Homogeneous rabbit immunoglobulin lacking group a allotypes: amino acid sequence analysis of the heavy chain

Antibacterial activity of lysozyme and lactoferrin is inhibited by binding of advanced glycation-modified proteins to a conserved motif

Why diabetes is associated with abnormally high susceptibility to infection remains unknown, although two major antibacterial proteins, lysozyme and lactoferrin, have now been shown to specifically bind glucose-modified proteins bearing advanced glycation end products (AGEs). Exposure to AGE-modified proteins inhibits the enzymatic and bactericidal activity of lysozyme, and blocks the bacterial agglutination and bacterial killing activities of lactoferrin. Peptide mapping revealed a single AGE binding domain in lysozyme and two AGE binding domains in lactoferrin; each domain contains a 17- to 18- amino acid cysteine-bounded loop motif (CX15-16C) that is markedly hydrophilic. Synthetic peptides corresponding to these motifs in lysozyme and lactoferrin exhibited AGE binding activity, and similar domains are also present in other antimicrobial proteins. These results suggest that elevated levels of AGEs in tissues and serum of diabetic patients may inhibit endogenous antibacterial proteins by binding to this conserved AGE-binding cysteine-bounded domain 'ABCD' motif, thereby increasing susceptibility to bacterial infections in the diabetic population.

Limited number of immunoglobulin VH regions expressed in the mutant rabbit "Alicia"

A unique feature of rabbit Ig is the presence of VH region allotypic specificities. In normal rabbits, more than 80% of circulating immunoglobulin molecules bear the VHa allotypic specificities, al, a2 or a3; the remaining 10% to 20% of immunoglobulin molecules lack VHa allotypic specificities and are designated VHa-. A mutant rabbit designated Alicia, in contrast, has predominantly serum immunoglobulin molecules that lack the VHa allotypic specificities (Kelus and Weiss, Proc. Natl. Acad. Sci. USA 1986. 83: 4883). To study the nature and molecular complexity of VHa- molecules, we cloned and determined the nucleotide sequence of seven cDNA prepared from splenic RNA of an Alicia rabbit. Six of the clones appeared to encode VHa- molecules; the framework regions encoded by these clones were remarkably similar to each other, each having an unusual insertion of four amino acids at position 10. This insertion of four amino acids has been seen in only 2 of 54 sequenced rabbit VH genes. The similarity of the sequences of the six VHa- clones to each other and their dissimilarity to most other VH genes leads us to suggest that the VHa- molecules in Alicia rabbits are derived predominantly from one or a small number of very similar VH genes. Such preferential utilization of a small number of VH genes may explain the allelic inheritance of VH allotypes.

Dynamic gene interactions in the evolution of rabbit VH genes: a four codon duplication and block homologies provide evidence for intergenic exchange

Two rabbit VHa-negative genes, RVH831 and RVH832, were isolated from a single genomic fragment selected by hybridization with the mouse VHIII gene S107V1. RVH831 is a pseudogene with a frameshift mutation in FR3 and a 19 bp deletion within the VH-D splice site. In contrast, RVH832 has an open reading frame and an intact VH-D splice site and thus may be functional. However, RVH832 displays a unique 4 codon duplication/insertion in FR1 that may be the result of an unequal exchange event between two ancestral VH genes. Sequence comparisons between these and other rabbit VH genes reveal patterns of shared blocks of nucleotide substitutions, suggestive of gene conversion. A high overall homology (greater than or equal to 73%) between the compared VH nucleotide sequences suggests that rabbit VH genes may not be organized in clearly divergent families or subgroups.

The structural and genetic basis for expression of normal and latent VHa allotypes of the rabbit

The immunoglobulin heavy chain variable regions of the rabbit are unusual in having genetically controlled, serologically detectable alternative forms, the VHa allotypes, as well as minor VH allotypes of the x, y and w groups. New insights into the probable structural basis for the VHa allotypes have come from re-examination of earlier protein sequence data in the light of newly deduced protein sequences derived from sequencing cloned cDNAs and genomic DNAs encoding VH regions. Here we review this sequence information, and define the allotype-correlated differences at seven positions in framework region 1 and 10 positions in framework region 3 that may lead to the serologically detectable allotypic determinants (allotopes). Most alternative amino acids at allotype-correlated positions can be derived from each other by single-base changes. Thus somatic mutations and/or gene conversion-like events must be considered along with other serological and genetic explanations for various reported observations of the production of latent VHa allotypes. The proximity of rabbit VH genes (approximately 3 kb apart) might enhance the likelihood of conversion-like events in both germline and somatic cells.

Organization of the diversity--joining region in rabbit immunoglobulin heavy chains as revealed by cleavage of a specific methionine residue in a100 allotype

Three anti-micrococcus antibodies of restricted heterogeneity have been isolated from the antisera of homozygous a100/a100 rabbits. Heavy chains contained an unusual methionine residue at position 87 that may correlate with the a100 specificity. From this position, the sequence of a stretch of 35-50 amino acid residues was determined, permitting the definition of variable (V), diversity(D), and joining (J) segments in rabbit Ig heavy chains, with their most probable boundaries. Rabbit D regions so defined appear to be highly variable, both in sequence and in length, which varies, in the heavy chains analyzed, between 6 and 11 residues. The J regions are highly homologous to the mouse J2 segment.