Use of Berlin EXCOR cannulas in both venovenous and venoarterial central extracorporeal membrane oxygenation configurations overcomes the problem of cannula instability while bridging infants and young children to lung transplant.

Objectives: Infants and young children awaiting lung transplantation present challenges that often preclude successful extracorporeal membrane oxygenation support as a bridge to transplantation. Instability of neck cannulas often results in the need for intubation, mechanical ventilation, and muscle relaxation creating a worse transplant candidate. With the use of Berlin Heart EXCOR cannulas (Berlin Heart, Inc) in both venoarterial and venovenous central cannulation configurations, 5 pediatric patients were successfully bridged to lung transplant. Methods: We performed a single-center retrospective case review of central extracorporeal membrane oxygenation cannulation used as a bridge to lung transplantation cases performed at Texas Children's Hospital between 2019 and 2021. Results: Six patients, 2 with pulmonary veno-occlusive disease (15-month-old male and 8-month-old male), 1 with ABCA3 mutation (2-month-old female), 1 with surfactant protein B deficiency (2-month-old female), 1 with pulmonary arterial hypertension in the setting of D-transposition of the great arteries after repair as a neonate (13-year-old male), and 1 with cystic fibrosis and end-stage lung disease, were supported for a median of 56.3 days on extracorporeal membrane oxygenation while awaiting transplantation. All patients were extubated after initiation of extracorporeal membrane oxygenation, participating in rehabilitation until transplant. No complications due to central cannulation and use of the Berlin Heart EXCOR cannulas were observed. One patient with cystic fibrosis developed fungal mediastinitis and osteomyelitis resulting in discontinuation of mechanical support and death. Conclusions: Novel use of Berlin Heart EXCOR cannulas for central cannulation eliminates the problem of cannula instability allowing extubation, rehabilitation, and bridge to lung transplant for infants and young children.

Application of organ dysfunction assessment scores following pediatric lung transplantation.

OBJECTIVE: Organ dysfunction (OD) after lung transplantation can reflect preoperative organ failure, intraoperative acute organ damage and post-operative complications. We assessed two OD scoring systems, both the PEdiatric Logistic Organ Dysfunction (PELOD) and the pediatric Sequential Organ Failure Assessment (pSOFA) scores, in recognizing risk factors for morbidity as well as recipients with prolonged post-transplant morbidity. DESIGN: Medical records of recipients from January 2009 to March 2016 were reviewed. PELOD and pSOFA scores were calculated on post-transplant days 1-3. Risk factors assessed included cystic fibrosis (CF), prolonged surgical time and worst primary graft dysfunction (PGD) score amongst others. Patients were classified into three groups based on their initial scores (group A) and subsequent trends either uptrending (group B) or downtrending (group C). Morbidity outcomes were compared between these groups. RESULTS: Total 98 patients were enrolled aged 0-20 years. Risk factors for higher pSOFA scores ≥ 5 on day 1 included non-CF diagnosis and worst PGD scores (p = .0006 and p = .03, respectively). Kruskal Wallis analysis comparing pSOFA group A versus B versus C scores showed significantly prolonged ventilatory days (median 1 vs. 4 vs. 2, p = .0028) and ICU days (median 4 vs. 10 vs. 6, p = .007). Similarly, PELOD group A versus B versus C scores showed significantly prolonged ventilatory days (1 vs. 5 vs. 2, p = < .0001). CONCLUSION: Implementing pSOFA scores bedside is a more effective tool compared to PELOD in identifying risk factors for worsened OD post-lung transplant and can be valuable in providing direction on morbidity outcomes in the ICU.

Cystic fibrosis lung transplant recipients 10 years of age or younger: Predisposing factors for end-stage disease.

