LGL leukemia and HTLV

Antibodies against endogenous retroviruses

The human genome harbors hundreds of thousands of integrations of ancient retroviruses, amassed over millions of years of evolution. To reduce further amplification in the genome, the host prevents transcription of these now endogenous retroviruses (ERVs) through epigenetic repression and, with evolutionary time, ERVs are incapacitated by accumulating mutations and deletions. However, several members of recently endogenized ERV groups still retain the capacity to produce viral RNA, retroviral proteins, and higher order structures, including virions. The retention of viral characteristics, combined with the reversible nature of epigenetic repression, particularly as seen in cancer, allow for immunologically unanticipated ERV expression, perceived by the adaptive immune system as a genuine retroviral infection, to which it has to respond. Accordingly, antibodies reactive with ERV antigens have been detected in diverse disorders and, occasionally, in healthy individuals. Although they are part of self, the retroviral legacy of ERV antigens, and association with and, possibly, causation of disease states may set them apart from typical self-antigens. Consequently, the pathogenic or, indeed, host-protective capacity of antibodies targeting ERV antigens is likely to be context-dependent. Here, we review the immunogenicity of typical ERV proteins, with emphasis on the antibody response and its potential disease implications.

Interplay between endogenous and exogenous human retroviruses

In humans, retroviruses thrive more as symbionts than as parasites. Apart from the only two modern exogenous human retroviruses (human T-cell lymphotropic and immunodeficiency viruses; HTLV and HIV, respectively), ~8% of the human genome is occupied by ancient retroviral DNA [human endogenous retroviruses (HERVs)]. Here, we review the recent discoveries about the interactions between the two groups, the impact of infection by exogenous retroviruses on the expression of HERVs, the effect of HERVs on the pathogenicity of HIV and HTLV and on the severity of the diseases caused by them, and the antiviral protection that HERVs can allegedly provide to the host. Tracing the crosstalk between contemporary retroviruses and their endogenized ancestors will provide better understanding of the retroviral world.

Not all LGL leukemias are created equal

Large Granular Lymphocyte (LGL) Leukemia is a rare, heterogeneous even more that once thought, chronic lymphoproliferative disorder characterized by the clonal expansion of T- or NK-LGLs that requires appropriate immunophenotypic and molecular characterization. As in many other hematological conditions, genomic features are taking research efforts one step further and are also becoming instrumental in refining discrete subsets of LGL disorders. In particular, STAT3 and STAT5B mutations may be harbored in leukemic cells and their presence has been linked to diagnosis of LGL disorders. On clinical grounds, a correlation has been established in CD8+ T-LGLL patients between STAT3 mutations and clinical features, in particular neutropenia that favors the onset of severe infections. Revisiting biological aspects, clinical features as well as current and predictable emerging treatments of these disorders, we will herein discuss why appropriate dissection of different disease variants is needed to better manage patients with LGL disorders.

High prevalence of low-allele-fraction somatic mutations in STAT3 in peripheral blood CD8+ cells in multiple sclerosis patients and controls

Somatic mutations have a central role in cancer, but there are also a few rare autoimmune diseases in which somatic mutations play a major role. We have recently shown that nonsynonymous somatic mutations with low allele fractions are preferentially detectable in CD8+ cells and that the STAT3 gene is a promising target for screening. Here, we analyzed somatic mutations in the STAT3 SH2 domain in peripheral blood CD8+ cells in a set of 94 multiple sclerosis (MS) patients and 99 matched controls. PCR amplicons targeting the exons 20 and 21 of STAT3 were prepared and sequenced using the Illumina MiSeq instrument with 2x300bp reads. We designed a novel variant calling method, optimized for large number of samples, high sequencing depth (>25,000x) and small target genomic area. Overall, we discovered 64 STAT3 somatic mutations in the 193 donors, of which 63 were non-synonymous and 77% have been previously reported in cancer or lymphoproliferative disease. The overall median variant allele fraction was 0.065% (range 0.007-1.2%), without significant difference between MS and controls (p = 0.82). There were 26 (28%) MS patients vs. 24 (24%) controls with mutations (p = 0.62). Two or more mutations were found in 9 MS patients vs. 2 controls (p = 0.03, pcorr = 0.12). Carriership of mutations associated with older age and lower neutrophil counts. These results demonstrate that STAT3 SH2 domain is a hotspot for somatic mutations in CD8+ cells with a prevalence of 26% among the participants. There were no significant differences in the mutation prevalences between MS patients and controls. Further research is needed to elucidate the role of antigenic stimuli in the expansion of the mutant clones. Furthermore, the high discovered prevalence of STAT3 somatic mutations makes it feasible to analyze these mutations directly in tissue-infiltrating CD8+ cells in autoimmune diseases.

