modulate inflammatory processes in the lesions. Platelets are instrumental in thrombosis and haemostasis, but also play important regulatory roles in immune response, inflammation, and angiogenesis. The present review summarises the current knowledge and/or understanding on how platelets regulate recruitment, activation, differentiation, and cytokine production of different CD4+ T cell subsets, as well as impacts of the platelet-CD4+ T cell interactions on atherogenesis. The research perspectives of platelet-CD4+ T cell interaction in atherosclerosis are also discussed.

Lack of Platelet-Activating Factor Receptor Attenuates Experimental Food Allergy but Not Its Metabolic Alterations regarding Adipokine Levels

Platelet-activating factor (PAF) is known to be an important mediator of anaphylaxis. However, there is a lack of information in the literature about the role of PAF in food allergy. The aim of this work was to elucidate the participation of PAF during food allergy development and the consequent adipose tissue inflammation along with its alterations. Our data demonstrated that, both before oral challenge and after 7 days receiving ovalbumin (OVA) diet, OVA-sensitized mice lacking the PAF receptor (PAFR) showed a decreased level of anti-OVA IgE associated with attenuated allergic markers in comparison to wild type (WT) mice. Moreover, there was less body weight and adipose tissue loss in PAFR-deficient mice. However, some features of inflamed adipose tissue presented by sensitized PAFR-deficient and WT mice after oral challenge were similar, such as a higher rate of rolling leukocytes in this tissue and lower circulating levels of adipokines (resistin and adiponectin) in comparison to nonsensitized mice. Therefore, PAF signaling through PAFR is important for the allergic response to OVA but not for the adipokine alterations caused by this inflammatory process. Our work clarifies some effects of PAF during food allergy along with its role on the metabolic consequences of this inflammatory process.

Microbial modulation of host immunity with the small molecule phosphorylcholine

All microorganisms dependent on persistence in a host for survival rely on either hiding from or modulating host responses to infection. The small molecule phosphorylcholine, or choline phosphate (ChoP), is used for both of these purposes by a wide array of bacterial and parasitic microbes. While the mechanisms underlying ChoP acquisition and expression are diverse, a unifying theme is the use of ChoP to reduce the immune response to infection, creating an advantage for ChoP-expressing microorganisms. In this minireview, we discuss several benefits of ChoP expression during infection as well as how the immune system fights back against ChoP-expressing pathogens.

In vitro suppression of lymphocyte activation in patients with seasonal allergic rhinitis and pollen-related asthma by cetirizine or azelastine in combination with ginkgolide B or astaxanthin

Novel strategies are evaluated for management of allergic rhinitis and asthma in patients co-afflicted with both disorders. It is hypothesized that the platelet activating factor receptor antagonist ginkgolide B (GB) and the carotenoid antioxidant astaxanthin (ASX) interact with antihistamines cetirizine dihydrochloride (CTZ) and azelastine (AZE) to potentiate their ability to downregulate potentially pathological immune activation. Peripheral blood mononuclear cells from asthmatics and healthy subjects, cultured 24 hours with 50 μg/ml phytohemaglutinin (PHA) or PHA plus each drug are analyzed by flow cytometry for expression of CD25+ or HLA-DR+ by CD3+ (T cells). Results are reported as stimulation indices for CD3+CD25+ (SICD3+CD25+) and CD3+HLA-DR+ (SICD3+HLADR+) cells in cultures treated with PHA alone, versus cultures treated with both PHA and drugs. Optimal suppression of activated cells was observed in cultures stimulated with ASX 10-6 M + CTZ 10-6 M (SICD3+CD25+, p = 0.016; SICD3+HLADR, p = 0.012); ASX 10-6 M + AZE 10-6 M (SICD3+CD25+, p = 0.012; SICD3+HLADR, p = 0.015); GB 10-6 M + CTZ 10-6 M (SICD3+CD25+, p = 0.024, SICD3+HLADR+, p = 0.019). Results demonstrate improved activity of antihistamines by 2 phytochemicals, suggesting dosing strategies for animal trials of ASX- or GB-augmented formulations for seasonal allergic rhinitis and asthma.

