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Toxicology and Applied Pharmacology 195 (2004) 331–338
Platelet activating factor receptor binding plays a critical
role in jet fuel-induced immune suppression
Gerardo Ramos,a,b Nasser Kazimi,a Dat X. Nghiem,a,b
Jeffrey P. Walterscheid,a,b and Stephen E. Ullricha,b,*
aDepartment of Immunology, The University of Texas MD Anderson Cancer Center, Houston, TX 77030, USAbThe Graduate School of Biomedical Sciences, Houston, TX 77030, USA
Received 3 June 2003; accepted 29 July 2003
Abstract
Applying military jet fuel (JP-8) or commercial jet fuel (Jet-A) to the skin of mice suppresses the immune response in a dose-
dependant 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.
D 2003 Elsevier Inc. All rights reserved.
Keywords: Jet fuel; Immune suppression; Platelet-activating factor; Prostaglandins
Introduction rather the product is refined to meet a series of perfor-
During the past decade, the United States Air Force and
the Air Forces of the NATO allies began a gradual
conversion to a new jet fuel, JP-8. Commercial and
military jet fuels are complex mixtures of hydrocarbons
whose exact compositions are not rigorously defined, but
0041-008X/$ - see front matter D 2003 Elsevier Inc. All rights reserved.
doi:10.1016/j.taap.2003.07.014
Abbreviations: BHT, beta-hydroxy toluene; c-PAF, carbamyl platelet-
activating factor; DTH, delayed-type hypersensitivity; JP-8, jet propulsion-
8 military jet fuel; Jet-A, commercial jet fuel; Oct Br., octylonium bromide;
PAF, platelet-activating factor.
* Corresponding author. Department of Immunology, The University of
Texas MD Anderson Cancer Center, PO Box 301402, Houston, TX 77030-
1903. Fax: +1-713-563-3357.
E-mail address: sullrich@mdanderson.org (S.E. Ullrich).
mance specifications (White, 1999). The new generation of
military jet fuel is refined to have a higher flash point, a
lower vapor pressure, and a lower freezing point than
previous fuels. The net result is a safer fuel, one that is
less combustible and more resistant to exploding than
previous fuels, and also, one that performs better at high
altitudes. The lower vapor pressure also minimizes evap-
orative loses during storage, a serious financial and logis-
tical concern.
Initial toxicological studies of JP-8 suggested that the
new fuel presented minimal adverse effects (Kinkead et al.,
1992; Mattie et al., 1991, 1995). However, as the conver-
sion to JP-8 progressed, reports of health problems by
personnel exposed to the fuel, especially engine mechanics,
fuel handlers, and flight line personnel, prompted further
examinations of JP-8 toxicity. It is now apparent that JP-
G. Ramos et al. / Toxicology and Applied Pharmacology 195 (2004) 331–338332
8 has subtle effects on the nervous system (Smith et al.,
1997), more serious effects on pulmonary function (Hays et
al., 1995; Pfaff et al., 1995; Robledo et al., 2000) and
induces dermal toxicity (McDougal et al., 2000; Monteiro-
Riviere et al., 2001). Furthermore, treating cell lines with
JP-8 induces DNA damage (Grant et al., 2001) and
oxidative stress (Boulares et al., 2002). Immune function
appears to be particularly sensitive to JP-8-induced toxicity.
The two major routes of human jet fuel exposure are
inhalation of aerosols and vapors and absorption through
the skin. Pulmonary (Harris et al., 1997a,b,c, 2000) and
dermal (Ullrich, 1999) exposure to JP-8 induced immune
suppression. Moreover, immune toxicity is noted much
before toxic effects are found in other organ systems
(Harris et al., 1997c).
One mechanism responsible for immune suppression
following dermal JP-8 exposure is the induction of immune
regulatory cytokines and biological response modifiers.
