Mucosa-associated lymphoid tissues

Abstract

The mucosal immune system is organized as a complex of two large pools of single immune-competent cells within the epithelium (lymphocytes: IEL) and lamina propria (lymphocytes: LPL) of the mucosa; the lymph nodes draining the mucosa; and the more or less organized lymphoid tissues associated with the mucosal epithelium. Mucosa-associated lymphoid tissue (MALT) is defined here as the organized lymphoid tissues associated with the mucosal epithelium. The body of humans and animals functions as a symbiotic ecosystem of cells and about a 10-fold higher number of microorganisms, which meet at mucosal barriers. The mucosal immune system plays an important role in this symbiotic relationship, maintaining a delicate balance of tolerating a health-promoting microbiome and allowing uptake of nutrients, while excluding potentially harmful pathogens. In turn, the microbiome is needed to ensure proper functioning of the immune system. Still, many aspects of mucosal immunity and their interrelationships are poorly understood, including the interplay with some non-lymphoid organs like the liver in gut immune functioning. MALT is included in most guidelines on safety evaluation of drugs, chemicals and food constituents. In view of the importance of the body’s mucosal immunity, it is advisable to examine not only MALT at the site of application or exposure like today’s common practice, but to include distant MALT as well. There is a need for best practices to select, sample and embed MALT for histopathologic evaluation, because these aspects can profoundly influence the evaluation of MALT pathology and the interpretation of responses against xenobiotics. Pathology reports on MALT and single mucosal immune cells are scarce, most likely due to the difficulty to dissect MALT or to properly evaluate MALT in situ, and the difficulty to sample LPL and IEL populations as FACS sorted suspensions. In addition, there may be high resilience in MALT against xenobiotic insults, exactly because of its importance as primary contact site to the outside world. Nevertheless MALT deserves specific attention. This chapter aims to present information on morphology and functioning of MALT to add in the assessment of changes in this important segment of our immune defense and homeostasis.