The patients with 2009 H1N1 infections showed lymphopenia, and its severity was associated with the severity of the illness in the early stages. Together this finding, the rapid improvement in clinical signs and the prompt resolution of severe pneumonic consolidations after immunemodulator (corticosteroid and IVIG) treatment suggest that the mechanisms of lung injury in this infection may be associated with the cell-mediated immune response of the host, rather than virus-induced cytopathies. Immunodetection of occult eosinophils in lung tissue biopsies may help predict survival in acute lung injury BACKGROUND: Acute lung injury (ALI) is a serious respiratory disorder for which therapy is primarily supportive once infection is excluded. Surgical lung biopsy may rule out other diagnoses, but has not been generally useful for therapy decisions or prognosis in this setting. Importantly, tissue and peripheral blood eosinophilia, the hallmarks of steroid-responsive acute eosinophilic pneumonia, are not commonly linked with ALI. We hypothesized that occult eosinophilic pneumonia may explain better outcomes for some patients with ALI. METHODS: Immunohistochemistry using a novel monoclonal antibody recognizing eosinophil peroxidase (EPX-mAb) was used to assess intrapulmonary eosinophil accumulation/degranulation. Lung biopsies from ALI patients (n = 20) were identified following review of a pathology database; 45% of which (i.e., 9/20) displayed classical diffuse alveolar damage (ALI-DAD). Controls were obtained from uninvolved tissue in patients undergoing lobectomy for lung cancer (n = 10). Serial biopsy sections were stained with hematoxylin and eosin (H&E) and subjected to EPX-mAb immunohistochemistry. RESULTS: EPX-mAb immunohistochemistry provided a >40-fold increased sensitivity to detect eosinophils in the lung relative to H&E stained sections. This increased sensitivity led to the identification of higher numbers of eosinophils in ALI patients compared with controls; differences using H&E staining alone were not significant. Clinical assessments showed that lung infiltrating eosinophil numbers were higher in ALI patients that survived hospitalization compared with non-survivors. A similar conclusion was reached quantifying eosinophil degranulation in each biopsy. CONCLUSION: The enhanced sensitivity of EPX-mAb immunohistochemistry uniquely identified eosinophil accumulation/degranulation in patients with ALI relative to controls. More importantly, this method was a prognostic indicator of patient survival. These observations suggest that EPX-mAb immunohistochemistry may represent a diagnostic biomarker identifying a subset of ALI patients with improved clinical outcomes. ALI encompasses a spectrum of pulmonary disorders that is often accompanied by life-threatening hypoxemic respiratory failure and diffuse bilateral pulmonary infiltrates. Moreover, the origins of ALI are often complex (e.g., pneumonia, sepsis, or left atrial hypertension) and not easily attributed to a defined cause [1] [2] [3] [4] [5] [6] . In its most dramatic clinical form, acute respiratory distress syndrome (ARDS), precipitous impairment of gas exchange is associated with a high mortality rate (38.5%), especially among elderly patients [7] . Therapy for most patients with idiopathic ALI is limited to supportive care and infection prevention [6, [8] [9] [10] . Select studies have suggested a benefit of corticosteroid therapy in a subset of patients with ALI/ARDS (e.g., [11] [12] [13] [14] ). However, large prospective studies have not supported a consensus opinion for the routine use of corticosteroids [15] [16] [17] [18] .