Cell-mediated immune response to human T-lymphotropic virus type I

Latency Reversing Agents: Kick and Kill of HTLV-1?

Human T-cell leukemia virus type 1 (HTLV-1), the cause of adult T-cell leukemia/lymphoma (ATLL), is a retrovirus, which integrates into the host genome and persistently infects CD4⁺ T-cells. Virus propagation is stimulated by (1) clonal expansion of infected cells and (2) de novo infection. Viral gene expression is induced by the transactivator protein Tax, which recruits host factors like positive transcription elongation factor b (P-TEFb) to the viral promoter. Since HTLV-1 gene expression is repressed in vivo by viral, cellular, and epigenetic mechanisms in late phases of infection, HTLV-1 avoids an efficient CD8⁺ cytotoxic T-cell (CTL) response directed against the immunodominant viral Tax antigen. Hence, therapeutic strategies using latency reversing agents (LRAs) sought to transiently activate viral gene expression and antigen presentation of Tax to enhance CTL responses towards HTLV-1, and thus, to expose the latent HTLV-1 reservoir to immune destruction. Here, we review strategies that aimed at enhancing Tax expression and Tax-specific CTL responses to interfere with HTLV-1 latency. Further, we provide an overview of LRAs including (1) histone deacetylase inhibitors (HDACi) and (2) activators of P-TEFb, that have mainly been studied in context of human immunodeficiency virus (HIV), but which may also be powerful in the context of HTLV-1.

HTLV-I infection: a dynamic struggle between viral persistence and host immunity

Human T-lymphotropic virus type I (HTLV-I) causes chronic infection for which there is no cure or neutralising vaccine. HTLV-I has been clinically linked to the development of adult T-cell leukaemia/lymphoma (ATL), an aggressive blood cancer, and HAM/TSP, a progressive neurological and inflammatory disease. Infected individuals typically mount a large, persistently activated CD8(+) cytotoxic T-lymphocyte (CTL) response against HTLV-I-infected cells, but ultimately fail to effectively eliminate the virus. Moreover, the identification of determinants to disease manifestation has thus far been elusive. A key issue in current HTLV-I research is to better understand the dynamic interaction between persistent infection by HTLV-I and virus-specific host immunity. Recent experimental hypotheses for the persistence of HTLV-I in vivo have led to the development of mathematical models illuminating the balance between proviral latency and activation in the target cell population. We investigate the role of a constantly changing anti-viral immune environment acting in response to the effects of infected T-cell activation and subsequent viral expression. The resulting model is a four-dimensional, non-linear system of ordinary differential equations that describes the dynamic interactions among viral expression, infected target cell activation, and the HTLV-I-specific CTL response. The global dynamics of the model is established through the construction of appropriate Lyapunov functions. Examining the particular roles of viral expression and host immunity during the chronic phase of HTLV-I infection offers important insights regarding the evolution of viral persistence and proposes a hypothesis for pathogenesis.

Altered expression of degranulation-related genes in CD8+ T cells in human T lymphotropic virus type I infection

Human T lymphotropic virus type I (HTLV-1) is the etiological agent of HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). CD8+ T cells may contribute to the protection or development of HAM/TSP. In this study we used SAGE methodology to screen for differentially expressed genes in CD8+ T cells isolated from HTLV-1 asymptomatic carriers (HAC) and from HAM/TSP patients to identify genes involved in HAM/TSP development. SAGE analysis was conducted by pooling samples according to clinical status. The comparison of gene expression profiles between HAC and HAM/TSP libraries identified 285 differentially expressed tags. We focus on cytotoxicity and cytokine-related genes due to their potential biological role in HTLV-1 infection. Our results showed that patients with HAM/TSP have high expression levels of degranulation-related genes, namely GZMH and PRF1, and of the cytoskeletal adaptor PXN. We found that GZMB and ZAP70 were overexpressed in HTLV-infected patients compared to the noninfected group. We also detected that CCL5 was higher in the HAM/TSP group compared to the HAC and CT groups. Our findings showed that CD8+ T cells of HAM/TSP patients have an inflammatory and active profile. PXN and ZAP70 overexpression in HTLV-1-infected patients was described for the first time here and reinforces this concept. However, although active and abundant, CD8+ T cells are not able to completely eliminate infected cells and prevent the development of HAM/TSP and, moreover, these cells might contribute to the pathogenesis of the disease by migrating to the central nervous system (CNS). These results should be further tested with biological functional assays to increase our understanding on the role of these molecules in the development of HTLV-1-related diseases.

