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Infection and Immunity, September 1999, p. 4720-4724, Vol. 67, No. 9
0019-9567/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.

Pneumococcal Surface Protein A Inhibits Complement Activation by Streptococcus pneumoniae

Anh-Hue T. Tu,1 Robert L. Fulgham,1 Mark A. McCrory,2 David E. Briles,1 and Alexander J. Szalai2,*

Department of Microbiology1 and Division of Clinical Immunology and Rheumatology,2 University of Alabama at Birmingham, Birmingham, Alabama 35294

Received 15 March 1999/Returned for modification 12 May 1999/Accepted 30 June 1999

Pneumococcal surface protein A (PspA) is a surface-exposed protein virulence factor for Streptococcus pneumoniae. In this study, no significant depletion of serum complement was observed for the serum of mice infected with pneumococci that express PspA. In contrast, in mice infected with an isogenic strain of pneumococci lacking PspA, significant activation of serum complement was detected within 30 min after infection. Also, the PspA-deficient strain but not the PspA-expressing strain was cleared from the blood within 6 h. The contribution of PspA to pneumococcal virulence was further investigated by using mice deficient for C5, C3, or factor B. In mice deficient for C3 or factor B, PspA-negative pneumococci became fully virulent. In contrast, in C5-deficient mice as in wild-type mice, PspA-deficient pneumococci were avirulent. These in vivo data suggest that, in nonimmune mice infected with pneumococci, PspA interferes with complement-dependent host defense mechanisms mediated by factor B. Immunoblots of pneumococci opsonized in vitro suggested that more C3b was deposited on PspA-negative than on PspA-positive pneumococci. This was observed with and without anticapsular antibody. Furthermore, processing of the alpha  chain of C3b was reduced in the presence of PspA. We propose that PspA exerts its virulence function by interfering with deposition of C3b onto pneumococci and/or by inhibiting formation of a fully functional alternative pathway C3 convertase. By blocking recruitment of the alternative pathway, PspA reduces the amount of C3b deposited onto pneumococci, thereby reducing the effectiveness of complement receptor-mediated pathways of clearance.

* Corresponding author. Mailing address: Division of Clinical Immunology and Rheumatology, Department of Medicine, University of Alabama at Birmingham, Birmingham, AL 35294-0006. Phone: (205) 975-6241. Fax: (205) 934-2126. E-mail: rheu022{at}uabdpo.dpo.uab.edu.

Infection and Immunity, September 1999, p. 4720-4724, Vol. 67, No. 9
0019-9567/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.


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