Affinity requirements for induction of sequential phases of human B cell activation by membrane IgM-cross-linking ligands.
The affinity of Ag interaction with a B cell's membrane IgM (mIgM) receptors has long been considered to play a critical role in the in vivo clonal selection of B lymphocytes. This study has examined a possible basis for this affinity selection at the level of Ag induction of sequential B cell activation phenomena, i.e., elevated membrane class II MHC expression (G0* excitation), G1 entry, and S phase entry. Functional experiments with model bivalent Ag, i.e., a group of murine mAb of diverse intrinsic binding affinities for human IgM, revealed that the minimal affinity requisites for inducing the above phenomena vary significantly. At a ligand concentration of 100 micrograms/ml, the induction of increased class II MHC expression, G1 entry, and S phase had minimal affinity thresholds of Ka approximately 0.2 to 2 x 10(6) M-1; approximately 7 x 10(6) M-1; and approximately 1 x 10(8) M-1, respectively. Pulsing studies revealed that whereas high affinity ligand was essential at later periods in the prolonged (greater than 24 h) signaling period that leads to S phase entry, mAb with significantly lower affinity were competent at signaling during the first 24 h. Because all but the lowest affinity ligand (Ka = 2 x 10(5) M-1) could effectively modulate mIgM, and furthermore, because B cells show a substantial increase in surface area during activation, it appears likely that one factor contributing to the higher affinity requirements for induction of late activation phenomena is a progressive decrease in the density of mIgM on the responsive B cells. These studies suggest that whereas only a small proportion of B cells, i.e., those with relatively high affinity for an antigenic epitope, will be triggered to clonally expand on encountering a paucivalent Ag in the absence of T cell help, a much wider spectrum of the B cell repertoire will be triggered to a state of partial activation. How the presence of ancillary T cells and cytokines may facilitate the full clonal expansion of these latter cells is discussed.
Mongini
P K
PK
Institute of Molecular Immunology, Hospital for Joint Diseases, New York, NY 10003.
Blessinger
C A
CA
Dalton
J P
JP
D000911:Antibodies, Monoclonal; D000915:Antibody Affinity / Q000276:immunology; D001402:B-Lymphocytes / Q000276:immunology*; D002454:Cell Differentiation; D016193:G1 Phase / Q000276:immunology; D006684:HLA-DR Antigens / Q000096:biosynthesis; D006801:Humans; D007140:Immunoglobulin Fab Fragments; D007075:Immunoglobulin M / Q000502:physiology*; D066298:In Vitro Techniques; D007700:Kinetics; D008213:Lymphocyte Activation / Q000276:immunology; D011940:Receptor Aggregation; D011947:Receptors, Antigen, B-Cell / Q000502:physiology*; D016192:Resting Phase, Cell Cycle / Q000276:immunology; D016196:S Phase / Q000276:immunology; D013601:T-Lymphocytes / Q000276:immunology; D013696:Temperature
D000911:Antibodies, Monoclonal; D006684:HLA-DR Antigens; D007140:Immunoglobulin Fab Fragments; D007075:Immunoglobulin M; D011947:Receptors, Antigen, B-Cell