Inherent potential for production of tumor necrosis factor-α by human intestinal macrophages
Macrophages play a crucial role in intestinal mucosal defence, forming dense subepithelial aggregates, particularly in the colon. One of their important bactericidal mechanisms is production of oxygen radicals but this may damage the intestinal epithelium, perhaps as an early step in inflammatory bowel disease (IBD). The potential for release of oxygen radicals from mucosal macrophages in IBD was measured and whether a difference exists between newly arrived (CD14+L1+) monocyte-like cells and resident macrophages (CD14(-)L1-), without or with additional priming in vitro, was investigated. Lamina propria mononuclear cells from six patients with IBD and five with a normal intestine were isolated with an ethylenediaminetetra acetic acid/collagenase/dispase technique and cultured for three days. The cells were tested with or without interferon gamma (200 U/ml) priming in the presence or absence of lipopolysaccharide (1 microgram/ml) for the last 48 hours in cultures. Samples from inflamed IBD mucosa depleted of CD14+ cells by immunomagnetic beads were compared with their undepleted counterparts and with samples from virtually normal mucosa from the same patients. The production of oxygen radicals was measured as the amount of reduced cytochrome C 2.5 hours after triggering with phorbol 12-myristate 13-acetate. The oxygen radical production in macrophages from moderately or severely inflamed mucosa was reduced by median 69% (range 22%-79%, p < 0.027) after depletion of CD14+ cells, reaching a level similar to that found for virtually normal samples from the same IBD patients. Furthermore, this production did not increase significantly in mucosal macrophages from normal reference mucosa and from virtually normal or inflamed IBD mucosa after priming with interferon gamma with or without addition of lipopolysaccharide. Upregulation of a respiratory burst in subepithelial resident macrophages os not a likely pathogenetic step in IBD. The increased oxygen radical production shown by macrophages from IBD lesions can, however, be ascribed to recently extravasated CD14+L1+ monocyte-like cells. Inhibition of extravasation of these reactive cells may form part of a therapeutic approach in the future.