A MONOVALENT C(MU)4-SPECIFIC LIGAND ENHANCES THE ACTIVATION OF HUMAN B-CELLS BY MEMBRANE IGM CROSS-LINKING LIGANDS

The ligand-receptor binding requirements for achieving full B cell act ivation through the membrane immunoglobulin (mlg) signaling pathway ar e relatively demanding, and mlg-antigen engagements which fall below t hese critical thresholds cause, at most, only the partial activation o f a cells. In an effort to resolve new means of enhancing the efficacy of mlgM-mediated signal transduction, as well as to further understan d the process by which mlgM-mediated signals are initiated, we have ex plored the mechanism for a previously reported synergy between certain mixtures of murine anti-lgM mAbs in eliciting human B cell DNA synthe sis. We here report that striking synergy occurs when any of several r elatively high affinity mAbs specific for diverse domains of mlgM are combined in culture with the relatively low affinity C(mu)4-specific l igand, mAb IG6. Although B cell activation was dependent upon the biva lency, and hence mlgM cross-linking potential, of the high affinity li gand, low affinity mAb IG6 could enhance the activation process when p resent as a monovalent Fab' fragment. This did not appear due to F(ab' )(2) contamination or Fab' aggregation, since IG6 Fab' preparations we re notably compromised in several other functions requiring ligand biv alency. Pulsing studies revealed that the C(mu)4-specific ligand exhib its its functional effects only when stimulatory mlgM receptor cross-l inks are being formed by bivalent ligands, and that IG6 Fab' enhanceme nt is most notable during the later interval of the prolonged mlgM sig naling process that leads to S phase entry. A unique region of the mem brane-proximal lgM domain may be important for Fab'-mediated enhanceme nt, since Fab' fragments that bind with higher affinities to distinct sites on C(mu)4 were not as effective at mediating this phenomenon, Se veral possibilities for the adjuvant effects of this C(mu)4-specific F ab' on B cell responses triggered by mlgM crosslinking ligands are dis cussed, including the possibility that IG6 Fab' influences the potenti al for mIgM dimer formation or interactions of mlgM with other signal- transducing molecules.