Response of Leucocyte Populations in the Ileal Peyer's Patch of Fetal Lambs Treated with Ferritin Per Os
Response of Leucocyte Populations in the Ileal Peyer's Patch of Fetal Lambs Treated with Ferritin Per Os
LENA H.M. RENSTROM
CHARLES McL. PRESS
" WENDY TREVELLA
THOR LANDSVERK
A combination of immunohistochemical techniques, a panel of monoclonal antibodies, and computer-assisted morphometric analysis was used to examine the response of the ileal Peyer's patch of fetal lambs 7 days after treatment with ferritin per os. Consistent with previous studies in fetal lambs that have reported the ileal Peyer's patch to be indifferent to antigen, the present study did not find any significant changes in the size of the predominantly B-cell dome/follicle compartment or the predominantly T-cell interfollicular area, nor were differences identified in the distribution of IgM-positive (+), CD4 +, and CD8+ cells in these two compartments. However, both compartments showed a significant increase (p < 0.05) in the percentage of area occupied by MHC II + cells and a significant decrease (p < 0.05) in the percentage of area occupied by CD44+ and B5+ cells. These changes show that the ileal Peyer's patch of fetal lambs is not indifferent to antigen and may represent the transition of a purely primary lymphoid organ to an organ that has both primary and secondary lymphoid functions.
Fetal sheep; ferritin; leucocytes; activation; MHC II; computer-assisted morphometric analysis
INTRODUCTION
In sheep, the ileal Peyer?s patch (PP) produces the
vast majority of circulating B cells
(Gerber et al.,
1986)
and is responsible for the generation of the
preimmune antibody repertoire
(Reynaud et al.,
1991)
. Active lymphopoiesis is present in PP from
about 110 days of gestation (length of gestation in
sheep is 150 days)
(Reynolds and Morris, 1983)
and
occurs in the absence of exogenous nonself
antigens. The sheep fetus is shielded from maternal
immunoglobulins and even small molecules by a
syndesmochorial placenta
(Brambell, 1970; Boyd et
al., 1976)
. The introduction of foreign substances
into this protected environment has shown that the
fetal lamb can mount an immune response to many
antigens including ovalbumin, ferritin, and killed
Brucella abortus organisms
(Silverstein et al., 1963;
Fahey and Morris, 1974, 1978)
. Although clear
evidence of histological changes was present in the
Corresponding author.
mesenteric lymph nodes and lamina propria of the
intestine of fetal lambs following exposure to an
antigen such as ferritin
(Husband and McDowell,
1975; Reynolds and Morris, 1984)
, changes in ileal
PP histology were not detected (Reynolds and
Morris, 1984). The ileal PP was considered indifferent to
foreign antigen, and this was argued as further
support for the antigen independence of
lymphopoiesis in this organ
(Reynolds and Morris,
1984)
. Recent studies of somatic hypermutation of
light-chain V genes in the ileal PP of fetuses and
lambs have shown that the hypermutation process
is also independent of the presence of external
antigens
(Reynaud et al., 1995)
.
However, the rate of B-cell proliferation following
the introduction of foreign antigen or indeed the
hypermutation process generating the repertoire
does not describe the resultant antibody repertoire.
Active lymphopoiesis in the ileal PP is accompanied
by massive cell death and it is estimated that only
5% of PP cells produced there are destined to leave
their site of production
(Reynolds, 1986; Motyka
and Reynolds, 1991)
. Thus, intense selection occurs
in the PP that shapes the preimmune antibody
the mean interfollicular area in the control group
was 9102.24+ 1871.39 xm2 compared with
5956.52+608.11 gm2 in the ferritin-treated group;
not significant) (Fig. 3).
The mean percentages of area occupied by the
leucocyte subpopulations identified by staining for
IgM, CD4, and CD8 in the two compartments were
also not significantly different when comparing the
ferritin-treated and control groups of fetal lambs
(Fig. 4). The percentage of areas occupied by T-cell
subpopulations showed a tendency to increase in
the dome/follicle and to decrease in the
interfollicular area. The most marked decrease in T-cell
subpopulations was in the percentage of area
occupied by the CD4 + cell population in the
interfollicular area (21.07+8.27% in the control group
compared with 5.70+2.00% in the ferritin-treated
group; not significant). Despite the widely different
percentages of areas occupied by IgM + cells in the
two lymphoid compartments of the ileal PP, both
percentages decreased in ferritin-treated lambs
(dome/follicle control 62.73 + 7.00% compared with
ferritin-treated 43.70+ 13.63%; interfollicular area
control 7.54 + 2.92% compared with ferritin-treated
3.83 + 1.26%; both not significant).
There was a significant increase in the mean
percentage of the area staining for MHC II in
both the dome/follicle (control 34.85+7.14% and
AREA
10
o00
<
<
<
lOO
80
60
40
20
50
40
30
20
10
50
40
30
20
10
IgM
D/F
Ferritin
IF
C D 4
D/F
IF
Control
D/F
Ferritin
IF
D/F
C (...truncated)