Demonstration of A Rabbit Cell Surface Ig That Bears Light Chain and VH, But Lacks µ-, α-, and γ-Allotypes-Rabbit IgD?

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Abstract Based on previous biochemical studies of rabbit sIg, we postulated that if rabbits possessed an IgD-like molecule, its heavy chain might co-electrophorese with the µ-heavy chain on reduced SDS-PAGE. Accordingly, we studied Ig from rabbit spleen cells radiolabeled biosynthetically or by cell surface iodination. Cell lysates were prepared and immunoprecipitation studies were performed by standard techniques. Sequential absorptions in which total removal of µ-allotype (n-locus) was followed by immunoprecipitation with anti-a(VH) or anti-b (kappa) showed residual band(s) with µ-relative mobility on SDS-PAGE under reducing conditions. Similar immunoprecipitations with anti-gamma (γ) or anti-alpha (α) reagents failed to precipitate the residual band with µ-mobility. The residual band was found in both radioiodinated and biosynthetically radiolabeled lysates. When direct immunoprecipitation was carried out in the absence of proteolytic enzyme inhibitors, the SDS-PAGE pattern of total Ig under reducing conditions frequently showed three peaks, corresponding to µ (MW ∼70,000), light chain (MW ∼23,000), and an unidentified heavy chain (MW ∼65,000). The second heavy chain peak was not immunoprecipitated with µ, γ, or α allotype sera. Thus, we have identified a “new Ig” in the rabbit that bears VH and light chain, and lacks µ, γ and α allotype markers. The molecule appears to be proteolytically labile, but has an undegraded heavy chain that co-electrophoreses with µ-chains on reduced SDS-PAGE. The similarities to published data on human and rat lymphocyte IgD strongly suggest, but do not formally prove, that we have identified the rabbit homologue of IgD.