The Proline-Rich Region of Pneumococcal Surface Proteins A and C Contains Surface-Accessible Epitopes Common to All Pneumococci and Elicits Antibody-Mediated Protection against Sepsis

Abstract

Pneumococcal surface protein A (PspA) and PspC of Streptococcus pneumoniae are surface virulence proteins that interfere with complement deposition and elicit protective immune responses. The C-terminal halves of PspA and PspC have some structural similarity and contain highly cross-reactive proline-rich (PR) regions. In many PR regions of PspA and PspC, there exists an almost invariant nonproline block (NPB) of about 33 amino acids. Neither the PR regions nor their NPB exhibit the alpha-helical structure characteristic of much of the protection-eliciting N-terminal portions of PspA and PspC. Prior studies of PspA and PspC as immunogens focused primarily on the alpha-helical regions of these molecules that lack the PR and NPB regions. This report shows that immunization with recombinant PR (rPR) molecules and passive immunization with monoclonal antibodies reactive with either NPB or PR epitopes are protective against infection in mice. PR regions of both PspA and PspC were antibody accessible on the pneumococcal surface. Our results indicate that while PspA could serve as a target of these protective antibodies in invasive infections, PspC might not. When antibody responses to rPR immunogens were evaluated by using flow cytometry to measure antibody binding to live pneumococci, it was observed that the mice that survived subsequent challenge produced significantly higher levels of antibodies reactive with exposed PR epitopes than the mice that became moribund. Due to their conservation and cross-reactivity, the PR regions and NPB regions represent potential vaccine targets capable of eliciting cross-protection immunity against pneumococcal infection.Pneumonia is the leading cause of mortality for children under the age of 5 years worldwide, and its most common etiology is Streptococcus pneumoniae (42). S. pneumoniae also cause otitis media and life-threatening meningitis. A 7-valent pneumococcal conjugate vaccine (PCV7) was introduced in the United States in 2000. PCV7 use reduced the number of cases of infections with vaccine capsular types in both immunized children (43) and nonimmunized individuals (18) in the same communities. But less than 5 years after the implementation of PCV7, reports of serotype replacement (increases in the number of invasive infections caused by strains of capsular serotypes not covered by the vaccine) began to appear (20,22,25,40). The observation of this serotype replacement within a few years after vaccine implementation and the fact that there are at least 91 capsular types (36) raise concerns about the long-term effectiveness of capsule-based vaccines and stress the need for continued development of effective, noncapsular serotype-dependent pneumococcal vaccines (2, 39).Surface proteins of pneumococci are important nonpolysaccharide vaccine candidates. Two of the more promising vaccine candidates are pneumococcal surface protein A (PspA) and pneumococcal surface protein C (PspC; also called CbpA). These two proteins have some similar structural features, and both proteins have ...

Discovery of Immunodominant B Cell Epitopes within Surface Pneumococcal Virulence Proteins in Pediatric Patients with Invasive Pneumococcal Disease

“…Furthermore, the fact that all epitopes reside within proteins that fulfill important non-redundant roles in bacterial virulence minimizes the possibility of negative selection or strain replacement under immune pressure (19). No immunodominant epitope was identified within the extensively studied PspA, a finding most likely explained by the existence of conformational epitopes instead of linear epitopes that could not be detected by the epitope mapping method used here or, alternatively, by the high primary sequence variability of PspA (20,21). All identified epitopes in our study were included within the large antigenic fragments defined previously by Giefing et al (8), who screened the whole pneumococcal genome against sera from adults following IPD.…”

“…Our finding that one of the two 6B isolates displayed an absence of surface binding could be attributed to the known variation in bacterial capsule thickness or to a lower expression of PhtE, PhtD, and CbpD proteins in this particular strain (38,39). Previous reports have demonstrated that surface-accessible epitopes may elicit antibody-mediated protection against pneumococcal disease (21,40,41), underlining the potential of our epitopes for the development of future vaccines. Additional studies evaluating the in vitro opsonophagocytic and in vivo protective efficacy of anti-peptide antibodies are required.…”

Free Sialic Acid Acts as a Signal That Promotes Streptococcus pneumoniae Invasion of Nasal Tissue and Nonhematogenous Invasion of the Central Nervous System

“…For our intranasal colonization model and intravenous infection model, mice were infected as previously described (28,35). A frozen inoculum containing known concentrations of viable bacteria was diluted in lactated Ringer's solution.…”

Evaluation of a Vaccine Formulation against Streptococcus pneumoniae Based on Choline-Binding Proteins

“…Since there is less cross-reactivity between families than within families, it has been proposed that a broad-coverage vaccine should be composed of at least one PspA of each of the two major families (22)(23)(24). Previous studies have shown that both the ␣-helical and prolinerich domains of PspA are able to elicit protection (24,25), but all phase I vaccine trials with the PspA antigen have been conducted using only the ␣-helical domain (11). Although a mixture of ␣-helical domains should be protective against most PspAs (22), recent evidence suggests that the proline-rich domain of PspA is even more cross-protective (25).…”

“…Previous studies have shown that both the ␣-helical and prolinerich domains of PspA are able to elicit protection (24,25), but all phase I vaccine trials with the PspA antigen have been conducted using only the ␣-helical domain (11). Although a mixture of ␣-helical domains should be protective against most PspAs (22), recent evidence suggests that the proline-rich domain of PspA is even more cross-protective (25). Vaccine immunogens should thus probably contain both of these domains.…”