The ability to expand tumour-infiltrating lymphocytes in vitro has been greatly enhanced by the use of antigen-independent mechanisms of immune cell costimulation. We have produced human, using the K562 cell line, and murine, using YAC-1 cells, artificial antigen presenting cells (aAPC) and demonstrate that these cell types stimulate murine lymphocyte populations in distinct ways. Using aAPC that have been transfected with CD137L (4-1BBL) and CD32 (FcRgammaII), as a means to bind anti-CD3 and anti-CD28 antibody, we found that CD4 cells preferentially expanded in vitro with K562 aAPC, while CD8 cells expanded with both K562 and YAC-1 aAPC. Co-stimulation mediated by CD137L on aAPC was superior to that mediated by anti-CD28 antibody. This was seen in both long and short-term expansion assays, and by the rapid induction of a CD8+ DX5+ population. DX5 serves, under these in vitro conditions, as a general marker for lymphocyte activation. In vivo, the superiority of CD137L was demonstrated by the induction of T helper 1 effectors seen in freshly isolated splenocytes from mice immunized with CD137L-expressing neuroblastoma tumour vaccines. The ability to stimulate a strong CD8 CTL response in vivo correlated with the induction of a DX5+ cell population in splenocytes with a memory-effector phenotype. The presence of this unique DX5+ cell population, phenotypically distinct with regards to CD69 and CD62L expression from DX5+ cells induced by aAPC in vitro, may be associated with the ability of CD137L to induce strong anti-tumour immunity.