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...