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Non-activated follicular lymphoma (FL) cells do not function as effective antigen-presenting cells in vivo. CD40 activation of FL cells up-regulates critical adhesion and co-stimulatory molecules, thereby inducing lymphoma-specific cytotoxic T cells in vitro. However, other evidence suggests that CD70 is another important co-stimulatory molecule involved in antigen dependent T-cell activation. Here, we showed that freshly isolated FL cells from eight diagnostic biopsies expressed intermediate to high levels of CD27, whereas only insignificant levels of CD70 were detected on their cell surface. Together with the low to intermediate expression of B7, these findings help to explain the poor antigen-presenting capacity of non-activated FL cells. Activation of FL cells by CD27 and CD40 induced a significantly higher alloantigen T-cell response than CD40 alone, whereas CD27 activation induced only a mostly insignificant T-cell proliferation. Both CD40 and CD27 + CD40 activation resulted in a high up-regulation of CD70 and B7 molecules, whereas CD27 activation up-regulated CD70 but not B7 expression on the surface of FL cells. Thus, expression of both CD70 and B7 co-stimulatory molecules appears to be essential for an efficient T-cell-mediated immune response. However, the molecules responsible for the significantly higher alloantigen T-cell response to FL cells activated through CD27 and CD40 remain to be identified. In summary, we conclude that CD27+CD70 represents another co-stimulatory pathway involved in T-cell-mediated immune responses to FL cells. Our findings suggest that several co-stimulatory pathways exist and should be taken into consideration to optimize antigen presentation for the generation of lymphoma-directed cytotoxic T cells for adoptive immunotherapy. [ABSTRACT FROM AUTHOR]