Introduction

Human T-lymphotropic virus type 1 (HTLV-1) was discovered in 1980 [1]. It is estimated that 15–20 million people are infected worldwide [2–4]. Endemic foci of infection have been reported in Japan, Melanesia, Iran, Central and West Africa, the Caribbean, and South America [5]. HTLV-1 is mainly transmitted by cell-to-cell contact [6, 7], since extracellular release of viral particles is a rare phenomenon in vivo, if it exists at all. Infected mothers may transmit the virus to their newborns through breast-feeding. Sexual intercourse and parenteral exposure may also contribute to inter-individual spread of HTLV-1 [2, 8, 9].

Although most HTLV-1-infected persons remain lifelong asymptomatic, approximately 3 % develop adult T cell leukemia/lymphoma (ATLL) and another 3 % develop HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) [10]. HAM/TSP is a chronic inflammatory disease of the central nervous system characterized by spastic paraparesis, sphincter dysfunction, and mild sensory disturbance in the lower extremities [11]. The reason why some HTLV-1-carriers develop HAM/TSP remains largely unknown, although immune-mediated mechanisms seem to be involved [12]. Clinical outcomes in HTLV-1 carriers most likely depend on an interaction between viral and host genetic factors [7]. In this regard, a high proviral load, specific viral variants and early life infection, such as perinatally from infected mothers, have all been associated with the development of clinical complications in HTLV-1 carriers [13–17].

Studies conducted more than a decade ago highlighted that specific human leukocyte antigen (HLA) haplotypes could influence the susceptibility to develop either ATLL or HAM/TSP by virtue of differences in host immune responsiveness to HTLV-1 specific antigens [18]. The presence of HLA-A*02 and/or HLA-Cw*08 was associated with a low HTLV-1 proviral load and a reduced risk of HAM/TSP [19, 20]. In contrast, HLA-B*07, HLA-B*54:01 and/or HLA-DRB1*01:01 were associated with and increased risk of disease [21–24]. Differences in methodologies to monitor HTLV-1 proviral load, criteria use to define HAM/TSP and demographics could all account for discordant results comparing those studies [25]. Herein, we investigate the potential association of HLA class I and II alleles with the development of HAM/TSP in a well characterized cohort of HTLV-1 individuals.

Patients and methods

The Spanish HTLV register records all reported cases of HTLV-1 and HTLV-2 infections in Spain since 1989. Up to January 2012, a total of 199 individuals with HTLV-1 infection had been reported in Spain. Twenty-five (13 %) had been diagnosed with HAM/TSP using well established criteria [17]. For the purpose of this study, only patients who had frozen peripheral blood mononuclear cells (PBMC) were chosen.

HTLV-1 proviral load was measured using an in-house real-time PCR method with primers and conditions reported elsewhere [26]. Briefly, DNA was extracted from 1x106 PBMC using QIAamp DNA Blood Mini kit (Qiagen, Hilden, Germany). Amplification was carried out using Taqman Universal Master Mix II (Applied Biosystems, Foster City, CA) with specific primers and probes targeting the pol region of HTLV-1. The HTLV-1 DNA copy number was normalized to the amount of cellular DNA by the quantification in parallel of the human albumin gene. Results were expressed as HTLV-1 DNA copies per 104 PBMC.

The analysis of HLA class I (A, B, C) and II (DRB1, DQB1) alleles was performed by PCR-SSO using the bead array technology at the Centro de Transfusiones de la Comunidad de Madrid in Spain. For the purpose of this study, alleles HLA-A*02 and HLA-Cw*08 that had previously been associated to protection from HAM/TSP [19, 20], were classified as HLA protectors. In addition, alleles HLA-B*07, HLA-B*54:01 and HLA-DRB1*01:01 that have been associated with disease susceptibility [21–24], were labelled as HLA pathogenic.

Statistical analysis

Results are given as proportions and median values. Comparisons were made using the Chi square test, with Fisher’s correction when appropriated. Differences were considered to be significant only when p values were lower than 0.05. All analyses were performed using SPSS version 11.0 (SPSS Inc., Chicago, IL).

Results

A total of 40 individuals with HTLV-1 infection had frozen PBMC and were examined. Twelve had been diagnosed with HAM/TSP, with the remaining 28 subjects asymptomatic HTLV-1 carriers. The median age of the study population was 46 years-old, and 57.5 % were females. Although most were from Latin America (80 %), 12.5 % were native Spaniards and 7.5 % were Africans. The main route of infection was heterosexual contact (at least 55 % of cases), with 12.5 % attributed to vertical transmission and 10 % to parenteral exposure. As shown in Table 1, no significant differences in demographics were seen when comparing HAM/TSP patients and asymptomatic HTLV-1 carriers, except for the proportion of native Spaniards. Two of the Spanish HAM/TSP patients acquired HTLV-1 infection following contaminated solid organ transplantation.