Pathogenesis and Treatment of T-Large Granular Lymphocytic Leukemia (T-LGLL) in the Setting of Rheumatic Disease

A complex relationship exists between rheumatic diseases and cancer. This delicate balance between chronic inflammation and malignant cell transformation in hematologic neoplasms has been observed, but is not well defined. Large Granular Lymphocyte (LGL) leukemia is at the intersection of a clonal lymphoproliferative disease, chronic inflammation, and autoimmunity. The association between rheumatoid arthritis (RA) and the spectrum of Felty’s Syndrome is well-known. Other rheumatic disorders have been reported including systemic lupus erythematosus (SLE), Sjogren’s Syndrome (SS), vasculitis, Behcet’s Disease (BD) and systemic sclerosis. The association between T-LGLL and rheumatic disease pathogenesis has been hypothesized, but has not yet been fully understood. Components of a shared pathogenesis includes chronic antigen stimulation, JAK-STAT pathway activation and overlap of various cytokines. We will summarize current knowledge on the molecular understanding between T-LGLL and rheumatic disease. There are many potential areas of research to help meet this need and lead to development of targeted therapeutic options.

Retrovirus insertion site analysis of LGL leukemia patient genomes

Background

Large granular lymphocyte (LGL) leukemia is an uncommon cancer characterized by sustained clonal proliferation of LGL cells. Antibodies reactive to retroviruses have been documented in the serum of patients with LGL leukemia. Culture or molecular approaches have to date not been successful in identifying a retrovirus.

Methods

Because a retrovirus must integrate into the genome of an infected cell, we focused our efforts on detecting a novel retrovirus integration site in the clonally expanded LGL cells. We present a new computational tool that uses long-insert mate pair sequence data to search the genome of LGL leukemia cells for retrovirus integration sites. We also utilize recently published methods to interrogate the status of polymorphic human endogenous retrovirus type K (HERV-K) provirus in patient genomes.

Results

Our data show that there are no new retrovirus insertions in LGL genomes of LGL leukemia patients. However, our insertion call tool did detect four HERV-K provirus integration sites that are polymorphic in the human population but absent from the human reference genome, hg19. To determine if the prevalence of these or other polymorphic proviral HERV-Ks differed between LGL leukemia patients and the general population, we used a recently developed tool that reports sites in the human genome occupied by a known proviral HERV-K. We report that there are significant differences in the number of polymorphic HERV-Ks in the genomes of LGL leukemia patients of European origin compared to individuals with European ancestry in the 1000 genomes (KGP) data.

Conclusions

Our study confirms that the clonal expansion of LGL cells in LGL leukemia is not driven by the integration of a new infectious or endogenous retrovirus, although we do not rule out that these cells are responding to retroviral antigens produced in other cell types. However, our computational analyses revealed that the genomes of LGL leukemia patients carry a higher burden of polymorphic HERV-K proviruses compare to individuals from KGP of European ancestry. Our research emphasizes the merits of comprehensive genomic assessment of HERV-K in cancer samples and suggests that further analyses to determine contributions of HERV-K to LGL leukemia are warranted.

Electronic supplementary material

The online version of this article (10.1186/s12920-019-0549-9) contains supplementary material, which is available to authorized users.

Clinical Features, Pathogenesis, and Treatment of Large Granular Lymphocyte Leukemias

Large granular lymphocytes (LGLs) are large lymphocytes with azurophilic granules in their cytoplasm. LGLs are either natural killer (NK) cells or T lymphocytes. Expansions of the LGLs in the peripheral blood are seen in various conditions, including three clonal disorders: T-cell LGL (T-LGL) leukemia, chronic lymphoproliferative disorders of NK cells (CLPD-NK), and aggressive NK-cell leukemia (ANKL). However, the monoclonal and polyclonal expansion of LGLs has been associated with many other conditions. The present article describes these LGL disorders, with special emphasis on the clinical features, pathogenesis, and treatments of the three above-mentioned clonal disorders.

