Interleukin-2 production by primary adult T cell leukemia tumor cells is macrophage dependent

Expression of miR-34a in T-Cells Infected by Human T-Lymphotropic Virus 1

Human T-lymphotropic virus 1 (HTLV-1) immortalizes T-cells and is the causative agent of adult T-cell leukemia/lymphoma (ATLL). HTLV-1 replication and transformation are governed by multiple interactions between viral regulatory proteins and host cell factors that remain to be fully elucidated. The present study investigated the impact of HTLV-1 infection on the expression of miR-34a, a microRNA whose expression is downregulated in many types of cancer. Results of RT-PCR assays showed that five out of six HTLV-1-positive cell lines expressed higher levels of miR-34a compared to normal PBMC or purified CD4+ T-cells. ATLL cell line ED, which did not express miR-34a, showed methylation of the miR-34a promoter. Newly infected PBMC and samples from 10 ATLL patients also showed a prominent increase in miR-34a expression compared to PBMC controls. The primary miR-34a transcript expressed in infected cell line C91PL contained binding motifs for NF-κB and p53. Pharmacological inhibition of NF-κB with Bay 11-7082 indicated that this pathway contributes to sustain miR-34a levels in infected cells. Treatment of infected cell lines with the p53 activator nutlin-3a resulted in a further increase in miR-34a levels, thus confirming it as a transcriptional target of p53. Nutlin-3a-treated cells showed downregulation of known miR-34a targets including the deacetylase SIRT1, which was accompanied by increased acetylation of p53, a substrate of SIRT1. Transfection of C91PL cells with a miR-34a mimic also led to downregulation of mRNA targets including SIRT1 as well as the pro-apoptotic factor BAX. Unlike nutlin-3a, the miR-34a mimic did not cause cell cycle arrest or reduce cell viability. On the other hand, sequestration of miR-34a with a sponge construct resulted in an increase in death of C91PL cells. These findings provide evidence for a functional role for miR-34a in fine-tuning the expression of target genes that influence the turnover of HTLV-1-infected cells.

HTLV-1 and innate immunity

Innate immunity plays a critical role in the host response to a viral infection. The innate response has two main functions. First, it triggers effector mechanisms that restrict the infection. Second, it primes development of the adaptive response, which completes the elimination of the pathogen or of infected cells. In vivo, HTLV-1 infects T lymphocytes that participate in adaptive immunity but also monocytes and dendritic cells that are major players in innate immunity. Herein, we will review the interplay between HTLV-1 and innate immunity. Particular emphasis is put on HTLV-1-induced alteration of type-I interferon (IFN-I) function. In vitro, the viral Tax protein plays a significant role in the alteration of IFN synthesis and signaling. Despite this, IFN-I/AZT treatment of Adult T-cell Leukemia/Lymphoma (ATLL) patients leads to complete remission. We will discuss a model in which exogenous IFN-I could act both on the microenvironment of the T-cells to protect them from infection, and also on infected cells when combined with other drugs that lead to Tax down-regulation/degradation.

Autocrine/paracrine cytokine stimulation of leukemic cell proliferation in smoldering and chronic adult T-cell leukemia

Adult T-cell leukemia (ATL), a heterogeneous disease, can be divided into smoldering, chronic, lymphoma, and acute types clinically. In addition to different clinical manifestations, different stages of ATL have different molecular signatures. Here, we demonstrated that smoldering/chronic ATL peripheral blood mononuclear cells spontaneously proliferated ex vivo in a cytokine (interleukin-12 [IL-12]/IL-9/IL-15)-dependent manner, while acute-type ATL peripheral blood mononuclear cells did not proliferate or proliferated independent of cytokines. Smoldering/chronic ATL cells produced IL-2 and IL-9 in 6-day ex vivo cultures. Interestingly, the addition of an anti-IL-2R-α monoclonal antibody profoundly inhibited IL-9 expression, suggesting optimal expression of IL-9 was dependent on IL-2 signaling in these patients. To determine whether there would be autonomous proliferation of ATL leukemic cells, we purified leukemic cells from patients with smoldering/chronic ATL. Purified leukemic cells cultured alone produced IL-2/IL-9, and the downstream Janus kinase/signal transducer and activator of transcription pathway was activated. However, the leukemic cells did not proliferate independently, but required coculture with autologous monocytes to induce proliferation. Moreover, interaction between leukemic cells and monocytes was contact dependent, and major histocompatibility complex class II expression may have contributed to this interaction. In conclusion, our data provide evidence that there is autocrine/paracrine cytokine stimulation of leukemic cell proliferation in patients with smoldering/chronic ATL that could be targeted for treatment.