Platelet-activating factor receptor is essential for the development of experimental cerebral malaria

Cerebral malaria is a severe form of the disease that may result, in part, from an overt inflammatory response during infection by Plasmodium falciparum. The understanding of the pathogenesis of cerebral malaria may aid in the development of better therapeutic strategies for patients. The immune response in cerebral malaria involves elevation of circulating levels of cytokines and chemokines associated with leukocyte accumulation and breakdown of the blood-brain barrier in the central nervous system. Platelet-activating factor (PAF) is a mediator of inflammation shown to orchestrate inflammatory processes, including recruitment of leukocytes and increase of vascular permeability. Using mice lacking the PAF receptor (PAFR(-/-)), we investigated the relevance of this molecule for the outcome and the neuroinflammatory process triggered by P. berghei ANKA, an experimental model of cerebral malaria. In PAFR(-/-) mice, lethality was markedly delayed and brain inflammation was significantly reduced, as demonstrated by histology, accumulation, and activation of CD8(+) T cells, changes in vascular permeability and activation of caspase-3 on endothelial cells and leukocytes. Similarly, treatment with the PAFR antagonist UK-74,505 delayed lethality. Taken together, the results suggest that PAFR signaling is crucial for the development of experimental cerebral malaria. Mechanistically, PAFR activation is crucial for the cascade of events leading to changes in vascular permeability, accumulation, and activation of CD8(+) T cells and apoptosis of leukocytes and endothelial cells.

Platelet-activating factor induces Th17 cell differentiation

Th17 cells have been implicated in a number of inflammatory and autoimmune diseases. The phospholipid mediator platelet-activating factor (PAF) is found in increased concentrations in inflammatory lesions and has been shown to induce IL-6 production. We investigated whether PAF could affect the development of Th17 cells. Picomolar concentrations of PAF induced IL-23, IL-6, and IL-1β expression in monocyte-derived Langerhans cells (LCs) and in keratinocytes. Moreover, when LC were pretreated with PAF and then cocultured with anti-CD3- and anti-CD28-activated T cells, the latter developed a Th17 phenotype, with a significant increase in the expression of the transcriptional regulator RORγt and enhanced expression of IL-17, IL-21, and IL-22. PAF-induced Th17 development was prevented by the PAF receptor antagonist WEB2086 and by neutralizing antibodies to IL-23 and IL-6R. This may constitute a previously unknown stimulus for the development and persistence of inflammatory processes that could be amenable to pharmacologic intervention.

Levocetirizine modulates lymphocyte activation in patients with allergic rhinitis

Levocetirizine, a second generation non-sedating antihistamine that blocks the H(1) histamine receptor, may exhibit immunoregulatory properties that augment its primary pharmacological mechanism. To investigate this possibility, 13 Kuwaiti seasonal allergic rhinitis (SAR) patients were treated with levocetirizine for four weeks in comparison with a 7-member placebo-treated control group, followed by clinical evaluation and flow cytometric analysis of peripheral venous blood for inflammatory cell and lymphocyte subpopulation profiles. Relative to the controls, levocetirizine-treated patients exhibited an expected reduction in early phase allergic symptoms, including sneezing (P<0.001), nasal itching (P<0.01), nasal congestion, and running nose (P<0.001); reduced percentages of eosinophils (P<0.05); and three subpopulations of activated T lymphocytes: CD4+CD29+, CD4+CD212+, and CD4+CD54+ (P<0.05). Levocetirizine treatment also correlated with a significant increase in the percentage of CD4+CD25+ T cells (P<0.001). The ability of levocetirizine to reduce percentage representation of cell phenotypes known to contribute to inflammatory tissue damage (eosinophils, CD4+CD29+, CD4+CD212+, and CD4+CD54+) and expand percentages of CD4+CD25+, which may include protective immunoregulatory (Treg) cells, indicates that the drug has pharmacological potential beyond the immediate effects of H(1) histamine-receptor inhibition. Although the present data does not define a therapeutic mechanism, the results reported here establish important trends that may be used to guide future mechanistic examination of immunoregulatory capacity of H(1) inhibitors.

Expression and localization of platelet-activating factor receptor in human nasal mucosa

BACKGROUND

Platelet-activating factor (PAF) has been thought to be a potent mediator of allergic rhinitis because PAF was recovered from the nasal lavage fluid of patients with allergic rhinitis after allergen provocation. Furthermore, PAF receptor antagonist attenuates the antigen-induced increase in nasal airway resistance and nasal vascular permeability in sensitized guinea pigs.

OBJECTIVE

To clarify the expression of PAF receptor in human nasal mucosa by investigating PAF receptor messenger RNA (mRNA) expression and its protein localization using polymerase chain reaction (PCR) and immunohistochemical analyses, respectively.