Increased seroreactivity to human T cell lymphoma/leukemia virus-related endogenous sequence-1 Gag peptides in patients with human T cell lymphoma/leukemia virus myelopathy

Previously, we had shown that although only 8% of patients with large granular lymphocytic leukemia (LGLL) were infected with human T cell lymphoma/leukemia virus (HTLV)-2, almost half had antibodies to HTLV Gag and Env peptides. Herein, we investigated whether this could be due to cross-reactive antibodies to two homologous peptides in the Gag protein of the endogenous retrovirus HTLV-related endogenous sequence-1 (HRES-1). In addition, we had previously shown that patients with HTLV neurodegenerative diseases had increased seroreactivity to homologous HERV-K10 endogenous retrovirus peptides. Hence, in this study we also examined whether these patients had increased seroreactivity to the aforementioned HRES-1 Gag peptides. Sera from 100 volunteer blood donors (VBD), 53 patients with LGLL, 74 subjects with HTLV-1 or 2 infection (58 nonmyelopathy and 16 myelopathy), and 83 patients with multiple sclerosis (MS) were evaluated. The HTLV-positive myelopathy (HAM) patients had a statistically increased prevalence of antibodies to both HRES-1 Gag peptides (81%) vs. the VBD (0%), LGLL patients (13%), and MS patients (1%), and the HTLV-positive nonmyelopathy subjects (21%). The data suggest that cross-reactivity to HRES-1 peptides could be involved in the pathogenesis of HAM. The difference between the VBD and LGLL patients was also statistically significant, also suggesting a possible association in a minority of patients.

Advances in diagnosis and treatment of large granular lymphocyte syndrome

PURPOSE OF REVIEW

Large granular lymphocyte (LGL) syndrome comprises a clonal spectrum of T-cell and natural killer (NK)-cell LGL lymphoproliferative disorders associated with neutropenia. This review presents advances in diagnosis and therapy of LGL syndrome.

RECENT FINDINGS

Due to the lack of a single unique genetic or phenotypic feature and clinicopathological overlap between reactive and neoplastic entities, accurate LGL syndrome diagnosis should be based on the combination of morphologic, immunophenotypic, and molecular studies as well as clinical features. For diagnosis and monitoring of LGL proliferations, it is essential to perform flow cytometric blood and/or bone marrow analysis using a panel of monoclonal antibodies to conventional and novel T-cell and NK-cell antigens such as NK-cell receptors and T-cell receptor β-chain variable region families together with TCR gene rearrangement studies. Treatment of symptomatic cytopenias in patients with indolent LGL leukemia is still based on immunosuppressive therapy. Treatment with purine analogs and alemtuzumab may be considered as an alternative option.

SUMMARY

Progress in understanding the pathogenetic mechanisms of these entities, especially resistance of clonal LGLs to apoptosis, due to constitutive activation of survival signaling pathways, has its impact on identification of potential molecular therapeutic targets.

Human T-cell leukemia virus type 3 (HTLV-3) and HTLV-4 antisense-transcript-encoded proteins interact and transactivate Jun family-dependent transcription via their atypical bZIP motif

Human T-cell leukemia virus types 3 and 4 (HTLV-3 and HTLV-4) are recently isolated retroviruses. We have previously characterized HTLV-3- and HTLV-4-encoded antisense genes, termed APH-3 and APH-4, respectively, which, in contrast to HBZ, the HTLV-1 homologue, do not contain a typical bZIP domain (M. Larocque É Halin, S. Landry, S. J. Marriott, W. M. Switzer, and B. Barbeau, J. Virol. 85:12673-12685, 2011, doi:10.1128/JVI.05296-11). As HBZ differentially modulates the transactivation potential of various Jun family members, the effect of APH-3 and APH-4 on JunD-, c-Jun-, and JunB-mediated transcriptional activation was investigated. We first showed that APH-3 and APH-4 upregulated the transactivation potential of all tested Jun family members. Using an human telomerase catalytic subunit (hTERT) promoter construct, our results also highlighted that, unlike HBZ, which solely modulates hTERT expression via JunD, both APH-3 and APH-4 acted positively on the transactivation of the hTERT promoter mediated by tested Jun factors. Coimmunoprecipitation experiments demonstrated that these Jun proteins interacted with APH-3 and APH-4. Although no activation domain was identified for APH proteins, the activation domain of c-Jun was very important in the observed upregulation of its activation potential. We further showed that APH-3 and APH-4 required their putative bZIP-like domains and corresponding leucine residues for interaction and modulation of the transactivation potential of Jun factors. Our results demonstrate that HTLV-encoded antisense proteins behave differently, and that the bZIP-like domains of both APH-3 and APH-4 have retained their interaction potential for Jun members. These studies are important in assessing the differences between HBZ and other antisense proteins, which might further contribute to determining the role of HBZ in HTLV-1-associated diseases. IMPORTANCE HBZ, the antisense transcript-encoded protein from HTLV-1, is now well recognized as a potential factor for adult T-cell leukemia/lymphoma development. In order to better appreciate the mechanism of action of HBZ, comparison to antisense proteins from other HTLV viruses is important. Little is known in relation to the seemingly nonpathogenic HTLV-3 and HTLV-4 viruses, and studies of their antisense proteins are limited to our previously reported study (M. Larocque É Halin, S. Landry, S. J. Marriott, W. M. Switzer, and B. Barbeau, J. Virol. 85:12673-12685, 2011, doi:10.1128/JVI.05296-11). Here, we demonstrate that Jun transcription factors are differently affected by APH-3 and APH-4 compared to HBZ. These intriguing findings suggest that these proteins act differently on viral replication but also on cellular gene expression, and that highlighting their differences of action might lead to important information allowing us to understand the link between HTLV-1 HBZ and ATL in infected individuals.

