The role of the mesenteric lymph on microcirculation injury during bacterial translocation

Background

Increasing evidence has implicated bacterial translocation (BT) as the main source of the so-called gut hypothesis of the pathogenesis of sepsis progressing to multiple organ failure. Others have shown that mesenteric lymph content in the course of BT promotes increased pulmonary permeability. In previous work we have shown significantly increased tumor necrosis factor alpha and lymphocytes in the mesenteric lymph during BT. In this study we examined the correlation between microcirculation injury and mesenteric lymph exclusion during the BT process.

Methods

Female Wistar rats were distributed in the following groups: BT, inoculation of 10 ml of 10¹⁰ cfu/ml Escherichia coli R-6 confined to the small intestine; BT-E, submitted to BT without the influence of the mesenteric efferent lymph (diverted away from systemic circulation by catheterization of the mesenteric lymph duct); BT-R, submitted to the same procedure as BT-E group followed by reinoculation of the collected lymph into the systemic circulation; BT-N, inoculation of lymph collected during BT into naïve animals. All animal mesenteric microcirculations were examined for 2 hours (n = 6/group) using an intravital microscope. The same number of BT-R and BT-N animals was injected with either lymph cells or lymph supernatants into the systemic circulation (immediately after light centrifugation of lymph collected during BT). A midline laparotomy was performed on Wistar rats under ketamine + hydroxychloral anesthesia (4:1). Rats were inoculated with 10 ml E. coli R-6 10¹⁰ cfu/ml by oroduodenal catheterization, which was confined to the small intestine by ligation of both the duodenum and ileum. All lymph samples were submitted to culture in MacConkey agar medium.

Results

All lymph cultures were negative. During BT, the onset of injuries in the mesenteric microcirculation was mainly focal hemorrhages in capillaries and small venules beginning around 30 min, which progressed quantitatively up to 2 hours. After 1 hour of translocation, focal thrombosis of capillaries and small and medium venules were observed. In contrast, in animals where the lymph was diverted away from systemic circulation by catheterization of the mesenteric lymph duct, no microcirculation injury occurred (BT-E group). The reinoculation of the collected whole lymph (BT-R) promoted similar injuries to the microcirculation as seen in the BT group in the same time period. Interestingly, the injection of whole lymph collected (negative culture for bacteria) from animals submitted to BT in the naïve animals (BT-N group) provoked similar mesenteric microcirculation damage within the same period. In addition, only lymph supernatant was able to promote microcirculation injuries in both the BT-R and BT-N groups. These findings allow us to speculate that BT-induced alterations in the mesenteric microcirculation are possibly due to the gut-associated lymphoid system activation by the BT process with the release of proinflammatory factor(s) and not due to the existence of bacteria in the lymph. Therefore, this might be the BT mechanism for the aggravation of a preexisting state of sepsis or for the installment of infectious disease. Ongoing experiments are in progress to better elucidate this hypothesis.