The anti programmed cell death-1 (PD-1) antibodies pembrolizumab and nivolumab 
have been recently licensed by the Food and Drug Administration for the 
treatment of advanced melanoma. Immune checkpoint inhibitors such as these can 
induce endocrine adverse events but autoimmune diabetes has not been described 
to date. However, there is a strong preclinical rationale that supports this 
autoimmune toxicity. We describe for the first time the case of an adult patient 
who developed autoimmune diabetes likely as a consequence of PD-1 inhibition 
with pembrolizumab. The presence of high serum titres of anti-glutamic acid 
decarboxylase antibodies together with a suggestive clinical presentation, age 
of the patient and preclinical data strongly support an autoimmune aetiology of 
the diabetes. Moreover, the patient was found to have a well-known high-risk 
human leucocyte antigen type for the development of type 1 diabetes in children, 
so the PD-1 inhibition is very likely to have triggered the autoimmune 
phenomenon. Our case suggests that insulin-dependent diabetes might be a rare 
but important anti-PD-1 immune-related adverse event.