Agriculture Reference
In-Depth Information
into inductive (PP and MLN) and effector (lamina propria, referred to immune cells
outside the PP, and intraepithelial lymphocytes) sites (Burkey
et al.
, 2009). The intestinal
epithelial cell monolayer provides a highly specialized physical and functional barrier
which facilitates selective transfer of luminal antigens to the underlying GALT designed
to maintain local tissue homeostasis (Figure 15.1).
The transcellular transport of bacterial antigen has been generally ascribed to microfold
cells overlying the follicle-associated epithelium of PP (Corr
et al.
, 2008). In addition,
microbial antigens can be sampled directly by dendritic cells which open tight junctions
between enterocytes to extend dendrites into the lumen (Rescigno
et al.
, 2008). Dietary
antigens present in the proximal intestine can be transported through enterocytes
by endocytosis at the apical membrane and transcytosed toward the lamina propria
(transcellular permeability) (Menard
et al.
, 2010). These antigens are released in the form
of amino acids or breakdown products at the basolateral pole of enterocytes (Figure 15.1).
Food proteins can also be processed by enterocytes and released as epithelial exosomes
which can diffuse in the basement membrane and interact with local immune cells. Passage
of small molecules is also permitted between enterocytes (paracellular permeability)
(Figure 15.1). Finally, antigen presenting cells [major histocompatibility complex (MHC)
class II molecules which are not present on porcine intestinal epithelial cells] go to the
draining MLN to generate effector T and B cells. Activated lymphocytes exit the node
via high endothelial venules, but not via efferent lymph vessels as in sheep and other
mammals (as porcine lymph nodes are inverted) (Binns
et al.
, 1990), and then passed
into the bloodstream and travel back to the lamina propria to carry out their specific
adaptive effector functions.
Porcine PP consist of discrete, organized lymphoid tissues located in the upper part of
the small intestine (jejunum and upper ileum), and of a large continuous patch along the
ileum, both responsible for the uptake of antigens and induction of mucosal immune
Maternal immunoglobulins
Transcellular transport
Paracellular transport
Maternal IgG
Dietary antigens
Maternal IgM
Small antigens
Bacterial antigens
Bacterial antigens
Lumen
Microbiota
Acidic pH
FcRn
Endosome
Tigh
junctions
Epithelial
barrier
Enterocyte
Lysosome
M cell
Dendrite
MIIC
Neutral ph
Degradative
route
DC
Exosomes
Non-degradative
route
Lamina propria
Peyer's patch
Figure 15.1. Molecular passages through the intestinal epithelium.
