Plasmid-Encoded Proinsulin Preserves C-Peptide While Specifically Reducing Proinsulin-Specific CD8+ T Cells in Type 1 Diabetes

A plasmid encoding proinsulin reduces the frequency of CD8+ T cells reactive to proinsulin while preserving C-peptide. Diabetes Trial Hits Its Mark Type 1 diabetes is an autoimmune disease where an inflammatory response destroys the insulin-producing cells of the pancreas. One way to block this response is through immunosuppression; however, it has proven difficult to target the specific autoreactive cells without suppressing the rest of the immune response. Now, Roep et al. demonstrate that an engineered plasmid that expresses proinsulin can preserve β cell function in type 1 diabetes patients. The authors randomized patients recently diagnosed with type 1 diabetes to receive various doses of either a proinsulin-expressing engineered plasmid or PBS vehicle. They observed no serious adverse events. The subjects in the experimental group had improved C-peptide levels—a readout of β cell function. The authors then examined the immune responses in these patients and found that there was a decrease in proinsulin-specific CD8+ T cells, but not unrelated CD8+ T cells. No difference was observed in cytokine production by CD4+ T cells. If these data hold true in larger studies, a plasmid encoding proinsulin could serve as a targeted means of immunosuppression for type 1 diabetes. In type 1 diabetes (T1D), there is an intense inflammatory response that destroys the β cells in the pancreatic islets of Langerhans, the site where insulin is produced and released. A therapy for T1D that targets the specific autoimmune response in this disease while leaving the remainder of the immune system intact, has long been sought. Proinsulin is a major target of the adaptive immune response in T1D. We hypothesized that an engineered DNA plasmid encoding proinsulin (BHT-3021) would preserve β cell function in T1D patients through reduction of insulin-specific CD8+ T cells. We studied 80 subjects over 18 years of age who were diagnosed with T1D within the past 5 years. Subjects were randomized 2:1 to receive intramuscular injections of BHT-3021 or BHT-placebo, weekly for 12 weeks, and then monitored for safety and immune responses in a blinded fashion. Four dose levels of BHT-3021 were evaluated: 0.3, 1.0, 3.0, and 6.0 mg. C-peptide was used both as an exploratory efficacy measure and as a safety measure. Islet-specific CD8+ T cell frequencies were assessed with multimers of monomeric human leukocyte antigen class I molecules loaded with peptides from pancreatic and unrelated antigens. No serious adverse events related to BHT-3021 were observed. C-peptide levels improved relative to placebo at all doses, at 1 mg at the 15-week time point (+19.5% BHT-3021 versus −8.8% BHT-placebo, P < 0.026). Proinsulin-reactive CD8+ T cells, but not T cells against unrelated islet or foreign molecules, declined in the BHT-3021 arm (P < 0.006). No significant changes were noted in interferon-γ, interleukin-4 (IL-4), or IL-10 production in CD4 T cells. Thus, we demonstrate that a plasmid encoding proinsulin reduces the frequency of CD8+ T cells reactive to proinsulin while preserving C-peptide over the course of dosing.