FIG. 43.13

How is antibody diversity generated?

BACKGROUND Humans produce an estimated 10 billion different antibodies. Antibodies are proteins, encoded by genes. However, there are only about 25,000 genes in the human genome. How then can humans (and other vertebrates) produce such a diversity of antibodies from a limited set of genes? This was one of the central problems in immunology until the 1960s and 1970s.

HYPOTHESIS In 1965, American biologists William Dreyer and J. Claude Bennett proposed a model in which there are many copies of two gene segments, which they called the V (variable) and C (constant) segments. An antibody gene is assembled by recombining a single copy of each of these segments. At the time, there was no direct experimental evidence to support this hypothesis.

EXPERIMENT In 1976, Japanese immunologists Nobumichi Hozumi and Susumu Tonegawa tested the Dreyer and Bennett hypothesis. Using mice as a model system, they isolated DNA from embryonic cells (before recombination was thought to occur) and from adult B-cell tumor lines that produce one type of antibody (after recombination was thought to occur). The procedure they followed was slightly different from what is described here but conceptually the same. DNA is cut by restriction enzymes, the fragments are separated on a gel and transferred to a filter paper, and light chain DNA (containing both V and C regions) is used as a radioactive probe to determine the size and positions of the V and C regions (Southern blot, Chapter 12).

RESULTS Hozumi and Tonegawa found that the V and C regions were far apart in the DNA of embryonic cells and close together in adult cells, suggesting that the gene segments were brought together during B cell differentiation.

CONCLUSION Antibody genes are assembled by recombination of individual gene segments, providing a mechanism for generating antibody diversity.

FOLLOW-UP WORK Further studies showed that there are multiple copies of several different gene segments, now called the V, D (diversity), J (joining), and C regions for heavy chains, and V, J, and C regions for light chains. These are recombined in unique patterns in different B cells, so each B cell produces one antibody and a population of B cells produces a diversity of antibodies.

SOURCES Dreyer, W. J., and J. C. Bennett. 1965. “The Molecular Basis of Antibody Formation: A Paradox.” Proceedings of the National Academy of Sciences 54:864–869; Hozumi, N., and S. Tonegawa. 1976. “Evidence for Somatic Rearrangement of Immunoglobulin Genes Coding for Variable and Constant Regions.” Proceedings of the National Academy of Sciences 73:3628–3632.