Classification of idiotypes

Idiotypes are generally defined by serologic reagents collectively designated as anti-idiotype antibodies. Several criteria were used to classify the idiotypes based on the type of interaction between idiotypes and anti-idiotype antibodies.

The immunochemical criterion used to classify the idiotype consists of the effect of antigen on the interaction between idiotype and anti-idiotype antibodies. In the case of some idiotypes, this reaction is inhibited by the antigen, whereas in other cases it is not. Because it is generally accepted that hyper-

variable regions make up the combining site of antibodies and amino acid residues are responsible for binding to antigens, antigen-inhibitable idiotypes are considered 'idiotypes associated with the combining site'.

The second category of idiotypes that interact with anti-idiotype antibodies are non-antigen inhibitable and are considered to be located outside the combining site, and are usually called 'framework associated idiotypes'.

Genetic criteria were used to classify the idiotypes at a clonal level. Individual idiotypes result from a unique somatic recombination event of immunoglobulin genes or as a consequence of somatic mutation. Individual idiotypes are expressed on the antigenic receptor of a single lymphocytic clone in a single individual of a given species and are not inherited.

Cross-reactive idiotypes (CRI), (IdX) are expressed on antibodies or T cell receptors (TCRs) with the same specificity from various individuals. This category of idiotypes is inherited in a mendelian manner and probably represents phenotypic markers of germline genes. Cross-reactive idiotypes were identified on antibodies with the same antigen specificities produced in various species: amphibians, mice, rats, guinea pigs, pigs, rabbits, monkey and humans. They have been identified on antibodies against foreign antigens (haptens, polysaccharides or proteins), cellular antigens (HLA-DR, H-2, erythrocyte antigens such as Rh, I or P), self antigens (DNA, Sm, thyroglobulin, basal membrane, collagen II), and cellular receptors (such as AChR, TSHR, insulin receptor, and (5 receptor).

In 1981, we classified the cross-reactive idiotypes into three major groups:

1. IdX linked to allotype. These idiotypes are expressed in all inbred or recombinant inbred strains which share the same allotypes of heavy or light chains. The restriction of the expression of the V region genes of immunoglobulin molecules by C genes is not fully understood.

2. Interstrain idiotypes. This category of idiotypes is expressed on various inbred strains or on a major fraction of individuals of an outbred species independent of genetic allelism of the constant region.

3. Interspecies idiotypes. These are cross-reactive idiotypes shared by antibodies displaying the same antigen specificity produced by individuals of various species. These IdX arc markers of germline genes that are conserved during evolution.

The same idiotypic determinants of antibodies with the same antigen specificity can be shared independently on various immunoglobulin classes or isotypes. This phenomenon is due to the ability of a given VH region gene to recombine with various constant region genes during clonal development, differentiation and switching of B cells subsequent to antigen stimulation.

The idiotypes were also classified based on their expression in the immunoglobulin repertoire, variation during the immune response, or their role in the immune response. With respect to their expression, the idiotypes can be classified into:

1. Dominant idiotypes expressed on the majority of antibodies exhibiting the same antigen specificity. As an example, one may cite T15 IdX expressed on antiphosphocholine antibodies, A5A IdX expression on ACHO-streptococcus A polysaccharide-specific or CRI of arsonate-specific antibodies. The dominance of certain idiotypes can be related to the phylogenetic selection of clones involved in the immune response against microbes.

2. Minor idiotypes expressed on a small fraction of antibodies produced in a response against a particular antigen.

3. Silent idiotypes, which are not expressed during a conventional immune response but which can be detected or even can become dominant subsequent to activation of clones by anti-idiotype antibodies or certain regimens of immunization.

From a functional point of view, the idiotypes can be classified into two major categories:

1. Conventional idiotypes, which can be defined by anti-idiotype antibodies obtained in various animals and which do not show variations during the immune response.

2. Regulatory idiotypes were described on antibodies specific for both foreign and self antigens. They are involved in the regulation of responses to antigen and may show variations during the immune response.

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