Platelet-activating factor, a molecular sensor for cellular damage, activates systemic immune suppression

Platelet-activating factor receptor gene polymorphism in Japanese patients with multiple sclerosis

We evaluated the association of the platelet-activating factor receptor (PAFR) gene polymorphism (A224D) with the susceptibility and severity of multiple sclerosis (MS) in a Japanese population. DNA was collected from 162 Japanese patients with clinically definite 'conventional' MS (MS) and 245 healthy controls. The missense mutation A224D that impairs PAF-PAFR signaling was determined by polymerase chain reaction restriction fragment length polymorphism. The frequency of the AD/DD genotypes was significantly higher in MS patients (21.0%) than in healthy controls (13.5%) (p=0.045; odds ratio (OR), 1.71; 95% confidence interval (CI), 1.01-2.89). Moreover, the frequency of D allele in MS patients (11.7%) was also significantly higher than those in healthy controls (6.9%) (p=0.019; OR, 1.78; 95% CI, 1.10-2.89). These findings suggest that the PAFR gene missense mutation has a relation to the susceptibility for MS.

Ultraviolet B radiation-induced immunosuppression: molecular mechanisms and cellular alterations

About 30 years ago, the discovery of the connection between UV radiation and the immune system triggered the field of photoimmunology. In that time, many aspects were studied, and a complex picture emerged. UV absorption results in multi-tiered molecular and cellular UV radiation-induced events, eventually affecting the immune system. The shorter wavelengths of the UV spectrum, i.e. UVB appear to be the most critical players for impairing immune reactions. This review summarizes and discusses UVB radiation-induced effects on the skin, considering the primary efferent molecular events following energy absorption of UVB radiation, ending with the various afferent cellular changes, such as induction of regulatory T cells.

Is antibody therapy of tumor compromised by infusion-related reactions?

Evidence suggests that amelioration of childhood immune thrombocytopenic purpura and some other autoimmune states by intravenous normal IgG is due to the following chain of events: (1) cross-linking of Fcgamma-receptors on blood effector cells; (2) release of mediators from these cells, often yielding an infusion-related reaction; (3) mediator-induced development of a cytokine field characterized by a mutually stabilizing Th2 polarization of CD4 T lymphocytes and alternative activation of macrophages; (4) selective quiescence of these macrophages towards targets coated with IgG autoantibody, due to increased expression of the macrophage Fcgamma-receptor IIB. In this paper it is postulated that in the field of antibody therapy of tumor, an undesirable delayed or absent subsidence of antibody-coated tumor is due to immunomodulation of the same type as yields amelioration of autoimmunity, and arising from a similar chain of events. If the postulate is correct the chain could usefully be broken at the level of mediator action, possibly by blocking that increased synthesis of prostaglandin E(2) which is catalyzed by the enzyme cyclooxygenase-2.

Human melanocyte biology, toxicology, and pathology

The human melanocytes of the skin, hair, eyes, inner ears, and covering of the brain provide physiologic functions important in organ development and maintenance. Melanocytes develop from embryonic neural crest progenitors and share certain traits with other neural crest derivatives found in the adrenal medulla and peripheral nervous system. The distinctive metabolic feature of melanocytes is the synthesis of melanin pigments from tyrosine and cysteine precursors involving over 100 gene products. These complex biochemical mechanisms create inherent liabilities for melanocytic cells if intracellular systems necessary for compartmentalization, detoxification, or repair are compromised. Melanocyte disorders may involve pigmentation, sensory functions, autoimmunity, or malignancy. Environmental factors such as ultraviolet radiation and chemical exposures, combined with heritable traits, represent the principal hazards associated with melanocyte disorders.

Regulation of immunity by lysosphingolipids and their G protein-coupled receptors

T and B lymphocytes, as well as endothelial cells, express distinctive profiles of G protein-coupled receptors for sphingosine 1-phosphate, which is a major regulator of T cell development, B and T cell recirculation, tissue homing patterns, and chemotactic responses to chemokines. The capacity of drugs that act on type 1 sphingosine 1-phosphate receptors to suppress organ graft rejection in humans and autoimmunity in animal models without apparent impairment of host defenses against infections suggests that this system is a promising target for new forms of immunotherapy.

