The role of nitric oxide in the neuropathology in soman intoxication

Interleukin-18 expression increases in response to neurovascular damage following soman-induced status epilepticus in rats

Background

Status epilepticus (SE) can cause neuronal cell death and impaired behavioral function. Acute exposure to potent acetylcholinesterase inhibitors such as soman (GD) can cause prolonged SE activity, micro-hemorrhage and cell death in the hippocampus, thalamus and piriform cortex. Neuroinflammation is a prominent feature of brain injury with upregulation of multiple pro-inflammatory cytokines including those of the IL-1 family. The highly pleiotropic pro-inflammatory cytokine interleukin-18 (IL-18) belongs to the IL-1 family of cytokines and can propagate neuroinflammation by promoting immune cell infiltration, leukocyte and lymphocyte activation, and angiogenesis and helps facilitate the transition from the innate to the adaptive immune response. The purpose of this study is to characterize the regional and temporal expression of IL −18 and related factors in the brain following SE in a rat GD seizure model followed by localization of IL-18 to specific cell types.

Methods

The protein levels of IL-18, vascular endothelial growth factor and interferon gamma was quantified in the lysates of injured brain regions up to 72 h following GD-induced SE onset using bead multiplex immunoassays. IL-18 was localized to various cell types using immunohistochemistry and transmission electron microscopy. In addition, macrophage appearance scoring and T-cell quantification was determined using immunohistochemistry. Micro-hemorrhages were identified using hematoxylin and eosin staining of brain sections.

Results

Significant increases in IL-18 occurred in the piriform cortex, hippocampus and thalamus following SE. IL-18 was primarily expressed by endothelial cells and astrocytes associated with the damaged neurovascular unit. The increase in IL-18 was not related to macrophage accumulation, neutrophil infiltration or T-cell appearance in the injured tissue.

Conclusions

These data show that IL-18 is significantly upregulated following GD-induced SE and localized primarily to endothelial cells in damaged brain vasculature. IL-18 upregulation occurred following leukocyte/lymphocyte infiltration and in the absence of other IL-18-related cytokines, suggesting another function, potentially for angiogenesis related to GD-induced micro-hemorrhage formation. Further studies at more chronic time points may help to elucidate this function.

Soman poisoning induces delayed astrogliotic scar and angiogenesis in damaged mouse brain areas

Gliotic scar formation and angiogenesis are two biological events involved in the tissue reparative process generally occurring in the brain after mechanically induced injury, ischemia or cerebral tumor development. For the first time, in this study, neo-vascularization and glial scar formation were investigated in the brain of soman-poisoned mice over a 3-month period after nerve agent exposure (1.2 LD50 of soman). Using anti-claudin-5 and anti-vascular endothelial growth factor (VEGF) immunostaining techniques on brain sections, blood vessels were quantified and VEGF expression was verified to appraise the level of neo-angiogenesis induced in damaged brain areas. Furthermore, glial scar formation and neuropathology were estimated over time in the same injured brain regions by anti-glial fibrillary acidic protein (GFAP) immunohistochemistry and hemalun-phloxin (H&P) dye staining, respectively. VEGF over-expression was noticed on post-soman day 3 in lesioned areas such as the hippocampal CA1 field and amygdala. This was followed by an increase in the quantity of mature blood vessels, 3 months after soman poisoning, in the same brain areas. On the other hand, massive astroglial cell activation was demonstrated on post-soman day 8. Reactive astroglial cells were located only in damaged cerebral regions where H&P-stained eosinophilic neurons were found. For longer experimental times, astroglial response slowly decreased overtime but remained detectable on post-soman day 90 in some discrete brain regions (i.e. CA1 field and amygdala) evidencing the formation of a glial scar. In this study, we discuss the key role of VEGF in the angiogenic process and in the glial or neuronal response induced by soman poisoning.

Effect of soman poisoning on populations of bone marrow and peripheral blood cells in mice

According to recent reports, brain lesions resulting from ischemia, mechanical injury or neurodegenerative diseases can be partially treated using bone marrow-derived stromal cell (BMSC) engraftment approaches. Nevertheless, for brain lesions resulting from organophosphate poisoning, nerve agents such as soman (pinacolyl methylphosphono-fluoridate) could affect blood and bone marrow (BM) micro-environments, thus preventing efficient BMSC migration and engraftment. It is therefore necessary to verify the hematologic response to soman exposure. To assess this issue, the survival of BM cells, in particular hematopoietic progenitor and precursor cells (HPC), as well as distribution of the different populations of peripheral blood cells, were investigated in soman-intoxicated mice. Nine-week-old adult male B6D2F1 mice were treated with 110 microg/kg soman and 5.0 mg/kg methyl nitrate atropine. BM and peripheral blood (PB) samples were collected 1, 4, 8 and 22 days after poisoning. Various parameters were determined such as PB cell counting or, for BM samples, myelogram, in vitro colony-forming cells and phenotypic flow cytometry analysis. On post-soman day 1, a significant decrease in numbers of white blood cells and an increase in erythrocyte and platelet counts were noted. On post-soman day 4, the number of HPC decreased significantly, probably due to reduction of the replication rate of these immature cells. However, the number of more immature cells (Sca1+/Lin- phenotype) remained unchanged. On post-soman day 8 and day 22, the number of monocytes and granulocytes in the blood had considerably increased, probably due to a strong inflammatory reaction in response to soman poisoning. In conclusion, PB cell and BM-derived HPC populations are affected by acute soman poisoning, suggesting particular care, mainly for graft kinetic aspects, during future development of autologous BM stem cell therapy strategy to treat nerve agent-induced brain damage.

Future considerations for the medical management of nerve-agent intoxication

The use of chemical warfare agents against civilians and unprotected troops in international conflicts or by terrorists against civilians is considered to be a real threat, particularly following the terrorist attacks on 11 September 2001 against the World Trade Center in New York and against the Pentagon in Washington, DC. Over the past 10 years, terrorists have been planning to use or have used chemical warfare agents on several occasions around the world, and the attacks in 2001 illustrate their willingness to use any means of warfare to cause death and destruction among civilians. In spite of new international treaties with strong verification measures and with an aim to prohibit and prevent the use of weapons of mass destruction, nevertheless, some countries and terrorist groups have been able to develop, produce, and use such weapons, particularly nerve agents, in domestic terrorist attacks or during warfare in international conflicts. This article reviews current medical therapy for nerve-agent intoxication and discusses possible future improvement of medical therapies. Present medical counter-measures against nerve agents are not sufficiently effective particularly in protecting the brain. Therefore, new and more effective countermeasures must be developed to enable better medical treatment of civilians and military personnel following exposure to nerve agents. Therefore, it is important with an enhanced effort by all countries, to improve and increase research in medical countermeasures, in the development of protective equipment, and in carrying out regular training of medical and emergency personnel as well as of military nuclear, biological, or chemical (NBC) units. Only then will nations be able to reduce the risk from and prevent the use of such weapons of mass destruction (WMD).