HTLV-1 Hbz protein, but not hbz mRNA secondary structure, is critical for viral persistence and disease development

HTLV-1 infected T cells cause bone loss via small extracellular vesicles

Adult T cell leukemia (ATL), caused by infection with human T cell leukemia virus type 1 (HTLV-1), is often complicated by hypercalcemia and osteolytic lesions. Therefore, we studied the communication between patient-derived ATL cells (ATL-PDX) and HTLV-1 immortalized CD4+ T cell lines (HTLV/T) with osteoclasts and their effects on bone mass in mice. Intratibial inoculation of some HTLV/T lead to a profound local decrease in bone mass similar to marrow-replacing ATL-PDX, despite the fact that few HTLV/T cells persisted in the bone. To study the direct effect of HTLV/T and ATL-PDX on osteoclasts, supernatants were added to murine and human osteoclast precursors. ATL-PDX supernatants from hypercalcemic patients promoted formation of mature osteoclasts, while those from HTLV/T were variably stimulatory, but had largely consistent effects between human and murine cultures. Interestingly, this osteoclastic activity did not correlate with expression of osteoclastogenic cytokine RANKL, suggesting an alternative mechanism. HTLV/T and ATL-PDX produce small extracellular vesicles (sEV), known to facilitate HTLV-1 infection. We hypothesized that these sEV also mediate bone loss by targeting osteoclasts. We isolated sEV from both HTLV/T and ATL-PDX, and found they carried most of the activity found in supernatants. In contrast, sEV from uninfected activated T cells had little effect. Analysis of sEV (both active and inactive) by mass spectrometry and electron microscopy confirmed absence of RANKL and intact virus. Viral proteins Tax and Env were only present in sEV from the active, osteoclast-stimulatory group, along with increased representation of proteins involved in osteoclastogenesis and bone resorption. sEV injected over mouse calvaria in the presence of low dose RANKL caused more osteolysis than RANKL alone. Thus, HTLV-1 infection of T cells can cause release of sEV with strong osteolytic potential, providing a mechanism beyond RANKL production that modifies the bone microenvironment, even in the absence of overt leukemia.

An HTLV-1 envelope mRNA vaccine is immunogenic and protective in New Zealand rabbits

Human T-cell leukemia virus type 1 (HTLV-1) is a retrovirus responsible for adult T-cell leukemia/lymphoma, a severe and fatal CD4+ T-cell malignancy. Additionally, HTLV-1 can lead to a chronic progressive neurodegenerative disease known as HTLV-1-associated myelopathy/tropical spastic paraparesis. Unfortunately, the prognosis for HTLV-1-related diseases is generally poor, and effective treatment options are limited. In this study, we designed and synthesized a codon optimized HTLV-1 envelope (Env) mRNA encapsulated in a lipid nanoparticle (LNP) and evaluated its efficacy as a vaccine candidate in an established rabbit model of HTLV-1 infection and persistence. Immunization regimens included a prime/boost protocol using Env mRNA-LNP or control green fluorescent protein (GFP) mRNA-LNP. After immunization, rabbits were challenged by intravenous injection with irradiated HTLV-1 producing cells. Three rabbits were partially protected and three rabbits were completely protected against HTLV-1 challenge. These rabbits were then rechallenged 15 weeks later, and two rabbits maintained sterilizing immunity. In Env mRNA-LNP immunized rabbits, proviral load and viral gene expression were significantly lower. After viral challenge in the Env mRNA-LNP vaccinated rabbits, an increase in both CD4+/IFN-γ+ and CD8+/IFN-γ+ T-cells was detected when stimulating with overlapping Env peptides. Env mRNA-LNP elicited a detectable anti-Env antibody response after prime/boost vaccination in all animals and significantly higher levels of neutralizing antibody activity. Neutralizing antibody activity was correlated with a reduction in proviral load. These findings hold promise for the development of preventive strategies and therapeutic interventions against HTLV-1 infection and its associated diseases.IMPORTANCEmRNA vaccine technology has proven to be a viable approach for effectively triggering immune responses that protect against or limit viral infections and disease. In our study, we synthesized a codon optimized human T-cell leukemia virus type 1 (HTLV-1) envelope (Env) mRNA that can be delivered in a lipid nanoparticle (LNP) vaccine approach. The HTLV-1 Env mRNA-LNP produced protective immune responses against viral challenge in a preclinical rabbit model. HTLV-1 is primarily transmitted through direct cell-to-cell contact, and the protection offered by mRNA vaccines in our rabbit model could have significant implications for optimizing the development of other viral vaccine candidates. This is particularly important in addressing the challenge of enhancing protection against infections that rely on cell-to-cell transmission.

The Pleiotropic Effects of YBX1 on HTLV-1 Transcription

HTLV-1 is an oncogenic human retrovirus and the etiologic agent of the highly aggressive ATL malignancy. Two viral genes, Tax and Hbz, are individually linked to oncogenic transformation and play an important role in the pathogenic process. Consequently, regulation of HTLV-1 gene expression is a central feature in the viral lifecycle and directly contributes to its pathogenic potential. Herein, we identified the cellular transcription factor YBX1 as a binding partner for HBZ. We found YBX1 activated transcription and enhanced Tax-mediated transcription from the viral 5' LTR promoter. Interestingly, YBX1 also interacted with Tax. shRNA-mediated loss of YBX1 decreased transcript and protein abundance of both Tax and HBZ in HTLV-1-transformed T-cell lines, as well as Tax association with the 5' LTR. Conversely, YBX1 transcriptional activation of the 5' LTR promoter was increased in the absence of HBZ. YBX1 was found to be associated with both the 5' and 3' LTRs in HTLV-1-transformed and ATL-derived T-cell lines. Together, these data suggest that YBX1 positively influences transcription from both the 5' and 3' promoter elements. YBX1 is able to interact with Tax and help recruit Tax to the 5' LTR. However, through interactions with HBZ, YBX1 transcriptional activation of the 5' LTR is repressed.