B Cell Growth Factor Distinct

The importance of murine monokines and lymphokines in T cell immune responses has been emphasized by the development o f T cell cloning technology (1) and the identification of cloned tumor lines that respond to or secrete these factors (2, 3). Such studies have shown that the monokine interleukin 1 (IL-1) 1 induces certain T cells to secrete interleukin 2 (IL-2), and that this lymphokine can maintain continuous in vitro proliferation of some T cell subsets. In contrast, the interaction of monokines and lymphokines with B cells is poorly understood, part ly because of the lack of assays for B cell function in which contaminat ing accessory cells capable of mediat ing secondary effects have been excluded. We have recently described a procedure for the long-term culture of normal mouse B lymphocytes in which induced supernatants from a T cell hybr idoma sustain slow proliferation of l ipopolysaccharide-activated B cells for periods of up to 12 mo (4). This procedure implied the existence of a T cellderived growth factor that interacts with activated B lymphocytes to maintain proliferation. Here we identify such a factor in induced supernatants from the mouse t hym om a EL4. This material is distinct from previously recognized factors, including IL-2.