Research for a retrovirus in myasthenic thymus and study for the pathogenesis

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By electron microscopy, we have demonstrated in retrovirus particles in cultured thymus cells (thymomas and thymus hyperplasias) of patients with myasthenia gravis. In most cases, these particles were detected in thymus epithelial cells when cultured with mitomycin C-treated human B-cells. For further characterization of these particles, virus-producer cell lines were established by co-culturing human T-lymphoblastoid cells with B-cell-stimulated human thymus cells. Mn^<++>-dependent RNA- directed DNA polymerase activity was detected at a density of 1.15-1.17 g/cm^3 on sucrose density gradients in the culture fluid of these co-cultured T-cells. We molecularly cloned the cDNA prepared by the detergent-activated endogenous polymerase reaction in the culture fluid of KK-ll cells, one of the virus-producer T-cell lines. The ^<32>p-labelled cloned cDNA hybridized to DNAs from freshly prepared hyman thymoma cells, in addition to KK-ll cell DNA. We sequenced about 400-1200 bp of seven cDNA clones, and the sequence of each cDNA was compared with that of all genes with known sequences present in EMBL data base. No striking similarity was observed when compared to other sequences. The nucleotide sequence of seven cloned cDNAs constantly revealed high homology (50-70%) with that of retroviruses such as HIV-l and equine infectious anemia virus (EIAV). These results indicate that cDNAs obtained here are new viral genes, possibly retroviral genes, derived from human thymus cells.