Brain white matter lesions in HAM/TSP: do they have any special meaning?

Human T-cell lymphotropic virus type I (HTLV-I)-associated myelopathy/tropical spastic paraparesis (HAM/TSP) is a disease that is clinically characterized by unremitting myelopathic symptoms, such as spastic paraparesis, lower back pain, bladder dysfunction, and mild sensory disorders of the limbs, which are symmetrical in most patients1. HTLV-I infection is a necessary condition for the development of HAM/TSP, but is insufficient on its own for this to occur. Risk factors play a pivotal role as disease “determinants”. High proviral load (PVL)>1% peripheral blood mononuclear cells, PBMCs)2,3, age, HTLV-I subtypes, and gender have been recognized as important risk factors. Interestingly, not only some host genetic factors have been identified as noteworthy risk factors, but also other factors have been identified as highly protective ones against the development of the disease4-6. Even though the pathological mechanisms for HAM/TSP so far remain incompletely understood, a great deal of knowledge has been gained in this regard. The majority (around 95%) of HTLV-I-infected individuals develop a strong, efficient, and protective CD8+ cytotoxic T-lymphocyte (CTL) response against the virus. It prevents an expansion of the infected target cells (CD4+ T cells), reduces the proviral load and keeps it under control, and therefore the disease does not develop (phase 0 of infection*). Nevertheless, when HTLV-I infection is combined with pivotal risk factors, the CTL protective response against HTLV-I will, at some point in time, gradually turn into a deregulated strong response to HTLV-I infected target cells. Consequently, an inflammatory reaction occurs, characteristically involving the watershed zones of the spinal cord and also, to a lesser extent, involving the brain7,8. In these regions, the vascular architecture leads to a slowing of the blood flow. The disease then starts and progresses over a period of years (phase 1 of infection*). After that, the patients enter into a stage characterized by spinal cord scarring and sequelae (phase 2 of infection*). Phase 1 starts when the target cells influenced by the HTLV-I tax protein promote expression of the ligands on their surface, which starts production of pro-inflammatory cytokines such as IFN-gamma, TNF-alpha and IL-2, and also of the chemoattractant factor MCP-1. On the other hand, IFN-gamma induces expression of adhesion molecules in the endothelial cells of the microvessels of the watershed zones. Therefore, this combined activity leads to chemotaxis, adhesion and migration, not only of the infected target cells, but also of the CTLs from the vascular compartment into the CNS tissue itself. As a consequence of IL-2 stimulus, CTLs proliferate and this leads to target cell apoptosis. Macrophages phagocytize apoptotic target cells, become infected and, under the influence of the HTLV-I Tax protein, produce MIP and other chemokines. This increases CTL degranulation and leads to greater apoptosis of the HTLV-I-infected target cells. Moreover, many other cytokines, chemoattractants and immunological molecules also play an important role in the HAM/TSP pathogenesis. Histopathologically, the disease affects the spinal cord, and mainly at the thoracic level, where the damage is most severe in the middle to lower regions. Degeneration is seen in the lateral corticospinal tract as well as in the spinocerebellar or spinothalamic tract of the lateral column7. Perivascular and parenchymal lymphocytic infiltration, along with foamy macrophages, astrocyte proliferation and fibrillary gliosis, is seen in such lesions9. The widespread loss of myelin and axons are noteworthy, particularly in the corticospinal tracts of the spinal MD, Infectologist, Neurologist and CSF Specialist, Fundação José Silveira, Salvador BA, Brazil.