Intranasal Immunization with Pneumococcal Polysaccharide Conjugate Vaccines with Nontoxic Mutants ofEscherichia coliHeat-Labile Enterotoxins as Adjuvants Protects Mice against Invasive Pneumococcal Infections

Abstract

Host defenses against Streptococcus pneumoniae depend largely on phagocytosis following opsonization by polysaccharide-specific immunoglobulin G (IgG) antibodies and complement. Since colonization of the respiratory mucosa is the first step in pneumococcal pathogenesis, mucosal immune responses may play a significant role. In addition to inducing systemic immune responses, mucosal vaccination with an effective adjuvant has the advantage of inducing mucosal IgA antibodies. The heat-labile enterotoxin (LT) ofEscherichia coli is a well-studied mucosal adjuvant, and adjuvant activity of nontoxic LT mutants has been demonstrated for several protein antigens. We investigated the immunogenicity of pneumococcal polysaccharide conjugate vaccines (PNC) of serotypes 1 and 3 in mice after intranasal (i.n.) immunization by using as an adjuvant the nontoxic LT mutant LT-K63 or LT-R72, which has minimal residual toxicity. Pneumococcal serotype-specific antibodies were measured in serum (IgM, IgG, and IgA) and saliva (IgA), and vaccine-induced protection was evaluated by i.n. challenge with virulent pneumococci of the homologous serotype. When administered with LT mutants, i.n. immunization with both conjugates induced systemic and mucosal immune responses, and serum IgG antibody levels were significantly higher than after subcutaneous immunization. All mice immunized i.n. with PNC-1 and LT mutants were protected against bacteremia and cleared the pneumococci from the lung 24 h after i.n. challenge; pneumococcal density correlated significantly with serum IgG antibody levels. Similarly, the survival of mice immunized i.n. with PNC-3 and LT mutants was significantly prolonged. These results demonstrate that i.n. vaccination with PNC and potent adjuvants can protect mice against invasive and lethal pneumococcal infections, indicating that mucosal vaccination with PNC may be an alternative vaccination strategy for humans.

Effects of LT‐K63 and CpG2006 on Phenotype and Function of Murine Neonatal Lymphoid Cells

“…We have demonstrated that both LT-K63 and CpG2006 can enhance primary pneumococcal PS and TT-specific Ab response in neonatal mice immunized with Pnc1-TT. This is in agreement with our previous studies on the effects of LT-K63 [10,27,28] and CpG2006 [34] on adult and neonatal Ab responses to Pnc1-TT, which still are highly age-dependant. In contrast, Kovarik et al [35] reported that CpG-ODN failed to enhance Ab responses in 2-weeks-old mice when administered along with pneumococcal PS conjugate, whereas CpG could enhance Ab responses to pneumococcal conjugate in adult mice.…”

IC31®, a Two‐Component Novel Adjuvant Mixed with a Conjugate Vaccine Enhances Protective Immunity against Pneumococcal Disease in Neonatal Mice

“…Groups marked with * have statistically higher Ab levels than the unimmunized group. The relationship between PPS1-specific Ab levels and protection against pneumococcal infection was highly significant in the experiments reported here, which is in agreement with our previous results [26,29]. This indicates that the PPS1-specific Ab are protecting the mice in this model, which is in agreement with PPS-specific Ab-mediating complement-dependent opsonophagocytosis being the main protective mechanism against pneumococcal infections [30,31].…”

“…Although all the groups receiving two immunizations are fully protected against pneumococcal infections 2 weeks after the second dose, there is a significant difference in Ab levels 1 week after the second immunization where the groups receiving low dose of IC31 Ò (at first or both immunizations) had significantly higher Ab levels than those receiving Pnc1-TT alone, suggesting that they would be fully protected earlier than mice receiving no adjuvant. Indeed, we have previously observed that PPS1-specific IgG Ab levels of log 2.5 EU ⁄ ml are protective against both blood and lung infection in adult animals [29] and those levels were already reached 1 week after the second immunization in neonatal mice primed with LD IC31 Ò and then boosted with either LD or HD IC31 Ò . The increased protection provided by IC31 when given with Pnc1-TT is comparable with what we have previously reported for LT-K63 [25].…”

Intranasal Cry1Ac Protoxin is an Effective Mucosal and Systemic Carrier and Adjuvant of Streptococcus pneumoniae Polysaccharides in Mice

“…Systemic capsular PS (CPS)‐specific IgG antibodies protect against invasive pneumococcal infections [51]. The IgG antibody response to PSs is predominantly restricted to a subset of IgG subclasses, such as IgG 3 in mice and IgG 2 in humans.…”

“…While the presence of type‐specific IgG and IgM antibodies is essential for the opsonization of pneumococci, there are few reports measuring mucosal IgA responses to pneumococcal vaccines, although it has been observed that immunity in these surfaces can play an important role in the protection against the colonization and illness caused by pneumococci [22, 51, 55–57]. In a recent work, Jakobsen et al .…”