immunology of pregnancy I & II

Question 1

Case study: A young woman had her 9th consecutive miscarriage. Her marriage broke down shortly afterwards. But within months of finding a new partner, she conceived again and the pregnancy went without a hitch.
Why do you think this happened?

Answer 1

Woman’s immune system took offence to the
first choice of partner
- over-reacting to the tissue carrying his genes and
expelling the fetus.
Infertility, recurrent miscarriage, premature
delivery and a dangerous complication of
pregnancy, pre-eclampsia, may be strongly
linked to immunological abnormalities.
Yet…..350,000 babies are successfully born every day
Half of the fetal genome derives from the father these are seen as foregin but, unlike a mismatched organ transplant, it isn’t normally rejected.
So how does the baby manage to avoid attacking father’s genes?

Question 2

How does the baby manage to avoid the mothers immune system attacking?

Answer 2

1.strong immune defence of mother
2.maternal antibodies ensure immune defence
=>The maternal/fetal interface is central to overcoming these problems. This interface occurs at the placenta.

Question 3

What are some immunological problems to solve during pregnancy?

Answer 3

1. Fetal tissue is half foreign – has to be protected from
   rejection
2. Mother’s immune defence must be sufficient during
   pregnancy to ensure survival
3. Fetus often immunologically immature at birth – must have maternal antibodies to ensure survival

Question 4

immunological abnormalities lead to …

Answer 4

-infertility
- recurrent miscarriage
-premature delivery
-complications in pregnancy
-pre-eclampsia

Question 5

Development of placenta

Answer 5

-blastocyst adheres to endometrium cells
-trophoblast cells differentiate into inner cytotrophoblast layer and outer synctiotrophoblast cells
-as the cytotrophoblast proliferates the newly formed cells migrate to synctiotrophoblast and lose their cell membranes forming a rapidly growing multinucleated mass
-cytotrophoblast secretes proteolytic enzymes and synctiotrophoblast sends out finger like projections allowing the blastocysts to be embedded into emndometrium
-lecunae (spaces) begin to form within the synctiotrophoblast
- As the synctiotrophoblast erodes the endometrium blood vessels and glands in the leucane becomes filled with maternal blood
- isolated lecuane fuse establishing early material placental circulation
- towards the end of week2 small projections of cytotrophoblast begin to project into the synctiotrophoblast forming primary chorionic villi
- early 3rd week extraembryonic mesoderm grows into the primary chorionic villi forming a lose connective tissue core (secondary chorionic villi )
- end of week 3 vessels form in the mesoderm in secondary chorionic villi transforming it into tertiary villi
-cytotrophoblasts from tertiary villi grow towards decidua basalis and spread across it to form cytotrophoblastic shell
- villi that are connected to decidua basalis through cytotrophoblastic shell are called anchoring villi , villi growing on the side of anchoring villi are called branch villi and serve as the main site of exchange between mother and fetus
- week 4 fetal blood flow(placenta) is established, deoxygenated blood is transported to the placenta from the fetus via umbilical arteries.
-umblical arteries send branches into chorionic villi and divide into capillary networks in terminal end
-CO2 and waste product is removed from fetal circulation across placental mebrane into the maternal blood in the inter villus space
- oxygen and nutrients are transported across from maternal blood to fetus through fetal capillaries
- oxygenated fetal blood travels through veins that converge to form a single umblical vein
-blood returns to the maternal circulation through endometrial vein forced out by the coming endometrial artery
-The placenta merges the fetus throughout development.
https://www.google.com/search?q=development+of+placenta&sxsrf=ALiCzsaQumKtytbuSk9acd8tfTm52ohtng:1668332830757&source=lnms&tbm=vid&sa=X&ved=2ahUKEwiZ8PmN8Kr7AhVLe8AKHQTGBvYQ_AUoAnoECAIQBA&biw=638&bih=721&dpr=2#fpstate=ive&vld=cid:6edab370,vid:bped-RVWsLk

Question 6

Where are the mother and babies cells in direct contact(maternal -fetal interface)?

Answer 6

1. syncytiotrophoblasts lining the chorionic villi and maternal blood in the intervillious space
2. invasive extravillious trophoblasts and and matenal blood in the spiral arteries
3. Invasive extravillious trophoblasts and infiltrated maternal immune cells in the decidua

1.syncytiotrophoblast layer covering the placenta is bathed in maternal blood (direct contact)
2. invading extravillious trophoblast head towards maternal spiral artery and come into contact with decidual immune cells -eg. NK, macrophage , these can potentially recognise the trophoblast cells which is fetally derived and express paternal antigen as foreign so trophoblast cells need to successfully bypass these infiltration of maternal immune cells so they can reach the spiral artery where they carry out important function.
3. invading trophoblasts come into contact with decidual vascular cells, to transform maternal spiral artery.

