Contrasting effects of interleukin 4 (IL-4) on the in vitro proliferation of human B cells is due to the presence of B cell subsets in different activation states.

Human tonsillar B lymphocytes proliferate in vitro after activation with immobilized anti-IgM antibodies in combination with interleukin 2 (IL-2) or interleukin 4 (IL-4). In addition to its proliferative effects, IL-4 is also able to inhibit B cell proliferation. An explanation for these different actions of the same protein is not as yet available. The susceptibility to the inhibitory action of IL-4 emerges at late time points after in vitro activation of the B cells, whereas IL-4 shows its growth promoting action during early stages of culture, indicating that activated IL-2 responsive B cells are the targets for IL-4 dependent inhibition. Freshly isolated tonsillar B lymphocytes also appear to be susceptible to IL-4 dependent suppression. Isotonic Percoll gradient centrifugation, that has been used by several groups as a standard procedure to isolate high density, resting used by several groups as a standard procedure to isolate high density, resting cells from tonsil B lymphocytes, does not result in separation of B cells with different in vitro proliferative response to IL-2 and IL-4. However, partial separation of cells with "resting" and "activated" phenotypes can be achieved by centrifugation in slightly hypotonic Percoll gradients. High-density fractions contain cells with a "resting" phenotype which after in vitro activation proliferate with IL-4 but not with IL-2. "Activated" B cells accumulate in the low-density fractions as concluded from the increased expression of transferrin receptors and HLA-DR molecules on these cells. However, these cells no longer proliferate in vitro with immobilized anti-IgM antibodies, possibly due to damage caused by the hypotonic treatment during centrifugation. Our data indicate that resting B cells when activated in vitro with immobilized anti-IgM antibodies proliferate primarily with IL-4 but not with IL-2. In contrast, about 15 hours after activation the cells become responsive to IL-2. At that stage, IL-4 becomes an inhibitory factor.