Characterization of Superantigen-Induced Clonal Deletion with a Novel Clan III-Restricted Avian Monoclonal Antibody: Exploiting Evolutionary Distance to Create Antibodies Specific for a Conserved VH Region Surface

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Abstract
Evolution of the Ab system has yielded three clans of VH region genes that are represented in almost every known higher species with an adaptive immune system. These clans are defined by sequence homologies primarily in highly conserved framework (FR) subdomains, which serve a scaffolding function maintaining the conformation of loops responsible for Ag binding. Structural analyses indicate that the VH FR1 and FR3 form a conserved composite exposed surface, which has been implicated in interactions with B cell superantigens. To directly investigate the expression of clan-defined supraclonal sets, we exploited the evolutionary distance of the chicken immune system and the selection power of phage display, to derive Abs diagnostic for clan III Ig. Using a specially tailored immunization and selection strategy, we created recombinant avian single chain Fv Abs specific for the clan III products, including those from the human VH3 family, and the analogous murine 7183, S107, J606, X24, and DNA4 families, and binding was competitive with natural B cell superantigens. The archetype, LJ-26, was demonstrated to recognize a clan-specific surface expressed in diverse mammalian, and also the Xenopus and chicken, immune systems. In flow-cytometric studies with LJ-26, we found that treatment of heterozygous T15i transgenic mice with a model B cell superantigen induced a clan III-restricted clonal deletion. These studies demonstrate the utility of a novel recombinant serologic reagent to study the composition of the B cell compartment and also the consequences of B cell superantigen exposure.