BACKGROUND: The largest age group among children and adolescents referred for lung transplantation for cystic fibrosis (CF) have been those in the pubertal or postpubertal age range. However, over 100 younger patients with CF have undergone lung transplantation over the last three decades in the United States. METHODS: We performed a retrospective review of our experience with 18 children with CF who underwent lung transplantation in our center before the age of 11 years and compared them to our older CF lung transplant recipients and our larger CF Center population. RESULTS: The transplant population was demographically distinct from our CF center in terms of ethnicity, country of origin, and insurance status. Other notable findings were a high prevalence of methicillin-resistant Staphylococcus aureus, a high prevalence of CF-related diabetes mellitus, and a high prevalence of consolidated lobar or whole lung disease. Posttransplant outcomes were comparable to those older than 10 years of age in our center until 5 years after transplant after which the younger cohort showed a superior enduring survival. CONCLUSIONS: In an era of increasingly effective medications modifying the natural history of CF, identification of risk factors for early severe lung disease in CF remains relevant to permit interventions to prevent or postpone the time of future lung transplantation.

Diagnosis and treatment of cryptococcal osteomyelitis in a pediatric lung transplant patient.

BACKGROUND: Asymptomatic pulmonary nodules may appear at any point after lung transplantation. The differential diagnosis is broad and includes serious life-threatening disease entities. METHODS: A retrospective case report of a single patient who developed a pulmonary nodule after lung transplantation. RESULTS: At 2 years post-transplant, an 11-year-old with cystic fibrosis was asymptomatic and had normal lung function. A single nodule was noted on surveillance chest CT scan. Initial evaluation was negative, but subsequently, he was diagnosed with cryptococcal osteomyelitis in a thoracic rib. He responded well to an extended course of antifungal therapy without loss of allograft function or infectious complications. CONCLUSION: Pulmonary nodules after lung transplantation may be a harbinger of serious complications. A systematic approach to evaluation and follow-up is recommended.

Infections Within the First Month After Pediatric Lung Transplantation: Epidemiology and Impact on Outcomes.

BACKGROUND: Despite successes in lung transplantation, with infection as the leading cause of death in the first year following lung transplantation, there remains a lag in survival compared with other solid organ transplants. Infections that occur early after transplantation may impact short- and long-term outcomes in pediatric lung transplant recipients (LTRs). METHODS: We performed a retrospective review of pediatric LTRs at a large quaternary-care hospital from January 2009 to March 2016 to evaluate both epidemiologic features of infection in the first 30 days post-transplantation and mortality outcomes. The 30 days were divided into early (0-7 days) and late (8-30 days) periods. RESULTS: Among the 98 LTRs, there were 51 episodes of infections. Cystic fibrosis (CF) was associated with early bacterial infections (P = .004) while non-CF was associated with late viral (P = .02) infections. Infection after transplantation was associated with worse survival by Kaplan-Meier analysis (P value log rank test = .007). Viral infection in the late period was significantly associated with 3-year mortality after multivariable analysis (P = .02). CONCLUSIONS: Infections in pediatric LTRs were frequent in the first 30 days after transplant, despite perioperative antimicrobial coverage. The association of 3-year mortality with late viral infections suggests a possible important role in post-transplant lung physiology and graft function. Understanding the epidemiology of early post-lung transplant infections can help guide post-operative management and interventions to reduce their incidence and the early- and long-term impact in this population.

Early evidence of pulmonary dysfunction in survivors of childhood Hodgkin lymphoma.

Survivors of childhood Hodgkin lymphoma (HL) are at risk for pulmonary late effects, but whether survivors also experience pulmonary dysfunction early off therapy is not well understood. We determined the incidence of pulmonary dysfunction in children/adolescents with HL at entry into survivorship, as well as risk factors related to this outcome. Survivors in clinical remission and with a pulmonary function test (PFT) obtained 2-6 years off therapy were included. Seventy-five of 118 subjects met eligibility criteria (mean age at diagnosis: 13 years, mean time off therapy: 40 months). Survivors of HL had a higher than expected incidence of pulmonary dysfunction at entry into survivorship (40/75 [53%] had an abnormal DLCO and/or a restrictive or obstructive impairment). Evidence for diffusion impairment was associated with female sex (odds ratio [OR] = 3.19, p = .04). Longitudinal follow-up studies are needed to determine if early evidence of pulmonary dysfunction predicts risk for later onset pulmonary outcomes.