Augmentation of chemotherapy-induced cytokine production by expression of the platelet-activating factor receptor in a human epithelial carcinoma cell line

In addition to their known cytotoxic effects, chemotherapeutic agents can trigger cytokine production in tumor cells. Moreover, many chemotherapeutic agents are potent pro-oxidative stressors. Although the lipid mediator platelet-activating factor (PAF) is synthesized in response to oxidative stress, and many epidermal carcinomas express PAF receptors (PAF-R) linked to cytokine production, it is not known whether PAF is involved in chemotherapeutic agent-induced cytokine production. These studies examined the role of the PAF system in chemotherapy-mediated cytokine production using a model system created by retroviral-mediated transduction of the PAF-R-negative human epidermal carcinoma cell line KB with the human PAF-R. The presence of the PAF-R in KB cells resulted in augmentation of the production of cytokines IL-8 and TNF-alpha induced by the chemotherapeutic agents etoposide and mitomycin C. These effects were specific for the PAF-R, as expression of the G protein-coupled receptor for fMLP did not affect chemotherapeutic agent-induced cytokine production. Moreover, ablation of the native PAF-R in the epithelial cell line HaCaT using an inducible antisense PAF-R strategy inhibited etoposide-induced cytokine production. Oxidative stress and the transcription factor NF-kappaB were found to be involved in this augmentative effect, because it was mimicked by the oxidant tert-butyl-hydroperoxide, which was blocked both by antioxidants and by inhibition of the NFkappaB pathway using a super-repressor IkappaBM mutant. These studies provide evidence for a novel pathway by which the epidermal PAF-R can augment chemotherapy-induced cytokine production through an NF-kappaB-dependent process.

Platelet-activating factor, a pleiotrophic mediator of physiological and pathological processes

Platelet-activating factor (PAF) is a potent proinflammatory phospholipid with diverse pathological and physiological effects. This bioactive phospholipid mediates processes as diverse as wound healing, physiological inflammation, apoptosis, angiogenesis, reproduction and long-term potentiation. Recent progress has demonstrated the participation of MAP kinase signaling pathways as modulators of the two critical enzymes, phospholipase A2 and acetyltransferase, involved in the remodeling pathway of PAF biosynthesis. The unregulated production of structural analogs of PAF by non-specific oxidative reactions has expanded this superfamily of signaling molecules to include "PAF-like" lipids whose mode of action is identical to that of authentic PAF. The action of members of this family is mediated by the PAF receptor, a G protein-coupled membrane-spanning molecule that can engage multiple signaling pathways in various cell types. Inappropriate activation of this signaling pathway is associated with many diseases in which inflammation is thought to be one of the underlying features. Inactivation of all members of the PAF superfamily occurs by a unique class of enzymes, the PAF acetylhydrolases, that have been characterized at the molecular level and that terminate signals initiated by both regulated and unregulated PAF production.

Platelet activating factor receptor binding plays a critical role in jet fuel-induced immune suppression

Applying military jet fuel (JP-8) or commercial jet fuel (Jet-A) to the skin of mice suppresses the immune response in a dose-dependent manner. The release of biological response modifiers, particularly prostaglandin E2 (PGE2), is a critical step in activating immune suppression. Previous studies have shown that injecting selective cyclooxygenase-2 inhibitors into jet fuel-treated mice blocks immune suppression. Because the inflammatory phospholipid mediator, platelet-activating factor (PAF), up-regulates cyclooxygenase-2 production and PGE2 synthesis by keratinocytes, we tested the hypothesis that PAF-receptor binding plays a role in jet fuel-induced immune suppression. Treating keratinocyte cultures with PAF and/or jet fuel (JP-8 and Jet-A) stimulates PGE2 secretion. Jet fuel-induced PGE2 production was suppressed by treating the keratinocytes with specific PAF-receptor antagonists. Injecting mice with PAF, or treating the skin of the mice with JP-8, or Jet-A, induced immune suppression. Jet fuel-induced immune suppression was blocked when the jet fuel-treated mice were injected with PAF-receptor antagonists before treatment. Jet fuel treatment has been reported to activate oxidative stress and treating the mice with anti-oxidants (Vitamins C, or E or beta-hydroxy toluene), before jet fuel application, interfered with immune suppression. These findings confirm previous studies showing that PAF-receptor binding can modulate immune function. Furthermore, they suggest that PAF-receptor binding may be an early event in the induction of immune suppression by immunotoxic environmental agents that target the skin.

