Human T-cell lymphotropic virus type 1 (HTLV-1) is a retrovirus etiologically linked to adult T-cell leukemia/lymphoma,1,2 HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP), and other inflammatory diseases.3 This virus is endemic in Japan, Caribbean Basin, and some countries in Latin America,4,5 with 5–10 million people infected worldwide.6,7 In Brazil, the highest prevalence of HTLV-1 is found in the North and Northeast, particularly in the cities of Belem, São Luiz, and Salvador.8

In areas endemic for retroviruses, the higher probability of occurrence of co-infections (HIV and HTLV-1, for example), with hepatitis C virus (HCV) may change some characteristics of the disease, such as an altered response to treatment,9 and especially in the pathogenesis of liver disease.9 Indeed, cell mediated immunity involved in the development and progression of liver disease associated with the interaction between HCV and HTLV-1 may contribute to changes in the natural history of the disease caused by these viruses, such as the development of hepatocellular carcinoma in co-infected subjects.10

In previous findings, HAM/TSP progression was associated with T-cell activation in the spinal cord, leading to an inflammatory process and demyelinization.11 A possible cause for higher immune activation could be the presence of higher DNA proviral loads (PVL) among HAM/TSP patients.12 Alternatively, those findings could be due to duration of disease, since we studied a broad range of TSP/HAM cases.13 In case of HTLV-1/HIV-1co-infection, down-regulation of T-cell proliferation, usually present with HIV infection, may not occur, a finding that could be related to the lower survival rate of such patients.14 Based on the consequences of co-infection on HAM/TSP development, we examined the possibility of an association among asymptomatic HTLV-1-co-infected subjects, increase of HTLV-1 DNA proviral load, and T-cell proliferation in a large cohort of HTLV-1-infected subjects in Sao Paulo city, Brazil.

The HTLV outpatient clinic from Institute of Infectious Diseases “Emilio Ribas” (IIER) has been following a cohort of 610 HTLV-1-infected patients for 19 years, starting in July 1997. For the purpose of this study, we recruited a total of 201 HTLV-1-infected subjects who were older than 18 years and remained in active follow-up from June 2011 to May 2012, and were clinically asymptomatic. The Ethical Review Board of the IIER approved the protocol (Number 13/2011), and a signed informed consent was obtained from all participants prior to study inclusion.

All 201 volunteers were asymptomatic according to neurological evaluation and were selected if laboratory results of HTLV-1 proviral load and T-cell proliferation were available and retrievable in the patient's record. Eligible patients were classified according to their serological status in four categories: 121 HTLV-1 asymptomatic monoinfected carriers; 32 HTLV-1/HCV, 29 HTLV-1/HIV-1, and 19 HTLV-1/HIV-1/HCV co-infected patients.

Blood samples were collected in acid-citrate-dextrose solution, and PBMC were separated by Ficoll density gradient centrifugation (Pharmacia, Uppsala, Sweden). Cells were washed with saline solution; cell number was adjusted to 2×106cells and then stored at −80°C. DNA was extracted using a commercial kit (Illustra Tissue and Cells GenomicPrep Mini Spin kit, Easton Turnpike, Fairfield, CA) according to manufacturer's instructions. After this procedure the DNA was stored at −80°C for later analysis.

Previous reports showed contradictory results regarding HTLV-1-HCV co-infection.20–22 If HTLV-1 could worsen the prognosis of HCV disease, as postulated by some authors, it would have been possible to see those poorer outcomes in a large cohort. In reality, we observed an increase in the number of co-infected subjects in our cohort over the years, making clear that co-infections have no impact on the clinical outcome of HTLV-1-infected patients. Neither T-cell proliferation nor DNA HTLV-1 proviral load were increased among HCV/HTLV-1 subjects compared to patients with HTLV-1/HIV-1 co-infection or HTLV-1/HIV-1/HCV. It seems that, even with HIV infection under control, some proteins of this virus cause a down-regulation of T-cell proliferation. On the last years, few new cases of HAM/TSP have been reported among HIV-1/HTLV-1co-infected subjects, despite the occurrence of 10 new cases on the first years of introduction of HAART.22,23

It should be pointed out the higher occurrence of co-infected cases in males. One could infer that males are more exposed to HCV and HIV infections, as intravenous drug use was more common in males in the 1990s.24 These co-infections could affect immunological parameters of HTLV-1-infected patients.

