A deadly virus no one talks about — and the HIV drugs that might stop it

A groundbreaking study co-led by Australian researchers has unveiled a promising new path forward, demonstrating that existing HIV drugs can effectively suppress the transmission of the Human T-cell leukemia virus type 1 (HTLV-1) in a living organism. Published in the prestigious journal Cell, this landmark research offers a beacon of hope for developing the first preventative treatments against a virus that disproportionately affects many First Nations communities worldwide, including those in Central Australia. Beyond prevention, the study, a collaborative effort between WEHI and the Peter Doherty Institute for Infection and Immunity (Doherty Institute), also identifies a novel drug target that could pave the way for eliminating HTLV-1 positive cells in individuals with established infections, thereby preventing disease progression and potentially offering a curative strategy.

Understanding HTLV-1: A Neglected Global Health Threat

HTLV-1, discovered in the early 1980s by Robert Gallo and his colleagues, was the first human retrovirus identified to cause cancer. It primarily infects T cells, a crucial type of immune cell responsible for fighting off infections. While the majority of infected individuals remain asymptomatic carriers for life, a significant proportion, typically 5-10%, develop severe, life-threatening conditions after a long latency period, often decades after initial infection. These debilitating diseases include Adult T-cell Leukemia/Lymphoma (ATL), a highly aggressive and often fatal cancer of the T cells, and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP), a chronic, progressive neurological disorder characterized by spasticity and weakness in the lower limbs. Other associated conditions include uveitis (eye inflammation), infective dermatitis, and a heightened susceptibility to opportunistic infections like Strongyloides stercoralis.

The global burden of HTLV-1 is substantial, with an estimated 10-20 million people infected worldwide, though this figure is likely an underestimate due to limited surveillance and diagnostic capabilities in many endemic regions. High prevalence rates are observed in specific geographical clusters, including southwestern Japan (particularly Kyushu), the Caribbean basin, parts of sub-Saharan Africa, South America (e.g., Brazil, Colombia), and aboriginal populations in Central Australia. Transmission occurs primarily through three routes: mother-to-child (predominantly via breastfeeding), sexual contact, and blood transfusion. The lack of widespread screening for HTLV-1 in blood donations in many countries, unlike HIV, contributes to its continued spread. Despite its serious health consequences and widespread prevalence, HTLV-1 has long been categorized as one of the world’s most neglected viruses, receiving considerably less research funding and public health attention compared to other blood-borne viruses like HIV.

A Decade of Breakthrough Research: The Australian-led Study

The current study represents a monumental leap forward, culminating a decade of dedicated research. Co-lead author and WEHI laboratory head Dr. Marcel Doerflinger underscored the profound significance of their findings, stating, "Our study marks the first time any research group has been able to suppress this virus in a living organism." This achievement is particularly critical because the insidious nature of HTLV-1 means symptoms can take decades to manifest. By the time an individual is aware of their infection, the immune system may have already sustained substantial, irreversible damage. Dr. Doerflinger emphasized, "Suppressing the virus at transmission would allow us to stop it before it has the chance to cause irreversible damage to immune function, leading to disease and a premature death."

A key enabling factor for this breakthrough was the development of a world-first humanized mouse model for HTLV-1. This sophisticated model, meticulously developed by the collaborative team, allowed researchers to study the virus’s intricate behavior within a living organism possessing a human-like immune system. The mice were transplanted with human immune cells, specifically T cells, which are the primary targets of HTLV-1 infection. Crucially, the researchers utilized both international HTLV-1 strains and Australia’s unique genetically novel HTLV-1c strain. This innovative model proved invaluable, demonstrating that both international and Australian strains were capable of causing severe diseases, including leukemia and inflammatory lung disease, in these human immune system mice.

The development of these humanized mouse models at WEHI was spearheaded by first author Dr. James Cooney and Professor Marc Pellegrini, a study lead author, WEHI Honorary Fellow, and Executive Director at the Centenary Institute. Prof. Pellegrini highlighted the dual importance of these models: "The mouse models were not only critical in identifying potential therapeutic targets, but also allowed researchers to understand how different strains of the HTLV-1 virus can alter disease symptoms and outcomes." This insight is particularly vital for understanding the distinct pathogenicity of the HTLV-1c strain prevalent in Australia.

