Immunological consequences of antigen sampling at mucosal surfaces, volume II

Mucosal surfaces such as those in the gastrointestinal and respiratory tracts are
continuously exposed to commensal and pathogenic microorganisms. The epithelial
layers lining these mucosal surfaces help provide an important first line of defense
against certain pathogens such as bacteria, viruses, parasites and their toxins. Specialized
cells within the mucosal epithelia play important roles in the initial sampling and
transportation of antigens across these surfaces to enable the mucosal immune system to
provide host defense by generating specific immunity against potential pathogenic agents.
At the same time, the mucosal immune system must also suppress immune responses, or
display tolerance to commensal microbes and food components. The six papers that were
published in the second volume of this Research Topic provide a useful update on the
progress that has been made in our understanding of the diverse cellular mechanisms that
mediate antigen sampling across mucosal epithelia. Some of these studies also highlight
how these processes may be affected by certain infections or aging.
M cells are a unique population of mucosal epithelial cells that can transport particulate
antigens acquired at their apical surface directly through to the specialized pocket beneath
their basolateral surface: a process known as transcytosis. While M cells were originally
identified in the intestinal tract (1), cells with M cell-like characteristics have since been
described in the mucosal surfaces of many other tissues including the airways (2) and eye
region (see Oya et al. below). Del Castillo and Lo provided an updated account of the
characteristics of M cells, along with a useful description of other cell types in the intestine,
oral cavity and thymus that share M cell-like features. Given this range of diversity the
authors argued that M cells, like many other immune cells, are not a homogeneous
population. Rather, they suggested that M cells represent a functional niche that can exist in
different environments, which in turn influences their role.