Interluekin-10 is found in the serum of JP-8-treated mice,
and neutralizing IL-10 activity with anti-IL-10 monoclonal
antibody blocked JP-8-induced immune suppression (Ull-
rich and Lyons, 2000). In addition, administering a selec-
tive cyclooxygenase-2 inhibitor to JP-8-treated mice
(Ullrich and Lyons, 2000), or mice treated with commercial
jet fuel (Jet-A), reversed jet fuel-induced immune suppres-
sion (Ramos et al., 2002). These findings imply that
prostaglandin E2 (PGE2) production is an essential early
step in the sequence of events that leads to immune
suppression.
The essential role that PGE2 plays in jet fuel-induced
immune suppression caused us to focus our attention on
platelet-activating factor (PAF; 1-O-alkyl-2-acetyl-sn-glyc-
ero-3-phosphocholine). PAF is a phospholipid inflammato-
ry mediator with a wide variety of biological effects. In
addition to platelets, PAF activates monocytes, mast cells,
and polymorphonuclear leukocytes. It also induces the
migration of granulocytes to sites of inflammation and is
mitototic for fibroblasts and lymphocytes (Prescott et al.,
2000). PAF plays a role in cell-to-cell communication in a
variety of different organ systems, including the vascular
system, the central nervous system, the endocrine system,
and the gastrointestinal tract. Cells responsive to PAF
express a seven transmembrane spanning G-coupled pro-
tein. Binding of PAF to its receptor induces a variety of
intracellular events, including the activation of the mito-
gen-activated protein kinase pathway, the activation of
phospholipases, and the biosynthesis of a variety of
cytokines and prostaglandins (Ishii and Shimizu, 2000).
Of particular interest are the findings that PAF-receptor
binding results in the up-regulation of cyclooxygenase-2
expression (Pei et al., 1998) and increased transcription of
cyclooxygenase-2 and IL-10 genes (Walterscheid et al.,
2002). These observations, coupled with the fact that
blocking PAF-receptor binding prevents the activation of
immune suppression by another dermal immunotoxin (UV
radiation) (Walterscheid et al., 2002), prompted us to
question whether PAF-receptor binding is an important
step in jet fuel-induced immune suppression.
Materials and methods
Animals. Specific pathogen-free female C3H/HeNCr
(MTV�) mice were obtained from the National Cancer
Institute Frederick Cancer Research Facility Animal Pro-
duction Area (Frederick, MD). The animals were main-
tained in facilities approved by the Association for
Assessment and Accreditation of Laboratory Animal Care
International, in accordance with current regulations and
standards of the United States Department of Agriculture,
Department of Health and Human Services, and National
Institutes of Health. All animal procedures were reviewed
and approved by the Institutional Animal Care and Use
Committee. Within each experiment, all mice were age
matched. The mice were 8–10 weeks old at the start of
each experiment.
Reagents and cell lines. The metabolically stable analogue
of PAF, carbamyl-PAF (c-PAF) and the specific PAF-recep-
tor antagonists, PCA-4248, CV-3988; trans-2,5-bis (3,4,5-
trimethoxyphenyl)-1,3-dioxolone (hereafter referred to as
dioxolone), and octylonium bromide (Oct Br) were pur-
chased from Biomol Research Labs, Plymouth Meeting, PA.
Stock solutions were prepared in 50% DMSO/PBS and
diluted further in PBS or tissue culture medium immediately
before use. Vitamin C, Vitamin E, and beta-hydroxy toluene
(BHT) were purchased from Sigma Chemical Co (St. Louis,
MO). The transformed murine keratinocyte cell, PAM 212,
was acquired from Dr. Stuart Yuspa, NCI. Tissue culture
medium was obtained from Gibco BRL (Grand Island, NY);
bovine calf serum was purchased from Hyclone Laborato-
ries (Logan, UT).
Application of jet fuels. Military jet fuel, JP-8 (lot #
3509) and commercial jet fuel (Jet-A) were acquired from
the Operational Toxicology Branch, Air Force Research
Laboratory, Wright Patterson Air Force Base, Dayton,
OH. The fuel was stored and used in a chemical fume hood.
