Role of serum amyloid P component in bacterial infection: protection of the host or protection of the pathogen

Abstract

Serum amyloid P component (SAP) binds to Streptococcus pyogenes, and we show here that it also binds to Neisseria meningitidis, including a lipopolysaccharide (LPS)-negative mutant, and to rough variants of Escherichia coli. Surprisingly, this binding had a powerful antiopsonic effect both in vitro and in vivo, reducing phagocytosis and killing of bacteria. Furthermore, SAP knockout mice survived lethal infection with S. pyogenes and rough E. coli J5, organisms to which SAP binds. The susceptibility of SAP(-/-) mice was fully restored by injection of isolated human SAP. However, SAP(-/-) mice were more susceptible than wild-type animals to lethal infection with E. coli O111:B4, a smooth strain to which SAP does not bind, suggesting that SAP also has some host defense function. Although SAP binds to LPS in vitro, SAP(-/-) mice were only marginally more susceptible to lethal LPS challenge, and injection of large amounts of human SAP into wild-type mice did not affect sensitivity to LPS, indicating that SAP is not a significant modulator of LPS toxicity in vivo. In contrast, the binding of SAP to pathogenic bacteria enabled them to evade neutrophil phagocytosis and display enhanced virulence. Abrogation of this molecular camouflage is thus potentially a novel therapeutic approach, and we show here that administration to wild-type mice of (R)-1-[6-(R)-2-carboxy-pyrrolidin-1-yl]-6-oxo-hexanoyl]pyrrolidine -2- carboxylic acid, a drug that inhibits SAP binding, significantly prolonged survival during lethal infection with E. coli J5.

Figure 1

Effect of SAP deficiency on survival of 129SV mice during bacterial infection. (a) E. coli O111:B4, 4.4 × 10⁸ organisms injected i.v. (b) S. pyogenes H350, 2.1 × 10⁵ organisms injected i.p. P values (log rank test) represent significant differences in survival between SAP^−/− and SAP^+/+ mice. Each result shown is typical of two to four identical experiments with each organism.

Figure 2

Effect of SAP deficiency on the survival of C57BL/6 mice during bacterial infection. (a) E. coli O111:B4, 10⁸ organisms injected i.v. (b) E. coli J5, 10⁹ organisms injected i.v. Each result shown in a and b is typical of at least two identical experiments with each organism. (c) E. coli J5, 10⁹ organisms injected i.v. at time 0; isolated human SAP, 10 mg per mouse, was injected i.p. at −2 h and 16 h. P values (log rank test) represent significant differences in survival between SAP^−/− and SAP^+/+ or SAP-reconstituted SAP^−/− mice.

Figure 3

Effect of inhibition of SAP binding on the survival of C57BL/6 mice during bacterial infection. All mice received an LD₉₀ of E. coli J5, 10⁹ organisms injected i.v. (a) The treated group received compound R at 10 mg per mouse by i.p. injection immediately before infection and at 4 h. (b) The treated group received compound R in doses of 5 mg per mouse by i.p. injection immediately before infection and 2 mg per mouse at hourly intervals up to 12 h. In both experiments the control group received injections of solvent alone at the same times. Note that the repeated manipulation and injections involved in b were associated with accelerated morbidity and mortality, but the treated group still showed significantly prolonged survival. P values (Wilcoxon test based on Kaplan–Meier analysis) represent significant differences in survival between treated and control groups.

Figure 4

Effect of SAP on immediate killing of E. coli J5 in SAP^−/− mice in vivo. E. coli J5 was preincubated with whole acute phase serum from wild-type SAP^+/+ mice, or from SAP^−/− animals, or with SAP^−/− acute phase serum reconstituted by the addition of isolated mouse SAP, and 10⁷ organisms were then injected i.p. into individual C57BL/6 SAP^−/− mice. After 15 min the peritoneal cavity was lavaged, and the numbers of bacteria surviving in each animal were determined. Horizontal lines indicate medians, and the P value (Wilcoxon rank sum test) indicates the significant difference between bacterial killing in the presence or absence of SAP.

Figure 5

Effect of SAP deficiency on survival after LPS challenge. (a) BALB/c mice received i.p. injections of 100 mg/kg of isolated pure human SAP, or equimolar amounts of isolated pure human CRP or serum albumin, followed after 20 min by 200 μg per mouse of S. typhimurium LPS, injected i.p. at time 0. (b) SAP^−/− or wild-type SAP^+/+ C57BL/6 mice received 10 mg/kg of whole LPS from E. coli O111:B4, about 200 μg per mouse, injected i.p. at time 0.