Human T-Cell Leukemia Virus Type I Tax Transactivates the Promoter of Human Prointerleukin-1β Gene Through Association With Two Transcription Factors, Nuclear Factor–Interleukin-6 and Spi-1

The human T-cell leukemia virus type I (HTLV-I), which infects a wide variety of mammalian cells including monocytes and macrophages, encodes a transactivating protein designated as Tax. We now report that Tax induces the human prointerleukin-1β (IL1B) gene promoter in monocytic cells. In our transient transfection assays using human THP-1 monocytic cells, a chloramphenicol acetyltransferase (CAT) construct containing the IL1B promoter sequence between positions −131 and +12 showed an approximately 90-fold increase in activity following cotransfection of a Tax expression vector. Moreover, Tax synergized with lipopolysaccharide (LPS) to induce the IL1B promoter activity. Analyses of specific nucleotide substitutions further indicated that the Tax-induced transcriptional activation requires two transcription factor binding motifs within the IL1B promoter; one is a binding site for nuclear factor (NF)-IL6 (CCAAT/enhancer binding protein β, C/EBPβ), which belongs to the basic region-leucine zipper (bZIP) family and the other for Spi-1 (PU.1), which is an Ets family protein found principally in monocytes, macrophages, and B lymphocytes. In electrophoretic mobility shift assays (EMSA) using in vivo THP-1 nuclear extracts, Tax expression in THP-1 monocytic cells significantly increased binding of the two factors to their target IL1B promoter sequences. However, in contrast to NF-IL6 and Spi-1, DNA binding activity of Oct-1, an ubiquitously expressed octamer-binding protein was not affected by Tax. Additional EMSA using in vitro translated proteins also showed that recombinant Tax enhances DNA binding of both of recombinant NF-IL6 and Spi-1 proteins. These data were supported by our glutathione S-transferase (GST)-pulldown data, which indicated that Tax physically interacts with the two proteins. Based on the results obtained from the present study, we conclude that the IL1B promoter is a Tax-responsive sequence as a result of ability of Tax to induce binding of NF-IL6 and Spi-1 to the IL1B promoter sequence through protein-protein interaction.

Autonomous and interleukin-2-responsive growth of leukemic cells in adult T-cell leukemia (ATL): a review of the clinical significance and molecular basis of ATL cell growth

Since the initial report of adult T-cell leukemia (ATL) in 1976, a number of investigators have described the basic biologic aspects of this disease. However, the precise mechanism of leukemogenesis remains unclear. Primary ATL cells demonstrate autonomous and IL-2 responsive growth in vitro. The autonomous growth of the cells is thought to be mediated by IL-2 in an autocrine manner, at least in part. These growth activities are related inversely to survival, and may be useful prognostic determinants. The viral Tax protein stimulates IL-2 and IL-2 receptor alpha expression via nuclear transfer factor NF-kappaB induction. We showed that marked activation of the Tax-NF-kappaB pathway is seen only in acute-type ATL patients. Recent studies show that mutations of p16 and p53 are also found in acute and lymphoma-type ATL. These appear to be late events in ATL leukemogenesis. The relationship between activation of Tax-NF-kappaB pathway and mutations of p53 and p16 genes is unknown. A few other genetic events may be involved in earlier stages of the entire process of ATL leukemogenesis, leading to smoldering and chronic-type ATL. These gene mutations may be accumulated by Tax protein during the long process from the time of HTLV-I infection to the onset of ATL.

Interleukin-2-mediated growth of leukemic cells in lymph nodes of patients with adult T-cell leukemia/lymphoma

We investigated the effect of interleukin-2 (IL-2) on tumor growth of primary adult T-cell leukemia/lymphoma (ATL) cells in biopsied lymph node cells obtained from 14 patients (seven [corrected] with acute-type disease, one with chronic-type disease and six [corrected] with lymphoma-type disease). Biological activity of IL-2 in culture supernatants of the cells was detected in six out of 12 cases. The IL-2 mRNA in the lymph node cells was detected in four out of nine patients by northern blotting. However, it was detected in all nine patients examined by reverse polymerase chain reaction (PCR) method. Lymph node cells from 12 out of 14 patients showed a high or moderate proliferative response to IL-2; the remaining two patients showed a slight response. These results suggest that malignant growth of primary tumor cells in lymph nodes may be associated with the IL-2-IL-2 receptor system in patients with ATL more frequently than had been previously thought.