Type 1 diabetes (T1D) is due to a loss of immune tolerance to islet antigens, such as glutamic acid decarboxylase 65 (GAD65), for which islet transplantation is a promising therapy. Therefore, the generation of tolerance aiming at both alloantigen and GAD65 will help therapeutic intervention greatly in T1D. In this study, we tested the effect of programmed death-1 ligands (PD-L1)-transfected dendritic cells (DC) loaded with GAD65 on the alloresponse and GAD65-reactive lymphocyte response. The DC2.4 cell line was transfected with PD-L1 and co-cultured with GAD65. BALB-c mice were primed, respectively, by intraperitoneal injection with GAD65, PD-L1-transfected- or non-transfected DC (PD-L1/DC or DC), and PD-L1-transfected- or non-transfected DC loaded with GAD65 (PD-L1/DC/GAD65 or DC/GAD65). Splenocytes of treated mice were isolated and restimulated in vitro with GAD65 or the various DC populations above being used as stimulators, respectively. In the mixed lymphocyte reaction, DC/GAD65 were able to stimulate both allogeneic and GAD65-reactive lymphocytes. However, PD-L1/DC/GAD65 were poorer than DC/GAD65 at activating the GAD65-reactive lymphocyte response. Further, although PD-L1/DC could inhibit the alloresponse, PD-L1/DC/GAD65 were more effective at down-regulating the GAD65-reactive lymphocyte response. More importantly, PD-L1/DC/GAD65-primed lymphocytes exhibited the weakest proliferation when again restimulated in vitro by PD-L1/DC/GAD65. Additionally, PD-L1/DC/GAD65 down-regulated interferon-gamma and up-regulated interleukin-10 production by activated lymphocytes. Therefore, combined stimulation in vivo and in vitro by PD-L1/DC/GAD65 could inhibit both the alloresponse and the GAD65-reactive lymphocyte response, which may contribute to controlling diabetes and islet transplant rejection.