Human T-lymphotropic virus type 1 (HTLV-1) causes the debilitating neuroinflammatory disease HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) in a minority of infected individuals, while the majority are lifelong asymptomatic carriers (ACs). The effectiveness of the CD8+ T-cell response to HTLV-1 is a major determinant of the HTLV-1 proviral load (PVL) and the associated HAM/TSP risk. Host genotype, particularly of HLA class I, strongly influences CD8+ T-cell effectiveness against HTLV-1. Furthermore, possession of the KIR gene 2DL2 was recently shown to enhance protective and detrimental HLA class I-associated immunity to HTLV-1 and HCV (Seich al Basatena et al., 2011). The proposed mechanism explaining this effect is a KIR expression-induced increase in activation thresholds, thus promoting survival. I investigated KIR expression and function on T-cells and NK cells in HTLV-1 infection, focussing on CD8+ T-cells and using new anti-KIR mAbs to analyse 2DL2 expression alongside four other KIRs.

This thesis presents evidence supporting the genetic findings and the KIR expression-induced CD8+ T-cell survival hypothesis. AC status was associated with expanded 2DL2+CD8+ T EM cell populations and 2DL2+ and 2DL3+CD8+ T EM cell frequencies were inversely correlated with PVL, consistent with a protective role for 2DL2 (and potentially 2DL3) expression on CD8+ T-cells in HTLV-1 infection. Rather than a generalised reduction in CD8+ T-cell function, specific KIR expression was associated with a shift in effector function profile towards a cytotoxic phenotype without cytokine production. This is consistent with raised activation thresholds and may suggest fine-tuning of CD8+ T-cell effector functions by KIRs, preserving killing without causing inflammation. I further speculate that 2DL2/3 could be a context-dependent modulator of CD8+ T-cell function, optimally tuning responses and contributing to CD8+ T-cell effectiveness. Together with the longevity of 2DL2/3+CD8+ T-cells in vivo, this hypothesis requires further testing, since it may be relevant to other persistent viral infections.