Applied molecular biology of sepsis

The effective treatment of sepsis and septic shock has remained elusive despite intense research efforts. The tools of molecular biology have been applied to the problem of sepsis in an attempt to design more rational, directed therapy. Cellular interactions with invading microorganisms begin a series of stimulation events within the cell. One of the important interactions is the binding of lipopolysaccharide (LPS) from gram-negative bacteria to the LPS binding protein, and then this complex binding to CD14 on monocytes. Cell stimulation occurs through activation of signal transduction pathways within the cell, many of which have been defined. These include the kinases that phosphorylate proteins, and phosphatases that dephosphorylate proteins. The next step after activation of the signal transduction pathways is stimulation of nuclear regulatory factors. One of the best characterized of these is nuclear regulatory factor kappa B (NF-kappa B), which is a trans activating element that binds to specific DNA nucleotide sequences to allow transcription of downstream elements. Many inflammatory mediators are located downstream of NF-kappa B so that activation of NF-kappa B causes upregulation of the inflammatory mediators. The cytokines have been identified as a group of mediators important in the pathogenesis of sepsis, because several studies have shown that higher levels are correlated with a worse outcome in patients. Additionally, in experimental animal models, inhibition of cytokines improves survival, and administration of exogenous, recombinant cytokines reproduces many of the pathophysiologic alterations observed in sepsis. Molecular biology has played a critical role in the understanding of sepsis by providing the tools to make the recombinant cytokines of sufficient purity and quantity for infusion into experimental animals. The cellular response for the production of cytokines occurs through classic protein chemistry, with the signal transduction inducing messenger RNA (mRNA) coding for the cytokines, which are then translated and secreted. The relative contribution of local versus systemic cytokine production is beginning to be appreciated, with several diseases showing substantially higher local cytokine levels. The cytokines exert their activity on other cells by binding to their specific cytokine receptors. These receptors are part of the immune response and may be shed from the cell surface. These soluble receptors bind to and inactivate the cytokines. Inhibition of cytokine activity has been hypothesized as a potential therapy for sepsis. This inhibition has been done with antibodies directed against either the cytokines themselves or their receptors. Naturally occurring cytokine inhibitors have been cloned and expressed by molecular biologists and used for treatment of sepsis and other diseases. Using molecular biology techniques, the murine antibodies have been "humanized" to reduce their immunogenicity. The measurement of cytokines is critically important to our understanding of their role in health and disease. Cytokines may be measured by either immunologic methods or biological assays. Molecular biology has made important contributions to our understanding of sepsis by precisely identifying some of the mediators and providing reagents for therapeutic use.