Immunoregulatory functions of paf-acether. VI. Inhibition of T cell activation via CD3 and potentiation of T cell activation via CD2

Associations of phase angle with platelet-activating factor metabolism and related dietary factors in healthy volunteers

Introduction

Phase angle (PA) is derived from bioelectrical impedance analysis (BIA). It reflects cell membrane function and decreases in disease. It is affected by inflammation, oxidative stress, and diet. Platelet-activating factor (PAF) is a potent inflammatory lipid mediator. Its levels, along with the activity of its metabolic enzymes, including CDP-choline:1-alkyl-2-acetyl-sn-glycerol-cholinephosphotransferase, acetyl-CoA:lyso-PAF-acetyltransferases, and PAF-AH/Lp-PLA2 are also related to dietary factors, such as the dietary antioxidant capacity (DAC). The aim of the study was to estimate whether the PAF metabolic circuit and related dietary factors are associated with PA in healthy volunteers.

Methods

In healthy subjects, PAF, its metabolic enzyme activity, and erythrocyte fatty acids were measured, while desaturases were estimated. Food-frequency questionnaires and recalls were used, and food groups, macronutrient intake, MedDietScore, and DAC were assessed. Lifestyle and biochemical variables were collected. DXA and BIA measurements were performed.

Results

Lp-PLA2 activity was positively associated with PA (rho = 0.651, p < 0.001, total population; rho = 0.780, p < 0.001, women), while PAF levels were negatively associated with PA only in men (partial rho = -0.627, p = 0.012) and inversely related to DAC. Estimated desaturase 6 was inversely associated with PA (rho = -0.404, p = 0.01, total sample). Moreover, the DAC correlated positively with PA (rho = 0.513, p = 0.03, women). All correlations were adjusted for age, body mass index, and sex (if applicable).

Conclusion

PA is associated with PAF levels and Lp-PLA2 activity in a gender-dependent fashion, indicating the involvement of PAF in cell membrane impairment. The relationship of PA with DAC suggests a protective effect of antioxidants on cellular health, considering that antioxidants may inhibit PAF generation.

Platelet Derivatives and the Immunomodulation of Wound Healing

Besides their primary role in hemostasis, platelets contain a plethora of immunomodulatory molecules that profoundly affect the entire process of wound repair. Therefore, platelet derivatives, such as platelet-rich plasma or platelet lysate, have been widely employed with promising results in the treatment of chronic wounds. Platelet derivatives provide growth factors, cytokines, and chemokines targeting resident and immigrated cells belonging to the innate and adaptive immune system. The recruitment and activation of neutrophils and macrophages is critical for pathogen clearance in the early phase of wound repair. The inflammatory response begins with the release of cytokines, such as TGF-β, aimed at damping excessive inflammation and promoting the regenerative phase of wound healing. Dysregulation of the immune system during the wound healing process leads to persistent inflammation and delayed healing, which ultimately result in chronic wound. In this review, we summarize the role of the different immune cells involved in wound healing, particularly emphasizing the function of platelet and platelet derivatives in orchestrating the immunological response.

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.

PAF and Haematopoiesis. I. 5-Fluoro-Uracil Induces PAF Production in Haematopoietic Organs of Rats

Haematopoietic organs of rats were examined for the presence of platelet-activating factor (PAF) and acetylhydrolase before and after treatment with 5-fluoro-uracil (5-FU) (200 mg/kg) a chemotherapeutic compound with apoptotic effects. PAF was reported in thymus, spleen and femoral bone marrow of rats with or without 5-FU. Although acetylhydrolase activity in organs was not affected by 5-FU treatment, elevated levels of PAF were observed in thymus and spleen. For the first time PAF is reported in haematopoietic organs of rats, strengthening in vitro data suggesting its role in the apoptotic processes in thymus, in the modulation of the immune response, and in the regulation of haematopoiesis.

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.

