BioWorld International Correspondent
LONDON - The prospects for an eventual septic shock treatment have improved with the discovery that blocking cellular receptors that detect the presence of bacteria can prevent a lethal shock-like syndrome in mice.
Carsten Kirschning, group leader at the Institute of Medical Microbiology, Immunology and Hygiene at the Technical University of Munich, Germany, told BioWorld International: "Most scientists currently believe that a main cause of septic shock following infection is the over-reaction of the immune system. Our paper is the first to show that one member of the family of molecules called Toll-like receptors plays an important role in mediating this over-activation of the immune system, which leads to sepsis and potentially fatal shock. We have now formally shown that, by blocking this receptor, we can prevent this type of lethality in a mouse model of septic shock."
The research is reported in a paper in the May 17, 2004, issue of Journal of Clinical Investigation. The paper is titled "Antagonistic antibody prevents Toll-like receptor 2-driven lethal shock-like syndrome."
Toll-like receptors (TLRs) are pattern-recognition molecules on cells such as macrophages and dendritic cells, which bind microbial products. They form a central part of the innate immune system.
Kirschning, together with Guangxun Meng and colleagues, was working on TLR2, known to be triggered by bacteria, including Listeria monocytogenes and Staphylococcus aureus. Kirschning said, "We wanted to prove that blockage of TLR2 might be interesting in the clinic."
The team took mice lacking the gene for TLR2 and gave it to them in order to generate antibodies against TLR2. They went on to manufacture monoclonal antibodies directed against TLR2.
The laboratory mouse model of septic shock involves injecting the mice with either lipopeptide or heat-inactivated bacteria. Similarly, when lipopeptide is added to murine macrophages in culture, the cells normally produce pro-inflammatory cytokines, which can be toxic at high concentrations.
Out of 12 monoclonal antibodies identified by the group, one was able to prevent murine macrophages reacting in that way.
When the group injected that antibody into mice and then challenged them with an otherwise lethal dose of either lipopeptide or heat-inactivated bacteria, the animals survived. Those injected with a control antibody did not.
Further experiments showed that the antibody also could prevent the release of pro-inflammatory mediators from human macrophages.
Writing in the Journal of Clinical Investigation, Kirschning and his colleagues concluded: "Our results implicate antibody-mediated TLR blockage on immune cells as a promising strategy for attenuation of potentially fatal host-response amplification in the course of acute infection."
Thomas Decker, of the Max Perutz Laboratories at the University of Vienna in Austria, writing in a commentary in the same issue, said TLRs "hold considerable promise for both agonistic and antagonistic therapeutic intervention."
Kirschning said much more work remains to be done before the findings can be applied in the clinic. The species of heat-inactivated bacteria he and his colleagues used in their experiments was Bacillus subtilis - hardly a major human pathogen.
"We chose B. subtilis because we knew that it activates the host immune system mainly via TLR2," Kirschning said. "But it is clear that TLR2-dependent cell activation is not going to be the only activation that is important in septic shock. There are 10 TLRs, all of which recognize different microbial components. TLR5, for example, recognizes flagellin, which is a component of salmonella species."
The group's next experiments will explore how to block further pattern-recognition receptors, such as other TLRs, and how to combine antibodies to block several receptors.
"We also want to work with clinically relevant bacteria, such as S. aureus," Kirschning added.