or s.c. i.n. Pnc1-TT immunization. Mucosal immunization was particularly efficient in neonates, as a single i.n. dose of Pnc1-TT and LT-K63 induced significantly higher PPS-1-specific IgG responses than s.c. immunization and was sufficient to protect neonatal mice against pneumococcal infections, whereas two s.c. doses were required to induce complete protection. In addition, i.n. immunization with Pnc1-TT and LT-K63 Rabbit polyclonal to HspH1 induced a vigorous salivary IgA response. This suggests that mucosal immunization with pneumococcal conjugate vaccines and LT-K63 may be able to circumvent some of the limitations of neonatal antibody responses, which are required for protective immunity in early life. Streptococcus pneumoniae(pneumococcus) is usually a major respiratory pathogen which enters the body through the respiratory mucosa (65) and may cause serious infections such as meningitis, pneumonia, and bacteremia, especially in young children and the elderly (4,32). It is also the most common cause of bacterial otitis media (20). The increase in resistance to antimicrobial brokers is an increasing problem worldwide (3,10), and infants are colonized very early by pneumococcus in countries where resistant strains are prevalent (28,35). To induce protection in early life, vaccines that rapidly induce protective immunity Roy-Bz are required, but the immaturity of the immune system in newborns makes it difficult to induce protective immune responses by vaccination. Preclinical immunization models using various protein antigens and DNA vaccines during the neonatal period have exhibited that induction of antigen-specific B- and T-cell responses (6,34,57,58) and protection against infections (57) may be achieved. However, early life responses frequently remain delayed and weaker than those elicited in immunologically mature hosts (56). The 23-valent pneumococcal polysaccharide (PPS) vaccine is usually immunogenic and protective in healthy adults (9,52), but PPS, which are T-cell-independent type 2 antigens (36,61), are not immunogenic in those of an early age (17). Immunization with PPS in adults induces limited class switching of activated B cells, no affinity maturation, and poor induction of memory cells. Thus, antibody responses to PPS are characterized by high levels of immunoglobulin M (IgM) and low levels of IgG that are primarily of the IgG2 subclass in humans (5,27) and of the IgG3 subclass in mice (42). A marked improvement in the immunogenicity of polysaccharide (PS) antigens has been achieved by conjugation of PS to various protein carriers (18,47,55), and several PPS-protein conjugate vaccines have confirmed immunogenic in infants and toddlers (2,12,53,59,70; S. T. Sigurdardottir, T. Gudnason, S. Kjartansson, K. Davidsdottir, K. G. Kristinsson, G. Ingolfsdottir, M. Yaich, O. Leroy, and I. Jonsdottir, Abstr. 40th Intersci. Conf. Antimicrob. Brokers Chemother., abstr. G-50, 2000), inducing immunologic memory (1,41; I. Jonsdottir, G. Ingolfsdottir, E. Saeland, K. Davidsdottir, M. Yaich, O. Leroy, and S. T. Sigurdardottir, Abstr. 40th Intersci. Conf. Antimicrob. Brokers Chemother., abstr. G-43, 2000) and reducing nasopharyngeal carriage of pneumococci (11,40). Efficacy against both invasive disease (7) and acute otitis media (19) in infants has been demonstrated. Accordingly, PPS-protein conjugate vaccines induce protective immune responses in various adult experimental animal models (21,22,30,31,51,66). Recent studies have shown that mucosal delivery of antigens induces humoral and cell-mediated immune responses in both mucosal and systemic compartments. Mucosal immunization is especially attractive for immunization against respiratory pathogens, since the first line of Roy-Bz defense in the upper respiratory tract is assumed to be due to IgA antibodies in mucosal secretions (62). Mucosal immunization with inactivated vaccines usually requires adjuvants. Of the many mucosal adjuvants under investigation, mutants ofEscherichia coliheat-labile enterotoxin (LT) are among the most Roy-Bz promising (14,45,46,69). The adjuvanticity of the two mutants LT-K63 (15,16) and LT-R72 (23) has been reported for various antigens and these molecules are ready to enter clinical trials (44). We previously exhibited that mucosal immunization of adult mice with PPS-protein conjugate vaccines and LT mutants induces protective immunity against lethal pneumococcal infections of several serotypes (30,31). However, little is known about mucosal immune responses in infants and neonates, and the potential advantage of LT mutants as adjuvants in immunization against pneumococcal contamination in early life remained to be explored. The aim of the present study was thus to investigate the early life immunogenicity of an experimental tetanus toxoid (TT) pneumococcal conjugate vaccine of serotype 1 (Pnc1-TT) following subcutaneous (s.c.) or intranasal (i.n.) immunization and the capacity of the nontoxic LT-K63 mutant to enhance vaccine responses. Early life immunization was performed both in 3- and 1-week-old mice, which best correspond to the state of immune maturation of human infants and newborns, respectively.