Analysis of Serum Cross-Reactivity and Cross-Protection Elicited by Immunization with DNA Vaccines against Streptococcus pneumoniae Expressing PspA Fragments from Different Clades

Abstract

Streptococcus pneumoniae is a major cause of disease, especially in developing countries, and cost-effective alternatives to the currently licensed vaccines are needed. We constructed DNA vaccines based on pneumococcal surface protein A (PspA), an antigen shown to induce protection against pneumococcal bacteremia. PspA fragments can be divided into three families, which can be subdivided into six clades, on the basis of PspA amino acid sequence divergence (S. K. Hollingshead, R. Becker, and D. E. Briles, Infect. Immun. 68:5889-5900, 2000). Since most clinical isolates belong to family 1 or family 2, PspA fragments from members of both of these families were analyzed. Vectors encoding the complete N-terminal regions of PspAs elicited significant humoral responses, and cross-reactivity was mainly restricted to the same family. DNA vaccines encoding fusions between PspA fragments from family 1 and family 2 were also constructed and were able to broaden the cross-reactivity, with induction of antibodies that showed reactions with members of both families. At least for the pneumococcal strains tested, the cross-reactivity of antibodies was not reflected in cross-protection. Animals immunized with DNA vaccines expressing the complete N-terminal regions of PspA fragments were protected only against intraperitoneal challenge with a strain expressing PspA from the same clade.

Fusion Proteins Containing Family 1 and Family 2 PspA Fragments Elicit Protection against Streptococcus pneumoniae That Correlates with Antibody-Mediated Enhancement of Complement Deposition

“…Nevertheless, cross-protection against family 2 strains was also dependent on the PspA clade; PspA1ABC-3AB was protective against the PspA3-containing strain, and anti-PspA1ABC-4B was protective against the PspA4-containing strain. Previous studies in our laboratory have shown that cross-reactivity of antibodies induced by DNA vaccination is restricted to the same family, while cross-protection against intraperitoneal challenge is restricted to the same clade (16). These similarities suggest that PspA hybrids are able to broaden protection when presented as either recombinant proteins or as DNA.…”

“…All cloning procedures were performed with Escherichia coli DH5␣ grown in Luria-Bertani medium supplemented with ampicillin (100 g/ml). DNA fragments encoding portions of the N-terminal regions of PspA clades 1, 3, and 4 were amplified by PCR from pTG-pspA1-4 or pTG-pspA3-2 (16). The primers used in this procedure are listed in Table1.…”

“…We have previously demonstrated that DNA vaccines that encode PspA family 1 and 2 hybrids induce protection against the homologous family. Immunization with family 1 PspA induced detectable protection against family 2, but family 2 DNA induced no protection against family 1 (16). An important defense mechanism against pneumococcal infections is opsonin-dependent phagocytosis, with activation of the classic and alternative complement pathways and deposition of C3b on the pneumococcal surface (7,11,12).…”

“…Surveillance studies show that PspA from families 1 and 2 are present in 94 to 99% of the isolates in different countries in the Americas (1,2,17,25), with prevalence rates of about 50% each (10,19,25). Previous studies have shown that the level of cross-reactivity among different PspA fragments roughly follows the degree of similarity among the amino acid sequences within the CDR, with a tendency for a higher degree of cross-reactivity among PspA fragments within the same family, rather than between families (16). On the basis of these data, it has been suggested that a PspA-based vaccine should contain at least one fragment from each of the two major families in order to broaden protection (10).…”

Optimized Immune Response Elicited by a DNA Vaccine Expressing Pneumococcal Surface Protein A Is Characterized by a Balanced Immunoglobulin G1 (IgG1)/IgG2a Ratio and Proinflammatory Cytokine Production

“…Our previous results showed that protective Th1 immune responses against pneumococcal infection in a mouse model can be achieved successfully by DNA vaccines using vectors expressing PspA (8,17). The PspA fragment used in this study comprises the N-terminal region through the prolinerich region without the signal sequence from a clade 3 (family 2) PspA and is expressed in a secreted form by a DNA vaccine vector (pSec-pspA3NS).…”