METHODS

Human turbinates were obtained after turbinectomy from 6 patients with nasal obstruction refractory to medical therapy. Total RNA was isolated from human nasal mucosa, and PAF receptor mRNA was detected in these tissues by using reverse transcriptase-PCR analysis. To identify the cells expressing PAF receptor protein, double immunostaining was performed using anti-PAF receptor antibody and monoclonal antileukocyte antibodies.

RESULTS

Reverse transcriptase-PCR analysis of total nasal RNA demonstrated the expression of PAF receptor mRNA. The immunohistochemical studies revealed that anti-PAF receptor antibody-labeled eosinophils, macrophages, neutrophils, mast cells, lymphocytes, vascular endothelial cells, epithelial cells, and submucosal glands in nasal mucosa.

CONCLUSIONS

These results may have important clinical implications for understanding the role of PAF receptor on upper airway diseases such as allergic and nonallergic rhinitis.

Delineation of the role of platelet-activating factor in the immunoglobulin G2 antibody response

Localized aggressive periodontitis (LAgP) is a chronic inflammatory disease characterized by severe destruction of periodontal tissues surrounding the first molars and incisors. LAgP subjects produce large amounts of immunoglobulin G2 (IgG2) antibody against oral pathogens, and this response is inversely correlated with the severity of disease. We previously demonstrated that platelet-activating factor (PAF) is required for optimal IgG2 responses. The present investigation was designed to determine the mechanism of IgG2 induction by PAF. Exogenous PAF acetylhydrolase suppressed approximately 80% of pokeweed mitogen-stimulated IgG2 production, confirming that PAF is essential for optimal responses. PAF-activated leukocytes produced gamma interferon (IFN-gamma), a Th1 cytokine that has been associated with IgG2 responses in previous studies. The monocyte-derived cytokines interleukin-12 (IL-12) and IL-18 are upstream of IFN-gamma production, and IgG2 production was suppressed by neutralizing antibodies against these proteins. In addition, PAF induced monocyte-derived dendritic cells (DC) but not macrophages (MPhi) to secrete IL-12 and IL-18. This observation was interesting because monocyte differentiation in LAgP subjects is skewed to the DC phenotype. Although other investigators have implicated IFN-gamma in IgG2 production, its precise role in this response is controversial. Our studies suggest that IFN-gamma induces isotype switching to IgG2 but only in concert with the Th2 cytokine IL-4. Thus, it appears that the unique PAF metabolism of LAgP monocytes or DC promotes Th1 responses that are essential for optimal IgG2 antibody production. As IgG2 antibodies opsonize oral bacteria and promote their clearance and destruction, these alterations in PAF metabolism may be essential for limiting disease severity in LAgP patients.

In vitro effects of astaxanthin combined with ginkgolide B on T lymphocyte activation in peripheral blood mononuclear cells from asthmatic subjects

This study was undertaken to identify novel approaches to pharmacological treatment of asthma. Here we hypothesize that the platelet-activating factor receptor antagonist ginkgolide B (GB) in combination with the antioxidant carotenoid astaxanthin (ASX) suppresses T cell activation comparably to two commonly-used antihistamines: cetirizine dihydrochloride (CTZ) and azelastine (AZE). Peripheral blood mononuclear cells from asthmatics, cultured 24 h with either 50 microg/ml phytohemaglutinin (PHA) or PHA plus selected dosages of each drug are analyzed by flow cytometry for CD25+ or HLA-DR+ on CD3+ (T cells). Results are reported as stimulation indices (SI) of %CD3+CD25+ cells or %CD3+HLA-DR+ cells in cultures treated with PHA alone versus these subpopulations in cultures treated with both PHA and drugs. Combinations of ASX and GB exhibited optimal suppression at 10(-7) M GB + 10(-8) M ASX for CD3+CD25+ (SI = 0.79 +/- 0.04, P = 0.001) and 10(-7) M GB + 10(-7) M ASX for CD3+HLA-DR+ (SI = 0.82 +/- 0.05, P = 0.004). In conclusion, suppression of T cell activation below fully stimulated values by GB, ASX, and their combinations was comparable and for some combinations better than that mediated by CTZ and AZE. These results suggest that ASX and GB may have application as novel antiasthmatic formulations.

