"The comprehensive testing plan that we have put in place to study patients' blood and bone marrow while they're undergoing this therapy is allowing us to be able to follow how the T cells are behaving in patients in real time, and guides us to be able to design more detailed and specific experiments to answer critical questions that come up from our studies," Kalos said.The CTL019 therapy eliminates all B cells that carry the CD19 cell receptor: healthy cells as well as those with leukemia. Patients can live without B cells, although they require regular replacement infusions of immunoglobulin, which can be given at home, to perform the immune function normally provided by B cells. The research team continues to refine their approach using this new technology and explore reasons why some patients may not respond to the therapy or may experience a recurrence of their disease. Grupp said the appearance of the CD19-negative leukemia cells in the second child may have resulted from her prior treatments. Unlike Emily, the second patient had received an umbilical cord cell transplant from a matched donor, so her engineered T cells were derived from her donor (transplanted) cells, with no additional side effects. Oncologists had previously treated her with blinatumomab, a monoclonal antibody, in hopes of fighting the cancer. The prior treatments may have selectively favored a population of CD19-negative T cells. "The emergence of tumor cells that no longer contain the target protein suggests that in particular patients with high-risk ALL, we may need to broaden the treatment to include additional T cells that may go after additional targets," added Grupp. "However, the initial results with this immune-based approach are encouraging, and may later even be developed into treatments for other types of cancer." Funding from the National Institutes of Health (grants 1RO1 CA165206, R01 CA102646 and R01 CA116660), the Leukemia and Lymphoma Society, and the Alliance for Cancer Gene Therapy supported this study.