Using three different procedures, immunomagnetic isolation, FACS and panning, each of the T4 cell, T8 cell, monocyte, and B cell population was fractionated from peripheral blood mononuclear cells (PBMC) obtained from 10 human T cell leukemia virus type I (HTLV-I)-associated myelopathy (HAM)/tropical spastic paraparesis (TSP) cases, 3 adult T cell leukemia and lymphoma (ATLL) cases and 9 HTLV-I-infected healthy carriers. DNA from each fraction was then subjected to the quantitative polymerase chain reaction. Although T4 cells possessed the highest copy number of HTLV-I in general, we also detected a significant amount of HTLV-I provirus in T8 cells, monocytes, and B cells. To confirm virus infection of the non-T cell populations, especially the monocyte fraction, we performed further experiments and the following results were obtained: (1) HTLV-I tax RNA expression was confirmed simultaneously in the RNA of monocytes that were negative for T cell receptor β chain (TCRβ) expression in vivo. (2) We cultured these monocytes for a short period of time and detected HTLV-I antigen expression in non-specific esterase-positive monocyte/macrophage cells. Our data indicate that HTLV-I has a broad host range in vivo and that monocytes or cells of monocyte lineage such as tissue macrophages might comprise a virus reservoir in vivo.
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