Clinico-radiologic features of pleuroparenchymal fibroelastosis in children.

BACKGROUND: Pleuroparenchymal fibroelastosis (PPFE) may be underdiagnosed clinically and radiographically in children with a remote history of cancer, leading to a delay in care and unnecessary lung biopsies. OBJECTIVE: To describe the characteristic clinical and radiologic findings of PPFE in a cohort of children to facilitate recognition and noninvasive diagnosis. MATERIALS AND METHODS: Clinical presentation, history of chemotherapy or radiation therapy, lung or bone marrow transplantation, and lung function testing and outcome were retrospectively extracted from the electronic medical records of eight children treated at our institution's pulmonary medicine clinic with histopathology confirmation of PPFE from 2008 to 2018. Two pediatric radiologists evaluated the chest imaging studies for the presence or absence of published radiologic findings of PPFE in adults, including platythorax, pneumothorax, upper lobe predominant pleural and septal thickening, and bronchiectasis. Platythorax indices were calculated from the normal chest CT exams of eight age- and gender-matched individuals obtained via the radiology search engine. RESULTS: The mean presentation age was 12.9 years (range: 7-16 years). Seven of the eight had a history of chemotherapy and radiation therapy for cancer. Three of the eight had undergone bone marrow transplantation and none had undergone lung transplantation. The mean time between chemotherapy, radiation therapy, and/or bone marrow transplantation and the presentation of PPFE was 8.4 years (range: 5.6-12.1 years). Most of the patients presented with dyspnea (63%), cough (50%) and/or pneumothorax (38%). The mean percentage of predicted FEV1 (forced expiratory volume in one second) was 14.1 (range: 7.7-27.5). All eight patients demonstrated platythorax, bronchiectasis, pleural and septal thickening (upper lobes in four, upper and lower lobes in four) and six had pneumothorax. Five underwent lung biopsies, four of whom developed pneumothoraces. CONCLUSION: Clinical and radiologic findings of pediatric PPFE are similar to those in adults, although a majority of the former have a history of treated cancer. Clinical presentation of restrictive lung disease, dyspnea, cough or spontaneous pneumothorax years after treatment for childhood cancer combined with platythorax, upper lobe pleural and septal thickening and traction bronchiectasis on chest CT establishes a presumptive diagnosis of PPFE.

Risk factors for infection after pediatric lung transplantation.

Although infection is the leading cause of death in the first year following pediatric lung transplantation, there are limited data on risk factors for early infection. Sepsis remains under-recognized and under-reported in the early post-operative period for lung transplant recipients (LTR). We evaluated the incidence of infection and sepsis, and identified risk factors for infection in the early post-operative period in pediatric LTRs. A retrospective review of medical records of LTRs at a large quaternary-care hospital from January 2009 to March 2016 was conducted. Microbiology results on days 0-7 after transplant were obtained. Sepsis was defined using the 2005 International Pediatric Consensus Conferencecriteria. Risk factors included history of recipient and donor infection, history of multi-drug resistant (MDR) infection, nutritional status, and surgical times. Among the 98 LTRs, there were 22 (22%) with post-operative infection. Prolonged donor ischemic time ≥7 hours, cardiopulmonary bypass(CPB) time ≥340 minutes, history of MDR infection and diagnosis of cystic fibrosis were significantly associated with infection. With multivariable regression analysis, only prolonged donor ischemic time remained significant (OR 4.4, 95% CI: 1.34-14.48). Further research is needed to determine whether processes to reduce donor ischemic time could result in decreased post-transplant morbidity.

