title: Mitogenicity of anti-Thy-1 monoclonal antibodies attributable to an Fc-dependent mechanism.
abstract: We analyzed the mechanism by which certain anti-Thy-1 monoclonal antibodies (mAb) activate T cells directly without additional stimuli. Using a panel of rat anti-Thy-1 antibodies which included more than 30 IgG2c mAb, we found that only the IgG2c isotype was able to induce a strong proliferative response in both resting T cells and a T cell lymphoma, suggesting that this form of T cell activation is isotype restricted and might be a consequence of a unique physico-chemical property of the IgG2c heavy chain. Results from surface distribution studies of Thy-1 molecules, following specific interactions with anti-Thy-1 antibodies of different isotypes, again showed that only IgG2c mAb formed Thy-1 aggregates of high valence on the surface of a T cell lymphoma, and such clustering always evoked a biological response. This led us to propose that IgG2c mAb have the inherent tendency to self-associate, probably through homophilic Fc-Fc contacts, and that this feature renders anti-Thy-1 mAb mitogenic. To prove this, we set up cross-inhibition studies with randomly selected mitogenic (IgG2c) and nonmitogenic (IgG2b) anti-Thy-1 mAb. The results clearly demonstrated that IgG2c antibodies enhance their own binding, analogous to the new form of antibody binding that was recently demonstrated between murine IgG3 mAb and a multivalent antigen. Confirmation of this was also provided by IgG2c-derived F(ab')2 fragments, which were unable to cause proliferation. Furthermore, masking the Fc part of cell-bound IgG2c mAb with a monomeric and thus non-aggregating IgG-binding protein A-derived fragment cancelled their mitogenic ability. Finally, induction of T cell proliferation appeared to be independent of cross-linking via Fc gamma R. The results support a model in which noncovalent intermolecular homophilic contacts of the Fc regions of the IgG2c isotype bring about effective aggregation of Thy-1 molecules, thereby stimulating the mitotic apparatus of the cell.
authors.affiliation: GSF-Forschungszentrum für Umwelt und Gesundheit GmbH, Institut für Immunologie, Munich, FRG.
authors.forename: U
authors.initials: U
authors.lastname: Kummer
authors.forename: J
authors.initials: J
authors.lastname: Haunschild
authors.forename: G
authors.initials: G
authors.lastname: Reisbach
authors.forename: H J
authors.initials: HJ
authors.lastname: Delecluse
authors.forename: S
authors.initials: S
authors.lastname: Thierfelder
mesh_terms: D000818:Animals; D000911:Antibodies, Monoclonal; D000954:Antigens, Surface; D002460:Cell Line; D002462:Cell Membrane / Q000276:immunology; D006825:Hybridomas / Q000276:immunology; D007141:Immunoglobulin Fc Fragments; D007074:Immunoglobulin G; D007132:Immunoglobulin Isotypes; D008213:Lymphocyte Activation; D008562:Membrane Glycoproteins; D051379:Mice; D008934:Mitogens; D051381:Rats; D013601:T-Lymphocytes / Q000276:immunology*; D018800:Thy-1 Antigens
chemical_list: D000911:Antibodies, Monoclonal; D000954:Antigens, Surface; D007141:Immunoglobulin Fc Fragments; D007074:Immunoglobulin G; D007132:Immunoglobulin Isotypes; D008562:Membrane Glycoproteins; D008934:Mitogens; D018800:Thy-1 Antigens