Host Defense Proteins and Peptides with Lipopolysaccharide-Binding Activity from Marine Invertebrates and Their Therapeutic Potential in Gram-Negative Sepsis

Sepsis is a life-threatening complication of an infectious process that results from the excessive and uncontrolled activation of the host's pro-inflammatory immune response to a pathogen. Lipopolysaccharide (LPS), also known as endotoxin, which is a major component of Gram-negative bacteria's outer membrane, plays a key role in the development of Gram-negative sepsis and septic shock in humans. To date, no specific and effective drug against sepsis has been developed. This review summarizes data on LPS-binding proteins from marine invertebrates (ILBPs) that inhibit LPS toxic effects and are of interest as potential drugs for sepsis treatment. The structure, physicochemical properties, antimicrobial, and LPS-binding/neutralizing activity of these proteins and their synthetic analogs are considered in detail. Problems that arise during clinical trials of potential anti-endotoxic drugs are discussed.

Ligand specificities and structural requirements of two Tachypleus plasma lectins for bacterial trapping

TPL (Tachypleus plasma lectin)-1 was purified by using a Sepharose column and TPL-2 was purified from an LPS-Sepharose (LPS coupled to Sepharose matrix) affinity column, as described previously [Chiou, Chen, Y.-W., Chen, S.-C., Chao and Liu (2000) J. Biol. Chem. 275, 1630-1634] and the corresponding genes were cloned [Chen, Yen, Yeh, Huang and Liu (2001) J. Biol. Chem. 276, 9631-9639]. In the present study, TPL-1 and -2 were produced in yeast, and the recombinant proteins secreted into the media were purified and characterized. The proteins show specific PGN (peptidoglycan)- and LPS-binding activity, suggesting a role in trapping Gram-positive and Gram-negative bacteria respectively in innate immunity. Using BIAcore assays, the dissociation constant for the TPL-1-PGN complex was measured as 8x10(-8) M. Replacement of Asn74, the N-glycosylation site of TPL-1, with Asp abolishes the PGN-binding affinity, whereas the unglycosylated TPL-2 N3D mutant retains LPS-binding activity. DTT (dithiothreitol) treatment to break disulphide linkages abrogates TPL-2 activity but does not interfere with TPL-1 function. Cys4 in TPL-2 may form an intermolecular disulphide bond, which is essential for activity. As a result, the TPL-2 C4S mutant is inactive and is eluted as a monomer on a non-reducing gel. TPL-2 C6S is active and forms a non-covalently linked dimer. A model describing TPL-2 binding with LPS is proposed. These two plasma lectins that have different ligand specificities can be used for the detection and discrimination of bacteria and removal of endotoxins.

The evolution of immune mechanisms

From early on in evolution, organisms have had to protect themselves from pathogens. Mechanisms for discriminating "self" from "non-self" evolved to accomplish this task, launching a long history of host-pathogen co-evolution. Evolution of mechanisms of immune defense has resulted in a variety of strategies. Even unicellular organisms have rich arsenals of mechanisms for protection, such as restriction endonucleases, antimicrobial peptides, and RNA interference. In multicellular organisms, specialized immune cells have evolved, capable of recognition, phagocytosis, and killing of foreign cells as well as removing their own cells changed by damage, senescence, infection, or cancer. Additional humoral factors, such as the complement cascade, have developed that co-operate with cellular immunity in fighting infection and maintaining homeostasis. Defensive mechanisms based on germline-encoded receptors constitute a system known as innate immunity. In jaw vertebrates, this system is supplemented with a second system, adaptive immunity, which in contrast to innate immunity is based on diversification of immune receptors and on immunological memory in each individual.Usually, each newly evolved defense mechanism did not replace the previous one, but supplemented it, resulting in a layered structure of the immune system. The immune system is not one system but rather a sophisticated network of various defensive mechanisms operating on different levels, ranging from mechanisms common for every cell in the body to specialized immune cells and responses at the level of the whole organism. Adaptive changes in pathogens have shaped the evolution of the immune system at all levels.

Interactions between coagulation and inflammation

The coagulation system evolved as a mechanism to limit the loss of vital elements from the internal milieu following mechanical injury to the circulatory system. The innate immune system developed as a rapid response system to detect and clear microbial invaders that have breached the integument of multicellular organisms. The coagulation and innate immunity systems coevolved from a common ancestral substrate early in eukaryotic development, and these 2 systems retain a highly integrated and coregulated circuitry of signals and control elements that defend the host following tissue injury and microbial invasion. Advances in the understanding of the molecular mechanisms that control clotting and inflammation have led to the successful development of recombinant human activated protein C for the treatment of human septic shock. Optimal use of this and other anticoagulant agents in sepsis will necessitate further basic research into the critical linkage between coagulation and innate immunity.