A novel human T-leukemia virus type 1 cell-to-cell transmission assay permits definition of SU glycoprotein amino acids important for infectivity.

Human T-leukemia virus type 1 (HTLV-1) envelope glycoproteins play a major role in viral transmission, which in the case of this virus occurs almost exclusively via cell-to-cell contact. Until very recently, the lack of an HTLV-1 infectivity assay precluded the determination of the HTLV-1 protein domains required for infectivity. Here, we describe an assay which allows the quantitative evaluation of HTLV-1 cell-to-cell transmission in a single round of infection. Using this assay, we demonstrate that in this system, cell-to-cell transmission is at least 100 times more efficient than transmission with free viral particles. We have examined 46 surface (SU) glycoprotein mutants in order to define the amino acids of the HTLV-1 SU glycoprotein required for full infectivity. We demonstrate that these amino acids are distributed along the entire length of the SU glycoprotein, including the N-terminus and C-terminus regions, which have not been previously defined as being important for HTLV-1 glycoprotein function. For most of the mutated glycoproteins, the capacity to mediate cell-to-cell transmission is correlated with the ability to induce formation of syncytia. This result indicates that the fusion capacity is the main factor responsible for infectivity mediated by the HTLV-1 SU envelope glycoprotein, as is the case for other retroviral glycoproteins. However, other factors must also intervene, since two of the mutated glycoproteins were correctly fusogenic but could not mediate cell-to-cell transmission. Existence of this phenotype shows that capacity for fusion is not sufficient to confer infectivity, even in cell-to-cell transmission, and could suggest that postfusion events involve the SU.