OBJECTIVE What determines the extent of tissue destruction during brain abscess formation is not known. Pyogenic brain infections cause destruction of brain tissue that greatly exceeds the area occupied by microbes, as seen in experimental studies, pointing to cytotoxic factors other than microbes in pus. This study examined whether brain abscess pus contains cytotoxic proteins that might explain the extent of tissue destruction. METHODS Pus proteins from 20 human brain abscesses and, for comparison, 7 subdural empyemas were analyzed by proteomics mass spectrometry. Tissue destruction was determined from brain abscess volumes as measured by MRI. RESULTS Brain abscess volume correlated with extracellular pus levels of antibacterial proteins from neutrophils and macrophages: myeloperoxidase (r = 0.64), azurocidin (r = 0.61), lactotransferrin (r = 0.57), and cathelicidin (r = 0.52) (p values 0.002-0.018), suggesting an association between leukocytic activity and tissue damage. In contrast, perfringolysin O, a cytotoxic protein from Streptococcus intermedius that was detected in 16 patients, did not correlate with abscess volume (r = 0.12, p = 0.66). The median number of proteins identified in each pus sample was 870 (range 643-1094). Antibiotic or steroid treatment prior to pus evacuation did not reduce the number or levels of pus proteins. Some of the identified proteins have well-known neurotoxic effects, e.g., eosinophil cationic protein and nonsecretory ribonuclease (also known as eosinophil-derived neurotoxin). The cellular response to brain infection was highly complex, as reflected by the presence of proteins that were specific for neutrophils, eosinophils, macrophages, platelets, fibroblasts, or mast cells in addition to plasma and erythrocytic proteins. Other proteins (neurofilaments, myelin basic protein, and glial fibrillary acidic protein) were specific for brain cells and reflected damage to neurons, oligodendrocytes, and astrocytes, respectively. Pus from subdural empyemas had significantly higher levels of plasma proteins and lower levels of leukocytic proteins than pus from intracerebral abscesses, suggesting greater turnover of the extracellular fluid of empyemas and washout of pus constituents. CONCLUSIONS Brain abscess pus contains leukocytic proteins that are neurotoxic and likely participate actively in the excessive tissue destruction inherent in brain abscess formation. These findings underscore the importance of rapid evacuation of brain abscess pus.
Keywords: FAM3C = family with sequence similarity 3C; GFAP = glial fibrillary acidic protein; MCEMP1 = mast cell–expressed membrane protein 1; MS = mass spectrometry; brain abscess; eosinophil; infection; macrophage; mast cell; neutrophil; subdural empyema.