1. Faecal excretion of the leucocyte primary granule component, myeloperoxidase, and of the secondary granule component, lactoferrín, were compared in inflammatory bowel disease and infective diarrhoea.
2. Faecal lactoferrín correlated with faecal myeloperoxidase in both inflammatory bowel disease (P = 0.0018; n = 32) and infective diarrhoea (P = 0.00013; n = 37), but inflammatory bowel disease was associated with a much higher faecal excretion of lactoferrín but lower excretion of myeloperoxidase than infective diarrhoea. As a consequence, the median ratio of lactoferrin/myeloperoxidase excretion (both expressed as ng/mg of protein) for inflammatory bowel disease was 7.5 (range 3.5–21.3) with similar values for ulcerative colitis (n = 18) and Crohn's disease (n = 14) compared with only 0.9 (range 0.4–2.3; P < 0.0001) for infective diarrhoea. In inflammatory bowel disease faecal lactoferrín and myeloperoxidase excretion remained increased even in clinical remission.
3. In subsequent immunohistochemical studies to assess the possible explanation for these findings, lactoferrín and myeloperoxidase were demonstrated within crypt abscesses and surface mucus, both in inflammatory bowel and in infective diarrhoea mucosal samples. There was a slight increase in the number of lactoferrin-containing cells in the mucosal samples from ulcerative colitis and in the submucosa of samples from Crohn's disease compared with infective diarrhoea, but these changes were not sufficient to account for the marked increase in faecal lactoferrín excretion in inflammatory bowel disease.
4. In all mucosal samples, including those from normal mucosa, lactoferrín was also shown to be contained within mast cells.
5. These results could best be explained by a different mechanism for leucocyte activation in inflammatory bowel disease compared with infective diarrhoea, and are compatible with selective secretion of secondary granule components, which include lactoferrín but not myeloperoxidase, as a result of leucocyte activation by N-formylated bacterial peptides in inflammatory bowel disease.