Uncovering the pathogenesis of large granular lymphocytic leukemia-novel STAT3 and STAT5b mutations

Large granular lymphocytic (LGL) leukemia is an incurable chronic disease, characterized by clonal expansion of cytotoxic T- or NK-cells in blood and bone marrow. Cytopenias (anemia, neutropenia) and autoimmune disorders such as rheumatoid arthritis are the most common clinical manifestations of LGL leukemia. Recently, somatic activating STAT3 gene mutations were shown to be specific for LGL leukemia with a prevalence of up to 70%. Analogous mutations in the STAT5b gene were seen in a smaller proportion of patients. These gain-of-function mutations are located in the SH2 domain of STAT3 and affect the phosphotyrosine-SH2 interaction required for dimerization of STAT3. The mutations increase the phosphorylation of STAT3 and STAT5b and enhance the transcriptional activity of the mutated proteins. STAT3 and STAT5b mutations can be used as molecular markers for LGL leukemia diagnostics, and they present novel therapeutic targets for STAT3 and STAT5b inhibitors, which currently are in development for treatment of cancer and autoimmune disorders.

Activating KIRs in Chronic Lymphoproliferative Disorder of NK Cells: Protection from Viruses and Disease Induction?

Human natural killer (NK) cells are functionally regulated by killer cell immunoglobulin-like receptors (KIRs) and their interactions with HLA class I molecules. As KIR expression in a given NK cell is stochastically established, KIR repertoire perturbations reflect a dominance of discrete NK-cell subsets as the consequence of adaptation of the NK-cell compartment to exogenous agents, more often represented by virus infection. Although inhibitory interactions between KIR and their cognate HLA class I ligands abrogate effector responses of NK cells, they are also required for the functional education of NK cell. The biology and molecular specificities of the activating KIRs are less well defined, and most interactions with presumed HLA class I ligands are weak. Interestingly, epidemiologic studies link activating KIR genes to resistance against numerous virus infections. Chronic lymphoproliferative disorder of NK cells (CLPD-NK) is an indolent NK cell disease characterized by a persistent increase of circulating NK cells (usually exceeding 500 NK cells/mm(3)). The mechanism through which NK cells are induced to proliferate during CLPD-NK pathogenesis is still a matter of debate. Accumulating data suggest that exogenous agents, in particular viruses, might play a role. The etiology of CLPD-NK, however, is largely unknown. This is likely due to the fact that not a single, specific agent is responsible for the NK cells proliferation, which perhaps represents the expression of an abnormal processing of different foreign antigens, sharing a chronic inflammatory background. Interestingly, proliferating NK cells are typically characterized by expression of a restricted pattern of KIR, which have been demonstrated to be mostly represented by the activating form. This finding indicates that these receptors may be directly involved in the priming of NK cells proliferation.

Increased seroreactivity to HERV-K10 peptides in patients with HTLV myelopathy

BACKGROUND

Previously, we had shown that persons infected with human T-cell lymphoma leukemia virus 1 or 2 (HTLV-1 or 2) had an increased prevalence of antibodies to a peptide in the Pol protein of the retrovirus HERV-K10, homologous to a peptide in HTLV gp21 envelope protein. The prevalence rate was higher in those with myelopathy vs. non-myelopathy. We have now extended our observations to a cohort restricted to North America in whom the diagnosis of HTLV myelopathy was rigorously confirmed to also test for reactivity to another HERV-K10 peptide homologous to the HTLV p24 Gag protein.

