Insulitis and lymphoid structures in the islets of Langerhans of a 66-year-old patient with long-standing type 1 diabetes.

Insulitis is a characteristic inflammatory lesion consisting of immune cell infiltrates around and within the pancreatic islets of patients with recent-onset type 1 diabetes (T1D). The infiltration is typically mild, both in terms of the number of infiltrating cells and the number of islets affected. Here, we present an unusual histopathological case study of a 66-year-old female patient with long-standing T1D, insulitis, and islet-associated lymphoid tissue. Most islets in the head of the pancreas of this patient were insulin-deficient, whereas the islets in the tail appeared normal. Insulitis was present in 0.84% of the insulin-containing islets and three islets had large lymphocytic infiltrates resembling tertiary lymphoid structures (TLS). Of note, this is the first description of potential TLS in the endocrine pancreas of a patient with T1D. Their association with a marked residual beta cell mass is of interest and may hint at new insights into disease progression and regulation of autoimmunity.