To investigate the in vitro regulation of IgA subclass synthesis, peripheral blood lymphocytes from healthy adults were cultured with the polyclonal B cell activator, pokeweed mitogen. Although 50% of the IgA plasma cells from a 7-d culture were positive for cytoplasmic IgA1 and 50% were positive for IgA2, less than 10% of the IgA released into the culture supernatant was IgA2. This discrepancy could not be explained by failure of the assay to detect in vitro synthesized IgA2, selective loss or destruction of IgA2 in culture media, delayed release of IgA2, or failure of IgA2 plasma cells to produce J chain. The results suggest that additional signals may be required for the differentiation of plasma cells into immunoglobulin-secreting cells.

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