Capacity of B cells to function as stimulators of a primary mixed leukocyte reaction.

The capacity of B cells to serve as stimulator cells for a primary mixed leukocyte reaction (MLR) was evaluated. Percoll-fractionated B cells were stimulated with lipopolysaccharide and dextran sulfate (L/D) or a B cell stimulatory factor (BSF-1)-containing culture supernatant, and then were fixed before being used as stimulator cells to more precisely define the state of activation associated with MLR stimulatory capacity. It was found that unstimulated B cells or B cells stimulated for 1 day with L/D or BSF-1 were incapable of initiating a primary MLR, whereas B cells incubated for 3 days in L/D were potent stimulators. The differential activity of 1 day L/D- and BSF-1-activated B cells compared with 3 day L/D-activated B cells was not related to the amount of the relevant MHC class I or class II alloantigens on these cell populations, because all three groups had large increments in MHC class II expression in the following order: BSF-1 greater than 3 day L/D greater than 1 day L/D, and had little difference in MHC class I expression. Also, all three populations were capable of stimulating both MHC class I- and class II-specific T cell hybrids. It was concluded that the capacity of 3 day L/D-activated cells to stimulate a primary MLR was due to the elaboration of necessary co-stimulator molecules. We evaluated whether interleukin 1 (IL 1) was the co-stimulator involved. That this was not the case was indicated by two findings. First, 3 day-activated L/D cells failed to express IL 1 activity as measured by a highly sensitive IL 1 assay that utilizes the T cell line D10.G4.1. Second, recombinant IL 1 added to MLR cultures containing 1 day L/D- or BSF-1 activated B cells failed to function as a co-stimulator. In contrast, the phorbol ester PMA was a potent co-stimulator in this system. We conclude from these experiments that appropriately activated B cells can function as stimulators of a primary MLR, and that they elaborate critical co-stimulator molecules, distinct from IL 1, that enable them to function in this regard.