Platelet-activating factor plays a role in the mechanism of major histocompatibility complex in T lymphocytes

In recent studies, using a swine model of single lung transplantation, we demonstrated that IRI alone increased MHC II expression in the host's peripheral T lymphocytes. The inhibition of increased MHC II expression with TCV-309, a specific platelet-activating factor (PAF) antagonist suggested that PAF might play a role in the mechanism of increased MHC II expression. The purpose of the current study was two fold: 1) to investigate the mechanism of PAF-induced increased expression of MHC II in T lymphocytes, 2) to determine whether a specific PAF-antagonist, TCV-309, is capable of inhibiting the increased expression in an in vitro system. This study was subdivided, using four in vitro conditions: 1) purified resting T cells, 2) purified proliferating T cells, 3) PBL treated with PAF, and 4) PBL preincubated with TCV-309 and treated with PAF. The level of MHC II on T cells were measured by two color flow cytometry analysis (swine anti-CD3, MHC II-DR-(beta)antibodies). Both MHC II intensity and the number of CD3+MHC+ T cells did not change in resting purified T cells once treated with PAF, Furthermore, MHC II intensity did not change in purified proliferating T cells treated with PAF. The number of CD3+MHC+ T cells, however, increased significantly (p<0.05) from day 1 to day 4 as compared with pre-treatment value (day 0) for purified proliferating T cells. Treatment of PBL with PAF (10(-7)M) resulted in a significant (p<0.05) increase in MHC II expression from day 2 to day 4 post-treatment. The number of CD3+MHC+ T cells in PBL, however, did not change significantly upon treatment with PAF. The results of this study indicated that PAF did not have a direct effect on increased MHC II expression in resting or proliferating purified T lymphocytes. However, the mechanism of PAF-induced increased expression of MHC II in T cells may be via an indirect pathway involving accessory cells. TCV-309, a specific PAF receptor antagonist, is capable of inhibiting this PAF-induced increased expression of MHC II in T cells.

Effects of Y-24180, a receptor antagonist to platelet-activating factor, on allergic cutaneous eosinophilia in mice

We examined the effects of Y-24180, a potent and long-acting antagonist to platelet-activating factor (PAF), on allergic cutaneous eosinophilia and cytokine production in the skin of mice. Mice sensitized actively with ovalbumin (OA) were challenged by an intradermal injection of OA solution. The number of inflammatory cells, including eosinophils and eosinophil peroxidase (EPO) activity reflecting eosinophil infiltration into the tissue increased in OA-challenged skin 12 hr after the challenge. The levels of interleukin-4 (IL-4) and IL-5 also increased significantly in the challenged skin 12 hr and 3-24 hr, respectively, but that of interferon-gamma (IFN-gamma) did not change. Then, we evaluated the effects of Y-24180, ketotifen, suplatast and prednisolone on the increase in EPO activity, IL-4 and IL-5. These drugs were orally administered once a day for 5 days beginning 4 days before the challenge. Y-24180 (10 mg/kg) and prednisolone (5 mg/kg) significantly suppressed these parameters. Suplatast did not affect EPO activity, but significantly decreased the levels of IL-4 and IL-5. Ketotifen had no effect on them. These results indicate that the inhibition of IL-4, IL-5 and PAF are required to suppress the cutaneous eosinophilia and Y-24180 contributes to the treatment of allergic cutaneous eosinophilia.

Platelet-activating factor and oxidized LDL induce immune activation by a common mechanism

Platelet activating factor (PAF) is a phospholipid with proinflammatory and thrombogenic properties, which has been implicated in inflammatory disorders including vasculitis and asthma. PAF-like compounds are present in oxidized LDL (oxLDL), which has been detected in the atherosclerotic lesion, where it may activate monocytes, macrophages, and T cells. OxLDL may therefore both initiate and perpetuate inflammatory reactions in the artery wall. Herein we demonstrate that PAF has the capacity to induce enhanced interferon gamma (IFN-gamma) secretion in peripheral blood mononuclear leukocytes (PBMCs), as does oxLDL. Both oxLDL- and PAF-induced IFN-gamma secretions were inhibited by a specific PAF-receptor antagonist, WEB 2170. PAF-like lipids in oxLDL could thus be responsible for oxLDL-induced activation of immune-competent cells. The effects of PAF and oxLDL were inhibited by antibodies to major histocompatibility complex class II and thus depend on accessory cells like monocytes. Both PAF and oxLDL induced tumor necrosis factor-alpha (TNF-alpha) synthesis in peripheral blood. PAF-mediated TNF-alpha production was inhibited by WEB 2170, whereas oxLDL-induced TNF-alpha was only partially inhibited. These findings indicate that both PAF and oxLDL have the capacity to induce TNF-alpha, which may increase atherogenesis due to its pleiotropic proinflammatory effects. Our findings suggest that the PAF receptor plays an important role in the inflammatory component of atherosclerosis.

