Induction of tolerance to immunogenic tumor antigens associated with lymphomagenesis in HOX11 transgenic mice

Phosphorylation of HOX11/TLX1 on Threonine-247 during mitosis modulates expression of cyclin B1

BACKGROUND

The HOX11/TLX1 (hereafter referred to as HOX11) homeobox gene was originally identified at a t(10;14)(q24;q11) translocation breakpoint, a chromosomal abnormality observed in 5-7% of T cell acute lymphoblastic leukemias (T-ALLs). We previously reported a predisposition to aberrant spindle assembly checkpoint arrest and heightened incidences of chromosome missegregation in HOX11-overexpressing B lymphocytes following exposure to spindle poisons. The purpose of the current study was to evaluate cell cycle specific expression of HOX11.

RESULTS

Cell cycle specific expression studies revealed a phosphorylated form of HOX11 detectable only in the mitotic fraction of cells after treatment with inhibitors to arrest cells at different stages of the cell cycle. Mutational analyses revealed phosphorylation on threonine-247 (Thr247), a conserved amino acid that defines the HOX11 gene family and is integral for the association with DNA binding elements. The effect of HOX11 phosphorylation on its ability to modulate expression of the downstream target, cyclin B1, was tested. A HOX11 mutant in which Thr247 was substituted with glutamic acid (HOX11 T247E), thereby mimicking a constitutively phosphorylated HOX11 isoform, was unable to bind the cyclin B1 promoter or enhance levels of the cyclin B1 protein. Expression of the wildtype HOX11 was associated with accelerated progression through the G2/M phase of the cell cycle, impaired synchronization in prometaphase and reduced apoptosis whereas expression of the HOX11 T247E mutant restored cell cycle kinetics, the spindle checkpoint and apoptosis.

CONCLUSIONS

Our results demonstrate that the transcriptional activity of HOX11 is regulated by phosphorylation of Thr247 in a cell cycle-specific manner and that this phosphorylation modulates the expression of the target gene, cyclin B1. Since it is likely that Thr247 phosphorylation regulates DNA binding activity to multiple HOX11 target sequences, it is conceivable that phosphorylation functions to regulate the expression of HOX11 target genes involved in the control of the mitotic spindle checkpoint.

Dysregulated expression of mitotic regulators is associated with B-cell lymphomagenesis in HOX11-transgenic mice

Dysregulated expression of the homeobox gene, HOX11 is a frequent etiologic event in T-cell acute lymphoblastic leukemias. HOX11-transgenic mice (IgHmu-HOX11Tg)-expressing HOX11 in the B-cell compartment develop B-cell lymphomas with extended latency. The latency suggests that additional genetic events are required prior to the onset of malignant lymphoma. We report the identification of 17 HOX11 collaborating genes, revealed through their propensity to be targeted in a proviral insertional mutagenesis screen. Seven integrations disrupted genes in mitotic spindle checkpoint control, suggesting that cells with elevated HOX11 expression are especially sensitive to dysregulation of chromosome segregation during mitosis. IgHmu-HOX11Tg primary B-lymphocyte cultures exposed to the aneugenic agents, colchicine and colcemid, exhibited increased incidences of chromosome missegregation as assessed by cytokinesis-block micronucleus assays. Additionally, IgHmu-HOX11Tg cultures were shown to exhibit aberrant bypass of spindle checkpoint arrest, as assessed by the increased presence of cycling cells determined by assessment of DNA content and by BrdU immunolabelling. Western immunoblotting revealed elevated expression of the mitotic effector molecules, cyclin A, cyclin B1 and cdc20 in IgHmu-HOX11Tg cultures. Moreover, spontaneously arising lymphoid neoplasms in IgHmu-HOX11Tg mice frequently exhibit aberrant expression of mitotic regulators, concomitant with increased development of micronuclei, abnormal mitotic checkpoint control and increased incidences of abnormal karyotypes when expanded in culture. Collectively, these findings indicate that abnormal regulation of spindle checkpoint control as a result of HOX11 overexpression leads to a heightened predisposition for development of aneuploidy, contributing to oncogenesis.

Tissue distribution of target antigen has a decisive influence on the outcome of adoptive cancer immunotherapy

Adoptive transfer of allogeneic T cells has unmatched efficacy to eradicate leukemic cells. We therefore sought to evaluate in kinetic terms interactions between T cells and allogeneic leukemic cells. T cells primed against the model B6(dom1) minor histocompatibility antigen were adoptively transferred in irradiated B10 (B6(dom1)-positive) and congenic B10.H7(b) (B6(dom1)-negative) recipients, some of which were also injected with EL4 leukemia/lymphoma cells (B6(dom1)-positive). A key finding was that the tissue distribution of the target epitope dramatically influenced the outcome of adoptive cancer immunotherapy. Widespread expression of B6(dom1) in B10 recipients induced apoptosis and dysfunction of antigen-specific T cells. Furthermore, in leukemic B10 and B10.H7(b) hosts, a massive accumulation of effector/memory B6(dom1)-specific T cells was detected in the bone marrow, the main site of EL4 cell growth. The accumulation of effector/memory cells in recipient bone marrow was EL4 dependent, and its kinetics was different from that observed in recipient spleen. We conclude that strategies must be devised to prevent apoptosis of adoptively transferred T cells confronted with a high antigen load and that local monitoring of the immune response at the site of tumor growth may be mandatory for a meaningful assessment of the efficacy of adoptive immunotherapy.