Interaction with FcgRIIB receptor determines inhibitory potential of antibody. Two monoclonal antibodies, K29 and K83, were compared for their ability to bind to the Fc receptor on activated B cells and to inhibit B-cell responses in an ELISPOT assay and in cotton rats. K29 and K83 antibodies both recognize different epitopes on MV-H and neutralize MV. K29 is of the IgG1 isotype and K83 of the IgG2a isotype. (A) To determine the inhibitory activity of K29 and K83, cotton rats were injected with a neutralization titer of 320 of antibody intraperitoneally and immunized subcutaneously with 105 pfu MV (Schwarz strain) one day later. Sera from immunized animals were tested by neutralization assay 7 weeks later. Whereas K83 suppressed the generation of neutralizing antibodies significantly (**P < .01), K29 did not. (B) Cotton rat B cells were activated by the addition of lipopolysaccharide to spleen cells overnight, purified by Ficoll gradient centrifugation, and stained for membrane-bound immunoglobulin (BCR). (C) These B cells also expressed FcγRIIB (CD32). (D) F(ab′)2-complexed K83 (black line) bound to cotton rat B cells, whereas F(ab′)2 fragments of F(ab′)2-complexed K83 did not (gray area). Similarly, F(ab′)2-complexed K29 did not bind to cotton rat B cells (gray line). (E) By ELISPOT, K29 was not able to suppress activation of MV-specific B cells, whereas K83 did (*P < .05; **P < .01). (F) For a competition ELISPOT, plates were preincubated with increasing amounts of K29 before addition of a constant amount of K83. Overall, K29 did not influence the inhibitory activity of K83.
