Are cross-reacting natural antibodies multispecific?

Human natural antibodies to antigens of Escherichia coli and Serratia marcescens were studied for cross-reactivity. The organisms were grown on synthetic media and extracted at 100 degrees C. The extracts were precipitated three times at 71% ethanol concentration and redissolved at the desired concentration. These preparations were referred to as Escherichia antigen (Ea) and Serratia antigen (Sa). They readily coated red blood cells (RBC), which then could be used for passive agglutination tests. Human serum selected for this study had strong agglutinins combining with both antigens (cross-reacting antibody) and rather weak agglutinins combining with Ea only or Sa only, a property that became obvious from absorption experiments. Absorption of the serum with RBC coated by Ea removed all activity for Ea and most of the activity for Sa, and the opposite effect was noted after absorption of the serum by RBC coated by Sa. The activity against both antigens could be recovered by elution of antibodies at 56 degrees C from RBC coated by either Ea or Sa. Significantly, inhibition of the serum or the eluate with soluble antigens gave strictly specific results in that Ea abolished only the reaction with RBC coated by Ea and did not influence the reaction of RBC coated by Sa, and the opposite result was obtained upon inhibition of the serum with Sa. These results strongly indicated to us that the cross-reacting antibodies under study were multispecific, i.e., had different antigen-reactive sites (haptophore groups) for Ea and Sa. Further evidence supporting this contention was obtained from a study in which the cross-reacting antibody was 'labelled' by a bacterial antigen. To this end, the tested serum was neutralized with Sa and then reacted with RBC coated by Ea. From these RBC, antibodies were eluted from which Sa was recovered. A 'mirror image' experiment was also conducted in which Ea was recovered from antibodies that were blocked by Ea and reacted with RBC coated by Sa.