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Induction of Acute Lung Injury after Intranasal Administration of Toxin Botulinum A ComplexCentre dEtudes du Bouchet (Defense Research Center), BP No. 3, 91710 Vert le Petit, France Correspondence: Address correspondence to: L. Taysse, Centre dEtudes du Bouchet, BP No. 3, Vert le Petit, 91710, France; e-mail:laurent.taysse{at}dga.defense.gouv.fr
The inhalation of aerozolized botulinum toxin may represent a potential significant hazard to both military and civilian personnel. Since the lung is the primary target organ for inhaled toxin, the investigation reported herein was conducted to examine lung function in mice exposed to botulinum toxin A complex by intranasal route. Data includes lethality, symptomatology, measurement of respiratory function (minute ventilation, respiratory frequency, and tidal volume), and histopathology of the lungs. The clinical signs of intoxication are similar to those observed in foodborne botulism. Plethysmography revealed severe impairment of all respiratory parameters tested from 7 hours postexposure. Severe lung lesions, possibly secondary to the intoxication, were observed in mice who survived 14 days after the toxin challenge. These included intra-alveolar hemorrhage and interstitial edema. Mice immunized by the pentavalent (ABCDE) toxoid were protected against the neurotoxin (4 LD50) as revealed by the decrease of lethality and severity of nervous signs of intoxication, but not against histopathological changes in the lungs. These effects are nonspecific and require further experiments in order to specify the relationships between the pathology and the inflammatory process in the lung due to mediators such as cytokines, and possibly permanent physiological sequelae.
Key Words: Botulinum neurotoxin histopathology lungs plethysmography vaccin toxicity inflammatory process
Clostridium botulinum produces 7 different neurotoxins, designated A to G, which are extremely potent protein toxins. Toxin types A, B, E, and F are mainly implicated in human diseases. Foodborne botulism accounts for most botulinum intoxication cases. However, exposure to toxins A to E through aerosols or in contaminated water is seriously considered a potential threat as a warfare or bioterrorist agent (Franz et al., 1993). Botulinum toxins inhibit the release of acetylcholine (Simpson, 1986) at neuromuscular junctions, resulting in flaccid muscle paralysis. Respiratory failure due to the paralysis of respiratory muscles is the most serious complication, resulting in death (Davis, 1993; Franz et al., 1993). These toxins also interfere with transmission at cholinergic parasympathetic terminals, producing autonomic symptoms: dry mouth and eyes, urinary retention, fluctuations of blood pressure, and impairment of autonomic control of heart rate. Recovery is slow, and the most common complaints are easy fatigue and dyspnea, although objective measurements of pulmonary function are usually normal. Many prophylactic and therapeutic countermeasures against botulism have been investigated since 1940. A new generation of botulinum vaccine is now under study: various laboratories are focused on the development of synthetic (Atassi et al., 1996; Atassi and Oshima, 1999) or recombinant vaccines against botulinum neurotoxins (Clayton et al., 1995; Gelzleichter et al., 1999; Clayton and Middlebrook, 2000). In this context, a pentavalent botulinum toxoid (PBT) directed against serotypes A to E (Siegel, 1988) is proposed to immunize specific at-risk populations, i.e., scientists in contact with botulinum toxins, and armed forces, which may be subjected to weaponized forms of the toxin. Inhalation appears to be the most likely route of exposure for the toxin if used as a terrorist weapon or warfare agent. However, this kind of poisoning seems to be particular. In testing the efficacy of vaccination against inhaled ricin in mouse (DaSilva et al., 2003), some remaining lesions in lungs were noticed even though the animals were well protected from death. The present study was conducted in order to examine whether botulinum toxin A complex, administered by intranasal route in mice, was able to induce lung alterations even if the animals were protected against the neurotoxic effects.
Animals Six-week-old male BALB/C mice, weighing 20–25 g at the time of exposure, were obtained from Charles River (Saint Aubain-les-Elbeuf, France). The animals were acclimatized to the laboratory conditions for at least 1 week before the experiment. They were housed 10 per cage under a 12:12-hour light/dark cycle (0600–1800 hours lights on). Food and water were available ad libitum. The principles of Laboratory Animal Care were followed in all experiments. The experimental design of the study was approved by our institutes Ethical Commitee.
