To make BKV-specific T cells (Viralym-B cells), small pieces of protein called peptides that come from BKV were mixed with blood cells from healthy donors. These peptides train a kind of white blood cell called T cells to recognize and kill cells that are infected with BKV. These T cells were then grown in special growth factors in special flasks in the lab. Once we made sufficient numbers of cells, we tested them to make sure they recognized cells infected by BK virus, and then we froze them.