TNF-related apoptosis-inducing ligand (TRAIL) exerts therapeutic efficacy for the treatment of pneumococcal pneumonia in mice

Apoptotic death of alveolar macrophages observed during lung infection with Streptococcus pneumoniae is thought to limit overwhelming lung inflammation in response to bacterial challenge. However, the underlying apoptotic death mechanism has not been defined. Here, we examined the role of the TNF superfamily member TNF-related apoptosis-inducing ligand (TRAIL) in S. pneumoniae–induced macrophage apoptosis, and investigated the potential benefit of TRAIL-based therapy during pneumococcal pneumonia in mice. Compared with WT mice, Trail^−/− mice demonstrated significantly decreased lung bacterial clearance and survival in response to S. pneumoniae, which was accompanied by significantly reduced apoptosis and caspase 3 cleavage but rather increased necrosis in alveolar macrophages. In WT mice, neutrophils were identified as a major source of intraalveolar released TRAIL, and their depletion led to a shift from apoptosis toward necrosis as the dominant mechanism of alveolar macrophage cell death in pneumococcal pneumonia. Therapeutic application of TRAIL or agonistic anti-DR5 mAb (MD5-1) dramatically improved survival of S. pneumoniae–infected WT mice. Most importantly, neutropenic mice lacking neutrophil-derived TRAIL were protected from lethal pneumonia by MD5-1 therapy. We have identified a previously unrecognized mechanism by which neutrophil-derived TRAIL induces apoptosis of DR5-expressing macrophages, thus promoting early bacterial killing in pneumococcal pneumonia. TRAIL-based therapy in neutropenic hosts may represent a novel antibacterial treatment option.Streptococcus pneumoniae is the most prevalent pathogen, and is responsible for causing community-acquired pneumonia in humans. Despite the fact that all clinically relevant serotypes of S. pneumoniae are susceptible against the most common antibiotics, S. pneumoniae remains a significant cause of morbidity and lethality worldwide (Welte et al., 2012). Therefore, the development of novel antibiotic-independent therapeutic strategies is urgently needed to decrease the disease burden associated with pneumococcal infections of the lung.Because of their pivotal role in bacterial phagocytosis and orchestration of innate immune responses to bacterial infections, alveolar macrophages represent the first line of lung protective immunity against inhaled S. pneumoniae (Calbo and Garau, 2010). Recruited neutrophils support macrophages in lung bacterial clearance during established pneumonia (Knapp et al., 2003; Herbold et al., 2010; Calbo and Garau, 2010), and resident alveolar and lung macrophages, along with inflammatory recruited exudate macrophages, critically contribute to resolution of lung inflammation (Knapp et al., 2003; Winter et al., 2007).An important feature of S. pneumoniae–induced lung infection is the rapid induction of apoptosis in alveolar macrophages within 24 h, resulting in a transient depletion of alveolar macrophages from distal airways (Paton, 1996; Rubins et al., 1996; Dockrell et al., 2003; Knapp et al., 2003; Maus et al., 2004, 2007; Winter et al., 2007; Taut et al., 2008; Hahn et al., 2011b). Inhibition of S. pneumoniae–induced macrophage apoptosis decreases lung pneumococcal clearance, thereby promoting invasive pneumococcal disease progression in mice (Dockrell et al., 2003; Marriott et al., 2005). Conversely, activation of apoptotic cascades in macrophages and neutrophils limits pathogen-driven inflammatory cascades during pneumococcal disease (Marriott et al., 2004, 2006). Moreover, phagocytosis of apoptotic macrophages by lung macrophages down-regulates the overall inflammatory response and decreases invasive disease progression of pneumococcal pneumonia (Fadok et al., 1998; Marriott et al., 2006). Together, these data suggest that macrophage apoptosis is protective in terms of limiting excessive proinflammatory responses during pneumococcal lung infections.The TNF superfamily member TNF-related apoptosis-inducing ligand (TRAIL) exhibits a complex ligand/receptor cross-talk (Schneider et al., 2003). In humans, four membrane-bound TRAIL receptors have been identified, of which TRAIL-R1 (DR4) and TRAIL-R2 (DR5) are apoptosis-inducing receptors, and TRAIL-R3 (DcR1) and TRAIL-R4 (DcR2) act as “decoy” receptors because of absent or nonfunctional death domains (Ashkenazi and Dixit, 1999). In mice, three decoy receptors, but only one death-mediating receptor for TRAIL, death receptor 5 (DR5), have been identified (Wu et al., 1999; Schneider et al., 2003). Previously, a role for caspases and TNF superfamily member Fas ligand has been established in lung infection models (Ali et al., 2003; Matute-Bello et al., 2005). More recently, there has been emerging evidence for a role of TRAIL to induce apoptosis in leukocyte subsets (Katsikis et al., 1997; Renshaw et al., 2003; Zheng et al., 2004; Lum et al., 2005; McGrath et al., 2011; Zhu et al., 2011), alveolar epithelial cells, and other host cell-types in models of LPS-induced acute lung injury, peritonitis (McGrath et al., 2011), as well as viral and bacterial infections (Zheng et al., 2004; Ishikawa et al., 2005; Hoffmann et al., 2007; Brincks et al., 2008, 2011; Stary et al., 2009; Cziupka et al., 2010; Zhu et al., 2011). These data collectively demonstrate that TRAIL plays a role in inducing apoptosis in different cell types in pulmonary inflammation and infection models.Despite the increased acknowledgment that TRAIL is a key player in several immune reactions within the lung, there are currently no data available regarding the role of TRAIL in macrophage apoptosis and disease progression in bacterial pneumonia induced by the major prototype lung-tropic pathogen, S. pneumoniae. Our data reveal a novel neutrophil-macrophage cross talk mechanism by which alveolar accumulating neutrophils responding to the infection secrete TRAIL that induces alveolar macrophage apoptosis and regulates bacterial killing subsequent to pneumococcal challenge. Importantly, we also show for the first time that treatment of neutropenic mice with agonistic anti-DR5 antibody compensates for the lack of neutrophil-derived TRAIL, and significantly improves survival of pneumococcal pneumonia. This finding may be of great interest for future antibiotic-independent immunomodulatory strategies in immunocompromised patients at risk of acquiring bacterial infections. The implications of these findings will be discussed.