Nitrile rubber-based gloves (Touch N Tuff, Fisher Scientific
Co.) were used in the place of normal latex gloves due to
their superior performance in preventing the penetration of
the fuels. The undiluted fuel (800 Ag/ml) was applied
directly to the shaved dorsal skin of the animals, as
described previously (Ullrich, 1999). The mice were held
individually in the hood for 3 h after exposure to prevent
cage mates from grooming and ingesting the fuel. Also, the
jet fuel was placed high up on the back of each mouse,
immediately behind the head to prevent the animals from
grooming themselves and ingesting the fuel. After 3 h, all
the residual fuel was either absorbed or evaporated and the
animals were returned to standard housing in a specific
pathogen-free barrier facility.
Fig. 1. PGE2 secretion by jet fuel-treated keratinocytes. Semi-confluent
keratinocyte cultures were treated with 0.1 Ag/ml of JP-8 or Jet-A for 1 h
at 37jC, in the presence or absence of PCA-4248, a specific PAF receptor
antagonist. The cells were then washed and re-plated in cRPMI medium.
Eighteen to 24 h later, supernatants were removed and PGE2 levels were
determined by ELISA. Results are expressed as pg/ml F SD. * indicates
a statistically significant difference ( P < 0.05) versus the Jet-A only-
treated control. # indicates a statistically significant difference ( P < 0.05)
versus the JP-8 only-treated control. A representative experiment is
shown, this experiment was repeated three times and similar results were
obtained each time.
plied Pharmacology 195 (2004) 331–338 333
Suppression of the elicitation of delayed-type hypersensi-
tivity by jet fuel exposure. On day 0, mice were immu-
nized by subcutaneous injection of 107 formalin-fixed
Candida albicans into each flank. Ten days later, each
hind footpad was measured with an engineer’s micrometer
(Mitutoyo, Tokyo, Japan) and then challenged by intra-
footpad injection of 50 Al of Candida antigen (Alerchek
Inc., Portland, ME). Eighteen to 24 h after challenge, the
thickness of each foot was re-measured, and the mean
footpad swelling for each mouse was calculated (left foot +
right foot H 2). The background footpad swelling (neg-
ative control) was determined in a group of mice that were
not immunized but were challenged. The specific footpad
swelling response was calculated by subtracting the back-
ground response observed in the negative controls from the
mean footpad swelling found in mice that were immunized
and challenged. There were five mice per group; the mean
footpad thickness for the group F the standard error of the
mean was calculated. Statistical differences between the
control and experimental groups was determined using a
one-way analysis of variance (ANOVA) followed by the
Dunnett’s multiple comparison test (GraphPad Software,
San Diego, CA). Percentage immune suppression was
determined by the following formula: % immune suppres-
sion = 1 � [specific footpad swelling of the jet fuel-
treated mice H specific footpad swelling of the positive
control] � 100. For the most part, the jet fuel was applied
to the dorsal skin of the mice 1 day before challenge.
Previous studies have indicated that applying the jet fuel
on days 6 through 9 post-immunization will suppress the
elicitation of delayed-type hypersensitivity (DTH) (Ramos
et al., 2002).
PGE2 analysis. Five million Pam 212 cells were sus-
pended in 5 ml of RPMI 1640 medium containing 10%
bovine calf serum and added to 100-mm tissue culture
dishes. Eighteen to 24 h later, the medium was removed;
the sub-confluent monolayer was washed with PBS and
treated with jet fuel, in the presence or absence of PCA-
4248. In initial experiments, keratinocyte monolayers were
treated with different concentrations of the jet fuel (2.0–
0.02 Ag/ml), and the cell viability was determined by the
MTT assay. In all subsequent experiments, we used the
highest dose of jet fuel (0.1 Ag/ml) that did not induce
overt cellular toxicity (data not shown). As positive con-
trols, the keratinocytes were exposed to 100 pmol of c-
PAF as described previously (Walterscheid et al., 2002).
One hour later, the medium was removed, the cells were
washed and resuspended in complete RPMI 1640. Back-
ground responses were measured in supernatants from cells
that were not treated with jet fuel, but were otherwise
treated in an identical manner. Eighteen to 24 h later, the
supernatants were collected and PGE2 levels were deter-
mined by a PGE2-specific ELISA according to the manu-
facturer’s instructions (Cayman Chemical, Ann Arbor MI).
