immunological CROSS-TALK IN infertility
Sathyabama, R. Sreeranjani*, M. Harsha, and P. Selvaraj
Department of Veterinary Physiology,
Veterinary College and Research Institute, Namakkal.
Tamil Nadu Veterinary and Animal Sciences University
Various physiological mechanisms ensure successful reproduction process in general. Immune response is modulated by the endocrine environment at multiple levels (hypothalamic-pituitary-ovario-uterine axis). At the time of implantation, a complex immunological interaction (cross-talk) takes place between the embryo and immune cells of the endometrium. This is important for successful implantation and for continuation of the pregnancy. This interaction is done through the exchange of cytokines. Cytokines are chemical mediators produced by T cells.
Natural Killer (NK) cells are known for their ability to kill tumour cells without any priming or prior activation. They secrete cytokines such as IFN γ and TNF α, which act on other immune cells like macrophage and dendritic cells to enhance the immune response. All NK cells originate from stem cells in the bone marrow and after maturation will either circulate in the blood or migrate and reside in tissues such as endometrium. They distinguish normal cells by recognizing surface molecules known as Major Histocompatibility Complex (MHC). NK cells are activated when their surface receptors bind to this specific antigen complex with the MHC. Therefore, they play an important role in pregnancy.
- Th1 (cytotoxic T cells/NK cells) produce cytokines that are pro-inflammatory such as interleukin 2, interferons and tumour necrosis factor alpha (TNF α), attack cells infected by viruses and tumour cells.
- Th2 (helper T cells) produce cytokines that oppose the effect of cytokines produced by Th1 (anti-inflammatory) such as interleukins. They also release growth factors that regulate other immune cells.
A fine balance between pro-inflammatory (Th1) and anti-inflammatory (Th2) cytokines at the maternal-foetal interface is required for the maternal tolerance of immunogenic fetus. The immune shift is tilted towards excess Th2 in successful pregnancy while excess Th1 may result in pregnancy failure. This altered maternal immune system during pregnancy makes the dam more susceptible to infections including autoimmune diseases.
Infertility is the diminished or absent capacity to produce viable offspring. The goal of a breeding program should be to have 90-95% of cows bred in a 65 day breeding season. Disorders of the immune system may lead to reproductive failure at different stages of reproductive process: unexplained infertility, recurrent IVF (invitro fertilization) or ICSI (intracytoplasmic sperm injection) failures and recurrent miscarriage.
Sperm (spermatozoa and the various secretions of the accessory glands) with its very complex antigenic system, respond to an antigenic stimulation with an immune reaction. They may behave as auto-antigens. The existence of a blood-testis barrier, a specialized inner Sertoli’s cell attachment, prevents all contact between the sperm and the immune system (modulators of antigenicity). Antigen mediated mechanisms include suppressor T-lymphocytes which partially mediate the normal state of immunologic unresponsiveness toward auto-antigens.
Antibodies are substances produced by B lymphocytes in response to pathogens such as bacteria. They circulate into the blood and body tissues. Antibodies normally protect the body from invasion by foreign bodies such as bacteria and viruses. For unknown reasons the body may develop antibodies to its own cells. The cervix is a site of sperm filtration, and uterine fluid has significant concentrations of IgG and IgA antibodies.
Generally, there are two types of immunologic cross-talk involved in conception and infertility:
- Autoimmune disorders are more common, implicated in >90% of immune-related infertility. Specifically, it means that dam’s immune cells are forming antibodies against tissue that is normal and part of their own body. This is an abnormal reaction that is associated with several infertility disorders.
- Alloimmune disorders, in contrast, involve the formation of antibodies against tissue associated with the male partner (e.g., paternal sperm proteins). Alloimmune problems are associated with less than 10% of implantation failure or recurrent pregnancy loss.
The following are some of the immune factors involved in infertility.
