Studies were carried out using HTLV-1 as a transforming agent for rabbit cells. Several methods have been successfully used to derive cell lines that have markers characteristics of T cells. These lines are currently being used to characterize and isolate the interleukin-2 receptor of the rabbit and are being used as targets to study HIV-1 infection in the rabbit. In addition to studies of the HTLV-1 cell line in vitro, rabbits infected with the HTLV-L infected human cell line, MT2, are being studied to determine whether the infection has any effect on circulating lymphoid cell populations or on immune function. Certain rabbit cell lines serve as targets for HIV-1 infection, a new line has been derived by transfection of the cell line RL-5 with a gene encoding human CD4. The transfectants were infected with HIV-1 but showed no substantial increase in any parameter related to HIV-1 infection. However, when supernatants from the MT2 cell line were placed on the transfected RL-5, there was evidence for extensive infection of the transfected cell line with HTLV-1; the parent line was examined by electromicroscopy and it was shown that a large percentage of the transfected cells produced virus like particles and, in addition, the virus appeared to be cytopathic which is not normal for HTLV-1 on rabbit cells. The protein produced by the HTLV-1 infected, transfected cells, as well as cells doubly infected with both HTLV-1 and HIV-1, are now being examined to determine whether there are pseudotype viral particles being produced by the transfected line. In other experiments, it had been shown that supernatants taken by rabbit macrophages infected with HIV-1 were negative for reverse transcriptase (RT) activity. In these cells other parameters for infection were positive. The activity giving rise to the RT negativity was shown to be a nuclease that can digest either substrates or products of the assay system. Attempts are being made to circumvent this activity in order to be able to use RT as a measure of cell infection. In addition to finding this activity in the rabbit macrophage line, it has also been found in the human cell line U937 and in adherent macrophages from both rabbit and human peripheral blood.
