In article <Roederer-1901951026170001 at b011-lucifer.stanford.edu>,
Mario Roederer <Roederer at Darwin.Stanford.Edu> wrote:
>Actually, the rejection would be mediated by foreign MHC! i.e., paternal
>MHC is incompatible (usually) with maternal, so why is the baby not
>rejected by Maternal T cells?
>>The answer is that the outer layer of the embryo consists of a cell type
>called trophoblasts. Trophoblasts are unique in that they do not express
>MHC! The downregulation of MHC expression is achieved by specific
>methylation of the genes encoding MHC. The lack of MHC expression keeps
>the embryo from being rejected, and leaves it susceptible only to
>Ig-mediated responses (i.e., mismatched RH).
>>mr
Actually, it has recently been found that trophoblasts do express
a class I molecule, dubbed HLA-G. No one is really sure how it functions
yet. However, HLA-G appears to be monomorphic (or possibly oligomorphic --
the populations surveyed for HLA-G aren't very large yet). It's possible
that the epitopes expressed on HLA-G are such that an allotypic response
is not generated.
I just pulled a review of this subject out of Medline (a bit old,
perhaps...):
Johnson PM: Immunobiology of the human placental trophoblast.
Experimental and Clinical Immunogenetics, 1993, 10(2):118-22.
On a related note, cells that fail to express (some or all) class
I molecules appear to be susceptible to attack by NK cells. A current
article that examines this issue is:
Litwin V; Gumperz J; Parham P; Phillips JH; Lanier LL.
NKB1: a natural killer cell receptor involved in the recognition of
polymorphic HLA-B molecules.
Journal of Experimental Medicine, 1994 Aug 1, 180(2):537-43.
So, in pregnancy, HLA-G may also function as a "dummy" class I
molecule that prevents NK cell lysis.
--
Unique ID : Ladasky, John Joseph Jr.
Title : BA Biochemistry, U.C. Berkeley, 1989
Location : Stanford University, Dept. of Structural Biology, Fairchild D-105
Keywords : immunology, music, running, Green
