A 26-year-old man has noted lumps in his neck that have been enlarging for the past 6 months. On physical examination, he has a group of enlarged, nontender right cervical lymph nodes. A biopsy of one of the lymph nodes microscopically shows macrophages, lymphocytes, neutrophils, eosinophils, and a few plasma cells. There are scattered CD15+ large cells with multiple nuclei or a single nucleus with multiple nuclear lobes, each with a large inclusion-like nucleolus. What is the most likely cell of origin with infectious agent for these large cells?