Trophoblast as the primary line of immune defense
Fetal protection begins with the physical barrier of the placenta, but the main mechanism is the specific expression of antigens on trophoblast cells. To avoid attack by maternal cytotoxic T lymphocytes, the expression of classical HLA-A and HLA-B molecules is completely blocked on villous cytotrophoblast and syncytiotrophoblast. This blockade occurs at the post-transcriptional level (due to the presence of specific peptides in the cytosol) and is not removed even by potent activators such as interferons.
Instead of classical antigens, the trophoblast expresses non-classical MHC-I molecules (primarily HLA-G and HLA-E). They exhibit limited polymorphism and perform a suppressive function:
- HLA-G binds to inhibitory LILRB1 receptors.
- HLA-E interacts with the CD94/NKG receptor complex.
The result of this interaction is a strict blockade of the cytolytic activity of maternal NK cells and γδ T lymphocytes.
Cellular microenvironment and local immunosuppression
The placenta creates a unique immunosuppressive microenvironment. Antigen-presenting cells (especially dendritic cells), which produce the enzyme indoleamine 2,3-dioxygenase, play a key role in this process. This enzyme breaks down tryptophan into kynurenine, creating a pronounced tryptophan deficiency in the tissues. Since tryptophan is critical for protein biosynthesis, its shortage suppresses the proliferation and activity of aggressive T cells.
The second line of defense is the induction of apoptosis in potentially dangerous lymphocytes. Trophoblast cells carry FasL and TRAIL molecules on their surface. Upon encountering effector T cells bearing Fas (CD95) or DR-5 receptors, they trigger programmed cell death of the lymphocytes, preventing them from reaching the fetus.
Dynamics of immunocompetent cells in the placenta
The maternal immune system does not ignore the fetus, but actively remodels to preserve it. This is reflected in the cellular composition of the decidua:
- Macrophages: Account for 10–20% of all placental leukocytes. They are in an activated state, but the synthesis of pro-inflammatory cytokines (IL-1, TNFα, IL-6) is strictly restricted. A surge in their production (e.g., during infection) can lead to fetal rejection.
- T helper cells: A shift in differentiation occurs. A weak systemic priority for a Th2 response is established. Locally within the placenta, Th1 cells (producers of IFNγ) are virtually absent, as their appearance is fraught with miscarriage.
- Regulatory T cells (Tregs): Their concentration in the maternal-fetal contact zone increases manifold (up to 14% of all CD4+ T lymphocytes). Together with Th3 and Tr1 cells, they secrete IL-10 and TGFβ, suppressing any signs of immune aggression.
Humoral immunity and the complement system
Maternal sensitization to fetal antigens inevitably occurs, as evidenced by the accumulation of antibodies in the serum of parous women. However, these antibodies perform a protective function by shielding fetal cells from destructive cellular immunity.
A pathological exception is hemolytic disease of the newborn, where maternal anti-Rh antibodies attack fetal erythrocytes that are sensitive to complement-dependent lysis.
To protect against the complement system (which could damage fetal tissues), the trophoblast synthesizes special inhibitors. The surface expression of CD46, CD59, and DAF molecules is sharply upregulated on its surface, inactivating complement components directly at the site of contact.
Class and exam questions
Why does the maternal immune system not reject the genetically foreign fetus?
The outer membrane of the placenta does not display classical MHC-I molecules on its surface, which T lymphocytes use to recognize "non-self". Simultaneously, the placenta secretes immunosuppressive cytokines and suppresses immunity via a local tryptophan deficiency.
Why does the placenta need non-classically expressed HLA-G molecules?
They do not present antigens; instead, they act as "brakes" for maternal natural killer (NK) cells and lymphocytes by binding to their inhibitory receptors and blocking their ability to kill fetal cells.
What is the immunological cause of labor?
Before labor, the number of regulatory T cells (Tregs), which have restrained the rejection reaction throughout pregnancy, sharply decreases in the placenta. Lifting this blockade triggers the immunological rejection of incompatible tissues, which is one of the triggers of labor.
What else to study on this topic
- Differences between membrane and soluble isoforms of the HLA-G molecule (including the G5 isoform)
- The role of ILT2 and ILT4 receptors on placental macrophages and dendritic cells
- Immunological mechanisms of spontaneous abortions and the role of Th1 cytokines
- Pathogenesis of hemolytic disease of the newborn (Rh conflict)
- Development and maintenance of autotolerance (basics of autoimmune pathology)
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