Anellovirus loads are associated with outcomes in pediatric lung transplantation.

Anelloviruses are DNA viruses ubiquitously present in human blood. Due to their elevated levels in immunosuppressed patients, anellovirus levels have been proposed as a marker of immune status. We hypothesized that low anellovirus levels, reflecting relative immunocompetence, would be associated with adverse outcomes in pediatric lung transplantation. We assayed blood samples from 57 patients in a multicenter study for alpha- and betatorquevirus, two anellovirus genera. The primary short-term outcome of interest was acute rejection, and longer-term outcomes were analyzed individually and as "composite" (death, chronic rejection, or retransplant within 2 years). Patients with low alphatorquevirus levels at 2 weeks post-transplantation were more likely to develop acute rejection within 3 months after transplant (P = .013). Low betatorquevirus levels at 6 weeks and 6 months after transplant were associated with death (P = .047) and the composite outcome (P = .017), respectively. There was an association between low anellovirus levels and adverse outcomes in pediatric lung transplantation. Alphatorquevirus levels were associated with short-term outcomes (ie, acute rejection), while betatorquevirus levels were associated with longer-term outcomes (ie, death, or composite outcome within 2 years). These observations suggest that anelloviruses may serve as useful biomarkers of immune status and predictors of adverse outcomes.

Risk and outcomes of pulmonary fungal infection after pediatric lung transplantation.

BACKGROUND: Prospective studies to determine associated risk factors and related outcomes for pulmonary fungal infection (PFI) after pediatric lung transplant (PLT) are lacking. METHODS: NIH-sponsored Clinical Trials in Organ Transplantation in Children enrolled PLT candidates, collecting data prospectively for 2 years post-transplant. Demographics, signs/symptoms, radiology, pathology and microbiology were collected. Analyses evaluated for PFI-related risks and outcomes. RESULTS: In 59 PLT, pre-transplant fungal colonization occurred in 6 donors and 15 recipients. Cystic fibrosis (CF) was associated with pre-transplant colonization (P < .01). Twenty-five (42%) PLT had 26 post-transplant colonizations (median = 67 days, range = 0-750 days) with Candida (13), Aspergillus (4), mold (6) or yeast (3). Post-PLT colonization was not associated with CF, age, or pre-PLT colonization. Thirteen PFIs occurred in 10 (17%) patients, 3 proven (Candida species) and 10 probable (Candida [3], Aspergillus [3], Penicillium [3], and mold [1]). Pulmonary fungal infection was preceded by post-PLT colonization with the same organism in 4 of 13 PFI, but post-PLT colonization did not predict subsequent PFI (P = .87). Older age at transplant was a risk for PFI (P < .01). No mortality was attributed to PFI. Prophylaxis use was not associated with decreased post-PLT colonization (P = .60) or PFI (P = .48). CONCLUSION: In PLT, PFI and fungal colonization are common but without associated mortality. Post-PLT colonization did not predict PFI. Optimal prevention strategies require additional study.

Waiting list outcomes in pediatric lung transplantation: Poor results for children listed in adult transplant programs.

BACKGROUND: Low case volume has been associated with lower survival after pediatric lung transplantation. Our aim was to analyze waitlist outcomes among pediatric lung transplant centers in the USA. METHODS: We studied a cohort of 1,139 pediatric candidates listed in the Organ Procurement and Transplantation Network for lung transplantation between 2002 and 2014. Of these candidates, 720 (63.2%) received a transplant. Candidates were divided into groups according to the clinical activity of the center of listing: high-volume pediatric (≥4 transplants per year); low-volume pediatric (<4 transplants per year); and adult (transplant volume predominantly in adults). We used multivariate Cox regression analysis to identify independent risk factors for waitlist mortality. We also determined the transplant rate-or likelihood of transplant after listing-over the study period. RESULTS: Fifty-eight percent of the children and adolescents were listed in adult centers where the resultant transplant rate was low-only 42% received a transplant compared with 93% in pediatric programs. Listing in an adult program was also the most significant risk factor for death on the waiting list (hazard ratio 15.6, 95% confidence interval 5.8 to 42.1). CONCLUSIONS: Most children (58%) are listed for lung transplantation in adult centers and have a reduced rate of transplantation and a greater chance of waitlist mortality.

