Spent culture medium from virulent Borrelia burgdorferi increases permeability of individually perfused microvessels of rat mesentery

Abstract

Background: Lyme disease is a common vector-borne disease caused by the spirochete Borrelia burgdorferi (Bb), which manifests as systemic and targeted tissue inflammation. Both in vitro and in vivo studies have shown that Bb-induced inflammation is primarily host-mediated, via cytokine or chemokine production that promotes leukocyte adhesion/migration. Whether Bb produces mediators that can directly alter the vascular permeability in vivo has not been investigated. The objective of the present study was to investigate if Bb produces a mediator(s) that can directly activate endothelial cells resulting in increases in permeability in intact microvessels in the absence of blood cells.

Methodology/principal findings: The effects of cell-free, spent culture medium from virulent (B31-A3) and avirulent (B31-A) B. burgdorferi on microvessel permeability and endothelial calcium concentration, [Ca(2+)](i), were examined in individually perfused rat mesenteric venules. Microvessel permeability was determined by measuring hydraulic conductivity (Lp). Endothelial [Ca(2+)](i), a necessary signal initiating hyperpermeability, was measured in Fura-2 loaded microvessels. B31-A3 spent medium caused a rapid and transient increase in Lp and endothelial [Ca(2+)](i). Within 2-5 min, the mean peak Lp increased to 5.6+/-0.9 times the control, and endothelial [Ca(2+)](i) increased from 113+/-11 nM to a mean peak value of 324+/-35 nM. In contrast, neither endothelial [Ca(2+)](i) nor Lp was altered by B31-A spent medium.

Conclusions/significance: A mediator(s) produced by virulent Bb under culture conditions directly activates endothelial cells, resulting in increases in microvessel permeability. Most importantly, the production of this mediator is associated with Bb virulence and is likely produced by one or more of the 8 plasmid(s) missing from strain B31-A.

Figure 1. Hydraulic conductivity, Lp, was measured based on the modified Landis technique in individually perfused microvessels.

Jv is the initial water flow per unit area of microvessel wall (S), which was calculated from the velocity of the marker cell (Vcell) after the vessel was occluded, the vessel radius (r), and the length between the marker cell position and the occlusion site (L).

Figure 2. Measurements of changes in endothelial [Ca²⁺]_i in an individually perfused microvessel.

The vessel was first perfused with albumin-Ringer solution containing Fura 2-AM for 45 min to load the endothelial cells of the microvessel wall. The vessel was then recannulated and perfused with albumin-Ringer solution for 10 min to remove fura 2-AM from the vessel lumen. Fluorescence intensity was collected through a rectangular diaphragm of the photometer (yellow area) under control conditions and after exposure to testing solutions.

Figure 3. PCR analysis of plasmids in strains B31-A and B31-A3.

Twenty-one pairs of specific primers were used to amplify all the circular and linear plasmids contained in the two strains. DNA markers (left lane) identify the sizes of amplified DNAs. “*” indicates the plasmids that are lost from B31-A strain. A) B31-A is missing lp25, lp28-1, lp28-4, lp36, cp32-6, cp32-7, lp5 and lp21, in addition to cp9. B) B31-A3 retains all twenty one plasmids except cp9, as expected .

Figure 4. Effects of B31-A3, B31-A spent medium and BSK-II control medium on Lp.

Relative changes in Lp are presented as Lp_test/Lp_control. Arrows indicate times at which perfusion with test solutions were initiated. A) One representative experiment showing a transient increase in Lp induced by B31-A3 spent medium. B) A representative experiment showing Lp is unaffected when vessels were perfused with BSK-II medium. C) Time course and magnitude changes in Lp from one representative paired experiment in which a venule was sequentially perfused with BSK-II medium, B31-A spent medium, BSK-II medium, then B31-A3 spent medium. An increase in Lp was only seen upon perfusion with B31-A3 spent medium. D) Summarized data of Lp changes induced by B31-A and B31-A3 spent medium; * indicates a significant increase (P<0.05) from negative control; † indicates a significant decrease (P<0.05) from B31-A3 spent medium.

Figure 5. Effects of Bb spent medium on endothelial [Ca²⁺]_i.

A) A representative venule was sequentially perfused with 5% albumin-Ringer (5% BSA Ringer), BSK-II medium, B31-A3 spent medium and 5% albumin-Ringer, showing no endothelial [Ca²⁺]_i changes upon exposure to BSK-II medium, while a transient increase in endothelial [Ca²⁺]_i was observed within 2–3 min after exposure to B31-A3 spent medium. B) Changes in endothelial [Ca²⁺]_i from one representative paired experiment, showing no significant changes in endothelial [Ca²⁺]_i during perfusion of B31-A spent medium, while a transient increase in endothelial [Ca²⁺]_i occurred upon perfusion of B31-A3 spent medium. Arrows indicate times at which perfusion with test solutions were initiated. C) Summarized data for the effects of spent medium from B31-A3 and B31-A on endothelial [Ca²⁺]_i; * indicates a significant increase (P<0.05) from negative control; † indicates a significant decrease (P<0.05) from B31-A3 spent medium.