Contribution of adjuvant to adaptive immune responses in mice against Actinobacillus pleuropneumoniae

The authors have previously demonstrated that adjuvant-mediated differences in early cellular responses to antigens significantly affect subsequent ad aptive immune responses. To investigate further the contribution of adjuvan t to adaptive immune responses, outer-membrane proteins (OMP) purified from the respiratory pathogen Actinobacillus pleuropneumoniae, given either alo ne (antigen group) or complexed with SAMA4 (vaccine group), were injected i ntradermally into groups of mice. Controls were given PBS, Inclusion of adj uvant did not significantly alter the kinetics of antibody responses agains t OMP in serum or respiratory tract washings (RTW) over 21 weeks. Re-exposu re to OMP at 21 weeks also induced identical recall responses in both immun ized groups. However, differences between the responses of the vaccine and antigen groups were apparent when sera and RTW were reacted against OMP and OMP-derived polysaccharide antigens (ODPA). Serum and RTW reactivity again st protein antigens was stronger in the vaccine group than in the antigen g roup. Serum and RTW from the vaccine group also reacted against a greater n umber of proteins than did the antigen group. Although serum reactivity aga inst ODPA was equivalent for both groups, RTW from the vaccine group reacte d only faintly against ODPA compared with the antigen group. The results su ggested that shifting of antibody reactivity away from polysaccharide antig ens toward protein antigens was an adjuvant-mediated effect, The rapid deat h of controls following intranasal inoculation confirmed that protection wa s ultimately dependent on the presence of specific antibodies in the serum and respiratory tract. However, since both groups responded equally to intr anasal infection with A, pleuropneumoniae. as seen by the rapid clearance o f bacteria from the lungs, the biological significance of any differences b etween the groups was unclear. Knowledge of the effects of adjuvants may pr ovide a rational basis for adjuvant selection and the ability to manipulate immunological outcomes more precisely.