Repurposing HIV Drugs: A Promising Prevention Strategy

One of the most exciting aspects of the study’s findings is the discovery that two antiviral therapies already approved and widely used to manage HIV and prevent AIDS—tenofovir and dolutegravir—can powerfully suppress HTLV-1. Tenofovir is a nucleoside reverse transcriptase inhibitor (NRTI), and dolutegravir is an integrase inhibitor. Both drugs target crucial enzymes involved in the replication cycle of retroviruses. Given that HTLV-1 is also a retrovirus, the researchers hypothesized that these drugs might show efficacy. The experimental results in the humanized mouse model confirmed this hypothesis, revealing significant suppression of HTLV-1.

Dr. Doerflinger expressed immense optimism about the direct translational potential of this discovery: "What’s most exciting is that these antivirals are already in use for millions of HIV patients, meaning there’s a direct path for the clinical translation of our findings. We won’t have to start from scratch because we already know these drugs are safe and effective. And now we’ve shown that their use can very likely be extended to HTLV-1." This significantly accelerates the timeline for potential clinical application, bypassing the extensive and costly safety trials typically required for entirely new drug compounds. The immediate implication is the possibility of using these drugs as a pre-exposure prophylaxis (PrEP) against HTLV-1 acquisition, mirroring the highly successful PrEP strategies employed for HIV prevention.

Beyond Prevention: Towards a Cure with Novel Drug Targets

The research extended beyond prevention, venturing into potential curative strategies. In another remarkable finding, the team discovered a novel approach to selectively eliminate HTLV-1 infected cells. When mice were treated with the aforementioned HIV drugs in combination with another therapy that inhibits a protein called MCL-1, human cells containing HTLV-1 were selectively killed. MCL-1 (Myeloid Cell Leukemia Sequence 1) is an anti-apoptotic protein, meaning it helps cells avoid programmed cell death. In many cancers, including some leukemias, MCL-1 is overexpressed, allowing rogue cells to survive and proliferate. By inhibiting MCL-1, the researchers were able to sensitize the HTLV-1 infected cells to cell death, even in the presence of HIV drugs.

This dual-pronged approach—suppressing viral transmission and eliminating infected cells—opens up unprecedented avenues for treatment. The research team is now actively leveraging precision RNA therapies to develop new ways to target MCL-1. Their goal is to establish combination treatments that can be clinically tested, with the belief that this strategy could offer a promising curative option for individuals already living with HTLV-1 infection and its associated diseases. Such a cure would not only halt disease progression but potentially eradicate the virus from the body, fundamentally changing the lives of millions.

The Unseen Burden: HTLV-1 in First Nations Communities

The study carries particular significance for First Nations communities globally, and especially in Central Australia, where the HTLV-1c strain is endemic and has a profound impact on health outcomes. Associate Professor Lloyd Einsiedel, a Clinician Scientist at the Doherty Institute and an Infectious Diseases Physician, has dedicated over a decade to providing clinical services in Central Australia and raising awareness about HTLV-1. His front-line work was instrumental in obtaining the human HTLV-1 samples necessary for developing the crucial mouse models.

Research by the University of Melbourne’s Professor Damian Purcell, Head of Molecular Virology at the Doherty Institute and co-lead author, further elucidated the unique characteristics of the Australian strain. Prof. Purcell’s team isolated the virus from First Nations donors and identified significant genetic differences between the HTLV-1c strains from Central Australia and the HTLV-1a strains found internationally. The new findings confirmed that both HTLV-1 strains cause disease in mice, but intriguingly, HTLV-1c exhibited more aggressive features. Encouragingly, the identified drug therapies (tenofovir and dolutegravir) were found to be equally effective against both strains, highlighting their broad potential.

Professor Pellegrini emphasized the importance of understanding these distinctions: "It’s long been hypothesized that differences in viral subtype may influence disease outcomes, but a lack of research into HTLV-1 has made it difficult for us to find the evidence needed to support this claim – until now. Our study provides critical insights that enable us to better understand the consequences of the distinct molecular make-up of the virus affecting our First Nations communities. This will further help us to investigate ways to create the tools needed to control the spread of this virus subtype." This targeted understanding is crucial for developing culturally appropriate and effective public health interventions.

A Chronology of Advocacy and Discovery

The journey towards these breakthroughs has been underpinned by years of relentless advocacy.

