Introduction: In the treatment of acute leukemia, much of the therapeutic effect of allogeneic stem cell transplantation is centered on engraftment of donor cells into the host, with a resultant graft versus leukemia effect. Work previously done by our group and others has shown the infusion of haploidentical stem cells in patients with acute leukemia results in a cytokine storm and exhibits an anti-tumor effect in the absence of engraftment. Whether the underlying clinical response was due to this cytokine storm or secondary to host or donor effector cell interactions was not determined. We present in vitro data collected from leukemia patients in which patient CD3+ cells are stimulated with randomly selected unrelated normal donor peripheral blood mononuclear cells (PBMC). The stimulated patient CD3+ cells are then collected and tested for their ability to lyse leukemia cells from the patient.

Methods: New leukemia patients were identified through standard pathologic diagnostic criteria. Patient samples were collected and separated into CD3+ and CD3- fractions. Patient CD3+ cells were then placed in a mixed lymphocyte culture and stimulated with mitomycin C treated unrelated normal donor PBMC. Cell proliferation was measured on Day 5. On Day 7, the stimulated patient CD3+ cells were collected and then cultured with Chromium 51 labeled CD3- cells, the bulk of which were leukemic blast cells. Cytolytic activity was measured through Cr51 release assays. Cell lysis was recorded as lytic units/million cells (LU/106), which is inversely related to the effector to target ratio at which 30% cell lysis occurs. Patient demographic data including sex and age in addition to leukemia type, cytogenetics, and molecular markers were recorded.

Results: 10 total patients were enrolled in the study; 8 AML, 1 CML, and 1 CMML. 9 patients underwent ex vivo CD3+ stimulation with 2 separate donor PBMC samples while 1 sample underwent only 1 donor stimulation for 19 total assays. The average white blood cell count was 57,640 cells/uL (range 7,100-336,500 cells/ul) with 4-80% peripheral blasts. 7 out of 19 (37%) assays showed leukemia cell cytolytic activity greater than 1 lytic unit. All 7 successful assays were derived from patients with AML (Table 1).

Conclusions: Patient CD3+ cells stimulated with random unrelated donor PBMC are able to generate a cytolytic anti-leukemia response in a donor dependant fashion. This cytolytic activity appears to be unrelated to host lymphocyte proliferation upon donor antigenic stimulation. Interestingly, of the seven assays with cytolytic activity greater than one lytic unit, four were performed in the two patients with known FLT3 ITD positive leukemia, which has a notoriously poor prognosis. Further studies to elucidate a mechanism of action for cellular therapy are warranted.

Citation Format: John L. Reagan, Loren D. Fast, Martha Nevola, Peter J. Quesenberry. Random unrelated donor peripheral blood mononuclear cells (PBMC) stimulate cytolytic host T cell activity against leukemia. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1252. doi:10.1158/1538-7445.AM2013-1252