Regulation of platelet-activating factor receptor expression in human B cells and B cell lines

We extended our previous data regarding the modulation of human platelet-activating factor receptor (hPAF-R) expression on human B cell lines as well as normal B cells. First, we showed that hPAF-R mRNA was present in B cell lines expressing membrane hPAF-R, but was absent from cell lines devoid of hPAF-R. Second, enhanced hPAF-R membrane expression induced in IM9 line by IL4 was preceeded by hPAF-R mRNA accumulation that was detectable by 8 h and which peaked at 24 h. Similar results were observed for 10 nM platelet-activating factor treatment, which increased hPAF-R mRNA content up to 120% at 48 h, whereas hPAF-R membrane expression was up-regulated by 130%. Third, our data indicate that functional hPAF-R are expressed on resting, as well as on activated, B cells and that B cell activation is required for maintaining hPAF-R membrane and mRNA expression. Thus, in normal B cells, as well as in B cell lines, transcriptional regulation and/or messenger stability control hPAF-R expression.

Expression of human platelet-activating factor receptor gene in EoL-1 cells following butyrate-induced differentiation

Platelet-activating factor (PAF) is a potent lipid mediator of allergic inflammation through its interaction with eosinophils. Expression of the PAF receptor is modulated by many agents, including those responsible for cell differentiation. We report here that differentiation of a human eosinophilic leukaemia cell line, EoL-1, by sodium n-butyrate is associated with induction of PAF receptor gene expression, as indicated by: PAF receptor mRNA accumulation; increases in the binding of [3H]WEB 2086, a PAF antagonist; analysis of cell-surface expression of PAF receptor protein using a monoclonal anti-(PAF receptor) antibody; and augmentation of PAF-induced increase in the intracellular concentration of calcium. Using cDNA cloning, the receptor expressed in EoL-1 cells was identified as 'Transcript 1', one of two transcripts which was previously reported from human genomic analysis (Mutoh, Bito, Minami, Nakamura, Honda, Izumi, Nakata, Kurachi, Terano and Shimizu (1993) FEBS Lett. 322, 129-134). The PAF-induced calcium response and phosphoinositide turnover were decreased by pertussis toxin (PTX) treatment, suggesting that these signals are coupled largely with PTX-sensitive G-protein(s) in EoL-1 cells. These systems may provide a useful experimental model with which to investigate the relationship between eosinophilic differentiation and PAF receptor induction, and the role of eosinophils in allergic responses.

Regulation of G protein-coupled receptor kinase subtypes in activated T lymphocytes. Selective increase of beta-adrenergic receptor kinase 1 and 2

Beta-adrenergic receptor kinase (beta ARK) is a serine-threonine kinase involved in the process of homologous desensitization of G-coupled receptors. beta ARK is a member of a multigene family, consisting of six known subtypes, also named G protein-coupled receptor kinases (GRK 1-6). In this study we investigated the expression of GRKs during the process of T cell activation, which is of fundamental importance in regulating immune responses. T cell activation was induced by exposing mononuclear leukocytes (MNL) to PHA and confirmed by tritiated thymidine incorporation measurement. A substantial increase of GRK activity (as measured by in vitro phosphorylation of rhodopsin) was found after 48 h (331 +/- 80% of controls) and 72 h (347 +/- 86% of controls) of exposure to PHA. A threefold increase of beta ARK1 immunoreactivity was found in MNL exposed to PHA for 72 h. Persistent activation of protein kinase C (PKC) by 10 nM 12-O-tetradecanoylphorbol-13-acetate (TPA) was able to increase beta ARK activity to the same extent as PHA, suggesting a PKC-mediated mechanism. The kinetic of beta-adrenergic-stimulated cAMP production was substantially modified in TPA and PHA-activated cells, indicating that the increased GRK activity resulted in an increased beta-adrenergic homologous desensitization. A three- to fourfold increase in GRK activity was also observed in a population of T cell blasts (> 97% CD3+) exposed to PHA for 48-72 h. A significant increase in beta ARK1 and beta ARK2 mRNA expression was observed 48 h after mitogen stimulation, while mRNA expression of GRK5 and GRK6 was not changed. In conclusion our data show that the expression of GRK subtypes is actively and selectively modulated according to the functional state of T lymphocytes.

