Serum antibody responses of juvenile and infant rhesus monkeys injected with Haemophilus influenzae type b and pneumococcus type 6A capsular polysaccharide-protein conjugates

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Abstract
Juvenile and infant rhesus monkeys were injected s.c. with saline solutions of H. influenzae type b (Hib) and pneumococcus type 6A (Pn6A) capsular polysaccharides conjugated to either tetanus toxoid (TT), horseshoe crab hemocyanin or cholera toxin (CT), and the antibody responses of the monkeys to both bacterial components were measured. All 3 Hib conjugates were immunogenic and elicited booster responses; their comparative immunogenicity was Hib-CT > Hib-TT > Hib-horseshoe crab hemocyanin. Hib alone did not elicit antibodies in the juveniles. Juveniles responded earlier and with higher levels of antibodies than did infants. TT, as well as diphtheria-tetanus toxoids-pertussis vaccine adsorbed injected concurrently at a separate site, increased both Hib and TT antibody responses in juveniles (P < 0.01). Concurrent injection of 5 Lf [limit of flocculation] of fluid TT with a nonimmunogenic 5-.mu.g dose in infants elicited levels of Hib antibodies comparable to those elicited by 50 .mu.g of Hib-TT. Hib antibodies elicited by the conjugates remained at protective levels in both juveniles and infants 2 mo. after the last injection, were bactericidal, and conferred passive immunity against bacteremia in infant rats. Passive immunization of juveniles with tetanus immune globulin before each injection of Hib-TT did not suppress Hib antibodies. Hib-TT and Hib-CT elicited increases of Hib antibodies of the IgM and IgG isotypes in the infants. The Pn6A-TT conjugate was considerably less immunogenic than the Hib-TT conjugate; only a few of the juveniles or infants responded with protective levels of Pn6A antibodies. Pn6A antibodies from responders conferred protection in mice against i.p. challenge with Pn6A organisms. TT antibodies were elicited in both juvenile and infant animals after 1 injection of 50 .mu.g of Hib-TT and in the infants injected with 5 .mu.g of Hib-TT plus 5 Lf of TT; 5 .mu.g of Hib-TT and Pn6A-TT in combination alone did not elicit TT antibodies. Hib-CT elicited CT antibodies in both juveniles and infants.