The limit of detection is approximately 30 pg/ml. Each
G. Ramos et al. / Toxicology and Ap
sample was measured in triplicate; the data are presented
as the mean F the standard deviation of the samples.
Statistical differences between the jet fuel-treated controls
and the PAF-receptor antagonist-treated groups were de-
termined by the use of the two-tailed Student’s t test
(GraphPad Software).
Results
Keratinocytes produce PGE2 following the application of jet
fuel
Previously, we reported that injecting jet fuel-treated
mice with a selective cyclooxygenase-2 inhibitor blocked
immune suppression (Ramos et al., 2002; Ullrich and
Lyons, 2000). These data imply that dermal jet fuel
treatment activates keratinocytes to secrete PGE2. To
directly test this hypothesis, keratinocyte cultures were
treated with jet fuel and PGE2 secretion was measured
(Fig. 1). Prostaglandin E2 was found in supernatants
isolated from keratinocytes cultures treated with JP-8 and
Jet-A. Because PAF-receptor binding up-regulates eicosa-
noid secretion by keratinocytes (Pei et al., 1998), we also
tested the role of PAF-receptor binding in jet fuel-induced
PGE2 secretion. To do this, keratinocytes were pretreated
with a specific PAF-receptor antagonist (PCA-4248; 1 h at
G. Ramos et al. / Toxicology and Applied Pharmacology 195 (2004) 331–338334
37jC), washed with PBS, and then treated with JP-8, Jet
A, or c-PAF. Induction of PGE2 release by JP-8, Jet A,
and/or PAF, coupled with the inhibition of jet fuel-induced
PGE2 secretion by the PAF-receptor antagonist (Fig. 1),
suggests that jet fuel treatment activates PGE2 secretion by
a PAF-receptor binding-dependent pathway.
PAF suppresses DTH in vivo
Next we wanted to determine if PAF suppresses the
elicitation of DTH in vivo (Fig. 2). Mice were injected with
c-PAF 1 day before challenge. As positive controls for
suppressor activity in this experiment, two other groups of
mice were treated with immunosuppressive doses of Jet-A
and JP-8 (Ramos et al., 2002). Injecting c-PAF suppressed
DTH in a dose-dependent fashion. Significant immune
suppression was observed in mice injected with 1 and 10
nmol of c-PAF. At the higher dose, the immune suppression
observed was indistinguishable from the suppression ob-
served in jet fuel-treated mice. These data indicate that PAF
can suppress the elicitation of DTH.
Platelet-activating receptor antagonists block jet
fuel-induced immune suppression
In the next series of experiments, we wanted to determine
whether blocking PAF-receptor binding would interfere
with jet fuel-induced immune suppression. Mice were
immunized with C. albicans on day 0, and treated with an
Fig. 2. PAF suppresses the elicitation of DTH. Mice were injected with c-
PAF or treated with JP-8 or Jet-A 1 day before challenge. The background
response (negative control) was measured in mice that were not immunized
but were challenged. The positive control was measured in mice that were
immunized and challenged. Results are expressed as means F SEM. The
asterisk (*) indicates a statistically significant difference ( P < 0.005) from
the positive control. A representative experiment is shown, this experiment
was repeated twice and similar results were obtained each time.
Fig. 3. A specific PAF receptor antagonist blocks JP-8-induced immune
suppression. Mice were treated with an immunosuppressive dose of JP-8
(A) or Jet-A (B) 1 day before challenge. Two hours before jet fuel
treatment, 10–200 nmol of PCA-4248 was injected into the peritoneal
cavity. Results are expressed as means F SEM. The asterisk (*) indicates a
statistically significant difference ( P < 0.01) from the positive control. A
representative experiment is shown, this experiment was repeated three
times and similar results were obtained each time.
immunosuppressive dose of JP-8 (Fig. 3A) or Jet-A (Fig.
