Correlation of cellular immune responses with protection against culture-confirmed influenza virus in young children

Abstract

The highly sensitive gamma interferon (IFN-gamma) enzyme-linked immunosorbent spot (ELISPOT) assay permits the investigation of the role of cell-mediated immunity (CMI) in the protection of young children against influenza. Preliminary studies of young children confirmed that the IFN-gamma ELISPOT assay was a more sensitive measure of influenza memory immune responses than serum antibody and that among seronegative children aged 6 to <36 months, an intranasal dose of 10(7) fluorescent focus units (FFU) of a live attenuated influenza virus vaccine (CAIV-T) elicited substantial CMI responses. A commercial inactivated influenza virus vaccine elicited CMI responses only in children with some previous exposure to related influenza viruses as determined by detectable antibody levels prevaccination. The role of CMI in actual protection against community-acquired, culture-confirmed clinical influenza by CAIV-T was investigated in a large randomized, double-blind, placebo-controlled dose-ranging efficacy trial with 2,172 children aged 6 to <36 months in the Philippines and Thailand. The estimated protection curve indicated that the majority of infants and young children with >or=100 spot-forming cells/10(6) peripheral blood mononuclear cells were protected against clinical influenza, establishing a possible target level of CMI for future influenza vaccine development. The ELISPOT assay for IFN-gamma is a sensitive and reproducible measure of CMI and memory immune responses and contributes to establishing requirements for the future development of vaccines against influenza, especially those used for children.

FIG. 1.

Rates of attack by culture-confirmed wild-type influenza virus A/H3N2, antigenically similar to the vaccine virus, for young children by treatment group. n, total number of evaluable children in each study group. CAIV-T 10^{^7}, 10^{7.0 ± 0.5} FFU per dose; CAIV-T 10^{^6}, 10^{6.0 ± 0.5} FFU per dose; CAIV-T 10^{^5}, 10^{5.0 ± 0.5} FFU per dose. One fluorescent focus unit is approximately equal to 1 50% tissue culture infective dose. Saline was used as the placebo.

FIG. 2.

Estimated curve of protection against culture-confirmed clinical infection with wild-type influenza virus versus cell-mediated immune responses for all subjects (pooled). P(protection) is the estimated probability of being protected against clinical culture-confirmed influenza virus infection. The estimated protection curve (solid line) is charted with 95% CI (dashed lines) against the magnitude of the CMI response as determined using the number of IFN-γ SFC per 10⁶ PBMC/ml.

FIG. 3.

Estimated curves of protection against culture-confirmed clinical infection with wild-type influenza virus versus cell-mediated immune responses by country. P(protection) is the estimated probability of being protected against clinical culture-confirmed influenza virus infection. The estimated protection curves for subjects from each of the two participating countries, Thailand and the Philippines, are charted against the magnitude of the CMI response as determined using the number of IFN-γ SFC per 10⁶ PBMC/ml.

FIG. 4.

Estimated curves of protection against culture-confirmed clinical infection with wild-type influenza virus versus cell-mediated immune responses for the four treatment groups. P(protection) is the estimated probability of being protected against clinical culture-confirmed influenza virus infection. The estimated protection curves for each of the four treatment groups are charted against the magnitude of the CMI response as determined using the number of IFN-γ SFC per 10⁶ PBMC/ml.