Abstract B22: Antitumor responses driven by alloreactivity.

Previous work done by our group had found that infusion of G-CSF mobilized haploidentical donor white blood cells at a dose of 1 – 2 x 108 CD3+ cells/kg into refractory cancer patients after 100 cGy total body irradiation resulted in a number of complete and partial responses in those patients with hematological malignancies. These responses occurred despite elimination of the donor cells by 2 weeks after infusion. The effector mechanism(s) responsible for these anti-cancer responses were not determined although it was noted that there was a cytokine storm appearing within a day following infusion of the cells. Alloreactive responses could be driving the anti-cancer responses using a variety of mechanisms. One possible mechanism is that the alloreactive response overrides inhibitory mechanisms present in the cancer patient allowing the existing cancer reactive effector cells to become activated and lyse cancer cells. Alternatively the alloreactive cytolytic T cells could be cross-reacting with the antigens expressed on the tumor cells. To test these possibilities, peripheral blood mononuclear cells (PBMNC) or the CD3+ T cells isolated from the PBMNC of newly diagnosed leukemic patients were mixed with mitomycin C treated fully allogeneic stimulator PBMNC (from 1 or 2 different normal controls chosen at random) or the CD3- cells from the leukemic patients. Their ability to proliferate in response to the allogeneic stimulation was tested on day five and their ability to lyse autologous leukemia cells after stimulation was tested on day seven. Ten total patients (8 AML, 1 CML, and 1 CMML) were enrolled in the study. The average white blood cell count was 57,640 cells/uL (range 7,100 – 336,500 cells/uL) with 4 -80% peripheral blasts. The results indicated that in 16/20 combinations, leukemic T cells were able to proliferate in response to allogeneic stimulators, however anti-cancer cytolytic activity was only detected in 9/19 evaluable combinations. These results demonstrated that alloreactive responses are able to induce anti-leukemic cytolytic responses by T cells from a subset of leukemic patients. One explanation for the inability of some combinations to generate anti-leukemic cytolytic activity could be the presence of inhibitory mechanisms which could not be overcome by the alloreactive responses. Immunophenotypic analysis of the leukemic T cells did reveal an increased number of T regulatory cells and increases in the expression of inhibitory receptors on the lymphocytes from some of the patients. Alternatively, it may be that the leukemic T cells had not been stimulated with the appropriate allogeneic antigens. An allogeneic stimulator pool comprised of cells from ten different normal donors was prepared and will be used to test whether increasing the number of allogeneic disparities in culture would enhance the ability of the leukemic T cells to generate anti-leukemic responses. These studies have suggested that cancer-reactive cells can be generated from leukemic patient T cells using alloreactive responses. To further test the role of these effector cells and identify additional effector cells, blood samples will be obtained from patients enrolled in the clinical protocol which is in the process of being restarted, having received FDA approval. Clearer definition and characterization of the anti-cancer effector cells should permit alterations to the protocol that would lead to enhanced anti-cancer responses in an increased number of refractory cancer patients. Citation Format: Loren D. Fast, John Reagan, Martha Nevola, Peter Quesenberry. Antitumor responses driven by alloreactivity. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology: Multidisciplinary Science Driving Basic and Clinical Advances; Dec 2-5, 2012; Miami, FL. Philadelphia (PA): AACR; Cancer Res 2013;73(1 Suppl):Abstract nr B22.