METHODS

Sera from 100 volunteer blood donors (VBD), 53 patients with large granular lymphocytic leukemia (LGLL), 74 subjects with HTLV-1 or 2 infection (58 non-myelopathy and 16 myelopathy) and 83 patients with multiple sclerosis (MS) were evaluated in ELISA assays using the above peptides.

RESULTS

The HTLV myelopathy patients had a statistically significant increased prevalence of antibodies to both HERV-K10 peptides (87.5%) vs. the VBD (0%), LGLL patients (0%), MS patients (4.8%), and the HTLV positive non-myelopathy subjects (5.2%).

CONCLUSION

The data suggest that immuno-cross-reactivity to HERV-K10 peptides and/or transactivation of HERV-K10 expression by the HTLV Tax protein may be involved in the pathogenesis of HTLV-associated myelopathy/tropical spastic paraparesis and spastic ataxia.

Animal models on HTLV-1 and related viruses: what did we learn?

Retroviruses are associated with a wide variety of diseases, including immunological, neurological disorders, and different forms of cancer. Among retroviruses, Oncovirinae regroup according to their genetic structure and sequence, several related viruses such as human T-cell lymphotropic viruses types 1 and 2 (HTLV-1 and HTLV-2), simian T cell lymphotropic viruses types 1 and 2 (STLV-1 and STLV-2), and bovine leukemia virus (BLV). As in many diseases, animal models provide a useful tool for the studies of pathogenesis, treatment, and prevention. In the current review, an overview on different animal models used in the study of these viruses will be provided. A specific attention will be given to the HTLV-1 virus which is the causative agent of adult T-cell leukemia/lymphoma (ATL) but also of a number of inflammatory diseases regrouping the HTLV-associated myelopathy/tropical spastic paraparesis (HAM/TSP), infective dermatitis and some lung inflammatory diseases. Among these models, rabbits, monkeys but also rats provide an excellent in vivo tool for early HTLV-1 viral infection and transmission as well as the induced host immune response against the virus. But ideally, mice remain the most efficient method of studying human afflictions. Genetically altered mice including both transgenic and knockout mice, offer important models to test the role of specific viral and host genes in the development of HTLV-1-associated leukemia. The development of different strains of immunodeficient mice strains (SCID, NOD, and NOG SCID mice) provide a useful and rapid tool of humanized and xenografted mice models, to test new drugs and targeted therapy against HTLV-1-associated leukemia, to identify leukemia stem cells candidates but also to study the innate immunity mediated by the virus. All together, these animal models have revolutionized the biology of retroviruses, their manipulation of host genes and more importantly the potential ways to either prevent their infection or to treat their associated diseases.

Seroreactivity to LGL leukemia-specific epitopes in aplastic anemia, myelodysplastic syndrome and paroxysmal nocturnal hemoglobinuria: results of a bone marrow failure consortium study

Large granular lymphocyte (LGL) leukemia is characterized by clonal expansion of antigen-activated cytotoxic T cells (CTL). Patients frequently exhibit seroreactivity against a human T-cell leukemia virus (HTLV) epitope, BA21. Aplastic anemia, paroxysmal nocturnal hemoglobinuria and myelodysplastic syndrome are bone marrow failure diseases that can also be associated with similar aberrant CTL activation (LGL-BMF). We identified a BA21 peptide that was specifically reactive with LGL leukemia sera and found significantly elevated antibody reactivity against the same peptide in LGL-BMF sera. This finding of shared seroreactivity in LGL-BMF conditions and LGL leukemia suggests that these diseases might share a common pathogenesis.

Human T-cell lymphotropic virus type 3 (HTLV-3)- and HTLV-4-derived antisense transcripts encode proteins with similar Tax-inhibiting functions but distinct subcellular localization

