Characterization of antibodies induced by immunization of mice with isoglobotrihexosylceramide (iGb3).

Isoglobotrihexosylceramide (iGb3), a well-characterized natural killer T cell ligand found in mammalian tissues, is also known as a glycosphingolipid that contains the human IgE epitope α-Gal (Galα1,3Gal) structure. Here, we analyzed the reactivity of several mice and human serum immunoglobulins against iGb3. Additionally, we isolated and characterized the variable region sequences of a monoclonal antibody that specifically recognizes iGb3. No IgE reactive with iGb3 was detected in sera from MRL/lpr mice, which are known to produce autoreactive antibodies, or in sera from healthy human donors. Furthermore, no induction of IgE and IgG was observed in the sera of mice immunized with iGb3; only IgM reactivity to iGb3 was detected. Further analysis of an anti-iGb3 monoclonal antibody generated from the splenocytes of an iGb3-immunized mouse revealed that the nucleotide sequences of the variable regions exhibited high homology to those of antibodies recognizing glycoconjugates containing Galα1,3 or Galα1,4 structures. These results indicate that the mouse genome harbors genes capable of encoding antibodies that recognize α-linked galactose-containing glycans, including iGb3, but that iGb3 is not sufficiently immunogenic to induce IgE in mammalian lymphocytes.