Fig. 1

CD26/DPP IV has enzymatic and costimulatory activities. The enzymatic activity of DPP IV cleaves peptides at their penultimate position, and the costimulatory activity of CD26 signals activation and proliferation of T cells (A). Upon skin transplantation from BALB/c into CD26k. o. mice on a C57BL/6 background, reduced levels of IgG1 antibodies were found. The number and function of CD4⁺ and CD8⁺ cells were decreased and reduced levels of pro-inflammatory cytokines such as IL-2, IFN-γ, and IL-6 were found. By contrast, IL-10 levels were enhanced and the number of regulatory T cells was increased. All of these findings lead to a delay in allograft rejection (B)