Bovine leukemia virus p24 antibodies reflect blood proviral load

Background

Bovine leukemia virus (BLV) is worldwide distributed and highly endemic in Argentina. Among the strategies to prevent BLV dissemination, a control plan based on the selective segregation of animals according to their proviral load (PVL) is promising for our dairy productive system. The objective of this work was to study the relationship between the blood PVL and the antibody level, in order to identify whether the individual humoral response, i.e. the anti-p24 or anti-whole-BLV particle, could be used as a marker of the blood level of infection and thus help to recruit animals that may pose a lower risk of dissemination under natural conditions.

Results

The prevalence of p24 antibodies on the 15 farms studied was over 66%. The prevalence of p24 and whole-BLV antibodies and PVL quantification were analyzed in all the samples (n = 196) taken from herds T1 and 51. ROC analysis showed a higher AUC for p24 antibodies than whole-BLV antibodies (Z_reactivity: 3.55, P < 0.001; Z_titer: 2.88, P < 0.01), and as consequence a better performance to predict the proviral load status in herd 51. No significant differences were found between the performance of p24 and whole-BLV antibodies in herd T1. A significant positive correlation was observed between PVL values and p24 antibody reactivity in both farms (r _T1 = 0.7, P < 0.001, r ₅₁ = 0.71, P < 0.0001). The analysis was extended to the whole number of weak p24 antibody reactors (n = 311) of the other 13 farms. The mean of high PVL reactors within weak p24 reactors was 17.38% (SD = 8.92). In 5/15 farms, the number of weak p24 reactors with high PVL was lower than 10%.

Conclusions

We found that the humoral response reflected the level of in vivo infection, and may therefore have useful epidemiological applications. Whereas the quantitative evaluation of blood proviral load using real-time PCR is expensive and technically demanding, the measurement of antibodies in blood by ELISA is relatively straightforward and could therefore constitute a cost-effective tool in a BLV control intervention strategy, especially in highly infected herds such as Argentinean dairy ones.

Multistability in a model for CTL response to HTLV-I infection and its implications to HAM/TSP development and prevention

Human T-cell leukaemia/lymphoma virus type I (HTLV-I) is a retrovirus that has been identified as the causative agent of HTLV-I-associated myelopathy/tropical spastic paraparesis (HAM/TSP) and other illnesses. HTLV-I infects primarily CD4(+) T cells and the transmission occurs through direct cell-to-cell contact. HAM/TSP patients harbor higher proviral loads in peripheral blood lymphocytes than asymptomatic carriers. Also, HAM/TSP patients exhibit a remarkably high number of circulating HTLV-I-specific CD8(+) cytotoxic T lymphocytes (CTLs) in the peripheral blood. While CTLs have a protective role by killing the infected cells and lowering the proviral load, a high level of CTLs and their cytotoxicity are believed to be a main cause of the development of HAM/TSP. A mathematical model for HTLV-I infection of CD4(+) T cells that incorporates the CD8(+) cytotoxic T-cell (CTL) response is investigated. Our mathematical analysis reveals that the system can stabilize at a carrier steady-state with persistent viral infection but no CTL response, or at a HAM/TSP steady-state at which both the viral infection and CTL response are persistent. We also establish two threshold parameters R(0) and R(1), the basic reproduction numbers for viral persistence and for CTL response, respectively. We show that the parameter R(1) can be used to distinguish asymptomatic carriers from HAM/TSP patients, and as an important control parameter for preventing the development of HAM/TSP.

