α₂-macroglobulin: an evolutionarily conserved arm of the innate immune system

Abstract

All animals and plants have immune systems that protect them from the diversity of pathogens that would otherwise threaten their survival. The different components of the immune system may inactivate the pathogens themselves or promote the inactivation and clearance of toxic products produced by the pathogens. An important category of virulence factors of bacterial and prokaryotic pathogens are the proteases, which act to facilitate the invasion of the pathogens and to promote their destructive growth in the host organism. The present review concentrates on the comparative biology of an evolutionarily conserved arm of the immune system, the protein, α₂-macroglobulin. α₂-Macroglobulin is an abundant protein of the plasma of vertebrates and members of several invertebrate phyla and functions as a broad-spectrum protease-binding protein. Protease-conjugated α₂-macroglobulin is selectively bound by cells contacting the body fluids and α₂-macroglobulin and its protease cargo are then internalized and degraded in secondary lysosomes of those cells. In addition to this function as an agent for protease clearance, α₂-macroglobulin binds a variety of other ligands, including several peptide growth factors and modulates the activity of a lectin-dependent cytolytic pathway in arthropods.

Introduction

The immune system is comprised of the array of tissues, cells and effector molecules that operate to limit the severity of attack by pathogenic parasites that have gained access to the internal milieu. Parasites may be unicellular or multicellular, prokaryote, eukaryote, or virus. The primary defense against pathogenic attack is the integument, which limits the penetration of foreign organisms into the internal spaces of a potential host. But once inside, a second line of defense is mediated by the diverse array of cells and soluble effectors comprising the immune system. In coelomate animals, the bulk of the immune system is found in the blood, presumably because the blood has ready access to all parts of the body and is best prepared to concentrate defense effectors at a site of pathogenic invasion.

α₂-Macroglobulin is an evolutionarily conserved element of the innate immune system whose best-characterized function is the clearance of active proteases from the tissue fluids. Proteases, whether of endogenous or exogenous origin, are capable of considerable mischief when free in the blood. They are important agents in a variety of human connective tissue diseases [1] and are important virulence factors contributing to the pathogenicity of prokaryotic and eucaryotic parasites [2], [3]. In response to protease challenge, animals have evolved a diverse array of protease inhibitors [4]. Approximately 3–5% of the protein content of the plasmas of the vertebrate, Homo [5], and the arthropod, Limulus [6], are protease inhibitors.

The protease inhibitors are of two fundamental classes, the active-site inhibitors, which bind to and inactivate the activate site of the targeted endopeptidase, and the α₂-macroglobulins, which react by a unique mechanism that involves the physical entrapment of the target protease within the folds of a molecule of α₂-macroglobulin. This entrapment process involves a reorganization of the molecule of α₂-macroglobulin that is initiated by its proteolytic cleavage by the target protease and that culminates with the protease molecule imprisoned within an internal pocket in the molecule of α₂-macroglobulin [7]. As far as we are aware, the α₂-macroglobulins are the only enzyme inhibitors that operate by a process of a physical entrapment of the target protein.

Mammalian plasma contains several members of the α₂-macroglobulin family of proteins, including α₂-macroglobulin, several closely related protease-binding proteins, and C3, C4 and C5 of the complement system [8]. C3, C4 and C5 are essential components of the complement system, which functions as an important immune effector in vertebrates [9]. Proteins of the α₂-macroglobulin family are abundant components of plasma of mammals [8], [10] and arthropods [11], comprising about 3% of the total plasma protein of humans [12] and with α₂-macroglobulin the third most abundant protein of the plasma of the American horseshoe crab, Limulus polyphemus [6].

The liver is the principal site of synthesis of α₂-macroglobulin in mammals [13]. In arthropods, the blood cells contain α₂-macroglobulin in the exocytotic secretory granules [14], [15], [16]. The source of the free α₂-macroglobulin in the plasma has not been determined for any invertebrate.