Aims To audit the cellular expression of the innate antibacterial enzyme lysozyme in colonic biopsies from a cohort of patients having microscopic colitis (MC—collagenous colitis (CC) or lymphocytic colitis (LC)). Results were compared with those recorded in patients with inflammatory bowel disease (IBD) of the colon (ulcerative colitis (UC) or Crohn's colitis).

Methods Fifty-five consecutive cases having biopsies from the left colon were investigated: 27 MC (14 CC and 13 LC) and 28 IBD (14 UC and 14 Crohn's colitis). Sections were stained with antilysozyme antibody. Twelve cases (3 CC, 3 LC, 3 UC and 3 Crohn's colitis) were challenged with the macrophage marker CD68 (clone PG-M1).

Results In MC, marked lysozyme expression in the colonic crypts was recorded in CC (p<0.05). The number of cases with metaplastic Paneth cells was higher in CC than in LC (p<0.05). In IBD, only active Crohn's colitis displayed marked lysozyme expression in the colonic crypts. Marked lysozyme immune-reactivity in subepithelial lamina propria mucosa (lpm) macrophages was found in LC (LC vs CC p<0.05).

Conclusions The increased production of the antibacterial enzyme lysozyme in CC and LC supports a bacterial aetiology for these two diseases. Lysozyme upregulation in different cell types (epithelial vs macrophages) supports the notion that CC and LC might be two different maladies.