Many studies suggest that with aging, immune capabilities gradually diminish, leading to a decrease in antibody production, cytokines, and various effector cells (1-4). In this study, we examined the effects of an immune-enhancing peptide on aged mice. P4, a 28-amino-acid cationic peptide derived from pneumococcal surface adhesin A (PsaA), is a eukaryotic cellular activator (10). Previously, we demonstrated that the cellular activation properties of P4 can be utilized to rescue severely ill young mice from fatal pneumococcal infection in the presence of pathogen-specific antibodies and active complement (8, 12). While P4 therapy was used to rescue young Swiss Webster mice (6 to 10 weeks old), we questioned its effectiveness in aged mice (11 and 15 months old).
Intranasal inoculation of mice with Streptococcus pneumoniae WU2 (serotype 3) and P4 therapy were done using protocols previously described, with minor modifications (12). Eleven-month-old BALB/c (n = 20) and 15-month-old Swiss Webster mice (n = 20) were infected intranasally with S. pneumoniae WU2 (~2.1 × 107 cells/mouse). Mice were monitored and visually scored twice daily for moribund characteristics as previously described (12). At 48 h postchallenge, 80% (16/20) were moribund. Moribund mice were divided into a control (n = 8) and a treatment group (n = 8). Two doses of P4 therapy with pathogen-specific antibody (intravenous immunoglobulin [IVIG]; Gamunex, Telecris, NC) and P4 were administered intravenously (postinfection) in the treatment group. Treated and untreated animals were monitored for 166 h, and the data computed for significant differences among various groups using a t test for paired samples for the means (MS Excel 2007).
Seventy-three percent of treated 15-month-old Swiss Webster mice survived with complete remission of symptoms, compared to 20% survival in the control group (P = 0.02) (Fig. (Fig.1).1). Ninety-five percent of the treated 11-month-old BALB/c mice survived, while only 45% of the mice from the control group survived (P = 0.0002) (Fig. (Fig.2).2). These findings are consistent with those of our previous studies where P4 therapy successfully rescued young mice from fatal pneumococcal infection (i.e., treated mice had an 80% survival rate and control mice had a 30% survival rate; P = 0.0002) (12).
In mice, P4 therapy augments innate immunity and treats severe bacterial infection in different age groups. The presence of pathogen-specific antibody, effector cells, and complement are the critical factors that determine the effectiveness of P4 therapy (11). The exact mechanism is not known. We speculate that polymorphonuclear neutrophils are the major innate immune component activated by the P4 peptide, as we have observed that mice treated with the neutrophil-depleting antibody RB6-8C5 (5) (after pneumococcal infection) failed to respond to in vivo P4 therapy (data not shown). While antibiotics are the first line of treatment for the elderly, antibiotic therapy can be challenging because of multiantibiotic-resistant strains, drug interactions with other medications, and side effects. Passive immune therapy supplemented with agents like P4 may address some of these concerns (6, 7, 9). New approaches are needed to augment passive immunization for infectious diseases, especially in the elderly. Immune enhancement with biomolecules such as the P4 peptide may provide a much needed thrust for patients to overcome severe infections.