Functional PAF receptors in glia cells: binding parameters and regulation of expression

Platelet activating factor is a unique phosphoglycerine which possesses a variety of biological functions exerting its biological effects via specific surface receptors. In the central nervous system, platelet activating factor has been suggested to play a role during injury especially in conditions of ischemia and trauma-induced neuronal damage. The specific cell populations expressing platelet activating factor receptor, however, have not been identified. In this study, the binding properties of platelet activating factor receptors in C6 glioma cells and primary cultures of astrocytes and oligodendrocytes were characterized by using the ligand [3H]WEB 2086. Early-passage glial cells which exhibit oligodendrocytic phenotype, expressed lower levels of [3H]WEB 2086 binding than either late-passage cells which exhibit astrocytic phenotypes or primary astroglia cells. No specific binding was observed in primary cultures of oligodendrocytes. The Bmax (136 +/- 15.3 fmol/mg protein) and Kd (29 +/- 3.2 nM) levels obtained for primary astroglia cells were similar to those described for other cell types. The expression of platelet activating factor receptor in early-passage glia cells was up-regulated by treatment with insulin which induces astrocytic differentiation. In contrast, db-cyclic AMP exerted an inhibitory effect on the level of platelet activating factor receptor in both early- and late-passage cells. The level of functional platelet activating factor receptor in C6 cells as measured by the ability of platelet activating factor to induce 45Ca2+ influx was increased in cells expressing astrocytic phenotypes and was decreased in db-cyclic AMP-treated cells. In accordance with lack of specific [3H]WEB 2086 binding, platelet activating factor did not induce a detectable response of Ca2+ influx in cultures of oligodendrocytes. This report provides the first direct demonstration of selective expression of functional platelet activating factor receptors and their properties in astroglia cells. The findings support the suggestion that platelet activating factor may play an important role as a mediator of injury and immune responses in the nervous system.

Agonist-induced up-regulation of platelet-activating factor receptor messenger RNA in human monocytes

Platelet-activating factor (PAF) is a potent inflammatory mediator and it actions are mediated via specific cell surface receptors which are coupled to G-proteins. PAF stimulates several functions in monocytes and may modulate the expression of its own receptor. To investigate the possible modulation of PAF receptor mRNA expression Northern blot analysis of total RNA from human monocytes was performed using the cDNA of human leukocyte PAF receptor as a probe. Following the addition of 100 nM PAF, there was a 2.0-fold increase in PAF receptor mRNA at 60 minutes after the stimulation, which was inhibited by pretreatment with the PAF receptor antagonist WEB 2086. This increase returned to control level at 120 and 180 min. The increase of PAF receptor mRNA was statistically significant for 10 nM to 1 microM of PAF, while 100 nM of lysoPAF did not increase PAF receptor mRNA levels. These results suggest that PAF receptor expression can be regulated by PAF itself at the transcriptional level.

Identification and functional characterization of platelet-activating factor receptors in human leukocyte populations using polyclonal anti-peptide antibody

Recently, the successful cloning of a receptor for platelet-activating factor (PAF), a lipid mediator of inflammation, was reported. Here we investigated the distribution and potential diversity of human PAF receptors (hPAF-Rs) among individual leukocyte populations by (i) hPAF-R mRNA transcription studies and (ii) analysis of cell surface expression of hPAF-R protein using a polyclonal anti-peptide antibody (anti-hPAF-R164-173). Northern blot analysis, flow cytometry, and immunoblotting with anti-hPAF-R antibody indicated that monocytic, neutrophilic, and B-lymphocytic cell lines all shared a similar hPAF-R species, whereas resting T-cell and natural killer cell lines failed to express detectable levels of either hPAF-R protein or mRNA. Peripheral blood leukocyte populations showed a distribution of hPAF-R cell surface expression similar to that of the corresponding cell lines. Furthermore, binding of anti-hPAF-R164-173 antiserum, purified IgG, or Fab and F(ab')2 fragments to the receptor of all investigated PAF-R-positive cell lines induced an increase in intracellular free calcium concentration. The characterization of the expression of a lipid ligand receptor using antibodies against an intrinsic portion of the receptor protein has, to our knowledge, never been reported previously.