3B) on day 9. Groups of mice were injected (intraperitoneal)
with a specific PAF-receptor antagonist, PCA-4248, 2
h before jet fuel treatment. As expected, treating the mice
with JP-8 or Jet-A caused significant immune suppression
(P < 0.01 vs. the positive control). Injecting 10 nmol of
PCA-4248 into the jet fuel-treated mice had no effect on
G. Ramos et al. / Toxicology and Applied Pharmacology 195 (2004) 331–338 335
immune suppression (P < 0.05 vs. the positive control).
When the mice received 100 nmol of PCA-4248, the DTH
reaction was reversed to a degree; however, it was still
significantly different from the positive control (P < 0.01).
Complete restoration of DTH was noted when the mice
were injected with 200 nmol of PCA-4248, in that the
response generated was not significantly different from that
found in the positive control.
To confirm these results, and to ensure that the effects we
see are not unique to PCA-4248, we repeated this experi-
ment using three other structurally diverse PAF-receptor
antagonists, CV-3988, dioxolone, and Oct Br (Corey et al.,
1988; Fernandez-Gallardo et al., 1990; Terashita et al.,
1985). Two hours before jet fuel exposure, the mice were
injected with 200 nmol of the PAF-receptor antagonist and
their effect on the suppression of DTH was measured. In
Fig. 4, panel A, the PAF-receptor antagonists were injected
into mice that were immunized, but not treated with jet fuel.
Injecting the PAF-receptor antagonists into non-jet fuel-
Fig. 4. Structurally diverse PAF receptor antagonists block jet fuel-induced
immune suppression. Mice received intraperitoneal injections of 200 nmol
of PCA-4248, CV-3988, dioxolane or Oct Br, 24 h before challenge. In
panel A, the mice received no further treatment. In panel B, the mice were
treated with Jet-A, 2 h after injection of the PAF receptor antagonists. In
panel C, the mice were treated with JP-8, 2 h after injection of the PAF
receptor antagonists. Results are expressed as means F SEM. The asterisk
(*) indicates a statistically significant difference ( P < 0.002) from the
positive control. A representative experiment is shown, this experiment was
repeated three times and similar results were obtained each time.
Fig. 5. Vitamin C reverses JP-8-induced immune suppression. Mice were
treated with JP-8 1 day before challenge. Two hours before JP-8 treatment,
the mice received intraperitoneal injections of 10–1000 nmol of Vitamin C.
Results are expressed as means F SEM. The asterisk (*) indicates a
statistically significant difference ( P < 0.002) from the positive control. A
representative experiment is shown, this experiment was repeated three
times and similar results were obtained each time.
treated mice had no effect on the immune response (P >
0.05 vs. the positive control). The effects of the structurally
diverse PAF-receptor antagonists on jet fuel-induced im-
mune suppression are found in Fig. 4, panels B and C. As
before, treating the mice with Jet-A (Fig. 4B) or JP-8 (Fig.
4C) suppressed DTH. No immune suppression was noted,
however, in mice treated with jet fuel and injected with any
of the PAF-receptor antagonists. These data indicate that
pharmacological blockade of the PAF receptor with any of
four structurally diverse receptor antagonists prevents jet
fuel-induced immune suppression.
Reversal of jet fuel-induced immune suppression with
anti-oxidants
Because dermal exposure to jet fuel induces oxidative
stress (Rogers et al., 2001), we wanted to determine whether
anti-oxidant treatment would reverse jet fuel-induced im-
mune suppression. Mice were immunized with C. albicans
on day 0 and treated with JP-8 on day 9. Two hours before
JP-8 treatment, the mice were injected i.p. with different
doses of Vitamin C. As shown in Fig. 5, we noted a dose-
dependent inhibition of JP-8-induced immune suppression.
At the lowest dose of Vitamin C injected (10 nmol), no
effect was noted (P < 0.002 vs. the positive control).
When, however, the mice were injected with 10- or 100-
fold more Vitamin C, no immune suppression was observed.
The DTH reaction in these mice was indistinguishable from
the positive control.
