T Helper Cell Control of B Cell Development and Isotype Expression

The supernatant of the T helper cell clone 52.3 (52.3-SN) was shown to induce polyclonal activation of resting B cells. 52.3-SN acts on most small B cells and through the allogeneic barrier. This supernatant induces cell size increase, RNA and DNA synthesis, and appearance of interleukin-2 and transferrin receptor. These results are interpreted as indicating the existence of a B Cell Activating Factor (BCAF) acting on resting B cells in an MHC-unrestricted way.

TH cells can be obtained in an intermediate state of activation where they secrete lymphokines leading to B cell proliferation and not the biological activities leading to plasmocyte development. TH cell clones can induce sIgG^? and sIgA^? unprimed B cells to switch and express all classes and subclasses of immunoglobulin. The bulk of the response consists of IgM. Among the non IgM isotypes, IgG₁ and IgA predominate. The supernatants prepared from TH cells reproduce these effects.