Intrapulmonary Delivery of Tumor Necrosis Factor Agonist Peptide Augments Host Defense in Murine Gram-Negative Bacterial Pneumonia

An effective host defense against lung bacterial infection is dependent primarily upon the rapid clearance of the organism from the respiratory tract, which is mediated by the influx and/or activation of phagocytic cells, including neutrophils (polymorphonuclear leukocytes [PMN]) and macrophages (27). The recruitment and activation of leukocytes in the setting of bacterial challenge is a complex and dynamic process which involves the coordinated expression of both pro- and anti-inflammatory cytokines (10–12, 14, 15). Tumor necrosis factor alpha (TNF) is a 17-kDa cytokine which is a critical component of an effective antibacterial host defense (2, 9, 13, 16). Specifically, TNF is a potent activator of both PMN and macrophages, leading to enhancement of protease release, stimulation of the respiratory burst, and induction of leukocyte and vascular adhesion molecule expression, which are essential for transmigration of these cells into sites of infection (7, 19, 24). Moreover, PMN and macrophage microbicidal activity is augmented by endogenous or exogenous TNF (22, 26). Finally, TNF is expressed in increased amounts in the airspaces of humans with bacterial pneumonia (21) and in the lungs of mice challenged with bacterial pathogens (9, 18). The inhibition of TNF has been shown to impair lung bacterial clearance and in some but not all studies to attenuate lung PMN influx in response to aerosolized or intratracheally (i.t.) administered Pseudomonas aeruginosa and Klebsiella pneumoniae (2, 9, 15, 18), resulting in markedly decreased survival in animals with bacterial pneumonia (18). However, TNF can mimic many of the detrimental pathophysiologic events that occur in sepsis and sepsis syndrome (1, 28), making it difficult to obtain the beneficial therapeutic effects of TNF without its cytotoxic consequences. Recently, a TNF agonist peptide composed of the 11 amino acids that constitute the site of binding of native human TNF to its receptors (referred to as TNF70-80) has been characterized (17, 23). Binding of TNF70-80 to TNF receptors (both p55 and p75) has been shown to mediate many leukocyte-activating effects of native TNF. Specifically, this peptide directly stimulates and primes neutrophils for enhanced protease release and respiratory burst, enhances neutrophil phagocytic activity, and augments neutrophil killing of Plasmodium falciparum in vitro and clearance of Plasmodium chabaudi in mice in vivo (17). In that study, TNF70-80 was associated with minimal toxicity when administered systemically, due in part to the fact that this peptide did not alter adhesive properties of the endothelium (17). The emergence of multidrug-resistant microbes in the immunocompromised host has made the treatment of bacterial infections of the lung increasingly difficult (3, 4, 6, 20, 25), underscoring the importance of the immune host defense in determining the eventual outcome of severe pneumonia. Given these clinical dilemmas, we performed this study to determine if the intrapulmonary and/or systemic administration of TNF70-80 was capable of augmenting host innate immunity in the setting of murine gram-negative pneumonia.