Porin A-specific antibody avidity in patients who are convalescing from meningococcal B disease.

Porin A (PorA), which determines the serosubtype of Neisseria meningitidis, is the main antigen of a candidate vaccine against serogroup B meningococci, which has been shown to induce high-avidity antibodies in children. We characterized the immune response of children after convalescing from meningococcal infection with a serosubtype P1.7-2,4 strain. Acute- and convalescent-phase sera of 21 children with meningococcal septic shock caused by strains with PorA subtype P1.7-2,4 were collected. The serum bactericidal antibody titers, IgG isotype distribution, and antibody avidity were measured. We determined whether the differences in avidity of anti-outer membrane vesicle antibodies were PorA specific. Serum bactericidal activity against H44/76 P1.7-2,4 was <4 in all convalescent sera. The IgG isotype distribution of the convalescent sera was dominated by IgG(1), followed by IgG(3), whereas no IgG(2) or IgG(4) was found. The geometric mean avidity index (GMAI) of convalescent sera measured against a strain with the identical subtype as the infective isolate was significantly higher than that against a strain with a heterologous PorA subtype or a PorA-negative mutant strain (57 versus 35 and 23%, respectively; p = 0.005 and p < 0.001). Geometric mean avidity titers were highest for P1.7-2,4, corresponding with the highest GMAI. The GMAI after invasive meningococcal disease was lower than after vaccination of healthy toddlers with a monovalent P1.7-2,4 outer membrane vesicle vaccine.