The Relationship Between Bacterial Endotoxin and Human B Cell-Activating Factor

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Abstract
The monokine, B cell-activating factor (BAF), has the property of stimulating murine B lymphocytes in the absence of T lymphocytes to make a humoral response in vitro to erythrocyte antigens. The factor is secreted by fresh human monocytes after stimulation with as little as 10-11 g/ml of bacterial endotoxin. It had previously been shown that commercial preparations of phytohemagglutinin stimulated BAF release, but subsequent analysis of this phenomenon showed that the BAF-inducing activity of the phytohemagglutinin preparations is resistant to heating (which reduces T cell mitogenicity) and is sensitive to the LPS inhibitor polymyxin B. Therefore, the BAF-releasing activity of the PHA preparations is probably due to contaminating LPS. Once it was realized that BAF release had only been observed in the presence of LPS, the question was raised whether BAF was a derivative of the added LPS. This seems unlikely since 1) BAF and LPS can be separated by ion exchange chromatography, 2) relative to its activity in the Limulus lysate test, BAF is at least 105 times more active than LPS as a B cell stimulator, 3) BAF is a poor mitogen compared to LPS, 4) BAF, but not LPS, loses activity in nonsupportive media, 5) the C3H/HeJ mouse responds to BAF but not LPS, and 6) polymyxin B inhibits LPS but not BAF. Finally, BAF is not a derivative of lipid A protein since lipid A protein (which resembles BAF by criteria 5 and 6) is not necessary to stimulate BAF secretion by monocytes.