Whole-body UVB (TL-01) or UVA-1 irradiation does not alter the levels of immunomodulatory cytokines in the serum of human volunteers

BACKGROUND/PURPOSE

Ultraviolet (UV) exposure of mammalian skin induces local and systemic immunosuppression. In mice it has been proposed that systemic immunosuppression is mediated by an UV-induced cytokine cascade involving systemic interleukin (IL)-4 and IL-10 and a reduction in IL-12 activity. To investigate whether there was a parallel mechanism in humans we examined the effect of whole-body narrowband ultraviolet B (UVB) (311-313 nm; TL-01) and ultraviolet A (UVA)-1 (340-400 nm) on serum cytokine levels.

METHODS/RESULTS

In a first study, five male psoriatic subjects were whole-body irradiated with three sessions of a standard UVB (TL-01) phototherapy regimen previously shown to cause downregulation of natural killer cell activity and T helper 1 (Th1) and Th2 cytokine production by peripheral blood mononuclear cells. Enzyme-linked immunoabsorbent assay (ELISA) of sera taken before and after the third session showed no effect of phototherapy on IL-10 and tumour necrosis factor-alpha (TNF-alpha). In a second study, five healthy subjects received three whole-body exposures of UVB (TL-01) and five other healthy subjects received three exposures of UVA-1 on alternate days (total 22 J/cm(2)). Blood samples were taken before the first irradiation and at 0, 4, 8, 12, 14, 24 and 48 h after the third irradiation. The sera were subsequently analysed for IL-10, IL-12, IL-8, IL-1beta and TNF-alpha, by ELISA. The levels of IL-1beta and TNF-alpha were below detection limits (<5 pg/ml), while no significant change in the levels of IL-10, IL-12 or IL-8 was detected as a result of either TL-01 or UVA-1.

CONCLUSIONS

It seems unlikely that a modulation in these circulating cytokines assessed in this study accounts for systemic UV-induced immunosuppression in human subjects.

Effects of a melanogenic bicyclic monoterpene diol on cell cycle, p53, TNF-alpha, and PGE2 are distinct from those of UVB

PURPOSE

Bicyclic monoterpene (BMT) diols are small-molecule compounds that mimic ultraviolet radiation (UVR) by inducing melanogenesis. The objective of this study was to compare the effects of 2,2-dimethyl-3-propanyldiol-norbornane (AGI-1140), a novel BMT diol, and ultraviolet B (UVB) on additional cellular responses.

METHODS

S91 mouse melanoma cells were treated with a range of concentrations of AGI-1140, and examined for induction of melanogenesis and nitric oxide (NO). The effect of AGI-1140 on dendrite outgrowth from human melanocytes was examined by quantitative microscopy. The effect of AGI-1140 and UVB on phosphorylation of p53 serine 15 in human keratinocytes was examined by Western blotting, while the release of tumor necrosis factor-alpha (TNF-alpha) and prostaglandin E2 (PGE2) was determined by enzyme-linked immunosorbent assay. The effects of AGI-1140 and UVB on cell cycle arrest of human melanocytes, keratinocytes, fibroblasts, and endothelial cells were compared using fluorescence-activated cell sorting.

