Vaccine-induced measles virus-specific T cells do not prevent infection or disease but facilitate subsequent clearance of viral RNA.

Infection with wild-type measles virus (MeV) induces lifelong protection from reinfection, and parenteral delivery of the live attenuated measles vaccine (LAV) also provides protection from measles. The level of neutralizing antibody is a good indicator of protection, but the independent roles of MeV-specific antibody and T cells have not been identified. In this study, macaques immunized with LAV through a nebulizer and a mouthpiece developed MeV-specific T-cell responses but not neutralizing antibodies. Upon challenge with wild-type MeV, these animals developed rashes and viremias similar to those in naive animals but cleared viral RNA from blood 25 to 40 days faster. The nebulizer-immunized animals also had more robust MeV-specific CD4(+) and CD8(+) T-cell responses than the naive animals after challenge, characterized by a higher number and better durability of gamma interferon (IFN-γ)-producing cells. Induction of MeV-specific circulating CD4(+) and CD8(+) T cells capable of producing multiple cytokines correlated with clearance of viral RNA in the nebulizer-immunized macaques. These studies demonstrated that MeV-specific T-cell immunity alone did not prevent measles, but T-cell priming enhanced the magnitude, durability, and polyfunctionality of MeV-specific T cells after challenge infection and correlated with more rapid clearance of MeV RNA. IMPORTANCE The components of vaccine-induced immunity necessary for protection from infection and disease have not been clearly identified for most vaccines. Vaccine development usually focuses on induction of antibody, but T-cell-based vaccines are also under development. The live attenuated measles vaccine (LAV) given subcutaneously induces both T cells and neutralizing antibody and provides solid protection from infection. LAV delivered to the upper respiratory tract through a nebulizer and mouthpiece induced a T-cell response but no neutralizing antibody. These T-cell-primed macaques demonstrated no protection from rash or viremia when challenged with wild-type MeV, but viral RNA was cleared more rapidly than in unimmunized animals. Thus, T-cell immunity did not protect from infection or acute disease but facilitated virus clearance during recovery. These studies demonstrate the importance and independent roles of T cells and antibody in protection and recovery from measles.