Immunoglobulin M memory B cell decrease in inflammatory bowel disease.

Memory B cells represent 30-60% of the B cell pool and can be subdivided in IgM memory and switched memory. IgM memory B cells differ from switched because they express IgM and their frequency may vary from 20-50% of the total memory pool. Switched memory express IgG, IgA or IgE and lack surface expression of IgM and IgD. Switched memory B cells derive from the germinal centres, whereas IgM memory B cells, which require the spleen for their survival and/or generation, are involved in the immune response to encapsulated bacteria. Since infections are one of the most frequent comorbid conditions in inflammatory bowel disease, we aimed to verify whether IgM memory B cell pool was decreased in Crohn's disease and ulcerative colitis patients.

PATIENTS & METHODS:
Peripheral blood samples were obtained from 22 Crohn's disease patients, 20 ulcerative colitis patients, 22 healthy controls and 18 splenectomized patients. To analyse peripheral blood lymphocytes, flow cytometry was performed using anti-CD19, anti-CD22, anti-CD27, anti-IgM, anti-IgD and anti-CD38 monoclonal antibodies.

RESULTS:
Circulating IgM memory B cells were significantly lower in Crohn's disease (median 7.1%, range 1.8-20.7) and ulcerative colitis patients (median 8.1%, range 2.1-18.8) in comparison to control subjects (median 14.0%, range 6.8-31.1). As expected, there was a highly significant difference in the proportion of IgM memory B cells between splenectomized patients (median 2.4%, range 0.9-6.9) and healthy controls. Crohn's disease patients with abscesses showed the lowest frequency of IgM memory B cells.

DISCUSSION:
Our findings show that peripheral IgM memory B cells are reduced in inflammatory bowel disease patients. Further studies are necessary to answer the question of whether high risk of infection (abscess development) is promoted by the reduction/depletion of IgM memory B-cell pool in inflammatory bowel disease.