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

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Abstract
A mouse hybridoma cell line, FluIgM-1, which secretes IgM specific for the hapten fluorescein (FLU) was developed [by fusing mouse spleen cells with mouse myeloma NS-1 cells] 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. Close parallels with the ECB of normal spleen AFC, in speed of onset and the dose of antigen required were indicated. 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; 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. FLU conjugates capable of causing blockade aggregated on the cell surface. The clearance of cell-associated antigen correlated with recovery from ECB; 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. Fc receptors were not present on hybridoma cells, excluding their involvement in induction of ECB.