Virology, pathogenesis, epidemiology and clinical management of HTLV-1 infection. Proceedings of the 30th HTLV European research network (HERN 2023)

Introduction

HTLV-1 is one of several retroviruses that infect humans. All produce lifelong infections and at least three cause overt disease. In the absence of antiretroviral therapy, progressive CD4+ T lymphocyte depletion leads to immunodeficiency and AIDS in almost all HIV-1+ individuals. This also occurs albeit more slowly in HIV-2 carriers. In contrast, HTLV-1 causes immortalization of CD4+ T cells, leading to lymphoproliferation and immune dysfunction. Whilst HTLV-1 infection remains asymptomatic in 90 % of carriers, emerging data demonstrate a broader impact on health and life-expectancy [1, 2]. The overt clinical manifestations in HTLV-1 infection are a subacute myelopathy (HAM, HTLV-1-associated myelopathy) which predominantly occurs in women, and an aggressive T-cell malignancy, adult T-cell leukemia/lymphoma (ATLL), which in Europe is equally distributed by sex. In addition, a wide range of immune/inflammatory conditions, including uveitis, thyroiditis, arthritis, and bronchiectasis may develop in HTLV-1 carriers [3].

The HTLV European Research Network (HERN) was originally funded by the European Commission three decades ago. After EU funding expired, most participant researchers continued working together and meeting periodically. On September 15–16, 2023, the 30th HERN conference was held in Madrid, Spain. Over fifty researchers from Europe and America convened for two days to present and debate updates on basic science, epidemiology, clinical management and therapeutics for patients with HTLV-1 infection. Here we summarise highlights from the six categories of presentations against what was previously known.

Epidemiology & public health

Antoine Gessain, from the Pasteur Institute in Paris, France, addressed the origin of the HTLV-1 retrovirus in humans from natural infection in African primates. HTLV-1 infection is a zoonosis like HIV, Mpox virus and Ebola. There are an estimated 10 million people infected with HTLV-1 worldwide, half of them in equatorial Africa [4]. Gabon and the Democratic Republic of Congo are the countries with the highest infection rate. Nigeria is the country with the largest number of infected individuals worldwide, given its large population of 215 million. Women in these countries exhibit increasing prevalence rates with age, reaching up to 10–25 % in elderly women. Despite Africa being the largest HTLV-1 reservoir worldwide, there is no screening of blood donors in the continent.

Many subjects infected with HTLV-1 are diagnosed late, after developing clinical manifestations. HTLV-1 screening of asymptomatic individuals is rarely performed in populations with high infection risk, including migrants from endemic areas in Western countries [5]. Many physicians and the general population have little knowledge about HTLV-1, in contrast to HIV, for which information is widely available.

Hotspots of HTLV-1 endemicity are distributed across all continents. High rates of HTLV-1 infection have been reported among aboriginal people in Australia, Equatorial Africa, Latin America and the Caribbean, northeastern Iran, and southwestern Japan [6, 7]. Romania and neighboring Moldova are the only European countries with a relatively high HTLV-1 seroprevalence. At least 5 new cases of ATLL are reported annually in Romania [8]. Molecular epidemiology studies have found that most infections in Romania were introduced recently and due to a HTLV-1a transcontinental variant most probably originated in South Africa or Mozambique [9].

Using distinct approaches, British and Spanish researchers provided updated estimates of the number of HTLV-1+ individuals in Europe. In England and Wales the estimate of 14,900 in 1991 was revised to 36,300 in 2021, based on the 2021 population census [10]. This doubling of cases of infection parallels the rise in the incidence of ATLL in the UK. Migration from HTLV-1 endemic regions, including Nigeria and Romania, contributed to the increase in the estimate of HTLV-1 in England and Wales during the last 30 years. Based on migrant populations, Spanish investigators updated HTLV-1 prevalence estimates for Italy, France, Spain and UK [11]. The largest HTLV-1 population was estimated in UK, with migrants from the Caribbean and West Africa being the most important contributors. In contrast, Latin American migrants were the most frequent in Spain. In France, persons from French West Indies as well as migrants from West Africa predominated. Finally, Italy had the lowest estimate of HTLV-1 infection among migrants.

The HTLV-1 Spanish register

A nationwide register was created in 1989. Main demographics, clinical symptoms/signs and laboratory findings are collected for each new case at baseline using one standardized case report form. All positive individuals are followed periodically. Members of the Spanish HTLV Network cover most of the laboratory facilities nationwide where this virus can be diagnosed, including public or private microbiology laboratories or blood banks [14, 15]. The group updates the national figures on a yearly basis and produces recommendations for screening and medical management. Up to the end of 2022, a total of 451 cases of HTLV-1 had been recorded in Spain. HAM had been diagnosed in 58 and ATLL in 35 patients [16]. The current incidence of HAM in Spain is of 2–3 new cases per year whereas it is slightly lower for ATLL (Figure 1). At least 8 individuals had been infected with HTLV-1 in Spain following blood transfusions or solid organ transplantation before screening became mandatory for transplants and targeted for blood donors [17].

