Agriculture Reference
In-Depth Information
15.2.5
Development of gut-associated lymphoid tissues in piglets and
bacterial colonization
The mucosal immune system is essentially absent in the neonatal piglet even though
systemic immune tissue is well developed (Inman
et al.
, 2005). Numerous studies showed
that development of the intestinal immune system is driven by exposure to microbiota
(Pabst
et al.
, 1988). The follicles of jejunal PP grow with age and are two times longer and
wider in specified pathogen-free and conventional pigs than in germ-free animals, thus
indicating an influence of the luminal microbiota (Barman
et al.
, 1997). These follicles
increase in size postnatally by a factor of 100 between day 1 and day 10. This development
is largely absent in germ-free animals (Rothkotter
et al.
, 1989). Moreover, in germ-free
pigs, B cells express preferentially surface IgM instead of IgA.
Mucosal immune cells populate the intestine of young pigs in a highly programmed
sequence (Vega-Lopez
et al.
, 1995). At birth, PP consist of very small immature follicles
surrounded by a small number of T cells and very few antigen presenting cells in the
lamina propria. From day 1 to 2 weeks of age, there is an influx of MHC class II+ dendritic
cells in the lamina propria with appearance of unusual T cells that express the CD2
surface marker but not CD4 or CD8 surface markers. The PP then begin to organize,
reaching a relatively adult architecture by 10-15 d of age. Between 2 to 4 weeks, mature
memory CD4+ T cells can be found in the core of the villi and small numbers of B cells
appear, expressing mostly IgM. From 5 weeks onward, CD8+ T cells enter the epithelium
and there is appearance of many IgA+ B cells in the crypt areas. By 7 weeks, the adult
immunological architecture is observed even though absolute numbers of cells continue
to increase as piglets grow (Rothkotter, 2009).
15.2.6
Passive immunity
The piglet is profoundly immunodeficient at birth and is highly dependent upon a supply
of specific and non-specific immune factors present in maternal colostrum and milk
for immune protection, development and survival (Salmon
et al.
, 2009). The gestating
sow presents a relatively impermeable epitheliochorial placenta meaning that newborn
piglets are hypo- or agammaglobulinemic. Colostrum is rich in IgM and IgG, compared
with plasma, due to a selective transudation of both isotypes of Ig at the end of gestation.
Immediately after birth, macromolecules from the intestinal lumen, such as Ig present in
the colostrum (Salmon, 2000) are taken up in a non-selective way (Westrom
et al.
, 1984).
Systemic immunity (IgG and IgM) is thereby transferred from the sow to its piglets.
A rapid closure of the gut for the uptake of macromolecules occurs within 24 to 36 h
postnatally. Within 24 h of birth, serum IgG concentrations in suckling piglets are often
similar to those of their dam. Whereas IgG and IgM reach the piglet bloodstream, the
majority of IgA remains in the gut lumen in order to inhibit the first adherence phase
of pathogen bacteria to enterocytes by masking surface receptors on these cells (Salmon
et al.
, 2009). Lymphocytes (coming indirectly from maternal GALT) migrating into the
mammary gland produce secretory IgA released into the sow's milk for maintenance
of humoral immunity in the offspring (enteromammary axis), explaining the same
