immunoglobulin m immunoglobulin m immunoglobulin m (igm) is one of several isotypes of antibody (also known as immunoglobulin) that are produced by vertebrates. igm is the largest antibody, and it is the first antibody to appear in the response to initial exposure to an antigen. in humans and other mammals that have been studied, plasmablasts residing in the spleen are the main source for specific igm production. immunoglobulin m(pentamer)protein typeantibody subunit namegenechromosomal locus immunoglobulin heavy chain µigh@ history in 1937, an antibody was observed in horses hyper-immunized with pneumococcus polysaccharide that was much larger in size than the typical rabbit γ-globulin, with a molecular weight of 990,000 daltons. in accordance with its larger size, the new antibody was originally referred to as γ-macroglobulin, and subsequently termed as igm—m for “macro”. the v domains of normal immunoglobulin are highly heterogeneous, reflecting their role in protecting against the great variety of infectious microbes, and this heterogeneity impeded detailed structural analysis of igm. two sources of homogeneous igm were subsequently discovered. first, the high molecular weight protein produced by some multiple myeloma patients was recognized to be a tumor-produced γ-macroglobulin and, because the tumor is a clone, the igm it produces is homogeneous. in the 1960s, methods were developed for inducing immunoglobulin-producing tumors (plasmacytomas) in mice, thus providing a source of homogeneous immunoglobulins of various isotypes, including igm (reviewed in). more recently, the expression of engineered immunoglobulin genes in tissue culture can be used to produce igm with specific alterations and thus to identify the molecular requirements for features of interest. structure immunoglobulins are composed of light chains and heavy chains. the light chain (λ or κ) is a protein of ~220 amino acids, composed of a variable domain, vl (a segment of approximately 110 amino acids), and a constant domain, cl (also approximately 110 amino acids long). the µ heavy chain of igm is a protein of ~576 amino acids, and includes a variable domain (vh ~110 amino acids), four distinct constant region domains (cµ1, cµ2, cµ3, cµ4, each ~110 amino acids) and a “tailpiece” of ~20 amino acids. the µ heavy chain bears oligosaccharides at five asparagine residues. the oligosaccharides on mouse and human igm have been partially characterized by a variety of techniques, including nmr, lectin binding, various chromatographic systems and enzymatic sensitivity (reviewed in). the structure of the oligosaccharides at each site varies in detail, and the predominant oligosaccharides – biantennary, triantennary, high mannose—differ among the sites. figure 1. schematic model of igm a) the µl heterodimer, sometimes called a halfmer, with variable (vh, vl) and constant region (cµ1, cµ2, cµ3, cµ4tp; cl) domains. the cysteines that mediate disulfide bonds between µ chains are shown as red arrowheads, so that a cysteine disulfide bond appears as a red double arrowhead (red diamond). b) the igm “monomer” (µl)2. the disulfide bonds between cµ2 domains are represented by a red double arrowhead. c, d) two models for j chain-containing igm pentamer that have appeared in various publications at various times. as in (b), the disulfide bonds between cµ2 domains and the disulfide bonds between cµ4tp domains are represented by a red double arrowhead; the cµ3 disulfide bonds are represented (for clarity) by long double-headed arrows. the connectivity, i.e., the inter-chain disulfide bonding of the µ chains, is denoted like electrical connectivity. in (c) the cµ3 disulfide bonds join µ chains in parallel with the cµ4tp disulfide bonds, and these disulfide bonds join µ chains in series with the cµ2 disulfide bonds. in (d) the cµ2 and cµ4tp disulfide bonds join µ chains in parallel and both types join µ chains in series with the cµ3 disulfide bonds. (figure reproduced with permission of the publisher and authors). the multimeric structure of igm is shown schematically in figure 1. figure 1a shows the “heterodimer” composed of one light chain, denoted l, and one heavy chain, denoted µ. the heavy and light chains are held together both by disulfide bonds (depicted as red triangles) and by non-covalent interactions. figure 1b shows two µl units linked by a disulfide bond in the cµ2 domains; this (µl)2 structure is often referred to as the igm “monomer”, as it is analogous in some ways to the structure of immunoglobulin g (igg). on the basis of its sedimentation velocity and appearance in electron micrographs, it was inferred that igm usually occurs as a “pentamer”, i.e., a polymer composed of five “monomers” 5, and was originally depicted by the models in figures 1c and 1d, with disulfide bonds between the cµ3 domains and between the tail pieces. also shown is that pentameric igm includes a third protein, the j chain. j chain (j for joining) was discovered as a covalently bonded component of polymeric iga and igm. the j chain is a small (~137 amino acids), acidic protein. as shown, the j chain joins two µ chains via disulfide bonds involving cysteines in the tailpieces. molecular requirements for forming polymeric igm it was initially expected that the j chain would be important for forming the polymeric immunoglobulins, and indeed polymerization of iga depends strongly (but not absolutely) on the j chain. in contrast, polymeric igm forms efficiently in the absence of the j chain. the predominant form of human and mouse igm is the pentamer. by way of comparison, the structure of igm from frogs (xenopus) is predominantly hexameric, igm from bony fish is predominantly tetrameric, and igm from cartilaginous fish (mainly sharks) is predominantly pentameric. although the pentameric form predominates in mice and humans, the hexameric form has also been observed. subsequent studies using recombinant dna expression systems indicated that the hexamer is a major form of mouse igm when the igm is produced under conditions where the incorporation of the j chain is prevented, either by producing igm in cells that lack the j chain or by producing igm with a µ heavy chain that lacks the cysteine in the tailpiece. in summary, hexameric igm never contains the j chain; pentameric igm can be formed so as to include or not include the j chain. an important difference between the µ and γ heavy chains is the availability of cysteines for forming disulfide bonds between heavy