RESULTS

Similar to UVB, AGI-1140 induced both melanogenesis and NO in melanoma cells. AGI-1140 also induced dendrite outgrowth from melanocytes, indicative of differentiation. However, whereas UVB induced G2 cell cycle arrest with phosphorylation of p53 at serine 15, AGI-1140 induced G1 cell cycle arrest without this phosphorylation. Additionally, unlike UVB, AGI-1140 did not increase the secretion of TNF-alpha or PGE2, mediators of UVB-induced immunosuppressive and inflammatory responses in the skin that may contribute to carcinogenesis.

CONCLUSION

This study shows that melanogenesis can be induced by AGI-1140 without many of the deleterious effects associated with UVB.

Cyclooxygenases in the skin: pharmacological and toxicological implications

Cyclooxygenase (COX), a prostaglandin-endoperoxide synthase (PTGS), catalyzes the formation of prostaglandins from arachidonic acid. Prostaglandins are lipid signaling mediators that play a central role in a broad range of diverse physiological and pathophysiological processes, including inflammation, reproduction, nocioception, and gastrointestinal protection. Inhibition of cyclooxygenase activity is the mechanism by which nonsteroidal antiinflammatory drugs (NSAIDS) exert their analgesic, antipyretic, antiinflammatory, and antithrombotic effects. COX is currently believed to exist in three isoforms. In this review, we provide a concise state-of-the-art description of the role of COX in pharmacology and toxicology of skin including its involvement in normal physiology, cutaneous inflammation, nociception, wound healing, and tumorigenesis. COX-dependent pathways influence keratinocyte differentiation, hair follicle development, and hair growth. The critical role of COX-2 in pathophysiology of skin is also addressed. COX-2 mediates inflammatory processes in skin, including inflammatory hyperalgesia and nociception, and administration of specific COX-2 inhibitors reduces edema, vascular permeability, and other markers of cutaneous inflammation. A number of studies in animal models and in humans show that COX-2 inhibitors possess cancer chemopreventive properties. Selective COX-2 inhibitors have a more favorable side-effect profile. Topical formulations of COX-2 inhibitors are being developed as a novel pharmacologic approach for the treatment of COX-2 mediated skin diseases.

Oxidized phospholipids as modulators of inflammation in atherosclerosis

PURPOSE OF REVIEW

This review will summarize recent evidence demonstrating that biologically active phospholipid oxidation products modulate inflammatory reactions.

RECENT FINDINGS

Structural identification of new biologically active oxidized phospholipids and the finding that they can also be formed at inflammatory sites other than the atherosclerotic lesion have expanded the potential role of these compounds in inflammation beyond atherogenesis. Various signaling pathways are induced by oxidized phospholipids, leading to the expression of inflammatory genes by mechanisms that differ from those mediated by the classic inflammatory agonists tumor necrosis factor or lipopolysaccharide. Furthermore, oxidized phospholipids can bind to pattern recognition molecules and thus potently influence inflammation and immune responses during host defense.

SUMMARY

During inflammatory processes biologically active lipid oxidation products accumulate that modulate the inflammatory process and may determine the fate and outcome of the body's reaction in acute inflammation during host defense. Oxidized phospholipids may induce and propagate chronic inflammatory processes; however, evidence is accumulating that cells and tissues respond towards these oxidatively formed stress signals also by activation of anti-inflammatory, cytoprotective reactions.

Platelet-activating factor antagonist WEB 2086 inhibits ultraviolet-B radiation-induced dermatitis in the human skin

It has been suggested that platelet-activating factor (PAF) plays a role in the pathomechanisms of various inflammatory diseases. In an experimental animal model we demonstrated earlier that a selective PAF receptor antagonist gel inhibits ultraviolet-B (UVB) light-induced edema in mouse ears. The goal of our present investigation was to determine whether locally applied WEB 2086, a selective PAF receptor antagonist, alters the dermatitis-causing effect of UVB light on human skin. We induced dermatitis in healthy volunteers by irradiating their skin with UVB light in increasing doses. The irradiated area was treated with WEB 2086 gel (3%) or with a placebo. Erythema was measured spectrophotometrically after 24 and 48 h. After both 24 and 48 h, the WEB 2086 gel significantly inhibited the UVB light-induced erythema at each radiation dose in comparison with the placebo. The PAF antagonist gel therefore proved to be effective against UVB-induced dermatitis. Our results may help to understand the relative importance of mediators in UVB-induced dermatitis and might perhaps pave the way to better therapeutic modalities in this condition.

