Despite the use of antibiotics active against secondary bacterial pathogens, pneumonia following influenza has a high mortality rate. Due to a steady increase in the number of antibiotic-resistant strains, the effectiveness of strategies using antimicrobials alone is likely to decrease over time. Alternative or adjunctive therapies are therefore necessary for routine treatment of complications of seasonal influenza and in preparation for the next influenza pandemic. In this paper, we report that combination therapy with IVIG and P4 peptide effectively rescues mice from a fatal secondary bacterial pneumonia without the use of antibiotics. An increase in phagocytic cells in the airways and a decrease in surface display of FcγR correlated with decreased inflammation in the lungs and clinical cure. The requirement of both P4 and IVIG for reductions in mortality suggests a model in which type-specific antibody binds pneumococci, P4 enhances transit of macrophages and neutrophils into the airways and activates FcγRs on these professional phagocytes, and a robust opsonophagocytosis response results, clearing the infection. The decrease in bacterial load, in turn, leads to a decrease in the requirement for new inflammatory cells within the lung parenchyma, reducing pneumonic consolidation and acute lung injury.
P4 is a novel immunomodulatory peptide derived from the pneumococcal surface adhesin A protein. It has previously been shown to activate various cell lines and increase the opsonophagocytosis capabilities of host cells in vitro
). This resulted in improved outcomes of primary pneumococcal pneumonia in mice (23
). In the setting of prior influenza virus infection, however, the inflammatory milieu is quite different during bacterial lung infections, with enhanced infiltration of inflammatory cells into both the lungs and the airways, increased bacterial loads, and hypercytokinemia (14
). Thus, we sought in this study to determine whether therapy with IVIG and P4 would remain effective during fulminant coinfections. Remarkably, two doses of IVIG followed by P4, administered 24 and 48 h following the onset of severe bacterial pneumonia, substantially improved mouse survival. By both bioluminescent imaging and culture, this correlated with a significant reduction in bacterial lung load. P4 or IVIG alone each had no effect, confirming that enhanced utilization of natural defenses, rather than direct killing by the peptide, was responsible for bacterial clearance. Since bacterial lysis by antibiotics is associated with worse outcomes in secondary bacterial pneumonia following influenza (8
), this alternate strategy may be superior to current practice either alone or as adjunctive therapy with antibiotics (16
). Although 80% survival represents a significant beneficial outcome in this model, we think it likely that earlier treatment, sustained treatment, or combination treatment with antibiotics would likely improve the survival advantage further.
The mechanism by which P4 exerts its salutary effects is of obvious interest. The finding of increased transit of phagocytes into the airways, where the pneumococcus exerts its pathogenic effects, suggests modulation of chemotaxis or trafficking. Once present at this site, the rapid clearance of bacteria associated with decreased surface display of FcγRI and FcγRII on these phagocytes implicates P4 in enhancing opsonophagocytosis. The pathogen-specific, antibody-dependent P4 effect at this stage implies that FcγR recognition of the Fc portion of antibodies bound to pneumococci assists in the process. The engagement of FcγRs has been shown to activate a self-limiting inflammatory response in macrophages (19
). Additionally, it has been shown that internalized antigen/IgG complexes are much more efficiently presented on major histocompatibility complex (MHC) class II molecules than internalized soluble antigens, most likely due to the specific binding to the surface FcγRs (11
). The limited proinflammatory response elicited by binding of the FcγRs and the independent cell-activating properties of P4 peptide may work collectively to restore the proper balance of pro- and anti-inflammatory cytokines and improve the function of professional phagocytes, leading to a more favorable outcome and effective treatment of secondary bacterial pneumonia following influenza.
In summary, the combined therapy of IVIG and P4 improves the outcome of influenza and secondary bacterial pneumonia caused by pneumococci. The accelerated clearance of pneumococci and reduced infiltration of inflammatory cells into the lung parenchyma likely contribute to this favorable outcome, through mechanisms involving both the altered trafficking and enhanced capacity of professional phagocytes. Historically, passive immunotherapy was clinically utilized in the treatment of pneumonia until the introduction of antibiotics. With the increasing failure of strategies involving direct killing of pneumonic pathogens to effect cure, particularly in the subset of patients with severe pneumonia, such as that engendered by prior influenza virus infection, a return to modified versions of passive immunotherapy may be indicated. We demonstrate here that an immunomodulatory strategy coupled with antibody therapy can be an effective tool in our armamentarium against severe pneumonia. Further exploration in preclinical models and clinical studies of humans is indicated.