The human T-cell lymphotropic virus (HTLV) retrovirus family is composed of the well-known HTLV type 1 (HTLV-1) and HTLV-2 and the most recently discovered HTLV-3 and HTLV-4. Like other retroviruses, HTLV-1 and HTLV-2 gene expression has been thought to be orchestrated through a single transcript. However, recent reports have demonstrated the unique potential of both HTLV-1 and HTLV-2 to produce an antisense transcript. Furthermore, these unexpected and newly identified transcripts lead to the synthesis of viral proteins termed HBZ (HTLV-1 basic leucine zipper) and APH-2 (antisense protein of HTLV-2), respectively. As potential open reading frames are present on the antisense strand of HTLV-3 and HTLV-4, we tested whether in vitro antisense transcription occurred in these viruses and whether these transcripts had a coding potential. Using HTLV-3 and HTLV-4 proviral DNA constructs, antisense transcripts were detected by reverse transcriptase PCR. These transcripts are spliced and polyadenylated and initiate at multiple sites from the 3' long terminal repeat (LTR). The resulting proteins, termed APH-3 and APH-4, are devoid of a typical basic leucine zipper domain but contain basic amino acid-rich regions. Confocal microscopy and Western blotting experiments demonstrated a nucleus-restricted pattern for APH-4, while APH-3 was localized both in the cytoplasm and in the nucleus. Both proteins showed partial colocalization with nucleoli and HBZ-associated structures. Finally, both proteins inhibited Tax1- and Tax3-mediated HTLV-1 and HTLV-3 LTR activation. These results further demonstrate that retroviral antisense transcription is not exclusive to HTLV-1 and HTLV-2 and that APH-3 and APH-4 could impact HTLV-3 and HTLV-4 replication.

HTLV infection among foreign pregnant women living in Spain

BACKGROUND

The overall seroprevalence of HTLV infection among pregnant women in Spain is below 0.02% and accordingly universal antenatal screening is not recommended. However, as the number of immigrants has significantly increased during the last decade, this population might warrant specific considerations.

OBJECTIVE

To evaluate the seroprevalence of HTLV infection among immigrant pregnant women living in Spain.

METHODS

From January 2009 to December 2010 a cross-sectional study was carried out in all foreign pregnant women attended at 14 Spanish clinics. All were tested for HTLV antibodies using a commercial enzyme-immunoassay, being reactive samples confirmed by Western blot or PCR.

RESULTS

A total of 3337 foreign pregnant women were examined. Their origin was as follows: Latin America 1579 (47%), North Africa 507 (16%), East Europe 606 (18%), Sub-Saharan Africa 316 (9%), North America and West Europe 116 (3.5%) and Asia and Australia 163 (5%). A total of 7 samples were confirmed as HTLV positive, of which 6 were HTLV-1 and 1 HTLV-2. HTLV-1 infection was found in 5 women coming from Latin America and 1 from Morocco. The only woman with HTLV-2 came from Ghana. The overall HTLV seroprevalence was 0.2%, being 0.3% among Latin Americans and 0.2% among Africans. It was absent among women coming from other regions.

CONCLUSIONS

The seroprevalence of HTLV infection among foreign pregnant women in Spain is 0.2%, being all cases found in immigrants from Latin America and Africa. Given the benefit of preventing vertical transmission, antenatal screening should be recommended in pregnant women coming from these regions.

HTLV-3/STLV-3 and HTLV-4 viruses: discovery, epidemiology, serology and molecular aspects

Human T cell leukemia/lymphoma virus Type 1 and 2 (HTLV-1 and HTLV-2), together with their simian counterparts (STLV-1, STLV-2), belong to the Primate T lymphotropic viruses group (PTLV). The high percentage of homologies between HTLV-1 and STLV-1 strains, led to the demonstration that most HTLV-1 subtypes arose from interspecies transmission between monkeys and humans. STLV-3 viruses belong to the third PTLV type and are equally divergent from both HTLV-1 and HTLV-2. They are endemic in several monkey species that live in West, Central and East Africa. In 2005, we, and others reported the discovery of the human homolog (HTLV-3) of STLV-3 in two asymptomatic inhabitants from South Cameroon whose sera exhibited HTLV indeterminate serologies. More recently, two other cases of HTLV-3 infection in persons living in Cameroon were reported suggesting that this virus is not extremely rare in the human population living in Central Africa. Together with STLV-3, these human viral strains belong to the PTLV-3 group. A fourth HTLV type (HTLV-4) was also discovered in the same geographical area. The overall PTLV-3 and PTLV-4 genomic organization is similar to that of HTLV-1 and HTLV-2 with the exception of their long terminal repeats (LTRs) that contain only two 21 bp repeats. As in HTLV-1, HTLV-3 Tax contains a PDZ binding motif while HTLV-4 does not. An antisense transcript was also described in HTLV-3 transfected cells. PTLV-3 molecular clones are now available and will allow scientists to study the viral cycle, the tropism and the possible pathogenicity in vivo. Current studies are also aimed at determining the prevalence, distribution, and modes of transmission of these viruses, as well as their possible association with human diseases. Here we will review the characteristics of these new simian and human retroviruses, whose discovery has opened new avenues of research in the retrovirology field.