Human interferon-gamma acts as a B cell growth factor in the anti-IgM antibody co-stimulatory assay but has no direct B cell differentiation activity.

In this study it is illustrated that recombinant human interferon-gamma (IFN-gamma) acts as a B cell growth factor (BCGF) in the anti-IgM antibody co-stimulatory assay. A monoclonal antibody that specifically inhibits the biological activities of IFN-gamma blocks its BCGF activity supporting the specificity of the IFN-gamma effect. Various IFN-gamma obtained from different sources displayed the same BCGF activity. Nonactivated B lymphocytes do not proliferate in response to IFN-gamma. IFN-gamma acts directly on B cells because highly purified B cells obtained after standard purification procedures coupled to cell sorting could still proliferate in response to IFN-gamma. Blood B lymphocytes were found to be more sensitive to the BCGF activity of IFN-gamma than B cells obtained from spleens or tonsils. The IFN-gamma-induced proliferation of B cells was short lasting when compared with that of recombinant IL 2 or BCGF containing T cell clone supernatants. B cells preactivated with either Staphylococcus aureus strain Cowan I (SAC) or optimal concentrations of anti-IgM antibodies coupled to beads did not proliferate in response to IFN-gamma, whereas they proliferated in response to IL 2 or T cell clone supernatants. IFN-gamma did not stimulate nor inhibit the proliferative response of human B lymphocytes stimulated with optimal concentrations of anti-IgM antibodies or SAC. Additionally none of the different IFN-gamma tested had B cell differentiation factor activity in the standard SAC assay. These results indicate that IFN-gamma sensitizes B cells to suboptimal mitogenic concentrations of anti-IgM antibody.