Prostaglandin E2 modulates dendritic cell function via EP2 and EP4 receptor subtypes

We have reported previously that PGE(2) inhibits dendritic cells (DC) functions. Because E prostanoid receptor (EPR) subtypes involved in this action are unknown, expression and functions of these receptors were examined in DC. Western blot and flow cytometry analyses showed that all EPRs were coexpressed in DC. In a dose-dependent manner, lipopolysaccharide (LPS) enhanced EP(2)R/EP(4)R but not EP(1)R/EP(3)R expressions. NS-398, a cyclooxygenase (COX)-2-selective inhibitor, suppressed LPS-enhanced EP(2)R/EP(4)R expression, suggesting that COX-2-issued prostaglandin E(2) (PGE(2)) modulates DC function through stimulation of specific EPR subtypes. Using selective agonists, we found that butaprost, an EP(2)R agonist, and PGE(1) alcohol, an EP(2)R and EP(2)R/EP(4)R agonist, inhibited major histocompatibility complex class II expression and enhanced interleukin-10 production from DC. However, no effect was observed with sulprostone and 17-phenyl-omega-trinor-PGE(2), selective agonists for EP(1)R and EP(1)R/EP(3)R, respectively. Treatment of DC with dibutyryl cyclic adenosine monophosphate (cAMP), an analog of cAMP, mimics PGE(2)-induced, inhibitory effects. Taken together, our data demonstrate that EP(2)R/EP(4)R are efficient for mediating PGE(2)-induced modulation of DC functions.

Platelet-activating factor promotes mucosal apoptosis via FasL-mediating caspase-9 active pathway in rat small intestine after ischemia-reperfusion

Platelet activating factor (PAF) is a proinflammatory lipid mediator for inflammatory response. It is unclear whether PAF is involved in the very complex process of ischemia-reperfusion (I/R) induced mucosal apoptosis in small intestine. Intestinal I/R was induced in rats intestine by 60 min occlusion of the superior mesenteric artery, followed by a 60 min reperfusion. I/R induced mucosal apoptosis and PAF activity but inhibited PAF-acetylhydrolase activity. Increases in interleukin-6 (IL-6) and decreases in IL-10 were observed. Western blot analysis showed that I/R induced expressions of platelet endothelial cell adhesion molecule-1 (PECAM-1) and Fas and Fas ligand (FasL) proteins, cleaved Bid, and enhanced the release of cytochrome c from mitochondria to activate caspase-9. Pretreatment of PAF antagonist BN-52021 attenuated these changes, except the increase in Fas. The results showed that I/R-inhibited mucosal PAF-acetylhydrolase activity resulted in an increase of activated PAF. The activated PAF increased the mucosal IL-6 and PECAM-1, enhanced the expression of FasL but not Fas, and led to the cleavage of Bid and the release of cytochrome c from mitochondria to activate caspase-9. This finding suggests that PAF promotes mucosal apoptosis after I/R in the rat small intestine partly through FasL mediating caspase-9 active pathway.

The epidermal platelet-activating factor receptor augments chemotherapy-induced apoptosis in human carcinoma cell lines

Most chemotherapeutic agents exert their cytotoxic effects in part through the induction of apoptosis. In addition, many chemotherapeutic agents are potent pro-oxidative stressors. Although the lipid mediator platelet-activating factor (PAF) is synthesized in response to oxidative stress, and many epidermal carcinomas express PAF receptors, it is not known whether PAF is involved in chemotherapeutic agent-induced apoptosis. These studies examined the role of the PAF system in chemotherapy-mediated cytotoxicity using model systems created by retroviral mediated transduction of the PAF receptor-negative human epidermal carcinoma cell line KB with the human PAF receptor (PAF-R) and ablation of the endogenous PAF-R in the carcinoma cell line HaCaT with a retroviral mediated inducible antisense PAF-R vector. The presence of the PAF-R in these models resulted in an augmentation of apoptosis induced by chemotherapeutic agents etoposide and mitomycin C but not by tumor necrosis factor-related apoptosis-inducing ligand or by C(2) ceramide. Oxidative stress and the transcription factor nuclear factor kappaB (NF-kappaB) are found to be involved in this augmentative effect because it was blocked by antioxidants and inhibition of the NF-kappaB pathway using a super-repressor form of inhibitor B. These studies provide evidence for a novel pathway whereby the epidermal PAF-R can augment chemotherapy-induced apoptotic effects through an NF-kappaB-dependent process.