Figure 1:

Annual incidence of HAM and ATLL in Spain.

Virology

Unlike many other viruses, including HIV-1, cell-free infection by HTLV-1 is rare, and HTLV-1-infected cells release few if any viable virus particles. HTLV-1 infectious spread requires cell-to-cell contact between an infected cell and an uninfected cell. One of the primary mechanisms for HTLV-1 transmission involves the formation of a virological synapse. Once an HTLV-1 virion enters a target cell, its RNA genome is reverse transcribed into dsDNA. During cell division, this dsDNA integrates into the new host genome, at which point it is termed a provirus. Once integrated, this provirus can be transcribed by cellular RNA polymerase II to express viral gene products.

Once HTLV-1 infection is established in a new subject, persistence of infection occurs predominantly without viral replication. After adaptive immunity develops, proliferation of infected lymphocytes occurs with division of cells containing the integrated provirus. This sustained proliferation, and the use of cell-to-cell spread when infection does occur, explains why there is no plasma viraemia, that is, extracellular viral particles in the blood. HTLV-1 primarily infects CD4+ T-cells; due to its widely available target receptors, it has the ability to infect a wide variety of cell types, but efficient propagation of the virus only occurs from infected T cells and (possibly) dendritic cells. The HTLV-1 envelope protein uses neuropilin-1 (NRP1) and heparan sulfate proteoglycan (HSPG) for attachment and binding to target cells. The binding between gp46 and HSPG/NRP1 allows a conformation change in gp46 that exposes a binding domain that interacts with glucose transporter-1 (GLUT-1), followed by viral/cell fusion and the release of HTLV-1 nucleocapsid into the cytoplasm. The chemokine receptor CCR4 and the adhesion molecule LFA-1 also facilitate cell-to-cell transmission of HTLV-1.

Charles Bangham provided new insights into the mechanisms of HTLV-1 persistence. Comparing cells infected in vivo with those infected in vitro, his team identified selective survival in vivo of clones with proviruses located near the nuclear lamina or the nucleolus [18, 19]. The same phenomenon was observed in selective clonal survival in the HIV-1 reservoir.

The HTLV-1 accessory protein p8 induces cellular protrusions through which HTLV-1 passes to newly infect cells. Andrea Thoma-Kress discussed how p8 is transferred to distinct cell types and could hamper immune responses towards HTLV-1-infected CD4+ T cells [20].

Roberto Accolla reported on the role of the viral HBZ protein in the development of leukemic cells. Recent findings from his group pointed to the unprecedented evidence, from nuclear interactome studies, of HBZ interaction with the RNA splicing and non-sense mediated RNA decay (NMD proteins), both basic functions of cell physiology, whose alterations is involved in cancer [21]. Furthermore, Greta Forlani from Accolla’s group highlighted that the nuclear localization of HBZ could be a hallmark of neoplastic transformation [22].

HBZ downregulates Tax expression, which activates the NFkB pathway and the release of the cytokine IL-10, both well-accepted primary steps of leukemogenesis. Thereafter, the Tax protein is detected in ATL cells in approximately 50 % of cases. Ali Bazarbachi presented new data demonstrating that primary cells from patients with ATL retain the ability to express very low levels of Tax protein and depend on Tax expression for NFkB activation and survival [23].

Pathogenesis

Most people infected with HTLV-1 remain free of HTLV-1 complications for many years or during their entire lives. Only 10 % develop overt clinical manifestations, typically a subacute myelopathy or a T-cell leukemia. Both conditions are serious and difficult to treat. Most patients with neurological disease end up in a wheelchair and very few patients with leukemia/lymphoma survive longer than one year [3].

Mother-to-child HTLV-1 transmission

In places with high HTLV-1 endemicity, transmission of the virus to newborns from infected mothers continues to be important. Children may acquire HTLV-1 when breastfeeding extends beyond 3 months. It has been suggested that maternal antibodies passively transmitted protect the baby until then. Researchers from Germany and from the UK presented data showing anti-viral properties of milk, impacting HTLV-1 transmission and replication in vitro. Overall, 20 % of infants born to mothers carrying HTLV-1 become infected. Most occur through breastfeeding, although anecdotal cases of intrauterine infection or at the time of vaginal delivery have been demonstrated. If breastfeeding is eliminated and elective cesarean section performed, there is hardly any vertical transmission of HTLV-1 [26].

The results of a Spanish study that examined the presence of antibodies against HTLV-1 in more than 7,000 pregnant women were presented. Four positive cases were identified, all from Latin America. Three were primigravida under the age of 23. They were advised against breastfeeding and to date no children have been infected. On the contrary, several cases of HTLV-1 infected children have already been reported in Spain after late diagnoses of HTLV-1 in their mothers [27].

