Immunoglobulin VH gene structure and diversity in Heterodontus, a phylogenetically primitive shark.

Abstract

A mouse VH probe has been used to identify and isolate VH homologs in a DNA library of Heterodontus francisci (horned shark). The complete nucleotide sequence of one VH gene, HXIA, has been determined and found to exhibit striking organizational homology and nucleotide identity with mammalian prototype VH genes. Metric analysis of the complete sequence is consistent with the early phylogenetic diversification of framework and complementarity-determining regions (CDR). Both the predicted amino acid sequence and the specific hybridization of the CDR2-specific, synthetic oligodeoxynucleotide probe in spleen mRNA suggest that HXIA is functionally expressed. A probe consisting of the entire coding region of this gene hybridizes with multiple components in Southern blot analysis of Heterodontus genomic DNA and together with the identification of additional unique VHj-A clones indicates that considerable complexity is associated with the germline VH gene family in a contemporary species that represents an early stage in the phylogenetic development of the vertebrates.The extraordinary complexity of the immunoglobulin genes and their associated recombination mechanisms (1) poses unique questions relating to the diversification and evolutionary stabilization of multigenic families. Our present understanding of the evolution of immunoglobulin VH diversity is based on both inter-and intraspecies comparisons of mouse, human (2-7), and reptilian (8-10) genes. Although inducible humoral immunity has been established in all representative vertebrate species, the elasmobranchs occupy the earliest level of evolutionary development in which immunoglobulins resembling mammalian antibodies have been unequivocally demonstrated (11). Heterodontus francisci, the earliest elasmobranch species studied to date, apparently lacks a heavy chain class shift mechanism and elicits hapten-specific antibody responses that lack interindividual variation and fail to undergo affinity maturation (12, 13). These latter findings may relate to a paucity of germline V genes as well as the absence of somatic mutation-recombination effects associated with the ontogenetic development of antibody-forming cells in higher vertebrates (1).As a first step in molecular genetic characterization of the biologically unique immune response exhibited by this species, a Heterodontus VH gene has been identified and characterized. The findings suggest that this modem representative of the phylogenetically early vertebrates possesses a remarkably complex germline VH gene pool. MATERIALS AND METHODSConstruction and Screening of a Heterodontus Genomic DNA Library. Partial Sau3A digests in the predominately 14-to 17-kb range were ligated (T4 DNA ligase; New England Nuclear) to isolated X47.1 (14) arms and packaged, and recombinant phage were amplified selectively in Escherichia coli Q359.1. When adjusted for nondigested, Sau3A-resistant DNA (-"20% of total), -1.5 x 106 recombinant phage, equivalent to 9 haploid genomes [3.3 x 109 base pairs (bp) per haploid genome, ...

Evolution of immunoglobulin genes: VH families in the amphibian Xenopus.

“…Many of the VH genes are functional, as a recent study (43) revealed that the frequency of pseudogenes in the VH1-3 families in Xenopus is -30%, comparable to that found in the human system (27). With (38), suggesting that it could have been a primordial VH gene or that there may have been selective pressure to maintain this family throughout evolution.…”

T-cell receptor gene homologs are present in the most primitive jawed vertebrates.

“…DNA cross-hybridization approaches have proven effective in identifying immunoglobulin gene homologs in lower vertebrates (28,34); however, outside of mammals, only the avian TCR 3-chain gene has been identified in this manner (6). The considerable sequence diversity among TCR genes in mammalian species and within the TCR multigene families in a single species (31) (28,35).…”

Induction of Cytotoxic T Lymphocytes for Immunotherapy of Breast Cancer

“…It is likely that TCR and MHC molecules evolved together early in the vertebrate Uneage. Genes that encode immunoglobulins were isolated firbm the horned shark in the 1980s (132). Interestingly, all three types of MHC loci found m higher vertebrates (class I, class IIA and class IIB) have been found in the shark (133).…”