Antibody structure and molecular immunology

Immunology's Coming of Age

This treatise describes the development of immunology as a scientific discipline with a focus on its foundation. Toward the end of the nineteenth century, the study of immunology was founded with the discoveries of phagocytosis by Elias Metchnikoff, as well as by Emil Behring's and Paul Ehrlich's discovery of neutralizing antibodies. These seminal studies were followed by the discoveries of bacteriolysis by complement and of opsonization by antibodies, which provided first evidence for cooperation between acquired and innate immunity. In the years that followed, light was shed on the pathogenic corollary of the immune response, describing different types of hypersensitivity. Subsequently, immunochemistry dominated the field, leading to the revelation of the chemical structure of antibodies in the 1960s. Immunobiology was preceded by transplantation biology, which laid the ground for the genetic basis of acquired immunity. With the identification of antibody producers as B lymphocytes and the discovery of T lymphocytes as regulators of acquired immunity, lymphocytes moved into the center of immunologic research. T cells were shown to be genetically restricted and to regulate different leukocyte populations, including B cells and professional phagocytes. The discovery of dendritic cells as major antigen-presenting cells and their surface expression of pattern recognition receptors revealed the mechanisms by which innate immunity instructs acquired immunity. Genetic analysis provided in-depth insights into the generation of antibody diversity by recombination, which in principle was shown to underlie diversity of the T cell receptor, as well. The invention of monoclonal antibodies not only provided ultimate proof for the unique antigen specificity of the antibody-producing plasma cell, it also paved the way for a new era of immunotherapy. Emil Behring demonstrated cure of infectious disease by serum therapy, illustrating how clinical studies can stimulate basic research. The recent discovery of checkpoint control for cancer therapy illustrates how clinical application benefits from insights into basic mechanisms. Last not least, perspectives on immunology progressed from a dichotomy between cellular-unspecific innate immunity and humoral-specific acquired immunity, toward the concept of complementary binarity.

Reflections on the clonal-selection theory

How do we account for the immune system's ability to produce antibodies in response to new antigens? It has been 50 years since F. Macfarlane Burnet published his answer to this question: the clonal-selection theory of antibody diversity. The idea that specificity for diverse antigens exists before these antigens are encountered was a radical notion at the time, but one that became widely accepted. In this article, Nature Reviews Immunology asks six key scientists for their thoughts and opinions on the clonal-selection theory, from its first proposal to their views of it today.

Molecular organization of 19 S calf thyroglobulin

Controlled freezing and thawing of 19 S calf thyroglobulin resulted in a specific and reproducible breakdown of the protein. Beside the elementary chain (300,000 Da), new, discrete bands are revealed by gel electrophoresis in sodium dodecyl sulfate under reducing conditions. These new species consist of a major peptide of 100,000 Da and several faster-migrating bands. Most of these polypeptides were purified by preparative gel electrophoresis and individually digested in formic acid with CNBr. A comparative gel electrophoresis under denaturating and reducing conditions of (i) the fragments obtained from the native protein, (ii) the electrophoretically purified elementary chain, (iii) the 100,000-Da peptide, and (iv) a smaller fragment (of about 50,000) was performed. It revealed a very close homology among the peptide maps of the intact 19 S, the elementary chain, and the 100,000-Da peptide. Furthermore, it was shown that the digestion products of the smaller fragment, present in the peptide map of the native protein, were absent in both the elementary chain and the 100,000-Da species. These results support the idea that calf thyroglobulin, even though it has an apparently complex molecular organization, contains structural motifs which are repeated in the elementary chain.

Evidence for homologous repeating segments within the elementary polypeptide chain of guinea pig thyroglobulin

In this paper we report on the peptide maps of the three polypeptides produced by reduction of denatured 19-S thyroglobulin from guinea pig. These maps were obtained by both chemical cleavage (CNBr) and by limited enzymatic proteolysis (Staphylococcus aureus SV 8 protease). Analysis of these peptides showed a strict correspondence among the electrophoretic bands produced by cleavage of the guinea pig thyroglobulin components. These results support the idea that thyroglobulin, even though it has an apparently complex molecular structure, contains polypeptide sequences which are repeated in the elementary chain. Since similar structures should correspond to similar functions, each of these regions may work as a functional domain for the biosynthesis of the thyroid hormones.

