The effectiveness and safety of mutant
Escherichia coli heat-labile enterotoxin, LT H44A (His to Arg substitution at position 44 from the N-terminus of the A1 fragment of the A subunit) as an adjuvant for nasal
influenza vaccine were examined. (1) When 0.2 microg of LT H44A, together with 0.2 microg of
influenza A/PR/8/34 virus (PR8, H1N1)
vaccine, was administered intranasally into BALB/c mice (twice, 4 weeks apart), anti-PR8
hemagglutinin (HA)
IgA and
IgG antibody (Ab) responses were induced at levels that were sufficient to provide either complete protection against
infection with a small volume of PR8 virus
suspension or partial protection against
infection with a lethal dose of the
suspension. The dose of the mutant LT and
vaccine used here (0.2 microg/ 20 g doses mouse) corresponded to the estimated dose per person, i.e. 0.1 mg/10 kg
body weight. (2) Using these vaccination conditions, no additional total
IgE Ab responses were induced. (3) The mutant was confirmed to be less toxic than the native LT when the toxicity was analyzed either using Y1 adrenal cells in vitro (1/483 EC(50)) or by an ileal loop test. (4) One hundred micrograms of the mutant, administered intranasally or intraperitoneally into guinea-pigs (Heartley strain, 0.3-0.4 kg), caused no
body-weight changes 7 days after administration, although 100 microg of the native LT administered intraperitoneally caused death in all guinea-pigs due to
diarrhea within 2 days. The
intranasal administration of 100 microg of the mutant resulted in almost no pathological changes in the nasal mucosa 3 days after administration. These results suggest that LT H44A, which can be produced in high yields in an E. coli culture (about 5 mg/l), could be used as one of the effective and safe adjuvants for nasal
influenza vaccine in humans.