Isolation of the Toxin A Complex The culture was incubated for 24 hours at 37°C in anaerobic condition without any agitation in glass bottles, each containing 0.5 L TGY medium. The TGY is a broth nutritive medium containing glucose and yeast extract to support the growth of Clostridium botulinum. It was then acidified by H2SO4 to pH 3.5. A heavy precipitate settled in 2–3 hours. It was collected by centrifugation (12.000 x g, 20 minutes, 4°C), washed twice with water, homogenized, and extracted with 0.05 M Na2HPO4/NaH2PO4 buffer pH 6. The solution was saturated with ammonium sulfate (35.1 g/100 ml) and stored at 4°C for 3 hours. The new precipitate was recovered by centrifugation (12,000 x g, 20 minutes, 4°C), dissolved in 0.05 M citrate buffer pH 5.5, and then dialyzed against 1 L of the same buffer for at least 18 hours at 4°C. The molecular cutoff of the dialysis tubing is 1 kDa. The solution was then centrifuged (12.000 x g, 20 minutes, 4°C) to remove insoluble material. The supernatant was loaded on a Q Sepharose Fast Flow column (25 ml, flow rate 5 ml/minute) equilibrated previously with 0.05 M citrate buffer (pH 5.5). The column was washed with the same buffer, and the collected toxin solution was saturated with solid ammonium sulfate (35.1 g/100 ml) and stored at 4°C for 3 hours. Then, the precipitate was processed as above and the supernatant was applied on a Sephacryl S-300 HR column (300 ml, flow rate 1 ml/minute), equilibrated, and then eluted with a 0.05 M citrate, 0.15 M NaCl buffer at pH 5.5. The second isolated peak contains the toxin. The toxin solution was concentrated at 20 µg/ml by ultrafiltration (Centripep Amicon 50,000 MWCO).
Toxic Activity
Toxic Activity in Immunized Animals
An additional group of mice (n = 10), receiving the vaccine, was used to assess immune status. Blood samples from these animals were collected from the retro-orbital sinus, under anesthesia (ketamine-diazepam mixture), at day 0, day 7, day 15, and day 30 to measure antibody concentration.
Antibody Titration
Signs and Symptoms
Breathing Electrodiagnostic Respiratory function was assessed by the measurement of respiratory frequency, tidal volume, and minute ventilation. Mice were placed in a modified Batelle tube system. This equipment allowed flow-breathing pattern monitoring of 8 restrained animals simultaneously. A calibrated pneu-motachograph (Fleisch No. 0000, Richmond, VA, USA) and a differential pressure transducer (DP-45, Validyne, North-bridge, CA, USA) were connected to the upper part of each plethysmograph. The signal was amplified and digitized (Dell Computer Corporation, CA, USA) with an input/output AS2 card at a sampling rate of 250 HZ Software (HEM 2.1 Notocord, Croissy sur Seine, France). Data files were imported into a worksheet (Excel 5.0, Microsoft Corp., CA, USA), and the mean ± SD of all the variables, for a given time, were automatically processed. Respiratory function was measured during 15-minute periods. Recordings were made the day before the exposure (control values) and, together with clinical observations, at 2, 5, 7, and 24 hours following the toxin challenge.
Histology
Data Analysis
Symptoms and Lethality The LD50 was found to be 21.1 ng/kg with a confidence interval at 95% (13.9–32.5 ng/kg; Table 3). Up to 17.3 ng/kg, symptoms generally developed 48 hours after nasal deposition (Table 2). The onset and the severity of the symptoms together with the number of affected animals were dose-dependent. Symptoms (score 1, Table 1) typically included prostration, inactivity, piloerection, polypsnea, a refusal to eat or drink, and ptosis in some cases. The animals remained in this state until death or, more often, returned to normal behavior on day 15. Doses between 17.3 and 30 ng/kg induced more severe dose-dependent signs of intoxication (score 2). Neurologic symptoms first concerned weakness of the cranial muscles progressively generalizing to the whole musculature and affecting locomotion. Neurophtalmological signs, anorexia, and labored respiration (bradypnea) were observed 48 hours after exposure. In severe cases (4LD50) the incubation period was as short as 7 hours.
Shortly after the onset of the initial symptoms described above, weakness progressed in a paralysis of the trunk musculature (including the muscles of respiration) and the limbs. Death was preceded by a severe respiratory depression. Indeed, minute ventilation measured at 2 and 5 hours after toxin exposure did not differ from control values (Figure 3a). However, 7 hours after toxin exposure, there was a significant and dose-dependent decrease of minute ventilation. This effect was especially noticed in animals receiving the highest dose of the toxin (4LD50): 26.4 ± 13.1 versus 37.1 ± 8 ml/min. Twenty-four hours following toxin administration, the minute ventilation was greatly decreased: 5.2 ± 2 ml/min versus 52.3 ± 16.9 ml/min. This effect, 24-hour postchallenge, was correlated with a significant decrease of tidal volume (Figure 3b) and respiratory frequency (Figure 3c) by 36.6 ± 11 and 41.6 ± 18.5%, respectively, compared to the reference values.
Death occurred progressively (Table 3): at a dose of 4LD50, 20% of the animals died within 7 hours after toxin challenge. This percentage reached 50% within 24 hours and jumped to 100% over a 48-hour postexposure period.
