SummaryHerpes virus saimiri (HVS) immortalizes T lymphocytes from a variety of primates and causes acute T cell lymphomas and leukemias in nonnatural primate hosts. Here we have analyzed the requirements for growth of three HVS-transformed human T cell lines. The cells expressed the phenotype of activated T cells: two were CD4 + , and one was CD8 + . All three cells responded to all allogeneic human cell lines tested with enhanced proliferation, production of interleukin 2 (Ib2), and increased expression of the I1:2 receptor. Binding of CD2 to its ligand CD58 was the critical event mediating stimulation because: (a) monoclonal antibodies (mAbs) to CD2 and to CD58, but not to a variety of other surface structures, blocked induced and spontaneous proliferation and Ib2 production; (b) only anti-CD2 mAbs were stimulatory if crosslinked; (c) a nonstimulatory cell was rendered stimulatory by CD58 transfection; and (d) the cells responded specifically to CD58 on sheep red blood cells. Growth of the cells required activation because cyclosporin A and FK506 blocked stimulator cell-induced II:2 production and proliferation as well as the spontaneous growth of the lines. Antibodies to the I1:2 receptor reduced proliferation of the cells and blocked 11:2 utilization. Taken together, these results show that HVS-transformed T cells proliferate in response to CD2-mediated contact with stimulator cells or with each other in an I1:2-dependent fashion. They suggest that HVS transforms human T cells to an activationdependent autocrine growth.
Herpes virus saimiri (HVS) is the prototype of the lymphotropic 3~-2 herpes viruses (rhadinoviruses). Natural hosts of the virus are the squirrel monkeys (Saimiri sciureus).By cocultivation with permissive monolayer cells, HVS can be isolated from PBL >80% of the population, and at least one in 106 T cells will yield virus (reviewed in reference 1). The virus persists in T cells for their lifetime. HVS is not pathogenic for squirrel monkeys, even after immunosuppression. In contrast to this benign behavior in its natural host, HVS has a high oncogenic potential in numerous other New World primates (1). It causes acute lymphoproliferative syndromes, leukemias, and lymphomas. After infection, the animals die within a few weeks from rapidly progressing neoplasias. Cell lines can be established from infected animals that initially produce virus in "~1-10% of the cells, but after prolonged cultivation the ability to produce virus is lost (2), HVS is able to transform T lymphoeytes of nonhuman primates as well as human PBL and thymocytes to continuous growth (2, 3). The molecular basis of T cell stimulation and transformation is unknown.In this study we show that transformation with HVS induces human T cells to respond to cell-cell contact with each other and with other cells leading to I1:2-dependent autocrine growth. The critical stimulating signal in this contact is given by binding of the CD2 molecule to its natural ligand. These HVS-transformed T cells are the first example of an activation-de...