Live-Attenuated Measles Virus Vaccine Targets Dendritic Cells and Macrophages in Muscle of Nonhuman Primates

Abstract

ABSTRACTAlthough live-attenuated measles virus (MV) vaccines have been used successfully for over 50 years, the target cells that sustain virus replicationin vivoare still unknown. We generated a reverse genetics system for the live-attenuated MV vaccine strain Edmonston-Zagreb (EZ), allowing recovery of recombinant (r)MVEZ. Three recombinant viruses were generated that contained the open reading frame encoding enhanced green fluorescent protein (EGFP) within an additional transcriptional unit (ATU) at various positions within the genome. rMVEZEGFP(1), rMVEZEGFP(3), and rMVEZEGFP(6) contained the ATU upstream of the N gene, following the P gene, and following the H gene, respectively. The viruses were comparedin vitroby growth curves, which indicated that rMVEZEGFP(1) was overattenuated. Intratracheal infection of cynomolgus macaques with these recombinant viruses revealed differences in immunogenicity. rMVEZEGFP(1) and rMVEZEGFP(6) did not induce satisfactory serum antibody responses, whereas bothin vitroandin vivorMVEZEGFP(3) was functionally equivalent to the commercial MVEZ-containing vaccine. Intramuscular vaccination of macaques with rMVEZEGFP(3) resulted in the identification of EGFP+cells in the muscle at days 3, 5, and 7 postvaccination. Phenotypic characterization of these cells demonstrated that muscle cells were not infected and that dendritic cells and macrophages were the predominant target cells of live-attenuated MV.IMPORTANCEEven though MV strain Edmonston-Zagreb has long been used as a live-attenuated vaccine (LAV) to protect against measles, nothing is known about the primary cells in which the virus replicatesin vivo. This is vital information given the push to move toward needle-free routes of vaccination, since vaccine virus replication is essential for vaccination efficacy. We have generated a number of recombinant MV strains expressing enhanced green fluorescent protein. The virus that best mimicked the nonrecombinant vaccine virus was formulated according to protocols for production of commercial vaccine virus batches, and was subsequently used to assess viral tropism in nonhuman primates. The virus primarily replicated in professional antigen-presenting cells, which may explain why this LAV is so immunogenic and efficacious.