THE ACYLATED FORM OF PROTEIN-D OF HAEMOPHILUS-INFLUENZAE IS MORE IMMUNOGENIC THAN THE NONACYLATED FORM AND ELICITS AN ADJUVANT EFFECT WHEN IT IS USED AS A CARRIER CONJUGATED TO POLYRIBOSYL RIBITOL PHOSPHATE

The nonacylated form of protein D (PDm) of Haemophilus influenzae has been shown to induce the production of antibodies that are bactericida l to homologous and heterologous nontypeable H. influenzae (NTHi) stra ins. In this study, immunization of rats with Lipoprotein D (LPD) indu ced higher levels of anti-protein D immunoglobulin G and A serum antib odies than immunization with PDm, and the bactericidal activities of s era from LPD-immunized rats were greater than those of sera from PDm-i mmunized rats, Immunization with LPD or PDm did not prevent the develo pment of acute otitis media (AOM) when rats were challenged with 10(4) CFU of an NTHi strain, However, on the eighth day of bacterial challe nge, 50% (5 of 10) of LPD-immunized rats had recovered from otitis med ia and 30% (3 of 10) had negative middle ear cultures, whereas only 30 % (3 of 10) of PDm-immunized rats had recovered, though none was cultu re positive, Immunization with an inactivated homologous bacterial str ain elicited 70% protection (i.e., 7 of 10 rats) in the rat otitis med ia model. LPD and PDm were also conjugated to the H. influenzae type b (Rib) capsular polysaccharide, polyribosyl ribitol phosphate (PRP), t o test protein D-conjugated PRP vaccine's potential for protection aga inst Rib infection, When two LPD-conjugated and two PDm-conjugated PRP vaccines, each containing a different protein concentration, and a te tanus toxoid-conjugated vaccine (ACT-RIB) were tested in the experimen tal model of rat otitis induced with a Rib strain (Minn A), both of th e LPD-conjugated and one of the PDm-conjugated vaccines induced signif icant protection from AOM, the level of protection being highest in an imals given the vaccine with the highest LPD content. Sera from these rats also manifested the highest anti-PRP and anti-LPD antibody levels and the highest bactericidal activities against a Rib strain and an N THi strain.