Table 1 Main baseline characteristics of the study population
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Patients with HAM/TSP had a median HTLV-1 proviral load greater than asymptomatic carriers (637 vs. 71 copies per 104 PBMC; p = 0.006). The prevalence of the different HLA alleles in the study population is depicted in Table 2. The HLA-B*5401 allele was absent in this population. No association between the presence of protective HLA alleles (HLA-A*02 and/or HLA-Cw*08) and HAM/TSP was found. Furthermore, no significant differences in HTLV-1 proviral load were recognized when comparing individuals with and without these HLA alleles (157 vs. 363 copies/104 PBMC; p = 0.3). In contrast, an association between pathogenic HLA alleles (HLA-B*07 and HLA-DRB1*01:01) and HAM/TSP was found (p = 0.039). Moreover, the subset of patients harbouring these alleles exhibited a greater HTLV-1 proviral load than patients with other HLA alleles (575 vs. 157 copies/104 PBMC; p = 0.027).

Table 2 Comparison of HLA alleles and HTLV-1 proviral load in the study population
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The country of origin of HAM/TSP patients harbouring HLA-B*07 and DRB*0101 was Spain (n = 2), Brazil (n = 1), Ecuador (n = 1) and the Dominican Republic (n = 1). According to the Allele Frequency net (http://www.allelefrequencies.net) no differences between rates of HLA-B*07 and HLA-DRB1*01:01 alleles should be recognized comparing Spaniards, Latin Americans and African populations (0–10 %) [24].

Discussion

HTLV-1 has spread worldwide but is endemic in some parts of Japan, Central and South America, and Sub-Saharan Africa [25]. More interestingly is the recognition of familial clusters of either ATLL or HAM/TSP, suggesting that host factors may influence the susceptibility to develop disease in HTLV-1 carriers. For this reason, the HLA allelic background has been considered by many investigators as a potential determinant of HTLV-1 disease outcome. Whereas HLA-B*07, HLA-B*54:01 and HLA-DRB*01:01 have been associated with increased susceptibility to HAM/TSP in Japan [21–23, 25], it has not been confirmed in France [26]. In our study, HLA-B*07 and/or HLA-DRB*01:01 were significantly more frequent in HAM/TSP patients than in asymptomatic HTLV-1 carriers. It must be noted that most of our HTLV-1 individuals (80 %), including those with HAM/TSP, came from Latin America. In contrast with others, we did not find any significant association between HAM/TSP and either older age or female gender. However, the relatively small size of our study population might have precluded recognition of any influence of demographics on HTLV-1-associated neurological disease. In contrast, the median HTLV-1 proviral load was significantly greater (~10-fold higher) in HAM/TSP patients than in asymptomatic carriers, as found in other studies [15, 17].

We did not find any evidence of a protective effect of HLA-A*02 or HLA-Cw*08 on HAM/TSP, as it was originally reported in Japan [25]. This is in agreement with more recent results obtained by others, especially in studies conducted in Europe [26, 28, 29]. More interesting was the recognition of a significant association between pathogenic HLA-B*07 and/or HLA-DRB*01:01 and high HTLV-1 proviral load, which hypothetically could account for the greater proportion of HAM/TSP in this subset of patients compared to HTLV-1 individuals carrying other HLA alleles.

This is the first study that has examined the distribution of HLA alleles in HTLV-1 individuals living in Spain. We acknowledge that our findings should be interpreted cautiously given the relative small size of our study population that reflects the low HTLV-1 prevalence in the country. It ranges from 0.01 % in pregnant women [30] to 0.1 % in hospital outpatient surveys [31]. A total of 199 individuals with HTLV-1 infection had been reported in Spain up to January 2012. Nearly 80 % were foreigners, mainly immigrants from regions in Latin America (57 %) and Africa (15 %), where HTLV-1 is endemic. Additionally, a total of 25 cases of TSP/HAM and 14 of ATLL had been diagnosed. Both HAM/TSP patients and asymptomatic HTLV-1 carriers were chosen from the national Spanish registry, with only the subset of individuals with available frozen PBMC included in the study. It should be highlighted that no significant differences were found in demographics when comparing the study population and the rest of patients, which might account for any unrecognized bias, with the only exception of the Spanish origin. This is probably because two out of the four Spanish HAM/TSP patients acquired HTLV-1 infection following a contaminated solid organ transplantation, which might have involved a large HTLV-1 inoculum [32]. In addition, the association of HLA DRB1*01:01 and HLA-B*07 alleles with a high HTLV-1 proviral load indirectly further supports our findings.

In summary, HLA alleles DRB1*01:01 and HLA-B*07 influence the susceptibility to develop HAM/TSP in HTLV-1 carriers. This effect seems to be largely mediated by an increased HTLV-1 proviral load. Thus, HLA testing in all HTLV-1 carriers and close monitoring of proviral load and neurological clinical manifestations in those with pathogenic HLA alleles are warranted.