Immunoglobulin variable regions in molecules exhibiting characteristics of innate and adaptive immune receptors

On Origin and Evolution of the Antibody Molecule

Simple Summary

Like many other molecules playing vital functions in animals, the antibody molecule possesses a complex structure with distinctive features. The structure of the basic unit, i.e., the immunoglobulin domain of very ancient origin is substantially simple. However, high complexity resides in the types and numbers of the domains composing the whole molecule. The emergence of the antibody molecule during evolution overturned the effectiveness of the organisms’ defense system. The particular organization of the coding genes, the mechanisms generating antibody diversity, and the plasticity of the overall protein structure, attest to an extraordinary successful evolutionary history. Here, we attempt to trace, across the evolutionary scale, the very early origins of the most significant features characterizing the structure of the antibody molecule and of the molecular mechanisms underlying its major role in recognizing an almost unlimited number of pathogens.

Abstract

The vertebrate immune system provides a powerful defense because of the ability to potentially recognize an unlimited number of pathogens. The antibody molecule, also termed immunoglobulin (Ig) is one of the major mediators of the immune response. It is built up from two types of Ig domains: the variable domain, which provides the capability to recognize and bind a potentially infinite range of foreign substances, and the constant domains, which exert the effector functions. In the last 20 years, advances in our understanding of the molecular mechanisms and structural features of antibody in mammals and in a variety of other organisms have uncovered the underlying principles and complexity of this fundamental molecule. One notable evolutionary topic is the origin and evolution of antibody. Many aspects have been clearly stated, but some others remain limited or obscure. By considering a wide range of prokaryotic and eukaryotic organisms through a literature survey about the topic, we have provided an integrated view of the emergence of antibodies in evolution and underlined the very ancient origins.

Gut CaVP is an innate immune protein against bacterial challenge in amphioxus Branchiostoma belcheri

The importance of calcium-binding proteins in immune response of vertebrates is determined, but whether they have the role in invertebrates is largely unknown. In the present study, phylogenetic analysis indicated that calcium vector protein (CaVP), a protein unique to amphioxus, shared 68% similarity in amino acid sequence with human and mouse calmodulin (CaM). CaVP cDNA was cloned into a bacterial vector pET-32a, and its His-tagged fusion protein was produced in Eschherichia coli cells (BL21). The recombinant CaVP was purified by Ni-NTA column and SDS-PAGE, and then utilized for antibody preparing. The prepared antibodies could recognize amphioxus CaVP with high specificity. Further analysis by Western blotting showed that CaVP was detected in muscle and humoral fluid of normal animals and appeared in gut of bacterial immunized or challenged amphioxus. Interestingly, gut CaVP was significantly higher in a healthy sub-group than a wounded sub-group post bacterial challenge. This response was detected strongly in immunization and challenge by the same Gram-negative bacterium Vibro parahaemolyticus and weakly in immunization by V. parahaemolyticus and then challenge by Gram-negative Aeromonas hydrophila, whereas no any feedback was found in immunization by V. parahaemolyticus and challenge by Gram-positive Staphylococcus aureus. These findings indicate the importance of gut CaVP in response to bacterial challenge.

Going adaptive: the saga of antibodies

Because of their extreme importance to human health, we probably know more about the structure and function of antibodies than practically any other molecule. Despite all the knowledge that has been accrued in the understanding of antibodies, modern approaches, especially comparative genomics, continue to yield novel findings regarding their underlying biology and evolution. In this review, we describe recent research that led to these revelations, and discuss the broad evolutionary implications of these findings. We have restricted our discussion to three vignettes. Considerable attention has been paid to the recent discovery that the teleost IgH locus is highly similar in organization to the Tcra-Tcrd locus, implicating an evolutionary common ancestor and parallels between the functions of B and T cells during development. Second, we discuss how a new type of antibody, recently discovered in jawless vertebrates, composed not of immunoglobulins but leucine-rich repeats, sheds new light on the overall forces driving evolution of all adaptive antigen receptors. Lastly, we discuss how accumulation of genomic sequences of various human subpopulations leads to better understanding of the directionality of antibody evolution. There is always more to learn from the unfolding saga of antibodies.