Hijacking Host Immunity by the Human T-Cell Leukemia Virus Type-1: Implications for Therapeutic and Preventive Vaccines

Abstract

Human T-cell Leukemia virus type-1 (HTLV-1) causes adult T-cell leukemia/lymphoma (ATLL), HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) and other inflammatory diseases. High viral DNA burden (VL) in peripheral blood mononuclear cells is a documented risk factor for ATLL and HAM/TSP, and patients with HAM/TSP have a higher VL in cerebrospinal fluid than in peripheral blood. VL alone is not sufficient to differentiate symptomatic patients from healthy carriers, suggesting the importance of other factors, including host immune response. HTLV-1 infection is life-long; CD4⁺-infected cells are not eradicated by the immune response because HTLV-1 inhibits the function of dendritic cells, monocytes, Natural Killer cells, and adaptive cytotoxic CD8⁺ responses. Although the majority of infected CD4⁺ T-cells adopt a resting phenotype, antigen stimulation may result in bursts of viral expression. The antigen-dependent "on-off" viral expression creates "conditional latency" that when combined with ineffective host responses precludes virus eradication. Epidemiological and clinical data suggest that the continuous attempt of the host immunity to eliminate infected cells results in chronic immune activation that can be further exacerbated by co-morbidities, resulting in the development of severe disease. We review cell and animal model studies that uncovered mechanisms used by HTLV-1 to usurp and/or counteract host immunity.

Keywords: ATLL; HAM/TSP; HTLV-1; HTLV-1-associated myelopathy/tropical spastic paraparesis; adult T-cell leukemia/lymphoma; human T-cell leukemia virus; immunology; vaccine.

Figure 1

HTLV-1 transmission occurs primarily through cell-to-cell contact. Three modes of transmission have been demonstrated: virological synapse, cellular conduits called tunneling nanotubes, and biofilm matrices. HTLV-1 viral proteins enable the evasion of host immunity and contribute to alterations in the innate and adaptive immune responses. Altered responses to chronic HTLV-1 infection lead to inflammation and T-cell exhaustion, and allow clonal expansion of infected cells. While the majority of individuals remain asymptomatic, a subset of infected individuals will progress to diseases such as Adult T-cell Leukemia/Lymphoma, HTLV-1-associated myelopathy/tropical spastic paraparesis, HTLV-1-associated uveitis, bronchiectasis, rheumatoid arthritis, and infective dermatitis.