A common finding in HTLV/HIV co-infection is an increased CD4 T-cell count without any additional immunological benefit to the patient,19,25–28 with the possibility of causing an undue delay in the start of antiretroviral treatment in co-infected patients.29 It is possible that earlier diagnosis, associated with better control of HIV infection with newer antiretroviral regimens may, have contributed to reduce the occurrence of HAM/TSP among HIV/HTLV co-infected patients. It has been noted that the CD8 T cells count was increased in all co-infected patients, both with HIV and HCV. This upregulation may be due to the presence of an increased viral load or to immune dysfunction caused by other, non-identified factors.30

Nevertheless, during the acute phase of HCV infection, a potent T-cell response would be an important factor for viral clearance.24,31 However, in the chronic phase the activation of CTLs leads to progressive liver damage. Thus, anti-HCV response could be impaired in HTLV-1-infected patients, the lysis of HCV-infected hepatocytes would be minor, causing a delay in the decision to perform liver biopsy and/or initiating anti-HCV treatment.9 The higher proliferative capacity could have decreased the potential for liver damage. In fact, HCV/HTLV-1 co-infected patients may have less immune mediated liver injury, but other factors associated with HTLV-1 can influence disease progression.32,33 Furthermore, HTLV-1/HCV co-infected patients showed less hepatic injury, and similar data shown in the Miyazaki Cohort Study indicated a possible negative interaction between HTLV-1 and HCV with respect to abnormal levels of ALT, with the prevalence of elevated ALT levels being lower in co-infected subjects than in subjects with HCV alone.34 In Bahia, one study showed that HIV/HTLV-1 and HIV/HCV co-infections could worsen the clinical related outcomes, but virologic and immunologic outcomes were similar. Liver function tests were more impaired in patients with more severe immunosuppression.35 This observation is probably due to an ineffective antiviral therapy for both diseases in the past.

As for HTLV/HIV co-infection, clinical stage at diagnosis should also be carefully considered in the interpretation of clinical studies involving HTLV/HIV/HCV co-infections, since a higher CD4 count due to HTLV-1 infection may lead to bias in the stratification of patients. Moreover, chronic hepatitis C has a prolonged course and the clinical follow-up may not be long enough to allow for definitive conclusions. Current studies have reported higher spontaneous clearance of HCV infection in patients with triple infection (HTLV/HIV/HCV) than in those with HIV/HCV or HCV only.24 In fact, HTLV-1 may modulate the immune response in HTLV/HIV co-infected patients by increasing the production of gamma-interferon and other pro-inflammatory cytokines related to Th1 type response. This mechanism could bring about an enhancement of the immune response against HCV, providing conditions for increased spontaneous clearance of HCV, which depends strongly on the endogenous production of interferons.

In conclusion, we observed a non significant increase of basal T-cell proliferation among HTLV-1 co-infected patients. However, HIV-1 co-infection may induce down-regulation of T-cell proliferation capacity. This interaction may be implicated in liver damage, worsening the prognosis of co-infected patients or, on the contrary, a higher spontaneous clearance of HCV infection in HTLV-1 co-infected patients. It is possible that a decreased T-cell proliferation can be a feasible strategy to avoid HAM/TSP development or to obtain an improvement of its symptoms, as seen with the use of corticosteroids. Further studies including more patients and longer observation periods are needed to clarify this matter.

TA performed the statistical analysis and wrote the main text; TM and MPMB performed the lab assays; AP, MH,JS, APO select the volunteers and discussed the main text; LAM and PJN revised the statistical analysis and discussion of text; JC conceived and read the final version of this manuscript.

The authors declare no conflicts of interest.