Considering the large Latin American population residing in Spain and its high birth rate, it has been estimated that around 30 new HTLV-1 infections occur each year by vertical transmission in Spain. This figure exceeds those of congenital syphilis, HIV or hepatitis B, all diseases for which there is a recommendation for antenatal screening. Given that prenatal screening for Chagas disease is already performed in Spain among pregnant women from Latin America, it would be convenient to add HTLV-1 screening to the same population [27]. An international study has concluded that antenatal HTLV-1 screening would be cost-effective or even cost-saving in many European countries [28], and much more so in HTLV-1 endemic regions [29]. Furthermore, more than 94 % of those living with HTLV-1 in the UK would support HTLV-1 antenatal screening [30].

Clinical manifestations & biomarkers

HTLV-1 prevention: prophylaxis & vaccines

Since HTLV-1 is a retrovirus like HIV, and many proteins are similar in both viruses, including reverse transcriptase, integrase, and protease, the possible efficacy of antiretrovirals in established HTLV-1 infection has been investigated. Overall, the results to date have been negative [42].

Once HTLV-1 infection is established in a new host, persistence of infection mostly occurs by proliferation of infected cells, although viral replication persists at a low level. This proliferation of infected lymphocytes by cell division explains why antiretrovirals do not reduce HTLV-1 proviral load in vivo. However, they do block infection in in vitro models, with inhibitory concentrations similar to those observed against HIV-1. The integrase inhibitors bictegravir, cabotegravir and dolutegravir are the most potent inhibitors of HTLV-1 transmission [43].

British and Italian researchers discussed the evidence supporting the use of antiretrovirals to prevent and manage HTLV-1 infection. Sexual contact being the predominant transmission route for HTLV-1, is there room to consider pre-exposure prophylaxis (PrEP) or post-exposure prophylaxis (PEP), resembling HIV-1 prevention strategies? The time window for PEP to have any expected potential inhibitory effect on HTLV-1 infection would be very early in the acute infection period, before clonal proliferation of infected cells replaces viral replication as the driver of proviral load. This is before 2–4 weeks after initial HTLV-1 exposure and based on HIV infection likely to be less than 72 h. Thus, only during the early phase, when transfer of virions throughout cell-to-cell contact via the virological synapsis occurs, antiretrovirals that block the full viral replication cycle might be effective (Figure 2). Based on in vitro data, PrEP with integrase inhibitors will provide susceptible cells the chances to block the HTLV-1 life cycle before proviral integration.

Figure 2:

Difference in the life cycle of human retroviruses.

Could antiretrovirals used as HIV PrEP or PEP work in uninfected persons at risk for HTLV-1 exposure? There are at least three target populations: (i) individuals having multiple sex partners, especially when they are from highly endemic regions; (ii) babies of infected mothers; and (iii) transplant recipients of HTLV-1+ donors. Advocates of PrEP for HTLV-1 consider that tenofovir would be the most convenient agent and that recruitment should be done in highly endemic regions. These researchers have estimated for the size of study populations but the difficulties in the design of trials to prove the benefit of antiretrovirals in these populations are acknowledged. An alternative to prospective, randomized trials would be to examine the HTLV-1 incidence in individuals on in HIV-1 PrEP programmes, including bimonthly cabotegravir, in regions with significant HTLV-1 prevalence [44, 45].

Conclusions

HTLV-1 is the second most prevalent human retroviral infection globally after HIV-1. Current estimates are of at least 10 million people living with HTLV-1 worldwide. The majority are asymptomatic, unaware of their infection and inadvertently able to transmit due to the paucity of testing. Once HTLV-1 is acquired, either sexually, vertically or parenterally, infection is lifelong. In contrast with HIV-1 infection, which causes a depletion of CD4+ T lymphocytes, HTLV-1 immortalizes infected CD4+ T-cells.

Nearly 10 % of HTLV-1 carriers develop overt clinical manifestations related to inflammation, such as, but not restricted to HAM, and malignancy in the form of ATLL. Worsened outcomes are also recognized with a range of co-infections [49] and associated to persistent immune activation and inflammation [1, 2]. Typically, HAM affects more frequently middle-aged women whereas ATLL occurs about a decade later, and in both sexes. A wide range of inflammatory conditions, including uveitis, thyroiditis and arthritis appear more frequently in individuals living with HTLV-1 than in the general population.

Although antiretrovirals are ineffective in established HTLV-1 infection, in vitro data support their potential as prophylaxis. Conversely, mogamulizumab, an anti-CCR4 monoclonal antibody, reduces HTLV-1 proviral load through semi-selective depletion of HTLV-1 infected cells, the majority of which express CCR4. Treatment with mogamulizumab infusion has shown efficacy in patients with HAM, with recent disease onset being a predictor of response.

The advent of new gene therapies and CRISPR editing has opened ways for targeting HTLV-1 proviral DNA. If not eradicative, these new treatment approaches could reduce proviral load, which ultimately predicts the risk of overt clinical disease [50]. Finally, the unique low genetic variability of HTLV-1 compared to many other RNA viruses, has encouraged the search for protective HTLV-1 vaccines [51]. However, the recognition that HTLV-1 spreads almost wholly by cell-to-cell contact through a virological synapse has diminished the initial enthusiasm for producing antibody-mediated vaccines [52].