A 42-year-old woman from rural Maharashtra presents with a 3-month history of polyuria, polydipsia, and a 4 kg weight loss. She reports no significant family history of diabetes. On examination, she is lean (BMI 19 kg/m²) and appears malnourished. Fasting blood glucose is 280 mg/dL, and HbA1c is 11.2%. C-peptide level is 0.8 ng/mL (normal 0.8–3.1). Serum ketones are positive. Islet cell antibodies (ICA) and glutamic acid decarboxylase (GAD) antibodies are both positive. What is the most likely pathological mechanism underlying her diabetes?