Abstract
This study examined whether secretory IgA (S‐IgA) antibodies (Abs) could confer cross‐protective immunity against infection with influenza B viruses of antigenically distinct lineages. Wild‐type or polymeric Ig receptor (pIgR)‐knockout (KO) mice were immunized by infection with different B viruses or by intranasal (i.n.) administration with different inactivated vaccines. Four weeks later mice were challenged with either the B/Ibaraki/2/85 virus, representative of the B/Victoria/2/87 (B/Victoria)‐lineage, or B/Yamagata/16/88 virus, representative of the B/Yamagata‐lineage. Three days after challenge, nasal wash and serum specimens were assayed for IgA and IgG Abs specific for challenge viral antigens and for protection against challenge viruses. In wild‐type mice, B/Ibaraki (or B/Yamagata) cross‐reactive IgA Abs were detected at higher levels when infected or immunized with homologous‐lineage viruses and at lower levels when infected or immunized with heterologous‐lineage viruses. There was a correlation between the amount of nasal cross‐reactive IgA Ab and the efficacy of cross‐protection with a homologous‐lineage virus. In mice lacking the pIgR, nasal cross‐protective IgA Abs were only marginally detected in vaccinated mice and an accumulation of IgA in the serum was observed. This reduction of nasal IgA was accompanied by inefficient cross‐protection against the B/Ibaraki (or B/Yamagata) virus infection. These results suggest that challenge viral‐antigen cross‐reactive S‐IgA in nasal secretions induced by i.n. infection or vaccination is involved in providing cross‐protection against challenge infection with virus within either the B/Victoria‐ or B/Yamagata‐lineage. J. Med. Virol. 74:328–335, 2004. © 2004 Wiley‐Liss, Inc.