A 25-year-old woman with a history of systemic lupus erythematosus presents with diffuse petechiae and fatigue. Physical examination demonstrates lymphadenopathy and splenomegaly Laboratory findings include normocytic anemia (hemoglobin =6.1 g/dL) and thrombocytopenia (30,000/mL). Which of the following is the most likely underlying mechanism in the development of thrombocytopenia in this patient?