Protective responses of an engineered PspA recombinant antigen against Streptococcus pneumoniae

Abstract

Streptococcus pneumoniae is a major pathogen in human respiratory tract which causes significant morbidity and mortality across from the world. Currently available vaccines are not completely effective and cannot cover all pathogenic strains so there is an important need to develop an alternative cost-effective vaccine, based on conserved protein antigens. Pneumococcal surface protein A (PspA) is one of interesting candidates for development of a serotype-independent vaccine against pneumococcal infections. PspA is grouped into two major families with five clades, and broad-reacting PspA-based vaccines should contain at least one functional fragment from each of the two families. In this study, we developed two immunogenic antigens based on recombinant PspA proteins that including the different antigenic regions of PspA from both two families. The cross-reactivity of antibodies elicited against two PspA proteins PspAB1-5 and PspA₄ABC and their role in complement deposition with three strains of pneumococci were tested. The protective effects of developed anti-PspA antibodies in mice in intranasal and intraperitoneal challenges were evaluated using a strain from clade 2. Sera from immunized mice with PspAB_1-5 in comparison with PspA₄ABC was able to deposit more C3 complement component on surface of pneumococci bearing diverse PspA from both families 1 and 2, and immunized mice with the PspAB_1-5 showed a higher protection than PspA₄ABC in pneumococcal challenges. The obtained results from this study indicate that a PspA-based antigen composed of B region from all clades in addition to conserved domains, can provide a significant protection against multiple strains of S. pneumoniae and may overcome the limitation of polysaccharide vaccines.

Keywords: Immune protection; Pneumococcal surface protein A (PspA); Streptococcus pneumoniae.

Fig. 1

Characterization of purified recombinant PspA fragments by SDS-PAGE (A) and western blotting (B). From left to right: lane 1: PspA₄ABC; lane 2: protein marker; line 3: PspAB_1-5.

Fig. 2

Binding of anti-PspA fragments antibodies to the pneumococcal surface with 3 different PspA clades. Sera from immunized mice with recombinant PspA4ABC and PspAB_1-5 were analysed by ELISA against pneumococcus strains with PspA fragments from clade 1 (ATCC 49619), clade 2 (ATCC 6305) and clade 5 (ATCC 700678). Non-immunized mouse serum was used as negative control.

Fig. 3

Complement deposition assay on 3 pneumococcal strains with anti-PspA fragments antibodies. Bacteria containing PspAs of clade 1 (ATCC49619), clade 2 (ATCC6305), and clade 5 (700678) were incubated with normal mouse serum and sera of immunized mice with PspA4ABC and PspAB_1-5. The obtained OD of complement deposited bacteria in ELISA is represented for each group. Non-immunized serum was used as negative control.