Altered Hypoxic-Adenosine Axis and Metabolism in Group III Pulmonary Hypertension.

Group III pulmonary hypertension (PH) is a highly prevalent and deadly lung disorder with limited treatment options other than transplantation. Group III PH affects patients with ongoing chronic lung injury, such as idiopathic pulmonary fibrosis (IPF). Between 30 and 40% of patients with IPF are diagnosed with PH. The diagnosis of PH has devastating consequences to these patients, leading to increased morbidity and mortality, yet the molecular mechanisms involved in the development of PH in patients with chronic lung disease remain elusive. Our hypothesis was that the hypoxic-adenosinergic system is enhanced in patients with group III PH compared with patients with IPF with no PH. Explanted lung tissue was analyzed for markers of the hypoxic-adenosine axis, including expression levels of hypoxia-inducible factor (HIF)-1A, adenosine A2B receptor, CD73, and equilibrative nucleotide transporter-1. In addition, we assessed whether altered mitochondrial metabolism was present in these samples. Increased expression of HIF-1A was observed in tissues from patients with group III PH. These changes were consistent with increased evidence of adenosine accumulation in group III PH. A novel observation of our study was of evidence suggesting altered mitochondrial metabolism in lung tissue from group III PH leading to increased succinate levels that are able to further stabilize HIF-1A. Our data demonstrate that the hypoxic-adenosine axis is up-regulated in group III PH and that subsequent succinate accumulation may play a part in the development of group III PH.

Bronchial artery revascularization and en bloc lung transplant in children.

BACKGROUND: Long-term success in pediatric lung transplantation is limited by infection and bronchiolitis obliterans syndrome (BOS). The bilateral sequential lung transplantation (BSLT) technique may result in airway ischemia leading to bronchial stenosis, dehiscence, or loss of small airways. En bloc lung transplant (EBLT) with bronchial artery revascularization (BAR) minimizes airway ischemia, thus promoting superior airway healing. BAR also allows for safe tracheal anastomosis, circumventing the need for bilateral bronchial anastomoses in small children. METHODS: This was a retrospective review of bilateral transplantations from 2005 to 2014. Both techniques were used in parallel. Redo and multiorgan transplants were excluded. RESULTS: There were 119 recipients comprising 88 BSLTs and 31 EBLTs. Follow-up time was 3 years (interquartile range, 1-5 years). Donor ischemic and cardiopulmonary bypass times were not different between techniques (p = 0.48 and p = 0.18, respectively). Degree of graft dysfunction and cellular rejection scores were not different (p = 0.83 and p = 0.93, respectively). There were 3 hospital deaths after BSLT and 2 after EBLT (p = 0.60). Overall survival was 61% for the BSLT group and 77% for the EBLT group (p = 0.54). Freedom from BOS was 71% in the BSLT group and 94% in the EBLT group (p = 0.08). On routine bronchoscopy, 57% BSLT and 16% EBLT patients had 1 or more airway ischemic findings (p < 0.0001). Multivariate analysis showed BSLT was associated with higher ischemic injury (relative risk, 2.86; 95 confidence interval, 1.3-6.5; p = 0.01) and non-airway complications (relative risk, 4.62; 95% confidence interval, 1.1-20.2; p = 0.04) but not airway reinterventions (p = 0.07). Airway dehiscence occurred in 3 BSLT patients. CONCLUSIONS: Pediatric EBLT with BAR can be safely performed without increasing operative or graft ischemic times. Airway ischemia and non-airway complications were significantly reduced when BAR was combined with tracheal anastomosis, potentially diminishing morbidity caused by anastomotic healing complications.

