FIG. 5.

Protection of immunized CD4⁺ T-cell-deficient mice against lethal PR8 virus challenge. CD4⁺ T-cell-deficient C57BL/6 mice i.m. immunized with inactivated PR8 virus were challenged intranasally with 10 times the LD₅₀ of live PR8 virus under anesthesia 4 weeks after boost. (a) Control, unimmunized CD4⁺ T-cell-deficient C57BL/6 mice. (b) CD4⁺ T-cell-deficient C57BL/6 mice immunized with inactivated PR8 virus. (c) CD8 T-cell-depleted CD4⁺ T-cell-deficient C57BL/6 mice immunized with inactivated PR8 virus.