Ultrastructural and immunologic characteristics of mouse x cattle xenogeneic hybridomas originating from bovine leukemia virus-infected cattle

Conservation and diversity in the ultralong third heavy-chain complementarity-determining region of bovine antibodies

A subset of bovine antibodies have an exceptionally long third heavy-chain complementarity determining region (CDR H3) that is highly variable in sequence and includes multiple cysteines. These long CDR H3s (up to 69 residues) fold into a long stalk atop which sits a knob domain that is located far from the antibody surface. Three new bovine Fab crystal structures have been determined to decipher the conserved and variable features of ultralong CDR H3s that lead to diversity in antigen recognition. Despite high sequence variability, the stalks adopt a conserved β-ribbon structure, while the knob regions share a conserved β-sheet that serves as a scaffold for two connecting loops of variable length and conformation, as well as one conserved disulfide. Variation in patterns and connectivity of the remaining disulfides contribute to the knob structural diversity. The unusual architecture of these ultralong bovine CDR H3s for generating diversity is unique in adaptive immune systems.

Somatic hypermutations and isotype restricted exceptionally long CDR3H contribute to antibody diversification in cattle

Antibody diversification in IgM and IgG antibodies was analyzed in an 18-month old bovine (Bos taurus) suffering from naturally occurring chronic and recurrent infections due to bovine leukocyte adhesion deficiency (BLAD). The BLAD, involving impaired leukocyte beta2 integrin expression on leukocytes, develops due to a single point mutation in conserved region of the CD18 gene resulting in substitution of aspartic acid128 with glycine (D128G). Twenty four VDJCmu and 25 VDJCgamma recombinations from randomly constructed cDNA libraries, originating from peripheral blood lymphocytes, were examined for the variable-region structural characteristics in IgM and IgG antibody isotypes. These analyses led to conclude that: (a) expression of exceptionally long CDR3H is isotype restricted to cattle IgM antibody; (b) VDJ recombinations encoding IgM with exceptionally long CDR3H undergo clonal selection and affinity maturation via somatic mutations similar to conventional antibodies; (c) somatic mutations contribute significantly to both IgM and IgG antibody diversification but significant differences exist in the patterns of 'hot spot' in the FR1, FR3 and CDR1H and, also, position-dependant amino acid diversity; and (d) transition nucleotide substitutions predominate over transversions in both VDJCmu and VDJCgamma recombinations consistent with the evolutionary conservation of somatic mutation machinery. Overall, these studies suggest that both somatic mutations and exceptional CDR3H size generation contribute to IgM and IgG antibody diversification in cattle during the development of immune response to naturally occurring chronic and multiple microbial infections.