T cell-induced suppression of IgG2ab expression in vivo leads to a large reduction of C gamma 2ab mRNA levels

In the T cell-induced suppression of IgG2ab expression, the level at which B cells are blocked in their development to IgG2ab-producing plasma cells was investigated. Although IgG2ab+ lymphocytes were barely detected in normal and IgG2ab-suppressed mice, intracellular IgG2ab was only detected in crude cell extracts from normal mice. B lymphocytes producing IgG2ab were revealed in T cell-depleted splenocytes from normal mice (86 +/- 15/10(6) cells), whereas corresponding cell preparations from IgG2ab-suppressed mice were completely free of such lymphocytes. However, in vitro stimulation of cell preparations from both normal and IgG2ab-suppressed mice with LPS plus rIFN-gamma resulted in IgG2ab production. Accounting for differences in spleen size between the two types of mice, these stimuli induced comparable cell proliferation and numbers of IgG2ab-producing lymphocytes. In addition, the level of IgG2ab production per cell was similar in the two types of stimulated cells. This demonstrates that normal and IgG2ab-suppressed mice have the same potential to generate IgG2ab-producing cells. By using a sensitive and specific RNase protection assay, C gamma 2ab transcripts were detected in total RNA preparations from IgG2ab-suppressed mice. The levels of C gamma 2ab gene expression in spleen were much lower (between 150 and 400 times less) in IgG2ab-suppressed mice than in normal mice. Taken together, our data suggest that B lymphocytes committed to IgG2ab production represent the target of CD8+ T cells, which we have previously shown to be required for suppression. The target B cells are very efficiently and rapidly silenced, as demonstrated by the absence of detectable serum IgG2ab and corresponding low levels of C gamma 2ab mRNA.