Hypoxia-induced deoxycytidine kinase contributes to epithelial proliferation in pulmonary fibrosis.

RATIONALE: Idiopathic pulmonary fibrosis (IPF) is a deadly lung disease with few therapeutic options. Apoptosis of alveolar epithelial cells, followed by abnormal tissue repair characterized by hyperplastic epithelial cell formation, is a pathogenic process that contributes to the progression of pulmonary fibrosis. However, the signaling pathways responsible for increased proliferation of epithelial cells remain poorly understood. OBJECTIVES: To investigate the role of deoxycytidine kinase (DCK), an important enzyme for the salvage of deoxynucleotides, in the progression of pulmonary fibrosis. METHODS: DCK expression was examined in the lungs of patients with IPF and mice exposed to bleomycin. The regulation of DCK expression by hypoxia was studied in vitro and the importance of DCK in experimental pulmonary fibrosis was examined using a DCK inhibitor and alveolar epithelial cell-specific knockout mice. MEASUREMENTS AND MAIN RESULTS: DCK was elevated in hyperplastic alveolar epithelial cells of patients with IPF and in mice exposed to bleomycin. Increased DCK was localized to cells associated with hypoxia, and hypoxia directly induced DCK in alveolar epithelial cells in vitro. Hypoxia-induced DCK expression was abolished by silencing hypoxia-inducible factor 1α and treatment of bleomycin-exposed mice with a DCK inhibitor attenuated pulmonary fibrosis in association with decreased epithelial cell proliferation. Furthermore, DCK expression, and proliferation of epithelial cells and pulmonary fibrosis was attenuated in mice with conditional deletion of hypoxia-inducible factor 1α in the alveolar epithelium. CONCLUSIONS: Our findings suggest that the induction of DCK after hypoxia plays a role in the progression of pulmonary fibrosis by contributing to alveolar epithelial cell proliferation.

Blockade of IL-6 Trans signaling attenuates pulmonary fibrosis.

Idiopathic pulmonary fibrosis (IPF) is a lethal lung disease with progressive fibrosis and death within 2-3 y of diagnosis. IPF incidence and prevalence rates are increasing annually with few effective treatments available. Inhibition of IL-6 results in the attenuation of pulmonary fibrosis in mice. It is unclear whether this is due to blockade of classical signaling, mediated by membrane-bound IL-6Rα, or trans signaling, mediated by soluble IL-6Rα (sIL-6Rα). Our study assessed the role of sIL-6Rα in IPF. We demonstrated elevations of sIL-6Rα in IPF patients and in mice during the onset and progression of fibrosis. We demonstrated that protease-mediated cleavage from lung macrophages was important in production of sIL-6Rα. In vivo neutralization of sIL-6Rα attenuated pulmonary fibrosis in mice as seen by reductions in myofibroblasts, fibronectin, and collagen in the lung. In vitro activation of IL-6 trans signaling enhanced fibroblast proliferation and extracellular matrix protein production, effects relevant in the progression of pulmonary fibrosis. Taken together, these findings demonstrate that the production of sIL-6Rα from macrophages in the diseased lung contributes to IL-6 trans signaling that in turn influences events crucial in pulmonary fibrosis.

The A2B adenosine receptor modulates pulmonary hypertension associated with interstitial lung disease.

