Demonstration of IgM antibodies of high affinity within the anti-Galα1-3Gal antibody repertoire

Background. Human anti-Gal alpha 1-3Gal IgG and IgM xenoantibodies can distinguish between very similar epitopes with a high degree of selectivity. Methods. Anti-Gal alpha 1-3Gal antibodies were affinity isolated using two separate Gal alpha 1-3Gal-based immunoadsorbents, Gal alpha 1-3Gal itself and Gal alpha 1-3Gal beta 1-4Glc. IgG and IgM were separated using a protein G column. Antibody purity was achieved by serial adsorption/elutions from the columns. By this means, different antibody fractions were prepared that contained either IgG or IgM, reactive with either Gal alpha 1-3Gal, Gal alpha 1-3Gal beta 1-4Glc, or both. The dissociation equilibrium constants (K-d) of these antibodies were then measured using an IAsys biosensor. Results and Conclusions. Sera from two individuals were used and K-d values for one IgG (fraction 1A) and two IgM (fractions 1B and 2A) fractions were obtained. The K-d for the IgG was 4.85 x 10(-7) M (fraction 1A). Far IgM, the K-d values were higher at 7.8x10(-10) M (fraction 1B) and 1.07x10(-10) M (fraction 2A). Natural anti-pig antibodies include high affinity IgM that continue to be produced without class switch. The B cell mechanism behind this is not known. It may be possible to exploit this mechanism in future xenotransplantation strategies.