• 1980s: HTLV-1 is discovered by Robert Gallo, identified as the first human retrovirus and linked to T-cell leukemia.
• Late 20th/Early 21st Century: Growing awareness of HTLV-1 prevalence in various global hotspots, including specific Indigenous communities in Australia, where the unique HTLV-1c strain is identified.
• Over a Decade of Advocacy (Pre-2021): Professor Purcell and Associate Professor Einsiedel, alongside the National Aboriginal Community Controlled Health Organization (NACCHO) HTLV-1 committee and the Australian Department of Health, tirelessly campaigned for greater recognition and action regarding HTLV-1. This advocacy highlighted the severe health disparities caused by the neglected virus.
• 2021: WHO Classification: These concerted efforts culminated in a major victory when the World Health Organization (WHO) formally classified HTLV-1 as a Threatening Pathogen to Humans. This critical classification was a direct result of the persistent advocacy, providing the necessary impetus for global public health bodies to address the virus.
• Post-2021: Following the WHO classification, formal WHO policies were developed to reduce international transmission, and clinical management guidelines for HTLV-1c in Central Australia were established under NACCHO leadership. This marked a significant turning point in global and national responses to HTLV-1.
• Current Study (Culmination of 10-year effort): The groundbreaking research by WEHI and Doherty Institute, which began approximately a decade prior, publishes its findings in Cell, demonstrating the efficacy of HIV drugs against HTLV-1 and identifying new drug targets.

Prof. Purcell reflected on the ongoing challenges despite these advancements: "Despite Australia’s high burden of HTLV-1, the virus and its associated diseases are still not notifiable in most states and true infection rates in the nation remain unknown." This lack of mandatory reporting hinders accurate epidemiological data collection, resource allocation, and targeted public health interventions.

Expert Perspectives and Collaborative Spirit

The collaborative nature of this research, spanning multiple institutions and disciplines, has been a cornerstone of its success. Dr. Doerflinger, Prof. Pellegrini, Prof. Purcell, and Assoc. Prof. Einsiedel represent a formidable team combining expertise in virology, immunology, clinical medicine, and public health advocacy. Their shared commitment to addressing this neglected disease has driven the extensive research effort.

The involvement of NACCHO is particularly significant, underscoring the importance of community engagement and leadership in addressing health challenges within Indigenous populations. Their advocacy ensured that the unique needs and perspectives of affected communities were central to policy development and research direction. The classification of HTLV-1 by the WHO in 2021, directly influenced by this advocacy, was a pivotal moment, elevating HTLV-1 from obscurity to a recognized global health priority.

Implications for Global Public Health and Policy

The findings of this study have profound implications for global public health and policy.

1. Immediate Clinical Translation: The fact that effective drugs are already approved and widely available for HIV dramatically shortens the path to clinical use for HTLV-1. This could mean a rapid rollout of prevention strategies.
2. Pre-Exposure Prophylaxis (PrEP) for HTLV-1: The potential for a PrEP strategy, similar to that for HIV, could revolutionize HTLV-1 prevention, particularly for at-risk populations such as individuals in endemic regions, sexual partners of infected individuals, and potentially pregnant women to prevent mother-to-child transmission.
3. Path to a Functional Cure: The identification of MCL-1 as a drug target and the promising results of combination therapies offer a tangible route towards developing a functional cure for HTLV-1, a prospect that was previously considered distant. This would significantly reduce the burden of ATL and HAM/TSP.
4. Addressing Health Disparities: For First Nations communities, where HTLV-1 has caused significant health disparities, these findings offer a renewed sense of hope for improved health outcomes and a reduction in disease prevalence.
5. Policy Advocacy: The study reinforces the urgent need for enhanced screening programs, improved surveillance, and making HTLV-1 a nationally notifiable disease in all affected countries. This would enable better data collection, targeted interventions, and equitable resource allocation.
6. Spotlight on Neglected Diseases: This success story could serve as a powerful exemplar, drawing more attention and funding to other neglected tropical diseases and viruses that disproportionately affect vulnerable populations.

Prof. Purcell articulated the broader vision: "People at risk from HTLV-1 deserve biomedical tools like those that provide game-changing therapeutic and prevention options for other blood-borne persistent viral infections, such as HIV. There is a real opportunity to prevent the transmission of HTLV-1 and end the diseases caused by these infections. Our research findings are a major leap forward in this."

The research team is already engaged in discussions with the pharmaceutical companies behind the HIV antivirals used in this study. The goal is to explore the possibility of including HTLV-1 patients in ongoing clinical trials. If these discussions are successful, it would pave the way for these drugs to become the first approved pre-exposure prophylaxis against HTLV-1 acquisition, marking a new era in the fight against this long-overlooked virus.

This pivotal research was generously supported by The Australian Center for HIV and Hepatitis Virology Research, The Phyllis Connor Memorial Trust, Drakensberg Trust, and the National Health and Medical Research Council (NHMRC), highlighting the crucial role of sustained funding in scientific discovery. The international scientific community and affected communities alike now look forward with anticipation to the clinical translation of these transformative findings.