ABSTRACT
BACKGROUND AND OBJECTIVES: In this study, we tested whether transgenic murine acute promyelocytic leukemia (APL) cells can be recognized and cleared by adaptive immune responses and/or vaccination strategies. DESIGN AND METHODS: Immunocompetent and SCID mice were examined for their ability to survive a challenge of APL cells. We also vaccinated immunocompetent mice with DNA vaccines encoding various portions of a bcr-1 PML-RARa fusion protein. RESULTS: In genetically compatible, immunocompetent animals, APL cells routinely engrafted and caused lethal leukemia; however, immunodeficient SCID mice required approximately 100-fold fewer APL cells to cause lethal disease. Massive doses of APL cells were efficiently eliminated in allogeneic recipients. Vaccination with a plasmid expressing a human PML-RARa cDNA conferred protection against leukemic cells in vivo; mice vaccinated with the human PML portion of the fusion gene demonstrated similar protection. Analysis of 10-mer peptides spanning the t(15;17) translocation-associated PML-RARa fusion breakpoint suggested that they were not involved in the generation of immune responses. INTERPRETATION AND CONCLUSIONS: These data show that tumor-specific immune clearance of APL cells does occur in mice. In this model system, the relevant immunogenic antigens may arise from the xenogenic PML portion of human PML-RARa, and not unique sequences derived from the breakpoint region. However, the study proves that APL cells are capable of being recognized and killed in vivo by adaptive immune responses, suggesting that therapeutic vaccines should be possible for this disease when relevant tumor-specific antigens are identified.
Subject(s)
Leukemia, Promyelocytic, Acute/diagnosis , Leukemia, Promyelocytic, Acute/immunology , Vaccines, DNA/therapeutic use , Animals , Antigens, CD34/analysis , Crosses, Genetic , Female , Leukemia, Experimental/diagnosis , Leukemia, Experimental/immunology , Leukemia, Experimental/therapy , Leukemia, Promyelocytic, Acute/therapy , Male , Mice , Mice, Inbred C3H , Mice, Inbred C57BL , Mice, SCID , Neoplasm Proteins/genetics , Neoplasm Proteins/immunology , Oncogene Proteins, Fusion/genetics , Oncogene Proteins, Fusion/immunologyABSTRACT
Transgenic mice expressing PML-RARalpha in early myeloid cells under control of human cathepsin G regulatory sequences all develop a myeloproliferative syndrome, but only 15% to 20% develop acute promyelocytic leukemia (APL) after a latent period of 6 to 14 months. However, this transgene is expressed at very low levels in the bone marrow cells of transgenic mice. Because the transgene includes only 6 kb of regulatory sequences from the human cathepsin G locus, we hypothesized that sequences required for high-level expression of the transgene might be located elsewhere in the cathepsin G locus and that a knock-in model might yield much higher expression levels and higher penetrance of disease. We, therefore, targeted a human PML-RARalpha cDNA to the 5' untranslated region of the murine cathepsin G gene, using homologous recombination in embryonic stem cells. This model produced a high-penetrance APL phenotype, with more than 90% of knock-in mice developing APL between 6 and 16 months of age. The latent period and phenotype of APL (including a low frequency of an interstitial deletion of chromosome 2) was similar to that of the previous transgenic model. Remarkably, however, the expression level of PML-RARalpha in bone marrow cells or APL cells was less than 3% of that measured in the low-penetrance transgenic model. Although the explanation for this result is not yet clear, one hypothesis suggests that very low levels of PML-RARalpha expression in early myeloid cells may be optimal for the development of APL in mice.
Subject(s)
Leukemia, Promyelocytic, Acute/genetics , Leukemia, Promyelocytic, Acute/physiopathology , Neoplasm Proteins/genetics , Oncogene Proteins, Fusion/genetics , Penetrance , Animals , Antigens, CD34/genetics , Antineoplastic Agents/pharmacology , Biomarkers , Cathepsin G , Cathepsins/genetics , Cell Differentiation/drug effects , Cell Differentiation/genetics , Chromosomes, Mammalian , Disease Models, Animal , Female , Gene Deletion , Gene Dosage , Humans , Male , Mice , Mice, Inbred C57BL , Mice, Transgenic , RNA, Messenger/analysis , Recombination, Genetic , Serine Endopeptidases , Tretinoin/pharmacologyABSTRACT
We previously developed a murine model of acute promyelocytic leukemia (APL) by using human cathepsin G gene regulatory elements to direct the expression of promyelocytic leukemia (PML)/retinoic acid receptor alpha (RAR alpha) and RAR alpha/PML fusion cDNAs to the early myeloid compartment of transgenic mice. To study the efficacy of noncytotoxic therapy in this animal model, cohorts of naive immunocompetent mice were inoculated with primary murine APL cells from a frozen tumor bank. Arsenic trioxide and liposomally encapsulated all-trans-retinoic acid (Lipo ATRA), alone or in combination, were administered for 21 days by i.p. injection using doses that yielded plasma levels similar to those observed in human APL patients treated with these agents. Lipo ATRA was highly effective in inducing durable molecular remissions in immunocompetent mice [C57BL/6 x C3H F(1) (B6C3HF1)]; arsenic therapy was much less effective, and did not clearly synergize with Lipo ATRA to increase the remission rate in immunocompetent mice. The survival of Lipo ATRA-treated severe combined immunodeficient (SCID) animals (lacking functional T and B cells) was inferior to that of immunocompetent B6C3HF1 recipients (40% vs. 88% survival at 1 y, P < 0.001). These data suggest that adaptive immunity cooperates with pharmacologic therapy to induce or maintain remissions in murine APL. It also implies that immunosuppressive anti-leukemia therapies could paradoxically blunt effective anti-leukemia immune responses that are important for clearing small numbers of residual tumor cells after chemotherapy-mediated cytoreduction.
