Evaluation of a novel therapeutic approach to treating severe pneumococcal infection using a mouse model

Abstract

P4, a 28-amino-acid peptide, is a eukaryotic cellular activator that enhances specific in vitro opsonophagocytic killing of multiple bacterial pathogens. In a previous study, we successfully recreated this phenomenon in mice in vivo by using a two-dose regimen of P4 and pathogen-specific antibodies, which significantly reduced moribundity in mice. For the present study, we hypothesized that the inclusion of a low-dose antibiotic would make it possible to treat the infected mice with a single dose containing a mixture of P4 and a pathogen-specific antibody. A single dose consisting of P4, intravenous immunoglobulin (IVIG), and ceftriaxone effectively reduced moribundity compared to that of untreated controls (n = 10) by 75% (P < 0.05) and rescued all (10 of 10) infected animals (P < 0.05). If rescued animals were reinfected with Streptococcus pneumoniae and treated with a single dose containing P4, IVIG, and ceftriaxone, they could be rerescued. This observation of the repeated successful use of P4 combination therapy demonstrates a low risk of tolerance development. Additionally, we examined the polymorphonuclear leukocytes (PMN) derived from infected mice and observed that P4 enhanced in vitro opsonophagocytic killing (by >80% over the control level; P < 0.05). This finding supports our hypothesis that PMN are activated by P4 during opsonophagocytosis and the recovery of mice from pneumococcal infection. P4 peptide-based combination therapy may offer an alternative and rapid immunotherapy to treat fulminant pneumococcal infection.

FIG. 1.

Survival of mice in various treatment arms and of control mice following exposure to S. pneumoniae serotype 3 (WU2). A single-dose i.v. injection of a mixture of P4 and gamma globulin (IVIG) with i.p. injection of ceftriaxone (Ceft) provided highly significant protection (100%; P < 0.05) compared to that of untreated controls. a, treatment administered.

FIG. 2.

Mice previously rescued (♦) with P4 combination therapy were reinfected with S. pneumoniae serotype 3(WU2) on day 28. A single dose of P4, gamma globulin (IVIG), and ceftriaxone was administered 2 days later. All animals (100%) were protected. a, treatment; b, reinfection.

FIG. 3.

An in vitro OPKA with peripheral blood PMNs isolated from mice 1 and 2 h postinfection or from uninfected mice was performed. Gamma globulin (IVIG) was used as the source for serotype-specific antibodies. The addition of P4 increased the opsonophagocytic killing of S. pneumoniae serotype 3 (WU2) by ≥80% over that by PMNs from control mice not receiving P4 (P < 0.05).