G. Ramos et al. / Toxicology and Applied Pharmacology 195 (2004) 331–338336
We extended these findings in the next experiment by
adding two additional anti-oxidants, Vitamin E and BHT,
and asking whether anti-oxidant treatment interferes with
Jet-A-induced immune suppression (Fig. 6). The data pre-
sented in panel A confirm that injecting the anti-oxidants
into non-jet fuel-treated mice has no suppressive effect. In
panel B, we examined the effect that Vitamin C, Vitamin E,
and BHT had on Jet-A-induced immune suppression. When
the mice were injected with 100 nmol of any one of the
three anti-oxidants, 2 h before Jet-A treatment, no immune
suppression was observed. The response found in these
mice was statistically similar to that found in the positive
control (P > 0.05). Similar results were obtained when 100
nmol of Vitamin C, Vitamin E, and/or BHT was injected
Fig. 6. Treatment with anti-oxidants blocks jet fuel-induced immune
suppression. Mice received intraperitoneal injections of 100 nmol of
Vitamin C, Vitamin E or BHT, 24 h before challenge. In panel A, the
mice received no further treatment. In panel B, the mice were treated with
Jet-A, 2 h after receiving the anti-oxidants. In panel C, the mice were
treated with JP-8, 2 h after receiving the anti-oxidants. Results are
expressed as means F SEM. The asterisk (*) indicates a statistically
significant difference ( P < 0.001) from the positive control. A
representative experiment is shown, this experiment was repeated three
times and similar results were obtained each time.
into JP-8-treated mice. All three anti-oxidants blocked JP-8-
induced immune suppression. Because oxidative stress also
stimulates the production of PAF and PAF-like oxidized
phospholipids (Lewis et al., 1988; Marathe et al., 1999) that
can signal through the PAF receptor (Davies et al., 2001;
McIntyre et al., 1999), these findings are consistent with the
data presented above suggesting a role for PAF-receptor
binding in jet fuel-induced immune suppression.
Discussion
The focus of the studies presented here was to deter-
mine the mechanisms underlying the activation of immune
suppression following the dermal application of military
and commercial jet fuels. Results from previous studies
imply that PGE2 production, presumably by jet fuel-treated
keratinocytes, is a critical early step in jet fuel-induced
immune suppression (Ramos et al., 2002; Ullrich and
Lyons, 2000). Here, we provide direct evidence that
PGE2 is secreted by jet fuel-treated keratinocytes. More-
over, we demonstrate that injecting jet fuel-treated mice
with PAF-receptor antagonists blocks jet fuel-induced
immune suppression. Furthermore, we show that treating
mice with anti-oxidants will block jet fuel-induced immune
suppression. These data indicate that PAF-receptor binding
is an essential early step in the cascade of events that leads
to jet fuel-induced immune suppression.
As mentioned above, jet fuels are complex chemical
mixtures, containing more than 200 aliphatic and aromatic
hydrocarbons. The precise composition of a jet fuel will
vary from batch to batch, as the fuel is refined to meet
performance specifications (White, 1999). Military jet fuel,
JP-8, is essentially commercial jet fuel (Jet-A) to which
three components are added, an anti-static agent, an anti-
corrosive agent, and an anti-freeze. Although the immune
suppressive effects of JP-8 are well documented (Harris et
al., 1997a,c, 2000; Ullrich, 1999; Ullrich and Lyons,
2000), less is known about the effect of Jet-A on the
immune response (Ramos et al., 2002). This is somewhat
surprising in view of the fact that commercial airlines in
the United States are estimated to use in excess of 68
million gallons of Jet-A a day (White, 1999). Our data
clearly demonstrate that commercial Jet-A induces immune
suppression. In addition to our previous results, showing
almost identical dose-response curves for JP-8 and Jet-A-
induced immune suppression (Ramos et al., 2002), the data
presented here, showing that both JP-8 and Jet-A activate
keratinocytes to secrete PGE2, coupled with the fact that
anti-oxidants and PAF-receptor antagonists interfere with
both JP-8 and Jet-A induced immune suppression, support
our previous conclusion that jet fuel-induced immune
suppression is a function of the base kerosene fuel, Jet-
A. What remains unresolved is the identity of the exact
hydrocarbon, or mixture of hydrocarbons, which activates
immune suppression.
