Objective: To assess B1a cell expression in the rectal mucosa of ulcerative colitis (UC) patients in comparison with healthy controls.

Methods: Rectal mucosa biopsies were collected from 15 UC patients and 17 healthy controls. CD5(+) B cells were analysed by three colour flow cytometry from rectal mucosal samples after mechanical disaggregation by Medimachine(®). Immunohistochemical analysis of B and T lymphocytes was also performed. Correlations between, on the one hand, rectal B1a cell concentrations and, on the other, erythrocyte sedimentation rate and C-reactive protein levels and clinical, endoscopic and histological disease activity indices were evaluated.

Results: Rectal B-lymphocyte (CD19(+)/CD45(+)) rate and concentration were higher in UC patients compared with those in healthy controls (47.85% ± 3.12% vs 26.10% ± 3.40%, P = 0.001 and 501 ± 91 cells/mm(2) vs 117 ± 18 cells/mm(2), P < 0.001); Rectal B1a cell density (CD5(+)CD19(+)) was higher in UC patients than in healthy controls (85 ± 15 cells/mm(2) vs 31 ± 6.7 cells/mm(2), P = 0.009). Rectal B1a cell (CD5/CD19(+)) rate correlated inversely with endoscopic classification (Rs = -0.637, P < 0.05).

Conclusions: B1a lymphocytes seem to be involved in the pathogenesis of UC, however, the role they play in its early phases and in disease activity, have yet to be defined.