Stimulation of helper/delayed-type hypersensitivity T cells by flavivirus infection: determination by macrophage procoagulant assay.

West Nile virus (WNV)-immune spleen cells produced an inducer of macrophage procoagulant activity (MPCA) on restimulation with WNV in vitro. This response was specific for WNV and depended on the presence of Thy 1+, L3T4+ and also Ia+ cells but not Lyt 2+ cells. It could be induced by culture with large amounts of non-infectious, u.v.-irradiated virus or with high doses of infectious virus. Inoculation of mice with low titres of infectious, but not with u.v.-irradiated, virus primed for this in vitro response. The time of maximum production of procoagulant-inducing factor was also the period of greatest [3H]thymidine uptake and release of interleukin-2 (IL-2)-like activity. In bulk cultures, MPCA-inducing factor was more readily quantified than IL-2. T cells of the helper/delayed-type hypersensitivity class are thus stimulated by WNV and this aspect of cell-mediated immunity can be readily assayed in vitro.