Development of pulmonary hypertension is a common and deadly complication of interstitial lung disease. Little is known regarding the cellular and molecular mechanisms that lead to pulmonary hypertension in patients with interstitial lung disease, and effective treatment options are lacking. The purpose of this study was to examine the adenosine 2B receptor (A(2B)R) as a regulator of vascular remodeling and pulmonary hypertension secondary to pulmonary fibrosis. To accomplish this, cellular and molecular changes in vascular remodeling were monitored in mice exposed to bleomycin in conjunction with genetic removal of the A(2B)R or treatment with the A(2B)R antagonist GS-6201. Results demonstrated that GS-6201 treatment or genetic removal of the A(2B)R attenuated vascular remodeling and hypertension in our model. Furthermore, direct A(2B)R activation on vascular cells promoted interleukin-6 and endothelin-1 release. These studies identify a novel mechanism of disease progression to pulmonary hypertension and support the development of A(2B)R antagonists for the treatment of pulmonary hypertension secondary to interstitial lung disease.

Synaptotagmin-2 controls regulated exocytosis but not other secretory responses of mast cells.

Mast cell degranulation is a highly regulated, calcium-dependent process, which is important for the acute release of inflammatory mediators during the course of many pathological conditions. We previously found that Synaptotagmin-2, a calcium sensor in neuronal exocytosis, was expressed in a mast cell line. We postulated that this protein may be involved in the control of mast cell-regulated exocytosis, and we generated Synaptotagmin-2 knock-out mice to test our hypothesis. Mast cells from this mutant animal conferred an abnormally decreased passive cutaneous anaphylaxis reaction on mast cell-deficient mice that correlated with a specific defect in mast cell-regulated exocytosis, leaving constitutive exocytosis and nonexocytic mast cell effector responses intact. This defect was not secondary to abnormalities in the development, maturation, migration, morphology, synthesis, and storage of inflammatory mediators, or intracellular calcium transients of the mast cells. Unlike neurons, the lack of Synaptotagmin-2 in mast cells was not associated with increased spontaneous exocytosis.

Stimulation of lung innate immunity protects against lethal pneumococcal pneumonia in mice.

RATIONALE: The lungs are a common site of serious infection in both healthy and immunocompromised subjects, and the most likely route of delivery of a bioterror agent. Since the airway epithelium shows great structural plasticity in response to inflammatory stimuli, we hypothesized it might also show functional plasticity. OBJECTIVES: To test the inducibility of lung defenses against bacterial challenge. METHODS: Mice were treated with an aerosolized lysate of ultraviolet-killed nontypeable (unencapsulated) Haemophilus influenzae (NTHi), then challenged with a lethal dose of live Streptococcus pneumoniae (Spn) delivered by aerosol. MEASUREMENTS AND MAIN RESULTS: Treatment with the NTHi lysate induced complete protection against challenge with a lethal dose of Spn if treatment preceded challenge by 4 to 24 hours. Lesser levels of protection occurred at shorter (83% at 2 h) and longer (83% at 48-72 h) intervals between treatment and challenge. There was also some protection when treatment was given 2 hours after challenge (survival increased from 14 to 57%), but not 24 hours after challenge. Protection did not depend on recruited neutrophils or resident mast cells and alveolar macrophages. Protection was specific to the airway route of infection, correlated in magnitude and time with rapid bacterial killing within the lungs, and was associated with increases of multiple antimicrobial polypeptides in lung lining fluid. CONCLUSIONS: We infer that protection derives from stimulation of local innate immune mechanisms, and that activated lung epithelium is the most likely cellular effector of this response. Augmentation of innate antimicrobial defenses of the lungs might have therapeutic value.

The mast cell-restricted tryptase mMCP-6 has a critical immunoprotective role in bacterial infections.

Although it has been shown that mast cell-deficient mice have diminished innate immune responses against bacteria, the most important immunoprotective factors secreted from activated mast cells have not been identified. Mouse mast cell protease 6 is a tetramer-forming tryptase. This serine protease is abundant in the secretory granules and is exocytosed upon bacterial challenge. Here we have described the generation of a mast cell protease-6-null mouse. Our discovery that mice lacking this neutral protease cannot efficiently clear Klebsiella pneumoniae from their peritoneal cavities reveals an essential role for this serine protease, and presumably its human ortholog, in innate immunity.