Low Grade B Cell Lymphoma of Gut-Associated Lymphoid Tissue (GALT): A Model of the Structure and Migration Pathways of the B Cell Component of Normal Human GALT

The histology, microenvironment and migratory pathways of low grade B cell lymphomas (LGBL) are thought to reflect those of normal lymphoid tissue of similar lineage. Striking clinical differences between LGBL of peripheral lymph nodes and those of the gastrointestinal tract serve to emphasize the lineage differences between mucosal and non-mucosal B cells and suggest that LGBL of gut-associated lymphoid tissue (GALT) could serve as a natural experimental model for the investigation of the properties of normal human GALT. The histology of LGBL of GALT parallels that of normal GALT and draws particular attention to the previously undescribed GALT marginal zone B cells which show close lineage homology with LGBL of GALT. These cells known as centrocyte-like (CCL) cells surround follicles, infiltrate the epithelium and frequently show plasma cell differentiation possibly in response to stimulation from the gut lumen. In LGBL of GALT the CCL cells have an intimate relationship with reactive follicles similar to that described for rat splenic marginal zone cells. LGBL of GALT are slow to disseminate to the periphery and can often be cured by local eradication. This suggests CCL cells do not enter the peripheral blood. However, the gastrointestinal tract remote from the primary tumour is occasionally selectively involved, a finding which supports circulation and homing as described in animals. Using a murine monoclonal anti-idiotypic antibody in one case of LGBL of GALT we have shown that CCL cells appear to enter the circulation and, in the absence of evident tumour, can be identified in the mucosae as well as the marginal zone of the spleen where they undergo plasma cell differentiation. These preliminary studies suggest that there are both discrepancies and parallels in the behaviour of human and animal GALT.