Regression Assay

In Epstein-Barr virus (EBV)-seropositive individuals, cell-mediated immunity against EBV can be monitored in vitro as the capacity of T-cell lymphocytes to inhibit virusinduced proliferation of autologous B cells. In vitro infection of peripheral blood mononuclear cells (PBMCs) from EBV-seronegative and EBV-seropositive donors is accompanied by the appearance of foci of proliferating cells expressing the EBV nuclear antigens (EBNAs) within 7–14 d postinfection. These cells continue to expand giving rise to EBV-transformed lymphoblastoid cell lines (LCLs) in cultures from EBVseronegative individuals. In corresponding cultures from EBV-seropositive donors, the initial proliferation is followed by a regression of growth, seen preferentially at high cell concentration, that culminates within 1 mo in the complete degeneration of the culture (1 ). This phenomenon has been termed regression or outgrowth inhibition and can be monitored by the regression assay. The strength of the regression can be quantitatively expressed as the minimal number of cells required for 50% growth inhibition.