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HA22 is an immunotoxin composed of an anti-CD22 variable fragment linked to a 38 kDa truncated protein derived from Pseudomonas exotoxin A. The mechanisms of cytotoxicity and resistance of HA22 against Acute Lymphoblastic Leukaemia (ALL) and Burkitt’s lymphoma were studied. Using a bone marrow mesenchymal cell culture assay to support ALL cell viability, I? investigated the in vitro cytotoxicity of HA22 against ALL blasts from newly diagnosed and relapsed patients. There was interpatient variability in sensitivity to HA22. There was no significant difference in HA22 sensitivity between diagnosis and relapse samples but peripheral blood ALL blasts were more sensitive to HA22 than those from bone marrow. The mechanisms of resistance to HA22 were studied, using cell lines as a model. The number of CD22 sites/ cell and the rates of immunotoxin internalisation did not affect HA22 cytotoxicity. HA22 mutants with resistance to lysosomal degradation and enhanced targeting to the endoplasmic reticulum had improved cytotoxicity. The role of apoptosis pathways proteins in HA22-mediated cell death was studied. Their role is complex but raised levels of the anti-apoptotic pathway protein Bcl-2 were found in the most resistant NALM6 cell line. Penetration of HA22 into Burkitt’s lymphoma masses was studied using a flow cytometric based method. HA22 rapidly penetrated into the lymphoma masses, however a barrier to further uptake is present which could not be overcome by the addition of adriamycin or taxol in the murine xenograft model. The ability of Acute Myeloid Leukaemia (AML) blasts to create an immunosuppressive niche was investigated using a cell line model and primary patient samples. AML blasts suppress T cell proliferation through altered arginine metabolism, dependent on the enzymes arginase II and iNOS. Small molecule inhibitors to arginase and iNOS restored T cell proliferation in vitro. AML further enhances its immunosuppressive niche by transforming surrounding monocytes into an M2-immunosuppressive phenotype, in an arginase dependent manner. The immunomodulatory protein Serum Amyloid A (SAA) was secreted by AML blasts, and leads to AML chemotaxis, IL-1production, and release of S100A9 protein. Finally, invariant Natural Killer T cells (iNKT) were shown to be cytotoxic to some AML blasts, in the presence of Galactosylceramide, and thus able to restore T cell proliferation. The results provide a strong rationale for the clinical testing of these novel immunotherapeutic and immunomodulatory strategies in patients with haematological malignancies.

No description available.