The Biological Origin of Antibody Diversity

Abstract

Antibody diversity has a compelling fascination for many scientists and over the years speculations have sometimes seemed more numerous than facts. Now the structural basis of antibody specificity is well defined. Amino acid sequences and recently three-dimensional structures of various immunoglobulins provide the most solid basis for discussing the origin of diversity. The novel pattern of variable (V) and Constant (C) regions of amino acid sequence has been resolved further to show the functional pattern of variability. Inheritance of separate V and C genes is accepted, but attempts to define more than one gene coding for each V region are considered here to be unnecessary. The pattern of variability is still best understood in terms of mutation and the presence or absence of various selective pressures. The major area of debate still hinges around the extent to which mutation and selection operate during evolution or somatically. Sequence data have now been generally interpreted to require multiple V genes carried in the germ line. A few individual VH genes have been mapped in close linkage to CH genes in the mouse. The apparent existence of three VH alleles in rabbits was a strong argument against multiple V genes. Now the three phenotypes have been shown to be due to alleles controlling the expression of three sets of VH genes all present on the same chromosome. That V-gene expression requires rejoining of V and C genes at the DNA level is now almost certain. Models for the joining process can draw on the precedents of transposable genetic elements, which are widespread in Nature. The total extent of antibody diversity remains a philosophical point. Estimates of the number of antibody molecules required for observed diversity are reduced by two recently documented proposals. Each antibody combining site apparently has many (estimated at 100) different specificities and most combinations of VH and VL regions probably form a viable site. A given combining site can be defined by its pattern of shared specificities. Several specific antibody repertoires have been measured and the size in each case is consistent with the stringency with which the specificity is selected. Repertoire size appears to be under genetic control, but there are problems in viewing the genotype through the veil of clonal selection. Molecular hybridization has been used recently in an attempt to count V and C genes directly. C genes are seen in DNA having nonreiterated sequences, as formal genetics predicts. Each V-region probe hybridizes at a similar rate to C-region probes. Interpretation of this result depends on the extent to which one V-region probe will reveal nonhomologous V genes. Previous estimates that many cross-hybridizing genes should have been seen if present are possibly exaggerated. It is argued here that the data are compatible with a germ-line gene for each probe studied. Maximum estimates for the number of germ-line genes are sufficient to account for antibody diversity...

Neutralizing and Non-neutralizing Monoclonal Antibodies to the E2 Glycoprotein of Semliki Forest Virus Can Protect Mice from Lethal Encephalitis

“…1 b); the UM 4.2 antibodies focused between pH 8 and 7.5 while the UM 5.1 antibodies focused between pH 7.2 and 6.7. This difference may be related to idiotypic differences between the two IgG2a clones (Williamson, 1976;Bloor et al, 1982). Ascitic fluids of UM 4.2 and UM 5.1 were tested for their ability to neutralize virus in a plaque reduction test.…”

Roy Cameron lecture. Control of antibody formation: certain uncertainties.

“…Murine VK gene segments appear to be organised in sets comprising about 10 per set, V genes within a set being more similar in sequence to one another than to V genes in other sets Valbuena et al, 1978). Analyses by molecular cloning and DNA sequencing, however, shows that there is both framework and hypervariable region diversity among V genes in the same set There have been many calculations of the size of the antibody repertoire, based on extrapolation from measurements of the size of individual repertoires of antibodies of a given specificity (Williamson, 1976 provides a large target size for mutation, and this must increase the pool of both germ line and somatic variability. (3) There is already evidence that aminoacid sequence diversity within certain V region groups may exceed the estimated number of V gene segments within the corresponding set (Tonegawa et al, 1977;Brack et al, 1978;Weigert et al, 1978).…”

“…The effect of negative selection, eliminating non-viable V region phenotypes, is evident. Negative selective pressure allows expression of all mutations, whether specificity determining or non-specificity determining, that do not interfere with the essential process of assembly, surface expression, and secretion of antibody molecules (Hood et al, 1974;Williamson, 1976 (Fig. 1).…”

Expression of IgD may use both DNA rearrangement and RNA splicing mechanisms

“…These data are difficult to reconcile with the CH gene deletion model because they imply that a VH gene can be expressed with the Cog gene without concomitant deletion of the C, gene. A number of mechanisms have been suggested to account for these observations, including the "copy-insertion" mechanism (14) in which a copy of the VH gene is joined to the C., gene while the original remains joined to the C,, gene, and differential RNA processing of a single transcript containing VH, C,,, and C5 genes (15). This paper reports experiments carried out to test these hypotheses.…”