We demonstrate a significantly high incidence of human T-cell leukemia virus type 1 (HTLV-1)-associated myelopathy (HAM)- or tropical spastic paraparesis (TSP)-like symptoms in WKA rats after injection with HTLV-1- producing MT-2 cells, while no symptoms were observed in F344 rats injected with MT-2 cells or in control WKA rats. Five of the eight (63%) WKA rats injected with MT-2 cells showed HAM/TSP-like paraparesis at 105 weeks of age, but none of seven MT-2-injected F344 rats or eight control WKA rats showed symptoms. This high incidence of HAM/TSP-like symptoms in WKA rats was statistically significant (P < 0.05). Six of the eight (75%) WKA rats injected with MT-2 cells showed HAM/TSP-like paraparesis at 108 weeks of age. HAM/TSP-like symptoms were also observed in one of the two WKA rats injected with HTLV-1-producing Ra-1 cells at 128 weeks of age. HTLV-1 provirus was detected in peripheral blood mononuclear cells in both WKA and F344 rats. The provirus was detected in the spinal cords of the HAM/TSP-like WKA rats that had severe neuropathological changes. WKA and F344 rats showed no significant difference in antibody response against HTLV-1 Gag antigen. However, the antibody response against the C-terminal half of gp46 HTLV-1 envelope protein was lower in WKA rats than in F344 rats. Pathological analysis of the HAM/TSP-like rats showed degeneration of the white matter of the spinal cord and peripheral nerves. These findings suggest that both the genetic background of the host and HTLV-1 infection are important in neuropathogenesis of HAM/TSP-like paraparesis in rats.
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