IMMUNOLOGY - Roitt I. - Mir 2000

Chapter 8. Sources of Diversity in Antigen-Recognizing Structures

IMMUNOGLOBULIN VARIABILITY

Immunoglobulin molecules consist of light and heavy polypeptide chains. Light chains belong to either the k or λ type. Theoretically, any light chain can pair with any heavy chain; therefore, the total number of possible antigen-binding sites of varying Specificity equals the product of the number of chains of one type and the number of chains of the other. For example, if there are 10⁴ different light chains capable of combining with any of 10⁴ different heavy chains, it is entirely possible to generate 10⁸ antibody molecules of distinct specificity. Each chain type possesses its own mechanism for enhancing diversity, as they are encoded by genes located on different Chromosomes (Fig. 8.3).

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Fig. 8.3. Numbers indicate the chromosomes on which the genes encoding various immunoglobulin Polypeptides, TCRs, and MHC products of humans and mice are located. Note that all loci are located entirely separately, except for the TCR δ-chain locus, which is situated within the GENES OF THE TCR α-chain locus.

Immunoglobulin polymorphism manifests as Variability in different parts of their molecules (Fig. 8.4). Let us first consider idiotypic variability, which arises from the diversity in The Structure of antigen-binding sites.

Fig. 8.4. All IMMUNOGLOBULINS are built from varying numbers of similar four-chain units and can exhibit Three types of variants:

1. Isotypic variants, resulting from the expression of germline genes present in all members of a given species and encoding heavy (μ, δ, γ, ε, α) and light (k and λ) chains, as well as "framework" amino acid residues within their V regions (subgroups).

2. Allotypic variants, resulting from intraspecific allelic variation.

3. Idiotypic variants, representing The Diversity of antigen-binding sites (paratopes) and caused, in particular, by the variability of hypervariable regions within V regions.

Wu and Kabat analyzed the Amino acid sequences of A large number of light and heavy chains from Antibodies of various specificities. Myelomas (monoclonal B-Cell tumors) served as the source of identical immunoglobulins. The results demonstrated that the variability of λ-type light chains is due to Amino Acid Substitutions in three hypervariable regions flanked by relatively invariant "framework" residues. Later, when it was established that these very hypervariable regions contact the antigen, they were named complementarity-determining regions (CDRs).

Initially, mouse CDRs were studied—a species characterized by a restricted diversity of λ chains (accounting for less than 5% of antibodies) due to a very small number of Vλ genes. For instance, out of 19 λ chains whose Amino Acid Sequence was determined, 12 were identical, while the remaining 7 differed from each other and from the prototype sequence by only a few residues (Fig. 8.5).

Fig. 8.5. Amino acid sequences of seven murine myeloma λ1 Proteins. Positions highlighted in yellow contain the same amino acid residues as the prototype sequence (MOPC 104E). Red indicates positions where substitutions occur. The number of specific nucleotide substitutions in the DNA required to produce the Observed changes in The amino acid sequence is shown on the right. The lower part of the figure shows a plot of amino acid residue variability at different positions of the light chains (according to Kabat and Wu), calculated as The ratio of the number of different residues at a given position to the frequency of the most common residue (cf. Chapter 6). Additional residues were found in some chains (at positions indicated by arrows), but they have been omitted here for better comparability.

Variable residues in heavy chains are similarly concentrated in three hypervariable regions (CDRs), flanked on both sides by nearly invariant "framework" regions (Fig. 8.6), which can be divided into groups based on similarity or identity (Fig. 8.7).

Fig. 8.6. Wu and Kabat's plot showing that variable amino acid residues are concentrated in three Regions of the heavy chain V region.

Fig. 8.7. The amino-terminal sequence (positions 1–65) of VhIII group heavy chains from six human myeloma proteins is compared in the diagram with the prototype TEI sequence. Residues identical to the prototype are colored yellow, while those differing at the same positions are colored dark red. Most differences between heavy chains belonging to the same group are concentrated in the complementarity-determining regions, CDR-1 and CDR-2. CDR-3 is not shown in the figure.