Inhalation Exposure to Surrogate Military Burn Pit Emissions Impairs Systemic Microvascular Function: Linking Pulmonary Insult and Diverse Peripheral Responses

Millions of U.S. troops and supporting personnel have been deployed to military bases in the Middle East. Essentially all personnel on military bases were exposed to the combustion emissions generated by open pit waste burning. Chronic multisymptom illness (CMI) is a term advanced to characterize the complex health effects of inhalation exposures to military burn pits (BPs). Because of the diversity of geography, environmental conditions, and deployment operations, it is very challenging to estimate the number of Veterans affected by CMI, but it has been reported to be in the range of ~ 40–60%. Despite this overwhelming number of patients, the underlying causes of CMI remain to be identified. The purpose of this study was to replicate BP combustion and deliver these representative emissions to a whole-body inhalation exposure chamber with Sprague Dawley rats. We hypothesized that because the microcirculation is a critical component of health, that normal microvascular function may be disrupted after BP inhalation exposures. A surrogate BP emission generator was used to combust mixtures of wood, rubber, plastic and jet fuel. Resultant emissions were complex mixtures of volatile organic chemicals, polyaromatic hydrocarbons, fine and ultrafine particles. The particle aerodynamic count median diameter was 113 nm with a geometric standard deviation of 2.21. The particle mobility diameter was 78.1 nm with a geometric standard deviation of 1.69. The aerosol mass-size size distribution had a mass median aerodynamic diameter of 288 nm with a geometric standard deviation of 1.72 nm. Rats were exposed for ~ 4 hr/d at BP emission concentrations of 15.4 ± 1.6 mg/m ³ , for 2, 3, or 6 days. Twenty-four hours later, the spinotrapezius muscle was prepared for intravital microscopy. Tissues were also harvested from different rats in these groups for thorough mechanistic analyses. After 3–6 days of exposure, endothelium-dependent arteriolar dilation was abolished. Adrenergic vasoconstrictor sensitivity was augmented by as much as 50% in the BP exposure groups. Bronchoalveolar lavage revealed robust pulmonary inflammation and cellular infiltration. High-performance liquid chromatography with plasma samples demonstrated significant increases (> 50%) in circulating xanthine oxidase, a known driver of oxidative stress, disruptor of vascular nitric oxide, and thus mediator of endothelial dysfunction. After 3 days of BP exposure, RNA sequencing tissue analyses revealed transcriptional markers of lung inflammation as well as an altered transcriptional immune response in both the lung and spleen. BP inhalation exposure also led to elevated RNA transcripts for the vascular growth factor Vegfa and the immune cell trafficking factor Icam1 in brain hippocampal tissue. These initial microvascular observations demonstrate disruption of typical function and mechanisms that may link pulmonary insult with diverse systemic syndromes characteristic with CMI in Veterans.