Robert Koch was the first person to demonstrate a strict causative relationship between a microorganism and disease. Before his pioneering work, the diagnosis of tuberculosis was a matter of judgment and dispute, described by a group of syndromes: miliary tuberculosis, caseous bronchitis, phthisis, scrofula, and bovine tuberculosis, with no universal agreement as to whether these were manifestations of the same condition. Since that time, Koch’s postulates have been applied to a variety of research fields, including atherosclerosis,18
and the gut lymph hypothesis.20
Acute lung injury, ARDS, and multiple organ failure are a group of syndromes caused by systemic hyperinflammation, in which morbidity is high and treatment remains supportive. The combined insults of trauma and hemorrhagic shock lead to the release of toxic inflammatory products from the gut into the circulation. Initially, loss of gut barrier function and bacterial translocation was the predominant explanation for the development of MOF.12
In a prospective clinical study exploring this hypothesis, however, our group did not find endotoxin in the portal circulation of trauma patients with MOF.3
Subsequent clinical studies have supported this finding.21,22
After the bacterial translocation mechanism failed to satisfy Koch’s postulates, the lymphatic system became the target of investigation as the primary conduit for delivering inflammatory factors from the ischemic gut into the circulation.
Koch’s postulates (adapted to the gut lymph hypothesis):
- The inflammatory mediator must be found in all organisms with the disease, and is not present in animals without the disease. A factor (or factors) in the PSML provokes acute lung injury following hemorrhagic shock, and in the absence of PSML due to ligation of the mesenteric duct, acute lung injury does not occur. 6,7
- The inflammatory mediator must be isolated from a diseased organism and maintain activity in vitro. PSML provokes PMN priming 23,24 and is toxic to human endothelial cells in vitro.25,26
- The inflammatory mediator should cause disease when introduced into a healthy organism. Injection of PSML provokes acute lung injury when transfused into a naïve animal.
- The inflammatory mediator must be re-isolated from the inoculated, diseased experimental host and identified as being identical to the original specific causative agent. With Koch’s 3rd postulate satisfied, future experiments are designed to test Koch’s 4th and final postulate.
Infusion of stored PSML into naïve rats has been shown to provoke ALI;27
however, lymph contains volatile peptides and lipids, sensitive to storage, freeze/thaw cycles, including clotting factors prone to contact activation. Concerned that processing the lymph may have altered its bioactivity, a method was developed to deliver PSML directly from the mesenteric duct from a T/HS rat into the jugular vein of a naïve rat, which provoked ALI. Real time transfusion of sham lymph resulted in lung injury similar to instrumented controls given normal saline only. This observation supports previous work that preshock lymph does not cause inflammation, and may even be protective against ALI.28
Because the lymphatic system is highly permeable, there is little, if any, exclusion of interstitial molecules.11
Mesenteric lymph contains a number of inflammatory products including arachidonic acid,29
cellular breakdown products, 32,33
and modified albumin.34
Bacteria and bacterial products are noticeably absent in PSML.35
Additionally, PSML demonstrates increased levels of anti-inflammatory cytokines21
and depletion of protease inhibitors.10
), an enzyme abundant in the mammalian gut has been implicated as the source of lipid mediators in the setting of hemorrhagic shock and sepsis.36
produces arachidonic acid, a precursor to a variety of inflammatory products found in the lymph, leukotrienes, thromboxane, and prostaglandins. Arachidonic acid is abundant in PSML, and leukotrienes are increased in the lungs of animals following T/HS, making these lipids the current focus of intensive investigation in our lab.37
Postshock mesenteric lymph may be toxic through a variety of mechanisms, including endothelial inflammation,6,26
pulmonary epithelial inflammation,42
and RBC hemolysis.43,32
In this study, we demonstrate that postshock mesenteric lymph is necessary and sufficient to provoke ALI in the naïve rat. These data further support a central mechanistic role of PSML in the pathogenesis of ALI. Although the postshock lung may also play a role, the cross-transfusion model will facilitate mechanistic investigation, compartmentalizing the gut-lymph versus ischemic lung contributions.