PAF and haematopoiesis. IV. Modifications of spleen and thymus PAF contents after a single dose of the chemotherapeutic drug 5-fluorouracil in mice

The spleen and thymus of mice were examined for the presence of PAF after injection of 5-fluorouracil (5-FU) (200 mg/kg). A significant increase of the spleen (P = 0.005) and thymus (P < 0.05) PAF concentrations was noted 48 h after 5-FU infusion. PAF levels in thymus are similar to those of controls from days 4 to 14. By contrast, spleen PAF significantly decreased (0.005 < P < 0.03) from days 7 to 14. Conversely, the 5-FU administration did not modify the spleen and plasma acetylhydrolase activity, suggesting that the variations of PAF levels in thymus and spleen were mainly due to differences of local PAF production. Thus, the chemotherapeutic drug 5-FU modulates in vivo PAF production in haematopoietic organs of mice. Considering the effects of PAF in the processes of B- and T-cell proliferation and functions, these results could be of importance for the role of PAF during human cancer therapy and haematopoiesis in vivo.

Evidence for an abnormal profile of interleukin-4 (IL-4), IL-5, and gamma-interferon (gamma-IFN) in peripheral blood T cells from patients with allergic eosinophilic gastroenteritis

Allergic eosinophilic gastroenteritis is characterized by elevated total IgE, specific IgE to food antigens, and eosinophilia of tissue and blood. Because the lymphokines IL-4, IL-5, and gamma-interferon, regulate IgE synthesis, and eosinophilopoiesis in vitro, we examined whether there is an imbalance in their production in allergic eosinophilic gastroenteritis. To explore this hypothesis, three adult patients with allergic eosinophilic gastroenteritis were studied. Flow cytometric studies of peripheral blood mononuclear cells from these patients did not reveal evidence of T cell activation or disturbance of T cell numbers or subsets. T cells were capable of normal mitogenic activation in vitro. IL-4 and IL-5 production were markedly elevated with mitogenic stimulation. Most IL-4 and IL-5 production was by CD4+ T cells. Synthesis of IL-5 by CD4+ T lymphocytes in three patients and CD8+ T lymphocytes in two patients occurred in the absence of mitogen. Mitogen-stimulated GM-CSF and gamma-interferon synthesis by CD4+ T cells was normal. Lymphokine mRNA in total cellular RNA derived from endoscopic biopsies was examined by reverse transcription/polymerase chain reaction. Mucosal biopsies from control subjects and most biopsies from allergic eosinophilic gastroenteritis patients contained less than 10(-8) micrograms IL-5 mRNA/1 microgram total cellular mRNA. gamma-Interferon mRNA was not detected by reverse transcription/polymerase chain reaction in biopsies from patients with allergic eosinophilic gastroenteritis but was present in controls. These lymphokine abnormalities are consistent with the elevated IgE and eosinophilia seen in allergic eosinophilic gastroenteritis and suggest that strategies targeting T lymphocytes may be efficacious in treatment of this disease.

Quantitation of IL-4 expression in small numbers of cells from mice

Interleukin-4 (IL-4) is an important T cell and mast cell product that participates in allergic and cytotoxic responses, as well as functions as a growth factor for B, T, and inflammatory cells. Studies of the expression of IL-4 by T cells present in inflammatory reactions would be facilitated by using polymerase chain reaction (PCR) coupled to reverse transcription of mRNA to amplify the small quantity of mRNA present in these cells. In order to use this method in a quantitative manner, a plasmid was constructed that contained a modified form of mouse IL-4 cDNA. This plasmid was transcribed to produce cRNA for this modified sequence. The cRNA was used as an internal standard for the reverse transcription and amplification of IL-4 transcripts in RNA samples from mouse thymocytes. Amplification of reverse-transcribed native IL-4 mRNA produced a 286 bp PCR product. Amplification of the reverse-transcribed standard RNA produced a 155 bp product, which reflected a deletion introduced into the original IL-4 cDNA sequence. Comparison of the amount of the 286 bp native product to the amount of 155 bp standard product enabled the quantitative determination of IL-4 expression in each sample. This method was used to demonstrate that platelet activating factor increases the expression of IL-4 in mouse thymocytes and in a mouse T cell line. The expression of IL-4 by thymocytes exposed to platelet-activating factor (PAF) may reveal an important link between inflammation and the maturation of T cells in the thymus.