Augmentation of staphylococcal alpha-toxin signaling by the epidermal platelet-activating factor receptor

Staphylococcal alpha-toxin is a cytolytic toxin secreted by many strains of Staphylococcus aureus that has proinflammatory and cytotoxic effects on human keratinocytes. alpha-toxin exerts its effects by forming a transmembrane pore that behaves like an ionophore for ions such as calcium. Because cellular membrane disruption with resultant intracellular calcium mobilization is a potent stimulus for the synthesis for the lipid mediator platelet-activating factor, the ability of alpha-toxin to induce platelet-activating factor production was assessed, and whether the epidermal platelet-activating factor receptor could augment toxin-induced signaling in epithelial cells examined. Treatment of the human keratinocyte-derived cell line HaCaT with alpha-toxin resulted in significant levels of platelet-activating factor, which were approximately 50% of the levels induced by calcium ionophore A23187. alpha-toxin also stimulated arachidonic acid release in HaCaT keratinocytes. Pretreatment of HaCaT cells with platelet-activating factor receptor antagonists, or overexpression of the platelet-activating factor metabolizing enzyme acetylhydrolase II blunted alpha-toxin-induced arachidonic acid release by approximately one-third, suggesting a role for toxin-produced platelet-activating factor in this process. Finally, retroviral-mediated expression of the platelet-activating factor receptor into the platelet-activating factor receptor-negative epithelial cell line KB resulted in an augmentation of alpha-toxin-mediated intracellular calcium mobilization and arachidonic acid release. These studies suggest that alpha-toxin-mediated signaling can be augmented via the epidermal platelet-activating factor receptor.

A novel host-parasite lipid cross-talk. Schistosomal lyso-phosphatidylserine activates toll-like receptor 2 and affects immune polarization

Schistosome infections are characterized by prominent T cell hyporesponsiveness during the chronic stage of infection. We found that schistosome-specific phosphatidylserine (PS) activated TLR2 and affected dendritic cells such that mature dendritic cells gained the ability to induce the development of IL-10-producing regulatory T cells. Using mass spectrometry, schistosomal lysophosphatidylserine (lyso-PS) was identified as the TLR2-activating molecule. This activity appears to be a unique property of schistosomal lyso-PS, containing specific acyl chains, because neither a synthetic lyso-PS (16:0) nor PS isolated from the mammalian host activates TLR2. Taken together, these findings provide evidence for a novel host-parasite interaction that may be central to long term survival of the parasite and limited host pathology with implications beyond parasitology.

