In this follow-up study, we evaluated the long-term effects of oral insulin on the development of type 1 diabetes and assessed the rate of progression to type 1 diabetes before and after oral insulin treatment was stopped.

Over the entire study and follow-up, individuals who did not develop type 1 diabetes (n = 198) had a significantly lower median ICA titer (80 vs. 160 Juvenile Diabetes Foundation Units, P < 0.0001), lower mean IAA titer (309.3 vs. 426.5 nU/mL, P = 0.02), lower proportion with ICA512 (45 vs. 60%, P = 0.004), higher mean first-phase insulin response (173.1 vs. 145.6 µU/mL, P = 0.002), higher mean C-peptide peak (5.7 vs. 5.1 ng/mL, P = 0.003), higher mean C-peptide AUC (530.1 vs. 470.7, P = 0.005) measured by OGTT, were more likely to be black or Hispanic (10.5 vs. 4.5%, P = 0.03), and were older at time of randomization in study (median age 11 vs. 9 years, P < 0.0001) compared with individuals who developed type 1 diabetes (n = 174).

Table 1 provides the baseline characteristics at the time of randomization into the DPT-1 study of the subjects eligible for the follow-up study by whether or not contact was achieved. Individuals contacted were more likely to be white (P < 0.0001) compared with those unable to be contacted.

Figure 2 shows the Kaplan-Meier curve from the start of the DPT-1 to the end of the follow-up for both the entire oral insulin population (Fig. 2A, n = 372) and subjects with a baseline confirmed IAA ≥80 nU/mL (Fig. 2B, n = 263). The overall median follow-up was 9.1 (Q1–Q3: 8.7–10.1) years. The annualized rate of type 1 diabetes development for the entire population was 7.4% per year in the oral insulin group and 8.2% in the placebo group (hazard ratio [HR] 1.125, 95% CI 0.837–1.511; P = 0.436). In subjects with a baseline confirmed IAA level ≥80 nU/mL, the annualized rate of type 1 diabetes development was 7.7% per year in the oral insulin group and 10.1% in the placebo group (HR 1.384, 95% CI 0.995–1.925; P = 0.052). Even after discontinuing oral insulin treatment, individuals with a confirmed IAA level ≥80 nU/mL who received oral insulin appeared to benefit overall by significantly delaying type 1 diabetes onset by 2.2 years.

Figure 3 breaks down the patient subgroups with confirmed IAAs ≥80 nU/mL into the following: time on (all subjects treated with oral insulin) and off (only subjects contacted through the follow-up) treatment for the oral insulin group (Fig. 3A) and time on (monitored on study receiving placebo) and off (only subjects contacted through follow-up and no longer monitored on study) treatment for the placebo group (Fig. 3B). There were 130 subjects who received oral insulin during the study, and 97 (75%) of those subjects were contacted in the follow-up; 133 subjects received placebo and 85 (64%) were contacted in the follow-up. The annualized rate of diabetes while on oral insulin was 6.2%; after treatment ended, the rate increased to 9.5%, a rate similar to the placebo group (HR 1.492, 95% CI 0.911–2.444; P = 0.110). Figure 3A clearly shows that the reduced rate of diabetes development depicted in Fig. 2 is only apparent during the time the subjects received oral insulin. During the time on oral insulin compared with after oral insulin treatment, type 1 diabetes onset was delayed by 3.9 years. After oral insulin ended, the median estimated survival for subjects treated was similar to placebo during the original trial (6.32 years for placebo vs. 6.94 years for oral insulin after oral insulin treatment ended). Figure 3B shows the proportion of the placebo group without diabetes while on study and off study. The annual type 1 diabetes rate on study was 10.4% and off study was 9.7% (HR 0.826, 95% CI 0.513–1.330; P = 0.431). The hazard rate remained relatively constant for subjects in the placebo group over the study and follow-up periods.

Figure 4A and B shows the proportion of subjects diabetes-free in the subgroup who did not have confirmed IAA ≥80 nU/mL for those who received oral insulin (Fig. 4A) or placebo (Fig. 4B). In Fig. 4A, the annualized type 1 diabetes rate for the time on oral insulin was 6.9 and 4.5% during the follow-up time after therapy was withdrawn (HR 0.817, 95% CI 0.330–2.022; P = 0.661). Figure 4B describes the proportion of subjects in the placebo group diabetes-free both during study and after study. The annualized rate was 2.7% during the study and 4.2% after study (HR 1.379, 95% CI 0.444–4.286; P = 0.577).

The mechanism through which oral insulin mediates the apparent treatment response is unclear, but tolerance induction through this route may operate via effects on the adaptive immune system (5). Several nonclinical studies of the response to oral antigen have identified the induction of T cells that are associated with, and can adoptively transfer, protection from autoimmune disease (6). These cells are typically antigen specific, are CD4⁺, secrete the immune suppressive cytokines interleukin (IL)-10 and transforming growth factor (TGF)-β (7,8), and have been termed T_H3 cells or regulatory T cells (T_regs) (9). In other settings in which antigen-induced tolerance was achieved via a mucosal route (intranasal), there is evidence of activation-induced cell death of responder T cells (10), a possibility that should be explored in future studies of oral antigen administration. Preliminary evidence has recently shown that these processes could be counterproductive, at least theoretically, if T_regs are also deleted or sequestered (M. von Herrath, personal communication) and that the balance of induction/deletion is strongly influenced by the dose (frequency), with more frequent dosing being more harmful (11–13). A further intriguing, and hitherto unexplained, finding of the oral insulin study has been the reliance of the therapeutic effect on the presence of high levels of IAAs. It is possible that the mechanistic pathway requires the induction of adaptive T_regs from cohorts of preexisting insulin-specific T cells, or perhaps that antigen presentation by insulin-specific B cells is important (14). In summary, the T-cell field has matured sufficiently to allow rational hypotheses as to the mechanistic effects of oral insulin to be proposed and examined experimentally.

In previous mouse model studies, such as those that influenced the initiation of the DPT-1 study (15–17), oral insulin therapy was successful in preventing type 1 diabetes onset; however, therapy was dose dependent, with a reduction in insulitis seen at lower doses (1 mg), disappearing at higher doses (5 mg) (18). Although the majority of human studies reported no beneficial effect (1,19,20), one study (21) that focused on evaluating time of intervention, antigen dosing, and administration interval reported that oral insulin given in low doses (1 mg) delays β-cell failure in new-onset patients >20 years of age. This double-blind placebo-controlled study randomized 191 subjects who were 5–60 years of age to receive 1 mg insulin dose, 10 mg insulin dose, or placebo. Ergun-Longmire et al. (21) demonstrated that age and duration of diabetes in combination with different immunological and metabolic markers have varying responsiveness to varying doses of oral insulin. The addition of the DPT-1 follow-up study finding that IAA titer is associated with drug therapy responsiveness will have a considerable impact on future human studies, such as those with antigen-specific immunotherapy, where understanding that variation in biomarker levels may have a large impact on immunotherapy success (22).