Ultrastructural characterization of microenvironmental elements associated with proximal colonic lymphoid tissue (PCLT) in the mouse

Abstract

The mucosal immune system is comprised of both diffuse and aggregated lymphoid components, and acts as the primary immune barrier for ingested antigens. The elaborate studies of Owen (1) and others have clearly demonstrated that a specialized population of antigen sampling Membranous cells (M cells) is present in the follicle-associated epithelium (FAE) overlying Peyer’s patches of the small intestine. However, in contrast to the antigen responsive role of secondary lymphoid tissue generally bestowed upon gastrointestinal associated lymphoid tissue (GALT), the terminal ileal Peyer’s patch has been shown to be a primary source of B lymphocytes in sheep (2,3) and pigs (4). We have recently described proximal colonic lymphoid tissue (PCLT), which is a lymphoid nodule in the proximal colon of mice and rats displaying many characteristics of a primary lymphoid tissue (5,6). The majority population of B cells found within PCLT display a relatively immature B cell phenotype (10–20% of which are CD5+), and appear unable to respond to mitogenic activation. In addition PCLT displays a unique steroid sensitivity among gut associated lymphoid tissues, losing up to 70% of its tissue weight after steroid treatment. Until now little attention has been focused on the stromal components associated with this unique tissue. Therefore it was the intent of these studies to define the non-lymphoid elements associated with PCLT in the mouse.