Page 1140 - Veterinary Immunology, 10th Edition

population. However, these cells are selectively dysfunctional.
VetBooks.ir They reject skin grafts but not the fetus. In effect they retain the

ability to protect the body while protecting the fetus.

Barton BM, et al. Pregnancy promotes tolerance to future offspring by programming
selective dysfunction in long-lived maternal T cells, J Leukoc Biol 101:975-987, 2017.

Up to 90% of pregnant mares make antibodies to foal MHC class
I molecules. Similar antibodies develop in multiparous sheep and
cattle. In some mouse strains, up to 95% of pregnant animals make

antibodies against fetal MHC molecules. Up to 40% of women make
antibodies to fetal MHC molecules after giving birth. The presence
of these antibodies has no adverse effect on the course of the
pregnancy. On the contrary, the maternal immune response may

actually stimulate placental function. In mice, hybrid placentas are
larger than the placentas of inbred animals, and females tolerant to
paternal antigens have smaller placentas than intolerant females.
Other studies show that mothers sensitized to paternal MHC

molecules have better fetal survival. This may be due to the
stimulatory effect of IL-3 and granulocyte-macrophage colony-
stimulating factor (GM-CSF) from maternal T cells on trophoblast
growth. It is of interest to note that in cattle there is a clear

association between retention of the placenta and its MHC class I
haplotype. MHC class I compatibility between a mother and her
calf increases the risk of a retained placenta, whereas MHC class II
compatibility has no effect. It has been suggested that the expulsion

of the placenta after birth may be due, at least in part, to an allograft
response.
Some antibodies made by the mother against fetal antigens may
coat placental cells, preventing their destruction by maternal T cells.

This blocking antibody can be eluted from the placenta and shown
to suppress other cell-mediated immune reactions against paternal
antigens, such as graft rejection. Absence of this blocking antibody
accounts for some cases of recurrent abortion in women.
Nevertheless, it can also be shown that totally immunodeficient

mice can have successful pregnancies.
The fetus does not depend entirely on maternal mechanisms for
its protection. The placenta is a source of many immunosuppressive

factors, including estradiol and progesterone and possibly also

1140