Sequential generation of antibody diversity during B-cell development

It has become increasingly apparent that generation of and variation in antigen-combining sites of antibodies occurs sequentially during B-cell development. Allelic and isotypic exclusion mechanisms ensure that a B cell produces antibody molecules having a single kind of combining site. A major reason for evolution of mechanisms which result in asynchronous formation of functional H and L chain genes may be the need for allelic and isotypic exclusion; but this may not be the only advantage of asynchronous formation of H and L chain genes. The evolution of mechanisms causing nonjunctional somatic mutation late in B cell development - only after antigen exposure apparently - may result from the biological advantages of: (1) 'fine tuning' of the combining site; (2) a response to an anti-idiotype regulatory network, or (3) expanded memory.

Structural correlates of immunoglobulin diversity

VL and VH domains, from different species and with widely different primary structures, interact with each other in the same way to create the globular FV region. Much of the FV is a highly conserved framework structure that is probably common to most, if not all, mammalian FV regions. The extensive contoured frontal surface of the FV is composed of highly variable polypeptide segments (Wu-Kabat complementarity-determining regions). These segments are derived from parts of VL, VH, JH gene products and most of the D gene product. This surface is currently considered to be the most likely location of the antigen-binding sites. The firm immunochemical data based on identification of contacting amino acids supporting this location are still, however, very fragmentary. VL and VH gene products form a large part of the potential antigen-reactive surface. Hence, combinations of different VL and VH gene products are the largest source of structural diversity. The JL and JH gene products have chiefly structural functions in maintaining domain architecture and controlling some interactions between VL and VH domains. The VL-J junction amino acid can provide unique structural properties in the deeper accesses of the potential antigen reaction surface. The VHD-JH junction is more superficial and could be, but has not yet been, directly implicated in antigen binding. The D gene product and the additional amino acids associated with the (VH-D-JH) rearrangement process do determine a substructural part of the potential antigen reactive surface. The D gene product (a connecting segment between two beta strands) can have many different secondary structures. Functionally, the D region product could interact with VL-CDR-1 amino acids to create a specific contour of the antigen reaction surface. Curiously, primary structural variations in H3 have not yet been directly implicated in antigen binding. Much remains to be learned about the role of VH-D-JH rearrangement in antibody diversity. The major genetic factors in creating structural diversity are the multiple VL and VH gene libraries. The gene rearrangement process provides a further amplification. Somatic mutations are yet another additional mechanism.

Structure of IgG and IgY molecules in ribosome-antibody complexes as studied by electron microscopy

The overall shape and dimensions of IgG (rabbit) and IgY (chicken) antibodies against ribosomal proteins have been studied in electron micrographs of ribosome-antibody complexes. The antibodies appear as Y-shaped molecules with an angle of about 90 degrees between their Fab arms. The length of one Fab arm amounts to about 10 nm. No differences between the IgG and IgY molecules could be detected electron microscopically. The data obtained on the shape of IgG and IgY correlate with those of earlier electron microscopic studies while the determined size of the Fab arms is in the range found by scattering methods.

Structure and function of the constant regions of immunoglobulins

Immunoglobulins are somewhat unusual in that they are biologically active proteins capable of carrying out several different functions. Thus, they can bind specifically with antigen, activate the complement system, mediate many cytotropic reactions, and act as antigen receptors on lymphocyte membranes.

Mouse immunoglobulin heavy chains are coded by multiple germ line variable region genes

The N-terminal 20 residues of 13 heavy immunoglobulin chains from myeloma protein of the BALB/c mouse are compared with the same residues of 15 other heavy chains described in the literature. Sixteen of 28 sequences are different from one another. These proteins fall into four major sets, with 18 of the proteins in the largest set being further divisible into at least five subsets. This pattern of diversity suggests there are at least eight germ line genes coding for the variable regions of mouse heavy chain. Many of the immunoglobulins from which these heavy chains are derived exhibit binding activity for various haptens. The differing hapten specificities are closely correlated with distinct primary amino-acid sequences.