Interaction of platelet-activating factor with interferon-gamma in the stimulation of interleukin-1 production by human monocytes

Cytokine gene regulation by PGE(2), LTB(4) and PAF

The initial response of the host to noxious stimuli produces a nonspecific inflammatory response. A more specific immune response is believed to be modulated by two classes of molecules: lipid mediators (PG, LT and PAF) and cytokines, synthesized by phagocytes and parenchyreal cells. In this review we discuss the increasing evidence of the interrelationship between eicosanoids, PAF and cytokines: IL-1 and TNF induce PG synthesis in various cells and PG, in turn, modulate cytokine production. We focused on the regulatory effects of LTB(4), PGE(2) and PAF on cytokine gene expression.

Dose-dependent agonist and antagonist effects of the platelet-activating factor analogue 1-palmitoyl-2-acetoyl-sn-glycero-3-phosphocholine on B lymphocytes

BACKGROUND

Platelet activating-factor (PAF), an ether-linked phospholipid, is a potent activator of B lymphocyte cell lines. The related ester-linked phospholipid, 1-palmitoyl-2-acetoyl-sn-glycero-3-phosphocholine (PAGPC), is synthesized by tissues important in B-cell development.

OBJECTIVES

We examined whether PAGPC was capable of influencing immunoglobulin synthesis in B lymphocytes and compared its action with that of PAF. We also examined the interaction of the two mediators as agonists or competitive antagonists.

METHODS

Ramos, an IgM-secreting immature B-cell line that expresses PAF receptor, was used in these experiments. The effect of PAF, PAGPC, or both mediators together on IgM secretion and anti-IgM-mediated apoptosis was measured.

RESULTS

Both PAF and PAGPC stimulated IgM production in Ramos cells in a dose-dependent fashion, with PAGPC being approximately three logs less potent than PAF. The effect of both mediators was inhibited by specific PAF receptor antagonists. Preincubation with suboptimal concentrations of PAGPC inhibited the ability of PAF to increase IgM secretion by Ramos cells. Additionally, preincubation with low concentrations of PAGPC prevented PAF from rescuing Ramos cells from apoptosis induced by cross-linking the B-cell receptor with anti-IgM antibodies. PAGPC caused PAF receptor desensitization because displacement of bound PAGPC with high concentrations of bovine serum albumin did not reverse its PAF antagonist effect.

CONCLUSIONS

PAF and PAGPC are biologically active phospholipids, but PAF is approximately 1000 times more potent. At high concentrations, PAGPC acts similarly to PAF, whereas at lower concentrations, PAGPC acts as a functional PAF antagonist. Because it is secreted at sites of inflammation and allergic reactions, PAGPC may be an endogenous regulator of the effects of PAF.

PAF and haematopoiesis: III. Presence and metabolism of platelet-activating factor in human bone marrow

Platelet-activating factor (PAF) is a phospholipid compound with major immunoregulatory activities. The present study shows that human bone marrow contains 576 +/- 39 pg PAF/ml (n = 35). Bone marrow-derived PAF exhibits the same biophysical and biological properties that synthetic PAF. PAF concentrations in bone marrow are correlated with the granulocyte (r = 0.4, P = 0.02) but not with the lymphocyte (r = 0.24, P = 0.17) and the monocyte (r = 0.12, P = 0.48) counts. In bone marrow PAF is inactivated by a plasma PAF acetylhydrolase activity (48.0 +/- 2.3 nmol/min per ml, n = 34). Experiments with [3H]PAF indicate that human bone marrow cells actively metabolize this potent molecule by the deacetylation-transacylation pathway. Results of this investigation indicate the permanent presence of significant amounts of PAF in bone marrow suggesting its putative involvement in the processes of bone marrow cell proliferation and maturation.