G. Ramos et al. / Toxicology and Applied Pharmacology 195 (2004) 331–338 337
Synthesis of PGE2 is a critical step in wound healing
and scar formation (Reno et al., 2001; Rys-Sikora et al.,
2000), the immune suppression associated with burns
(Schwacha et al., 2002), the immune suppression induced
by some chemical carcinogens (Andrews et al., 1991), and
the sunburn and immune suppression caused by UV
radiation (Rhodes et al., 2001; Shreedhar et al., 1998).
We suggest that a common mechanism underlying PGE2
synthesis in all these examples of dermal trauma is
activation via the PAF receptor. Cells that secrete PAF,
such as keratinocytes, do not contain preformed stores of
the molecule, but rapidly synthesize PAF in response to
cellular stress. Secreted PAF then binds to PAF receptors
on target cells, inducing downstream effects such as
calcium flux, activation of the MAP kinase cascade, and
activation of phospholipase-A2 to cleave arachidonic acid
from the cell membrane. An acetyl residue is then trans-
ferred from Acetyl-CoA to the free hydroxyl of arachi-
donic acid to form biologically active PAF. In addition,
arachidonic acid servers as a substrate for PAF-induced
cyclooxygenase-2. This series of events ultimately results
in the secretion of PGE2 and a variety of inflammatory and
immune regulatory cytokines (Pei et al., 1998). Manipu-
lating this pathway may have practical implications. For
example, the systemic immune suppression associated with
burns may be blocked by PAF-receptor antagonists, in a
manner similar to the inhibition of immune suppression we
describe here.
Perhaps the most successful public health campaign of
the 20th century was the widespread use of vaccination to
prevent the morbidity and mortality due to infectious dis-
eases. Previously, we demonstrated that dermal jet fuel
exposure suppresses secondary immune reactions (Ramos
et al., 2002). We believe that this is a very relevant model to
study jet fuel-induced immune toxicity. Jet fuel-induced
immune suppression of the elicitation of DTH suggests that
jet fuel exposure has the potential to weaken resistance to
infectious disease afforded by previous immunization. This
may be particularly relevant in situations such as combat,
where stress and fatigue, coupled with jet fuel-induced
immune suppression, may contribute to a weakened im-
mune response.
Unfortunately, minimizing jet fuel exposure may not be
easy. As mentioned above, the active moieties within jet
fuel (both JP-8 and Jet-A) that induce immunotoxicity
remain unknown. So removing them from the blend of
hydrocarbons that constitute jet fuel is impossible at the
present time. Spills during refueling, contact with residual
fuel during fuel tank and/or engine maintenance, fuel leaks,
and exposure of flight line personnel to un-burnt fuel
during ‘‘cold’’ engine starts are common ways in which
Air Force and commercial airline employees encounter jet
fuel. Because risk avoidance is probably impossible, it may
be more useful to attempt to block the induction of immune
suppression. Previously, we demonstrated that blocking
PGE2 production, with selective cyclooxygenase-2 inhib-
itors, very effectively blocked jet fuel-induced immune
suppression (Ramos et al., 2002; Ullrich and Lyons,
2000). In this study, we targeted an event up-stream of
PGE2 production, PAF-receptor binding, and saw similar
results. Although the safety of PAF-receptor antagonists in
humans is unknown, blocking jet fuel-induced immune
suppression with anti-oxidants, such as Vitamins C and
E, has the potential to be useful in protecting humans
against jet fuel-induced immunotoxicity.
Acknowledgments
The views and opinions expressed here are those of the
authors and do not reflect the official policy or position of
the United States Air Force, The Department of Defense,
or the US Government. This work was supported by grants
from the United States Air Force Office of Scientific
Research (F49620-02-0121) and the National Cancer
Institute (CA 075575). Gerardo Ramos was supported by
the USAF Institute of Technology. The animal facilities at
the MD Anderson Cancer Center are supported in part by
a core grant from the NCI (CA 16672).
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