Identification of platelet-activating factor acetylhydrolase II in human skin

Platelet-activating factor acetylhydrolases are a family of specialized phospholipase A2 enzymes. They serve an anti-inflammatory function by converting the proinflammatory autocoid, PAF, into biologically inactive lyso-PAF, by the removal of the sn-2 acetyl group of this glycerophospholipid. Similarly, platelet-activating factor acetylhydrolases can also degrade oxidatively modified sn-2 polyunsaturated-fatty-acid-containing phospholipids, which are toxic to cells. Platelet-activating factor acetylhydrolase II is a recently cloned member of this family of specialized phospholipases. Consistent with a potential role of this intracellular enzyme in protecting membrane phospholipids against oxidative stress, platelet-activating factor acetylhydrolase II has been shown to translocate from cytosol to membranes in response to pro-oxidative stressors, and overexpression of this enzyme decreases the cytotoxic effects of these agents. The objective of this study was to assess whether platelet-activating factor acetylhydrolase II is involved in protecting skin against oxidative stress. Platelet-activating factor acetylhydrolase II protein was demonstrated in human skin by immunohistochemistry, with the highest levels of the enzyme found in sebaceous glands and lesser amounts in epidermal keratinocytes. Treatment of epidermal cells with t-butylhydroperoxide or ultraviolet B radiation resulted in platelet-activating factor acetylhydrolase II translocation from cytosol to membranes. To assess the role of this enzyme in epidermal function, a recombinant retroviral strategy was used to overexpress platelet-activating factor acetylhydrolase II in the human keratinocyte-derived cell line HaCaT. Overexpression of platelet-activating factor acetylhydrolase II protected HaCaT cells against apop tosis induced by oxidative stressors t-butylhydroperoxide and ultraviolet B radiation. Similar levels of apoptosis, however, were seen in both control and platelet-activating-factor-acetylhydrolase-II-over expressing HaCaT cells in response to C2 ceramide. These studies demonstrate the presence of platelet-activating factor acetylhydrolase II in a restricted pattern in human skin, and provide evidence that this specialized phospholipase is involved in protecting this organ against oxidative stress through the degradation of oxidatively modified bioactive phospholipids.

Ultraviolet radiation-induced immunosuppression of delayed-type hypersensitivity in mice

Ultraviolet (UV) radiation present in sunlight plays a critical role in the initiation and promotion of nonmelanoma skin carcinogenesis and immune suppression. The immune suppressive effects of UV have been identified as a risk factor for skin cancer induction. For these reasons, scientists have focused on elucidating the mechanisms of UV-induced immune suppression to better understand the pathogenesis of skin cancer induction. A hallmark of UV-induced immune suppression is the generation of antigen-specific suppressor T cells. These suppressor cells have been shown to suppress antitumor immunity as well as other cell-mediated responses such as delayed-type hypersensitivity (DTH) reactions. Due to the excessive cost and time involved in traditional UV carcinogenic experiments, scientists have opted to use UV-induced suppression of DTH reactions as a surrogate model. DTH has been, and continues to be, a widely used assay system to measure in vivo immune function. Although somewhat unsophisticated by today's standards, this assay has great advantages because it presents a fast, inexpensive, and reliable model system to help dissect the mechanisms involved in UV-induced immune suppression. Furthermore, the murine model of DTH enables scientists to perform additional procedures, such as adoptive transfer studies with suppressor T cells, which are currently unavailable with human subjects.

Dermal application of jet fuel suppresses secondary immune reactions

Applying military jet fuel (JP-8) to the skin of mice activates systemic immune suppression. In all of our previous experiments, JP-8 was applied to immunologically naïve mice. The effect of jet fuels on established immune reactions, such as immunological memory, is unknown. The focus of the experiments presented here was to test the hypothesis that jet fuel exposure [both JP-8 and commercial jet fuel (Jet-A)] suppresses established immune reactions. Mice were immunized with the opportunistic fungal pathogen Candida albicans and, at different times after immunization (10 to 30 days), various doses of undiluted JP-8 or Jet-A were applied to their skin. Both the elicitation of delayed-type hypersensitivity (DTH) (mice challenged 10 days after immunization) and immunological memory (mice challenged 30 days after immunization) were significantly suppressed in a dose-dependent manner. Dermal exposure to either multiple small doses (50 microl over 4 days) or a single large dose (approximately 200-300 microl) of JP-8 and/or Jet-A suppressed DTH to C. albicans. The mechanism by which dermal application of JP-8 and Jet-A suppresses immunological memory involves the release of immune biologic response modifiers. Blocking the production of prostaglandin E(2) by a selective cyclooxygenase-2 inhibitor (SC 236) significantly reversed jet fuel-induced suppression of immunologic memory. These findings indicate, for the first time, that dermal exposure to commercial jet fuel (Jet-A) suppresses the immune response. In addition, the data reported here expand on previous findings by suggesting that jet fuel exposure may depress the protective effect of prior vaccination.