Question 7

syncytiotrophoblast layer covering the placenta is bathed in maternal blood

Answer 7

-multi-nucleated layer which arises from fused cytotrophoblasts. It forms a barrier and performs endocrine functions as well as gas and nutrient exchange from maternal blood

Question 8

What are the extravillious trophoblasts?

Answer 8

• the extravillious trophoblast are differentiated fetal cells which invade into the maternal decidua to transform maternal spiral arteries.

Question 9

Spiral arteries remodeling

Answer 9

• spiral arteries are tightly coiled and spiral in nature of non pregnant women.
  -in pregnant women they supply blood to the intervillous space.
• as fetus grows high volume of blood needs to be supplied to meet demands
  -this is brought about by remodelling the spiral artery to to change from high resistance - low flow arteries to high flow - low resistance vessels.
• this occurs early in pregnancy alongside decudilisation

Question 10

Trophoblast independent remodelling stage

Answer 10

• spiral artery remodelling
  -some immune cells have a role too which explains why large infiltration immune cells are needed
  -part of trophoblast independent remodelling is carried out by signals carried by immune cells in the decidua

Question 11

Decuidalisation

Answer 11

Refers to the functional and morphological changes that occur within the endometrium to form the decidual lining into which the blastocyst implants
The process involves recruitment of immune cells , ie leukocytes and the transformation of endometrial fibroblast cells (ESCs) into decidual stromal cells (DSCs)

Question 12

Trophoblast dependent remodelling

Answer 12

-extravillious trophoblasts reach the spiral artery they cause loss of vascular smooth muscle layer and temporary loss of endothelial layer, the lumen of the spiral artery becomes much larger and loses its contractile properties and allows much larger volume of blood to enter the intravillious space.
-Trophoblast express markers of endothelial cells and replace the endothelium cells lining the spiral arteries so fetal cells will be in direct contact with maternal blood.

Question 13

how do both of these types of trophoblast evade the immune response- immunological tolerance?

Answer 13

1. physical separation of maternal and fetal tissues
2. antigenic immaturity of fetal tissue
3. mother is immunologically inert

Question 14

Which immune cells are present in the maternal - fetal interface - decidua?

Answer 14

• > 40% decidual cells are leukocytes in early pregnancy, leukocytes include NK cells, macrophages, T and B cells
• Of these>40%, approximately 70% are NK cells (subpopulation of cytotoxic lymphocytes) which function by cell killing or cytokine production
• approximately 20% are macrophages
• T and B cells make up the remaining 10%

Question 15

What immune cells are present in the intervillious and spiral arteries?

Answer 15

immune cells present are the same as immune cells circulating in peripheral maternal blood.

Question 16

How are decidual natural killer cells(dNK) different to peripheral blood (pbNK) immune cells of mother ?

Answer 16

• their pattern of receptor expression is unique and they are identified by CD5 and CD16
  -they have been identified as being essential to pregnancy in the mouse and they play a role in human decidual remodelling through cytokines which they secrete
  -dNK is in the decidua it is no longer a scary killer cell but its role is to produce cytokines that encourage important events of placentation such as trophoblast invasion

Question 17

macrophages make up 20% if immune cells in the decidua…

Answer 17

• second most abundant immune cells
• M2 like > M1 like
  -M1 phenotype are pro inflammatory - secrete cytokines and growth factors
  -M2 are antiinflammatory in nature and secrete interleukin 10 and VEGF
• so decidual macrophages are more anti inflammatory

Question 18

1. Is there a physical separation of maternal and fetal tissues ?

Answer 18

-* Fetus separated from the mother by the fetal trophoblast cells
* Fetal and maternal circulation is separated
* Maternal cells cannot reach the fetus
=>But in humans, IgG can cross into the fetal blood via a placental transport mechanism.
Therefore IgG directed against fetal antigens could also be transferred
=> fetus is separated from the mother but fetal trophoblast cells are not separa

Question 19

Why doesnt IgG crossing into the fetal blood via placenta not harm the baby(bc baby antigens will be recognised as foreign)?

Answer 19

=>Most fetal blood group and histocompatibility antigens are widely distributed on the
fetal cells and tissues - IgG would be diluted out.
=>Many fetal antigens are also present as soluble forms in the fetal blood and amniotic
fluid - IgG would be mopped up by free soluble antigen.

