In acute promyelocytic leukemia (APL), all-trans-retinoic acid (ATRA) treatment induces granulocytic maturation and complete remission of leukemia. MicroRNA are known to be critical players in the formation of the leukemic phenotype. In this study, we report downregulation of the miR-181a/b gene cluster in APL blasts and NB4 leukemia cells upon ATRA treatment as key event in the drug response. We found that miR-181a/b expression was activated by the PML/RARα oncogene in cells and transgenic knock-in mice, an observation confirmed and extended by evidence of enhanced expression of miR-181a/b in APL patient specimens. RNAi-mediated attenation of miR-181a/b expression in NB4 cells was sufficient to reduce colony forming capacity, proliferation and survival. Mechanistic investigations revealed that miR-181a/b targets the ATRA-regulated tumor suppressor gene RASSF1A by direct binding to its 3’UTR. Enforced expression of miR-181a/b or RNAi-mediated attenuation of RASSF1A
inhibited ATRA-induced granulocytic differentiation via regulation of the cell cycle regulator cyclin D1. Conversely, RASSF1A overexpression enhanced apoptosis. Lastly, RASSF1A levels were reduced in PML/RARα knock-in mice and APL patient samples. Taken together, our results define miR-181a and miR-181b as oncomiRs in PML/RARα-associated APL, and they reveal RASSF1A as a pivotal element in the granulocytic differentiation program induced by ATRA in APL.
Tumor Escape from CD4+ T Cells
Tumors cells can escape cytotoxic CD8+ T cells by preventing MHC I display of tumor antigens. It is unknown how tumors evade CD4+ T-cell responses, but because many tumor cells lack MHC II expression, novel mechanisms would be required. We have investigated this issue in a model in which MHC IINEG myeloma cells secrete a monoclonal Ig containing a V region L chain (VL) epitope recognized by CD4+ T cells. Infiltrating macrophages process and present the secreted tumor antigen to Th1 cells, resulting in induction of macrophage cytotoxicity and apparent rejection of the tumor. Despite long-term tumor protection in VL-specific T-cell receptor transgenic mice, we here describe that some myeloma cells persisted in a dormant state and, eventually, formed expanding tumors. Escape tumor cells maintained their secretion of complete (H+L) monoclonal Ig with unchanged sequence, while secretion of surplus free L chain was severely diminished. Although free L chains were efficiently processed and presented by tumor-infiltrating macrophages to CD4+ T cells, complete (H+L) monoclonal Ig was not. Forced overexpression of free L chain secretion reinstated tumor rejection. These results show that tumors can escape CD4+ T-cell–mediated rejection by impairing indirect presentation of tumor antigen by infiltrating macrophages. This occurs through a novel mechanism of immunoediting, in which modulation of the quaternary structure of the secreted tumor-specific antigen reduces its immunogenicity. Cancer Res; 75(16); 1–11. ©2015 AACR.
Tumors escape CD4+ T cell-mediated immunosurveillance by impairing the ability of infiltrating macrophages to indirectly present tumor antigens
Tumors cells can escape cytotoxic CD8+ T cells by preventing MHC I display of tumor antigens. It is unknown how tumors evade CD4+ T cell responses, but since many tumor cells lack MHC II expression, novel mechanisms would be required. We have investigated this issue in a model where MHC IINEG myeloma cells secrete a monoclonal Ig containing a V region L chain epitope (VL) recognized by CD4+ T cells. Infiltrating macrophages process and present the secreted tumor antigen to Th1 cells, resulting in reciprocal induction of macrophage cytotoxicity and apparent rejection of the tumor.
Despite long-term tumor protection in VL-specific T-cell-receptor-transgenic mice, we here describe that some myeloma cells persisted in a dormant state, and eventually formed expanding tumors. Escape tumor cells maintained their secretion of complete (H+L) monoclonal Ig with unchanged sequence, while secretion of surplus free L chain was severely diminished. While free L chains were efficiently processed and presented by tumor infiltrating macrophages to CD4+ T cells, complete (H+L) monoclonal Ig was not. Forced overexpression of free L chain secretion reinstated tumor rejection. These results show that tumors can escape CD4+ T cell-mediated rejection by impairing indirect presentation of tumor antigen by infiltrating macrophages. This occurs through a novel mechanism of immunoediting, in which modulation of the quaternary structure of the secreted tumor specific antigen reduces its immunogenicity.