Identification of monocyte chemotactic factors in supernatants of ovalbumin-stimulated lymphocytes from patients with atopic dermatitis who are sensitive to hen's egg

Monocyte chemotactic activities in supernatants of ovalbumin (OA)-stimulated peripheral blood mononuclear cell (PBMC) cultures were studied in patients with atopic dermatitis (AD) who were sensitive to hen's egg. The monocyte chemotactic activities in hen's egg-sensitive AD patients were significantly higher than those of non-atopic healthy controls and patients with immediate allergic symptoms. However, the monocyte chemotactic activities were not detected in bovine serum albumin-stimulated PBMC culture supernatants in patients with AD who were sensitive to hen's egg, but not to cow's milk. Furthermore, there was significant correlation between the monocyte chemotactic activities and proliferative responses of PBMCs to OA in hen's egg-sensitive AD patients, whereas there was no significant correlation between the monocyte chemotactic activities and radioallergosorbent test values. These results suggest that PBMCs stimulated with food antigens produce monocyte chemotactic factors which relate to the pathogenesis of AD in food-sensitive AD patients and that the pathogenesis of AD may be related to cell-mediated immune responses.

Tumor necrosis factor release by murine macrophages stimulated by the cytotoxic ether lipid 1-O-hexadecyl-2-O-methyl-SN-glycero-3-phosphorylcholine (ET-18-O-OCH3)

The cytotoxic ether lipid 1-O-hexadecyl-2-O-methyl-SN-glycero-3-phosphorylcholine (ET-18-O-OCH3) is a structural analog of the mediator of inflammation platelet-activating factor (PAF). Recent studies demonstrated that ET-18-O-OCH3 activates human monocytes selectively at non-cytotoxic concentrations. The current studies determined the capacity of ET-18-O-OCH3 to stimulate release of TNF alpha by murine peritoneal macrophages. Macrophage receptors for ET-18-OCH3 and PAF were also assessed. ET-18-O-OCH3 and PAF stimulated TNF alpha release by resident BALB/c macrophages in the presence of LPS but not in the absence of this co-factor. In contrast, both ET-18-O_OCH3 and PAF stimulated TNF alpha release by thioglycollate-elicited macrophages in the absence of LPS although release was greater in the presence of this co-stimulus. Optimal stimulation of TNF alpha release occurred at 10(-14)-10(-11) M ET-18-O-OCH3 and PAF. Elicited macrophages and splenic macrophages from C57Bl/6 mice, unlike those from BALB/c mice, did not respond to 10(-15)-10(-8) M ET-18-O-OCH3 or PAF without or with LPS. Scatchard analysis of [3H]PAF binding to elicited BALB/c macrophages revealed the existence of high affinity receptors for PAF. In contrast, there was no evidence for receptors for ET-18-O-OCH3. ET-18-O-OCH3 did not compete with PAF for binding; macrophage activation by ET-18-O-OCH3 was not stereospecific; and, binding studies using [3H]ET-18-O-OCH3 did not reveal saturable binding characteristic of binding to specific receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Human PAF receptor gene expression: induction during HL-60 cell differentiation

Platelet-activating factor is a potent lipid mediator of inflammation and immune regulation. Its numerous biological activities are mediated through specific receptors on the plasma membranes of responsive cells. The expression of such receptors may be modulated by various agents, including those responsible for cell differentiation. Here, we demonstrate that differentiation of the human promyelocytic leukemia cell line HL-60 by 1 alpha,25(OH)2 vitamin D3 towards the macrophage phenotype is associated with induction of PAF receptor gene expression: PAF receptor mRNA accumulation correlates with the induction and development of specific PAF responsiveness as assayed by [Ca2+]i fluxes. Our studies suggest that PAF responsiveness parallels macrophage differentiation and that PAF receptor expression can be regulated at the transcriptional level.

Platelet-activating factor stimulates expression of IL-1 beta mRNA in THP-1 cells

A human monocytic cell line (THP-1) was used to study the effects of PAF (platelet-activating factor) on the expression of IL-1 beta mRNA. THP-1 cells were incubated with 10 pM PAF in the presence or absence of 0.1 microgram/mL endotoxin for 4 hr, after which cytoplasmic RNA was extracted and subjected to Northern hybridizations. PAF, alone and in combination with endotoxin, caused an increase in mRNA levels for IL-1 beta. The magnitude of the effects of PAF on IL-1 beta mRNA levels matched closely the effects seen at the level of protein synthesis, suggesting that the effects of PAF on IL-1 beta release may result largely from its effects on IL-1 beta mRNA levels.