Question 20

2.Is there antigenic immaturity of fetal tissues?

Answer 20

fetal tissue is antigenically immature
histocompatibility antigens are targets for rejection
MHC haplotypes inherited from both parents and are co-dominantly expressed

Question 21

What are different class of MHC?

Answer 21

class Ia= HLA-A, HLA-B, HLA-C
class Ib = HLA-E, HLA-F, HLA-G
class II = HLA-DP, HLA-DQ, HLA-DR

Question 22

Whats expressed in trophoblast cells?

Answer 22

1. syncytiotrophoblast - lack both MHC class I and II antigens
2. Extravillious trophoblasts lack class II but express an unusual combination of MHC class I antigens - HLA-C, HLA-E and HLA-G (non- classical) => expressing cells that can be recognised by maternal antibodies

Question 23

3. Is the mother immunologically inert?

Answer 23

=> maternal blood in pregnancy is able to respond immunologically to the fetus and fetal cells are dectable in maternal blood BUT
=> Pre- sensation to paternal antigen does not prevent pregnancy
=> There is neither a generalised or specific depression of maternal immune responsiveness
=> The quality of the maternal immune response may be what differs

Question 24

Theories of immune evasion in the placenta

Answer 24

-role for natural killer cells in the decidua
- selective local induction of programmed cell death in maternal immune cells
- alteration in the cytokines balance
- local indolemaine 2,3 dioxygenase synthesis
- complement regulatory proteins

Question 25

Decidual natural killer cells

Answer 25

dNK cella are different to peripheral blood (bb) NK cells
Their pattern of receptors expression is unique and they are identified by CD56hiCD16lo
They have been identified as being essential to pregnancy in the mouse and they may play a role in human decidual remodelling through the cytokines which they secrete
So why don’t they attack the fetus?

Question 26

How does expression of HLA-C, E and G by EVT help immune evasion?

Answer 26

by binding to receptors on NK cells

NK cell receptors
-killer - cell immunoglobulin-like receptors (KIRs)
-CD94/NKG2 receptor (LILRs)
- leukocyte immunoglobulin like receptors (LILRs)

There are both inhibitory and activating members of these families of receptors

Question 27

Trophoblasts interacting with NK cells

Answer 27

Binding of HLA class I molecules to inhibitory NK cell receptors inhibits the cytotoxic action of NK cell, therefore the trophoblast is not attacked

Inhibitory NK receptor :
- CD94/ NKG2A
-KIR2DL/S1
-LILRB-1

Trophoblast
HLA-E
HLA-C
HLA-G

Question 28

Experimental evidence

Answer 28

Inhibitory receptors are expressed at higher levels in uterine NK cells than peripheral blood NK cells
* HLA-E has higher affinity for the inhibitory receptor than the activating receptor
* More uNK cells found in women with a history of recurrent pregnancy loss

Trophoblast HLA molecules can also bind to activating NK cell receptors
may alter the NK cytokine repertoire
may contribute to how the trophoblast behaves.

Question 29

HLA- G

Answer 29

Question 30

role for soluable HLA- G?

Answer 30

Soluble HLA-G can be released from trophoblasts
* In vitro studies have shown that sHLA-G can induce apoptosis in maternal T cells
* May be an additional way of protecting trophoblasts from attack
IVF: an association between the presence of soluble human leukocyte antigen G
(sHLA-G) in human embryo culture supernatants (ES) and implantation success

Question 31

local induction of programmed cell death in maternal immune cells

Answer 31

Experimental evidence that trophoblasts can induce programmed cell death
(apoptosis) in maternal immune cells.

Apoptosis
Cell shrinks, nucleus reorganises, DNA fragments, membranes bleb and cell fragments into membrane bound apoptotic bodies.
Regulation of apoptosis depends on a balance between pro- and anti-apoptotic factors.

Mechanisms
* Fas-Fas L
* TRAIL-TRAIL R

Question 32

Maternal immune cells : The Th1/Th2 balance in pregnancy

Answer 32

T cells differentiate into Th1 or Th2 cells in response to signals given during antigen presentation.

Th1 type reaction in placenta mainly generates inflammatory responses, activates T cells
and NK cells and is correlated with miscarriages.
eg IFNg, IL-2

Th2 type reaction generates non-inflammatory reactions that are consistent with the
survival of the fetus.
eg IL-4, IL-6, IL-10, IL-13

Trophoblasts may be producing cytokines and hormones that promote a Th2 balance.

Question 33

Maternal T cells

Question 34

Resistance index (RI)

Answer 34