chains. in the case of the γ heavy chain, the only inter-γ bonds are formed by cysteines in the hinge, and accordingly, each γ chain binds to only one other γ chain. by contrast, the cµ2 and cµ3 domains and the tailpiece each include a cysteine that form a disulfide bond with another µ chain. the cysteines in the cµ2 domains mediate the formation of monomeric igm (µl)2. the tailpiece along with the included cysteine is necessary and sufficient for the formation of polymeric immunoglobulins. that is, deleting the tailpiece from the µ heavy chain prevents the formation of polymeric igm. conversely, cells expressing a γ heavy chain that has been modified to include the tailpiece produce polymeric igg. the role of the cysteine in the cµ3 domain is more subtle. figures 1c and 1d represent possible models for pentameric igm. in these models, each µ chain is envisaged to bind two other µ chains. however, neither model alone can fully account for the structure of polymeric igm. for example, the model in figure 1c predicts that the disulfide bond between the cµ2 domains is essential for making disulfide-bonded polymeric igm. the model in figure 1d predicts that the disulfide bond between the cµ3 domains is essential. disulfide bonded, polymeric, igm can still be made if any one of the three cysteines is absent. in the context of models in which each µ chain interacts with only two other µ chains, these results suggest that some molecules are like figure 1c and some like figure 1d. however, the availability of three cysteines for inter-µ chain bonding suggests that the µ chains might each bind three other µ chains, as illustrated in figure 2. in the same spirit, figure 2c presents a model for a j chain-containing pentamer that reflects evidence that the j chain joins µ chains that are not joined to other µ chains by the cysteines in the cµ3 domains. these and other models, both regular and irregular are discussed elsewhere. figure 2. some alternative ways of linking µ chainsa, b) these figures depict two of many possible models of inter-µ chain disulfide bonding in hexameric igm. as in figure 1, the cµ2 disulfide bonds and the cµ4tp disulfide bonds are represented by a red double arrowhead, and the cµ3 disulfide bonds are represented by the long double-headed arrows. in both models a and b each type of disulfide bond (cµ2-cµ2; cµ3-cµ3; cµ4tp-cµ4tp) joins µ chains in series with each of the others. methods for distinguishing these and other models are discussed in reference .c) this representation of pentameric igm illustrates how the j chain might be bonded to µ chains that are not linked via cµ3 disulfide bonds pentameric igm is typically represented as containing a single j chain per polymer, but in actuality the measurements of j chain stoichiometry have ranged from one j molecule per polymer to three j molecules per polymer. the wide range might be due to technical problems, such as incomplete radiolabeling or imprecisely quantitating an ouchterlony line. however, the variation might also be due to heterogeneity in the igm preparations, i.e., the various preparations might have differed substantially in their content of j-containing and j-deficient polymers. tertiary and quaternary structure of the µ constant region individual c2, c3, and c4tp domains were generated independently in e. coli and then studied using a range of approaches, including sedimentation rate, x-ray crystallography, and nmr spectroscopy, to obtain insight into the detailed three-dimensional structure of the chain. the domains of the heavy chain, like those of other immunoglobulins, have the distinctive overlaying -sheets of seven strands, which are stabilized by intra-domain disulfide linkages. overall, the igm constant region has a "mushroom-like" shape, with the c2-c3 domains forming a disk similar to the mushroom's head and the c4tp domains protruding like a short stem. function igm interacts with several other physiological molecules: igm can bind complement component c1 and activate the classical pathway, leading to opsonization of antigens and cytolysis. igm binds to the polyimmunoglobulin receptor (pigr) in a process that brings igm to mucosal surfaces, such as the gut lumen and into breast milk. this binding depends on the j chain. two other fc receptors that bind igm—fcα/µ-r and fcµ-r -- have been detected. fcα/µ-r, like pigr, binds polymeric igm and iga. fcα/µ-r can mediate endocytosis, and its expression in the gut suggests a role in mucosal immunity. fcµ-r (formerly known as toso/faim3) binds igm exclusively and can mediate cellular uptake of igm-conjugated antigen. inactivation of the corresponding genes in knock-out mice produces a phenotype, but the physiological functions of these receptors are still uncertain regulation of the immune response specific immunoglobulins that are injected into animals together with their antigen can influence the antibody response to this same antigen. endogenous antibodies produced after a primary immunization can also affect the antibody response to a booster immunization, suggesting that similar effects occur during physiological conditions. the ”regulatory” effects can be either positive or negative. that is, depending on the type of antigen and the isotype of the antibody, the effect can be suppression or enhancement of the antibody response. such effects are well illustrated by experiments involving immunization with xenogenic (foreign) erythrocytes (red blood cells). for example, when igg is administered together with xenogenic erythrocytes, this combination causes almost complete suppression of the erythrocyte-specific antibody response. this effect is used clinically to prevent rh-negative mothers from becoming immunized against fetal rh-positive erythrocytes, and its use has dramatically decreased the incidence of hemolytic disease in newborns. in contrast to the effect of igg, antigen-specific igm can greatly enhance the antibody response, especially in the case of large antigens. thus, when igm specific for erythrocytes is injected into animals (including humans) together with erythrocytes, a much stronger antibody response to the erythrocytes is induced than when erythrocytes are administered alone. several lines of evidence indicate that the ability of igm to activate complement is required for its enhancing effect. that is, igm-mediated enhancement does not occur in animals that have been depleted for complement component c3, nor in mutant animals lacking complement receptors 1 and