Specific antigens inhibit plasmacytoma cells bearing phosphorylcholine-binding myeloma proteins

Cross-linking of surface immunoglobulins and endocytosis of antigen are not sufficient to suppress antibody production of two hybridoma cell lines

The two cell lines Lev 1-3 and AS3, which secrete monoclonal anti-levan antibody and express surface membrane immunoglobulins that are capable of binding the levan, were cultured for 24 h or 48 h with levan of various molecular weights (5 x 10(4), 2 x 10(5) or 2 x 10(7) daltons). The production of antibody was measured by a plaque-forming cell assay for AS3 cells and by biosynthetic labelling for Lev 1-3 cells. Surface immunoglobulins were detected by a rosette-forming cell assay. By using immunoenzymatic procedures, we observed that levan of 5 x 10(4) and 2 x 10(5) daltons was intensely endocytosed by the cells and localized mainly in the cellular center, whereas levan of 2 x 10(7) daltons remained mostly associated with the plasma membrane and formed surface aggregates. After transfer of cells in levan-free medium, it was shown that internalized levan could persist several days inside the cells without being degraded. Viability, growth, antibody synthesis and secretion were not modified in cells cultured with levan, whatever the molecular weight of the antigen used. Our results show that cross-linking of surface immunoglobulins by multivalent antigen or endocytosis of antigen do not appear to be sufficient to induce a suppression of antibody production or membrane immunoglobulin expression.

Mechanism of effector-cell blockade. I. Antigen-induced suppression of Ig synthesis in a hybridoma cell line, and correlation with cell-associated antigen

A mouse hybridoma cell line, FluIgM-1, which secretes IgM specific for the hapten fluorescein (FLU) was developed to allow detailed analysis of the effector-cell blockade (ECB) phenomenon, in which contact of antibody-forming cells (AFC) with specific antigen results in marked reduction of antibody secretion. Treatment of hybridoma cells with highly substituted FLU conjugates (e.g., Flu20gelatin) resulted in inhibition of plaque formation. The data indicated close parallels with the ECB of normal spleen AFC, both in speed of onset and the dose of antigen required. The inhibition of antibody secretion was confirmed with a biosynthetic-labeling procedure which demonstrated that this was a result of reduced Ig synthesis. The inhibitory effect appeared to be confined to antibody synthesis, in the total protein synthesis, DNA synthesis, and cell-doubling times were unaffected. The association of FLU conjugates with the cells during and following ECB was studied directly using fluorescence microscopy and the fluorescence-activated cell sorter. These experiments showed that FLU conjugates capable of causing blockade aggregated on the cell surface, that the clearance of cell-associated antigen correlated with recovery from ECB, and that at all times when cell associated antigen was detectable, a portion remained bound to the cell surface and was susceptible to enzymatic removal. The latter observations supported previous findings suggesting that ECB was mediated by extracellular antigen. The direct observation of aggregates of antigen on the surface of blockaded cells is consistent with a mechanism involving cross-linking of Ig receptors. Finally, Fc receptors were not present on hybridoma cells, excluding their involvement in induction of ECB.