Immune response to avian leukosis virus determined by the cell-mediated focus-reduction (CEMFOR) assay

A cell-mediated focus-reduction (CEMFOR) assay was used to determine the sequential development of cell-mediated immunity to avian oncornaviruses and the nature of the cells participating in this immune reaction. Peripheral blood leukocytes of Leghorn chickens that had regressed Rous sarcoma virus-induced tumors or were immune to avian leukosis virus had CEMFOR activity. The response was biphasic early in the infection. Peripheral blood leukocytes from nonimmune chickens or viremic, immunologically tolerant chickens did not have CEMFOR activity. Sera from leukosis virus-immune chickens blocked CEMFOR activity. The early CEMFOR response was mediated by T-cells. The late response was mediated by an adherent cell population possibly augmented by the presence of T-cells.