Platelet Activating Factor Inhibits Both the Proliferation and the Differentiation of Activated B Lymphocytes in Response to Interleukin-2

The role of the platelet activating factor in human B lymphocyte responses to Interleukin-2 was examined and compared with that of Interleukin-4 by assessing the ability of this molecule to modulate proliferation and differentiation. Highly purified B lymphocytes were prestimulated for 48 h with Staphylococcus aureus Cowan Strain I and then were cultured with Interleukin-2 alone or in combination with either Interleukin-4 or the platelet activating factor and the proliferation (after 3 days) and the immunoglobulins (IgG and IgM) production (after 7 days) were evaluated. When SAC-activated B lymphocytes were preincubated overnight with PAF (0.0001 to 1 μM) or with IL-4 (1 to 100 U/ml) both the IL-2-induced proliferation and immunoglobulins secretion were inhibited. This inhibition was not a reflection of a decreased expression of the IL-2 receptor (CD25) because this expression was not modified on SAC-activated B lymphocytes after preincubation with either PAF or IL-4. Moreover, this suppression effect was not the result of a delayed response to IL-2.

The PAF-induced suppression was overcome in the presence of PAF antagonists (BN 52021 and BN 50730) but was not modified in the presence of a neutralizing anti-IL-4 antiserum. On the other hand, the IL-4 mediated suppression was totally reversed in the presence of the neutralizing anti-serum and only marginally reversed in the presence of the PAF antagonists. These results indicate that both PAF and IL-4 may exert a number of immunoregulatory actions on human B lymphocyte proliferation and differentiation. They interfere with the stimulation of activated B lymphocytes by IL-2 and could play an important immunoregulatory role in the determination of isotypic regulation in the specific humoral responses.

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

Activated human T cells via the CD2 or the CD3 pathways exhibited a higher capacity than resting T lymphocytes to incorporate and metabolize [3H]pafacether (paf) at 37 degrees C. Resting T lymphocytes lacked specific binding capacity for paf, yet high-affinity paf receptors (paf-R) were induced on CD3- or CD2-dependent activation. This up-regulation in the number of paf-R became apparent by day 1 of culture, reached a maximum of about 25,000 sites cell by days 4 to 6 and subsequently declined. Interestingly, human recombinant interleukin-2 in a dose-dependent manner prevented the decrease of high-affinity paf-R expression on T cells. By contrast, the receptor affinity was constant throughout the culture period. Thus, paf-R at different stages of T cell activation were indistinguishable with respect to receptor-ligand interaction, and differed only in their number. Together, these data demonstrate that after activation human T cells develop membrane high-affinity paf-binding sites. They also suggest for the first time that expression of the paf-R are coupled to T cell activation and/or differentiation.

Immunoregulatory functions of paf-acether. VI. Dual effect on human B cell proliferation

The role of paf-acether (paf), a phospholipid cytokine, in the modulation of human B cell function was investigated. Paf, from 1 x 10(-5) M to 10(-6) M, decreased B cell proliferation induced by both phorbol myristate acetate (PMA) and anti-IgM antibodies (anti-IgM Ab). By contrast, 1 x 10(-7) M to 1 x 10(-9) M paf enhanced PMA triggered, but not anti-IgM triggered B cell proliferation. B cell proliferation was modulated between 24 and 72 hr of culture indicating that the effect of paf did not merely reflect a shift in proliferation kinetics. Interestingly, paf also enhanced the spontaneous proliferation of a Burkitt lymphoma-derived B cell line, Raji, which suggests that paf can directly act on B cells. The modulatory effect of paf on peripheral blood B cells was independent of PMA concentration, yet the effect on Raji cells was dependent upon cell density. The data suggest that paf is a potent modulator of B cell function, and may be involved in the control of humoral immune response.