Conga y los movimientos políticos posteriores

Immunoglobulin heavy (H) and light (L) chain genes are encoded by distinct gene segments residing on separate chromosomes; the H chain locus is located on human chromosome 14, the kappa (k) chain locus on chromosome 2

and the lambda (%) chain locus on chromosome 22. The H chain variable region is comprised of a variable (V h ), diversity (D h ) and joining (Jh) genes. The L

chain is comprised of Vk and Jk or and J% and does not contain D genes (Sakano et al 1980, Schable and Zachau 1993). Immunoglobulin genes exist as functional gene segments that are able to express light and heavy chain polypeptides or as pseudogenes which are unable to be expressed. In humans, each heavy and light chain locus contains between 100-200 V genes, 5-6 J segments and 30 D gene segments (Cook et al 1995). The heavy and light chains of the immunoglobulin molecule are formed by rearrangement of multiple genes. The first rearrangement occurs in the heavy chain locus and begins by joining of a D segment to a J segment. This is followed by rearrangement of a V gene segment to the DJ unit (Honjo and Habu 1985). During rearrangement, non

template derived nucleotides may be inserted at the VD or DJ junctions of the heavy chain. This addition of N sequences is mediated by enzyme terminal deoxynucleotidetransferase. Additional diversity in the antibody structure results from imprecise VDJ joining resulting in junctions of variable length. The heavy chain constant region sequences are encoded in several exons and are later joined to the variable region gene by RNA splicing after transcription. Once the heavy chain is generated, the light chain gene segments can rearrange by joining of the V segment to a J segment to form a complete light chain variable region gene. The light chain constant region is encoded on a separate exon and is later joined to the variable region gene by RNA splicing after transcription. Rearrangement occurs in the K locus first and proceeds to the lambda locus only if both

rearranged K alleles are unable to code for a functional protein (Korsmeyer 1981).

N sequence rearrangement occurs rarely in light chain rearrangement (Heller et al 1987). Rearranged heavy and light chains are first expressed as IgM antibody molecules but can associate with other constant region genes in a process called class switching (Gritzmacher 1989). Each B cell expresses only one heavy and light chain through a process called allelic exclusion in which a productive rearrangement at one heavy and one light chain locus inhibits rearrangement at the other loci. If a non productive rearrangement occurs at the first locus, then the Ig genes on the other locus undergo rearrangement. Prior to antigen exposure, different genetic mechanisms contribute to diversity of antibody repertoire such as random associations of different V(D)J gene segments along with junctional diversity produced by N sequence insertion and imprecise joining of gene segments and random pairing of heavy and light chain chains.

Additional diversity in the antibody molecules can occur by the form of somatic point mutations in an immune response (French et al 1989). Somatic point mutations are single nucleotide substitutions that can occur throughout the heavy and light chain variable regions but are most numerous in the CDRs. Due to degeneracy of the genetic code, point mutations may not result in any change in the amino acid level and are referred to as silent mutations (S). Alternatively, these mutations may result in a change at the amino acid level and a decreased or increased affinity for the triggering antigen or in the acquisition of new antigenic specificities. These are termed replacement mutations (R). Only B cells making high affinity antibodies to the eliciting antigen are selected for proliferation by a process called affinity maturation. This selection may be often reflected in a greater than random R/S ratio in the complementarity determining regions (CDRs) or in a random or lower than random R/S ratio in the framework regions (FRs) of the selected antibodies (Schlomchik et al 1987).

The human Vh, D and Jh segments have been mapped to band 14q32.3 of

chromosome 14 (Croce et al 1979, Kirsch et al 1982). A complete map of the functional Vh locus at 14.q32.3 has now been produced (Matsuda et al 1993, Cook et al 1994). The total length of the locus is 1100 kb and it contains 95 genes of which 51 are functional genes. There are about seven Vhfamilies (VhI -

7). Two Vh sequences may be defined as members of the same family if they shared sequence identity at 80% or more (Wu and Kabat 1970, Pascual and Capra 1991). Vh families 1,3 and 4 are the largest Vh families. Genes from different families are not clustered together but are interpersed throughout the locus. The human Dh locus contains 30-50 gene segments and consists of a 9kb unit

repeated five times (Siebenlist et al 1981, Matsuda et al 1988, Ichihara et al 1988). A further Dh gene, DQ 52, lies within the Jh locus (Ravetch et al 1981).

Each 9kb unit contains each of six Dh families; DM, DLR, DXP, DA, DK and DN arranged in that order. Members of the same family are more closely homologous to each other than to members of other families. In their studies, Corbett et al (1997) have determined the complete nucleotide sequence of the human immunoglobulin D segment locus on chromosome 14q32.3 and identified a total of 27 D segments, which include nine new segments. Their studies also indicate that the human antibody repertoire is created by VDJ recombination involving 25 of these 27 D segments, extensive processing at the V-D and D-J junctions and use of multiple reading frames. Corbett et al (1997) also proposed a two number naming system for the D segments whereby the first number indicated the family and the second number indicates the relative position of the segment in the locus from Vh to Jh- The human Jh locus contains six functional genes, JhI to Jh6 and three pseudogenes (Ravetch et al 1981).