TNFSF13B and B-cell chronic lymphocytic leukemia: To test the functional effects of the tumor microenvironment model, peripheral blood mononuclear cells from CLL patients (PBMCs; here referred to as CLL cells) were co-cultured with fibroblasts expressing GFP-APRIL, GFP-BAFF and GFP-APRIL + CD40L for 24 h.