Maternal-infant transfer of influenza-specific immunity not detectable by haemagglutination inhibition.

Evidence was sought for the transfer from mother to infant mouse of influenza-specific immunity other than that associated with haemagglutination inhibition (HI) specific antibody. Data were analysed for infant mice born to three groups of mothers: influenza immunized mothers with high levels of HI antibody. influenza immunized mothers in whom influenza-specific HI antibody was suppressed to undetectable levels by passive administration of antibody prior to influenza immunization, and non-immunized control mothers. At 4 weeks of age, infants were given either a lethal or a non-lethal influenza challenge. After a lethal influenza challenge, infants of immunized mothers with HI antibody showed no mortality. Infants of immunized mothers in whom HI antibody was suppressed showed a mortality of 8 to 33%, which was significantly lower than the 71% mortality found in infants of non-immunized control mothers (P less than 0.001). The lower mortality in infants of immunized mothers without HI antibody appeared to be associated with breast feeding, but could not be attributed to the transfer from mother to infant of a local immune response, an influenza specific cytotoxic response, or a secondary antibody response after non-lethal influenza infection. When sera were additionally tested for anti-influenza IgG by ELISA, this lower mortality was found to correlate with anti-influenza serum IgG measurable by ELISA but not detectable by HI. Protective immunity in infants undetectable by HI could be attributable to very low levels of antibody detected by more sensitive ELISA.