Symptoms in Protected Animals
Mice immunized by PBT vaccine and challenged with botulinum neurotoxin A (4 LD 50) were free of symptoms during the entire observation period (Table 2), excepting 1 animal showing a significant piloerection at 7 hours. No alterations of the measured respiratory parameters were detected, at least during the period considered (Figure 3). No fatal case was noticed prior to scheduled euthanasia (over a 14-day postex-posure period, Table 3).
Histology
There is little reference in the literature to the cellular effects or mode action that toxins may have in the lung. No findings relevant to the effects of botulinum toxin on the respiratory tract of humans are available, because such occurrence cannot take place under normal conditions. Only 3 cases of laboratory intoxications caused by inhalation of dust that contained botulinum toxin A were reported. Very minute amounts of toxin were apparently sufficient to trigger the classical disease (Holzer, 1962) before the admittance of the patients to the hospital, where they received an antitoxin serum. They were discharged on the 9th day. The report only mentioned a subsequent slow convalescence, without further description. Some animal experiments examined botulism by inhalation (Franz et al., 1993; Gelzleichter et al., 1999; DaSilva et al., 2003). Rhesus monkeys exposed to an approximately 2000-mouse intraperitoneal lethal dose (50 ng/kg) of aerosolized liquid botulinum toxin (Franz et al., 1993) died 2–4 days after exposure to the toxin. Clinical signs included intermittent ptosis, severe weakness, mouth breathing, serous nasal discharge, rales, salivation, and dyspnea before death. No histological change directly attributable to botulinum toxin was observed. However, mild-to-moderate subacute submucosal tracheitis, possibly secondary to the intoxication, was noticed in 2 of the 4 animals that died (Franz et al., 1993). In our experiment, signs of intoxication were rather similar. They first included piloerection and ptosis 7 hours after toxin exposure. This last symptom was generally attributed to the symmetrical cranial nerve impairment affecting the bulbar musculature resulting from a toxin-induced blockade of the voluntary motor and autonomic cholinergic junctions (Habermann and Dreyer, 1986). Then, severe muscular weakness, dysphagia, and dyspnea were observed in all the mice 24 hours after challenge. Death occurred 7 hours to 1 week after the toxin deposit. Ventilatory failure and dyspnea due to the paralysis of respiratory muscles are usually responsible for death in botulinum intoxication in humans (Tacket and Rogawski, 1989) and characterise the ultimate symptom in the progress of pathology. Apart from clinical observations, no data were available for the evaluation of respiratory function after inhalation of botulinum toxins. In our experiment, plethysmo-graphic evaluation revealed a significant modification 7 hours after toxin exposure, although clinical examination was usually normal. Twenty-four hours after the intoxication, all pneumophysiological parameters measured (minute ventilation, tidal volume, and frequency) were affected. At this state of the disease, dyspnea was clinically observed and a few animals were already dead. These observations are in good accord with the fact that botulinum toxin A produces a more severe disease in terms of skeletal muscle weakness and need for ventilatory support. Numerous studies have documented the ability of pentavalent botulinum toxoid (PBT) to induce protection against lethal effects (Gelzleichter et al., 1999). Our data suggest that, with regard to intranasal exposure, the antitoxin titer was sufficient to prevent all signs of intoxication when animals received 4LD50. No modifications of respiratory parameters were recorded in the PBT-treated group, suggesting that neurotransmission (mainly cholinergic) was fully protected. In this context, the recording of pneumophysiological parameters may be useful to assess immunotherapy against botulism as shown in many infectious models (De Hennezel et al., 2001).
It seems that many inhaled toxins could induce inflammatory-immune processes (Paddle, 2003) as a patho-physiological lung defense response. In the present experiment, animals exposed to the BoNT/A complex showed pulmonary histological changes with edematous lung injury. These data, together with the fact that lung pathology was also present in animals protected against neurological expression of botulism by vaccine, could suggest inflammatory processes independent of the toxic agent. Inhaled toxicants frequently induce an inflammatory response in the lungs. Such alterations were particulary observed after ricin (Rippy et al., 1991) or abrin (Griffiths et al., 1995a, 1995b) inhalation. It is well known that a number of cytokines, such as tumor necrosis factor (TNF In conclusion, the present data indicates that botulinum toxin administered by the intranasal route induces lung lesions in addition to its neurotoxic action. This pneumopathology corroborates the previous findings with ricin and staphylococal enterotoxin B. However, the relationship between the histological and physiological data needs further investigations in order to determine whether the observed effects are mainly due to the BoNT/A complex or are also shared by free BoNT/A. Futhermore, the long-term influence on pulmonary function during recovery must also be investigated.
We thank Mr. Guillot, Mr. Desforges, Mr. Morio, and Mr. Cocher for their technical assistance. This research was supported by grants from the Defense Ministery DGA/STTC/SH.
Toxicologic Pathology, Vol. 33, No. 3,
336-342 (2005)
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