A monovalent C{micro}4-specific ligand enhances the activation of human B cells by membrane IgM cross-linking ligands

The ligand-receptor binding requirements for achieving fullB cell activation through the membrane immunoglobulin mIg signalingpathway are relatively demanding, and mIg-antigen engagementswhich fall below these critical thresholds cause, at most, onlythe partial activation of B cells. In an effort to resolve newmeans of enhancing the efficacy of mIgM-medlated signal transduction,as well as to further understand the process by which mIgM-mediatedsignals are initiated, we have explored the mechanism for apreviously reported synergy between certain mixtures of murineanti-IgM mAbs in eliciting human B cell DNA synthesis. We herereport that striking synergy occurs when any of several relativelyhigh affinity mAbs specific for diverse domains of mlgM arecombined in culture with the relatively low affinity Cµ4-specificligand, mAb IG6. Although B cell activation was dependent uponthe bivalency, and hence mIgM cross-linking potential, of thehigh affinity ligand, low affinity mAb IG6 could enhance theactivation process when present as a monovalent Fab' fragment.This did not appear due to F(28ab')² contamination or Fab' aggregation,since IG6 Fab' preparations were notably compromised in severalother functions requiring ligand bivalency. Pulsing studiesrevealed that the Cµ4-specific ligand exhibits its functionaleffects only when stimulatory mIgM receptor cross-links arebeing formed by bivalent ligands, and that IG6 Fab' enhancementis most notable during the later interval of the prolonged mIgMsignaling process that leads to S phase entry. A unique regionof the membrane-proximal IgM domain may be important for Fab'-mediatedenhancement, since Fab' fragments that bind with higher affinitiesto distinct sites on Cµ4 were not as effective at mediatingthis phenomenon. Several possibilities for the adjuvant effectsof this Cµ4-speclfic Fab' on B cell responses triggeredby mIgM crosslinking ligands are discussed, including the possibilitythat IG6 Fab' influences the potential for mIgM dimer formationor interactions of mIgM with other signal-transducing molecules. Received 13 October 1994, accepted 9 November 1994.