Characterisation of mucosal lymphoid aggregates in ulcerative colitis: immune cell phenotype and TcR-gamma delta expression

Background and aims-A histopathological feature considered indicative of ul cerative colitis (UC) is the so-called basal lymphoid aggregates. Their rel evance in the pathogenesis of UC is, however, unknown. We have performed a comprehensive analysis of the immune cells in these aggregates most likely corresponding to the lymphoid follicular hyperplasia also described in othe r colitides. Methods-Resection specimens of UC and normal colon were analysed by immunom orphometry, immunoflow cytometry, and immunoelectron microscopy, using a la rge panel of monoclonal antibodies. Results-(1) In all cases of UC, colonic lamina propria contained numerous b asal aggregates composed of lymphocytes, follicular dendritic cells, and CD 80/B7.1 positive dendritic cells. (2) CD4(+)CD28(-) alpha beta T cells and B cells were the dominant cell types in the aggregates. (3) The aggregates contained a large fraction of cells that are normally associated with the e prthelium: that is, gamma delta T cells (11 (7)%) and alpha(E)beta(7)(+) ce lls (26 (13)%). The gamma delta T cells used V delta 1 and were CD4(-)CD8(- ). Immunoelectron microscopy analysis demonstrated TcR-gamma delta internal isation and surface downregulation, indicating that the gamma delta T cells were activated and engaged in the disease process. (4) One third of cells in the aggregates expressed the antiapoptotic protein bcl-2. Conclusions-Basal lymphoid aggregates in UC colon are a consequence of anom alous lymphoid follicular hyperplasia, characterised by abnormal follicular architecture and unusual cell immunophenotypes. The aggregates increase in size with severity of disease, and contain large numbers of apoptosis resi stant cells and activated mucosal gamma delta T cells. The latter probably colonise the aggregates as an immunoregulatory response to stressed lymphoc ytes or as a substitute for defective T helper cells in B cell activation, gamma delta T cells in the aggregates may be characteristic of UC.