The current conjugate pneumococcal vaccines, in which capsular polysaccharides are bound to either diphtheria or tetanus toxoid, are immunogenic and efficacious in children and prevent disease caused by the serotypes whose capsule types are in the vaccine (2
). However, as these vaccines do not cover the full spectrum of invasive pneumococcal serotypes, temporal and geographic changes in serotype frequency associated with IPD exist. The pneumococcus is able to replace its capsule type through natural transformation, and children remain at risk of infection by nonvaccine serotypes. Moreover, the possibility of serotype shift, where the nonvaccine serotypes acquire an ecological advantage for increased carriage prevalence and, concomitantly, disease, remains real (8
A possible solution to this problem is either replacement of the carrier toxoid with a conserved and highly antigenic single pneumococcal protein, thus providing serotype-independent protection, or alternatively, if a single antigen is insufficient, creation of a multivalent protein vaccine that eschews the capsular polysaccharide entirely. At this time, it is not clear which approach is best or which pneumococcal protein(s) should be included in any revised vaccine formulations. To address these issues, detailed molecular epidemiology is required to assess the frequency and distribution of various pneumococcal determinants in invasive clinical isolates. This is the first large study analyzing the prevalence of psrP in pneumococcal isolates from children with IPD and healthy nasopharyngeal carriers. The results of our study are in agreement with published data regarding the function of this new pneumococcal virulence factor and provide clues to the forces responsible for the spread of the pathogenicity island encoding PsrP, psrP-secY2A2, among different serotypes and clones.
The increased frequency of the gene encoding PsrP in clinical isolates from individuals with pneumonia compared with the frequency in those isolated from the nasopharynges of healthy carriers or children with bacteremia is consistent with published findings showing that PsrP is exclusively a lung cell adhesin and that it does not play a role in the nasopharynx during colonization or in the bloodstream in an intraperitoneal model of sepsis. These data also suggest that PsrP alone would protect against only 60% of strains that are capable of causing pneumonia. Thus, these findings indicate that, at best, PsrP could be a single component of a multicomponent vaccine formulation. The inclusion of a second or third protein that protects against bacteremia and whose coverage helps to cover the ~40% of invasive isolates that lack PsrP would be required.
Given that psrP
was found to be predominantly associated with antimicrobial-susceptible isolates, it can be inferred that its inclusion within any vaccine would not serve as a mechanism to decrease the incidence of existing multidrug-resistant pneumococcal isolates. It also suggests that the extensive use of antimicrobials within the community is not responsible for promoting the preponderance of clonotypes that carry psrP
. Counter to the latter view, we have recently shown that PsrP mediates the formation of bacterial aggregates within the lungs and the formation of more dense biofilms in vitro
). As psrP
is predominantly found in antimicrobial-susceptible isolates and bacterial aggregates and biofilms are considered to be more resistant to antimicrobials, it is a distinct possibility that in the absence of a dedicated antimicrobial resistance mechanism, PsrP confers resistance to susceptible isolates in vivo
through enhanced biofilm formation. Thus, antimicrobial pressures may be serving to maintain psrP
within susceptible clonotypes. To test this hypothesis, ongoing experiments are testing the resistance of these PsrP-mediated aggregates to antimicrobials.
When we stratified the incidence of psrP
in children with pneumonia by age, we found that 65.4% of pneumococcal strains that caused disease in children greater than 2 years of age carry this virulence gene. This rate was significantly higher that the rate found in strains that cause disease in young children. One possible explanation for this may be the serotype distribution of psrP. psrP
was found to be predominant in serotypes not covered by the 7-valent conjugate vaccine. In the prevaccine era, the 7 serotypes included in the vaccine were most prominent in Europe. Thus, the discrepancy in age might be explained by the fact that the infant nasopharynx is first occupied by these 7 serotypes and is then filled with those from the nonvaccine serotypes at a later age. Studies by Melegaro and colleagues would support this explanation (10
). However, in our study, this is not a valid explanation because PCV-7 serotypes were not found to be predominant in either younger children or older children (36 of 189 [19%] in children less than 2 years of age versus 34 of 252 [13.5%] in children ≥2 years of age; P
= 0.1). Thus, other reasons, which are not yet clear, must explain why strains that carry psrP
cause IPD at a later age than those that do not. Importantly, the fact that psrP
is found predominantly in the PCV-7 nonvaccine serotypes suggests that its inclusion would expand coverage beyond that of the current vaccine. However, this is less so for the PCV-10 and PCV-13 formulations due to the inclusion of serotypes 1 and 19A, the strains of which have a high frequency of psrP
. Of note, the reported prevalence of psrP
among PCV-7 serotypes may be skewed by the fact that relatively few PCV-7 serotype strains were isolated in our study. It would therefore be interesting to test a collection of clinical isolates archived prior to the introduction of the vaccine. Such a study would determine if the current 60% incidence of psrP
in pneumonia clinical isolates was due to serotype replacement (i.e., positive selection of nonvaccine serotypes with clones that carry psrP
) or if psrP
has been prevalent all along among serotypes and clones that frequently cause pneumonia.
Finally, we observed the presence of psrP
in certain clonotypes and its absence in others. For example, all strains of serotype 1 with CC306 were positive for psrP
, while only 1 of 18 strains with ST304 was positive. ST306, together with ST304, ST228, and others, belongs to lineage A of serotype 1, which is the major lineage detected in North America and Europe (3
). According to some epidemiological studies ST306 has been related to several outbreaks of invasive pneumococcal disease (9
) and the emergence of pleuropneumonia in the vaccine era (15
), while ST304 has not. The high prevalence of PsrP, a lung cell bacterial adhesin, in ST306 strains could be associated with this fact. It is not clear why psrP
would be present in some isolates but not others; however, this suggests that other pneumococcal virulence determinants compensate for the absence of PsrP. Thus, detailed comparative genomic analyses of invasive clonotypes within the same serotype containing psrP
and not containing psrP
may be warranted to identify the compensatory determinants that are responsible for disease and, moreover, to determine if their inclusion along with PsrP in a multicomponent vaccine would enhance coverage.
In conclusion, psrP is highly prevalent in our clinical collection and is mainly present in strains isolated from older children with pneumonia. The distribution of psrP seemed to be strongly associated with antimicrobial sensitivity, non-PCV-7 serotypes, and specific clonotypes of pneumococci. These data support the potential use of PsrP as a protective antigen in the design of a next generation of protein-based combination vaccines. However, the data also indicate that additional components that fill the bacteremia and serotype niche not covered by PsrP are required.