Relevance of a pre-existing measles immunity prior immunization with a recombinant measles virus vector.

Measles virus (MV) vectors are promising candidates for designing new recombinant vaccines since the parental live vaccines have a well-known safety and efficacy record. Like all viral vectors, the MV vector efficacy in inducing a protecting immune answer could be affected by the pre-existing immunity among the human population. In order to determine the optimal immunization route and regimen, we mimicked a MV pre-immunity by passively administrating MV neutralizing antibodies (MV-nAb) prior intramuscular (i.m.) and/or intranasal (i.n.) immunization with recombinant MV expressing the SIV-gag antigen (rMV-SIVgag). Our results revealed that 500 mIU of MV-nAb allowed the induction of a humoral and cellular immune response against the vector and the transgene, while higher titers of the MV-nAb were significantly inhibitory. In a prime-boost regimen, in the presence of MV-nAb, the intranasal-intramuscular (i.n.-i.m.) or intramuscular-intramuscular (i.m.-i.m.) routes induced higher humoral immune responses against the vector and the transgene (SIV-gag). In naive animals, cellular immune response was significantly higher by i.m. immunization; however, MV pre-immunity did not seem to affect the cellular immune response after an i.n. immunization.   In summary, we show that a pre-existing immunity of up to 500 mIU anti-MV neutralizing antibodies had little effect on the replication of rMV and did not inhibit the induction of significant humoral and cellular immune responses in immune-competent mice.