J-chain expression is more prominent in immunoglobulin A2 than in immunoglobulin A1 colonic immunocytes and is decreased in both subclasses associated with inflammatory bowel disease.

Paired immunofluorescence staining demonstrated reduced J-chain positivity of both immunoglobulin A1 (IgA1)- and IgA2-producing cells in colonic mucosa from patients with ulcerative colitis and Crohn's colitis compared with controls (p less than 0.002). J-chain expression was generally higher in IgA2 than in IgA1 immunocytes. The median proportion in normal mucosa was 100% for IgA2 vs. 88% for IgA1 (p less than 0.005); in ulcerative colitis, 69% vs. 46% (p less than 0.004); and in Crohn's colitis, 74% vs. 46% (p less than 0.004). Taken together with the overall IgA-subclass distribution, however, these results showed that the proportion of J-chain-positive IgA2 cells in the total IgA-cell population was lower for ulcerative colitis (20%) and Crohn's colitis (32%) than for normal mucosa (63%) (p less than 0.002). In relation to the total J-chain-positive IgA-cell population, which contributes to the secretory IgA system, an increased proportion (p less than 0.002) belonged to IgA1 in ulcerative colitis (61% vs. normal, 27%), whereas IgA2 was reduced (39% vs. normal, 73%). Similar but smaller trends were noted in Crohn's colitis. The disease-associated reduction of J chain might be compensated by the previously reported twofold numeric increase of IgA cells in colitis. Our study, therefore, did not suggest that the secretory IgA-cell system was quantitatively impaired in inflammatory bowel disease.