Proviral load and immune markers associated with human T-lymphotropic virus type 1 (HTLV-1)-associated myelopathy/tropical spastic paraparesis (HAM/TSP) in Peru

Human T-lymphotropic virus type 1 (HTLV-1) is the aetiological agent of HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). The objective of this study is to identify which ex vivo and in vivo markers are associated independently with HAM/TSP in a Peruvian population. Eighty-one subjects (33 men/48 women) were enrolled: 35 presented with HAM/TSP, 33 were asymptomatic HTLV-1 carriers (ACs) and 13 were HTLV-1-seronegative controls (SCs). Ex vivo markers included T cell proliferation and Th1 [interferon (IFN)-gamma], Th2 [interleukin (IL)-4, IL-5], proinflammatory [tumour necrosis factor (TNF)-alpha] and anti-inflammatory (IL-10) cytokine production in non-stimulated peripheral blood mononuclear cell (PBMC) cultures. In vivo CD4(+) T cell count, markers of Th1 [interferon-inducible protein (IP)-10] and Th2 (sCD30) activity in plasma and HTLV-1 proviral load in PBMCs were also evaluated. In univariate analysis, several markers, including T cell proliferation, IFN-gamma, IP-10, sCD30 and proviral load were associated with HAM/TSP, but in a multiple logistic regression analysis only the proviral load remained associated significantly with disease manifestation [adjusted OR 9.10 (1.24-66.91)]. Our findings suggest that HAM/TSP is associated primarily with proviral load, whereas the observed association with some immune markers seems secondary.

The HTLV-I p30 Interferes with TLR4 Signaling and Modulates the Release of Pro- and Anti-inflammatory Cytokines from Human Macrophages

Whereas adaptive immunity has been extensively studied, very little is known about the innate immunity of the host to HTLV-I infection. HTLV-I-infected ATL patients have pronounced immunodeficiency associated with frequent opportunistic infections, and in these patients, concurrent infections with bacteria and/or parasites are known to increase risks of progression to ATL. The Toll-like receptor-4 (TLR4) activation in response to bacterial infection is essential for dendritic cell maturation and links the innate and adaptive immune responses. Recent reports indicate that TLR4 is targeted by viruses such as RSV, HCV, and MMTV. Here we report that HTLV-I has also evolved a protein that interferes with TLR4 signaling; p30 interacts with and inhibits the DNA binding and transcription activity of PU.1 resulting in the down-regulation of the TLR4 expression from the cell surface. Expression of p30 hampers the release of pro-inflammatory cytokines MCP-1, TNF-alpha, and IL-8 and stimulates release of anti-inflammatory IL-10 following stimulation of TLR4 in human macrophage. Finally, we found that p30 increases phosphorylation and inactivation of GSK3-beta a key step for IL-10 production. Our study suggests a novel function of p30, which may instigate immune tolerance by reducing activation of adaptive immunity in ATL patients.

Enhancement of LFA-1-mediated T cell adhesion by human T lymphotropic virus type 1 p12I1

Cell-to-cell transmission of retroviruses, such as human T lymphotropic virus type 1 (HTLV-1), is well documented, but the roles of viral regulatory or other nonstructural proteins in the modulation of T cell adhesion are incompletely understood. In this study we tested the role of the HTLV-1 accessory protein, p12(I), on LFA-1-mediated cell adhesion. p12(I) is critical for early HTLV-1 infection by causing the release of calcium from the endoplasmic reticulum to activate NFAT-mediated transcription. We tested the role of this novel viral protein in mediating LFA-1-dependent cell adhesion. Our data indicated that T cells expressing a mutant HTLV-1 provirus that does not produce p12(I) mRNA (ACH.p12(I)) exhibited reduced LFA-1-mediated adhesion compared with wild-type HTLV-1-expressing cells (ACH). Furthermore, the expression of p12(I) in Jurkat T cells using lentiviral vectors enhanced LFA-1-mediated cell adhesion, which was inhibited by the calcium chelator BAPTA-AM, the calcium channel blocker SK&F 96365, and calpeptin, an inhibitor of the calcium-dependent protease calpain. Similar to the intracellular calcium mobilizer, thapsigargin, the expression of p12(I) in Jurkat T cells induced cell surface clustering of LFA-1 without changing the level of integrin expression. Our data are the first to indicate that HTLV-1 p12(I), in addition to enhancing T cell activation, promotes cell-to-cell spread by inducing LFA-1 clustering on T cells via calcium-dependent signaling.