The reactivity pattern of hemagglutinin-specific clonotypes from mice immunized as neonates or adults with naked DNA

In contrast to adult mice immunized with influenza A virus strain WSN and plasmid expressing WSN hemagglutinin (HA) gene which developed primary and secondary anti-HA antibody responses, mice immunized as neonates with virus failed to produce anti-HA antibodies while those immunized with plasmid developed weak primary but strong secondary responses. Analysis of the frequency of HA-specific B clonotypes as well as their reactivity pattern (RP) showed that viral or genetic immunization of adults increased the frequency of clonotypes which exhibit broad RP. The most striking observation of our study is that immunization of neonates with plasmid leads to increased synthesis of anti-HA antibodies as well as to an increased frequency of clonotypes exhibiting an adult-like RP. In contrast, neonatal immunization with virus caused a long-lasting unresponsiveness and the few clonotypes stimulated in vitro exhibited only a monoreactive pattern. Isotype patterns of mAb are also diversified in the case of mice immunized with plasmid as neonates. Rapid replacement of neonatal with adult clonotypes may explain the significant survival of the mice immunized with plasmid and challenged 1 or 3 months later with lethal doses of virus.