- Autoimmune disease such as lupus erythematosis, hypothyroidism (Hashimoto’s disease), rheumatoid arthritis, etc.
- A pregnancy is recognised as foreign because the male Human Leukocyte Antigen (HLA) is different from that of the female. The dam makes blocking antibodies that attach to and camouflage the placenta (protecting antibodies). If the HLA is too similar to that of the dam, the embryo will not be protected because it will not be able to differentiate, leading to lack of blocking antibodies to protect the fetus and the pregnancy may fail.
- Antiphospholipid antibodies syndrome (APAS): Phospholipids are present on the cell membranes of all cells; they are glue molecules which play an important role in embryo implantation. Antiphospholipid antibodies (anticardiolipin and Lupus anticoagulant) cause the dam’s blood to clot quickly cutting off blood supply to the foetus.
- Inherited thrombophilias (Factor V Leiden, prothrombin mutation, protein C, protein S, antithrombin deficiency etc) are associated with an increased risk of recurrent miscarriage and probably recurrent IVF failure.
- A dam may develop antibodies (antinuclear antibodies, ANA) to her own foetus’s DNA or DNA breakdown products.
- Excess or hyperactive NK cells (more than 12% of NK cells) in blood can damage the fetal cells leading to recurrent abortions.
- Formation of antisperm antibodies
Mechanisms of immunological infertility
Approximately 5% of cases of unexplained infertility are of immunological origin and are linked to the presence and/or formation of ASA.
Anti-sperm antibodies can be present in either or both partners. It can be present either in the blood or in the genital tract secretions such as cervical mucus and ejaculate. There are different types of antibodies e.g., IgG, IgA and IgM. Anti-sperm antibodies in the ejaculate will make the sperm ineffective by making them stick together and preventing them from being released. In the female, anti-sperm antibodies may interfere with the process of sperm transport and fertilization.
The production of ASA is usually greater in males. The aetiology of ASA formation is unknown in both males and females. However, it has been hypothesized that ASA formation is associated with genital infection, testicular trauma, varicocele, vasectomy etc., in males, and, trans-tubal passage of sperm within peritoneal cavity, pelvic infective and inflammatory diseases, hydrosalpinx etc., in females.
The blood-testis barrier may be breached by a variety of mechanisms resulting in the exposure of sperm antigen to the immune system which could result in an inflammatory reaction and ASA formation. Immunosuppression is another potential mechanism wherein greater CD4: CD8 ratio represents T cell suppressor function in the testis. This prevents an immune response to the sperm and ultimately leads to ASA formation.
The possible mechanisms in ASA mediated conception failure may include:
- Spontaneous agglutination in ejaculation, thus preventing the sperm progression in the female genital tract.
- Immobilisation: Free spermatozoa, charged with antibody, are retained at the level of the cervical secretion (Shaking phenomenon- immobilisation of the spermatozoa in the cervical secretion). The anti-sperm IgA and not anti-sperm IgG is responsible for the penetration reduction of spermatozoa in cervical mucus and for the “shaking phenomenon” in the Sperm Cervical Mucus Contact (SCMC)-test
- Impaired penetration: The anti-spermatozoal antibodies mask some antigens, so preventing penetration of the spermatozoa into the ovule. It is an impaired interaction with zona pellucida when antibodies (ASA) occupy sperm receptor sites-ZP3, blocking sperm binding and poor egg penetration.
- Expulsion: Contact of spermatozoa with antibody fixed to the uterine tissue is followed by secretion of histamine and this encourages the expulsion of an implanted egg.
- Phagocytosis: Increased phagocytosis of sperm by macrophage and polynuclear neutrophils of the female reproductive tract is another mechanism. The immune response to the postpartum uterine bacterial infection is dependent on the influx of polymorphonuclear inflammatory cells from the peripheral circulation, and to a lesser extent on humoral antibody.
- Blocking of complement inhibitors: Normally, the expression of complement inhibitors on the outer plasma membrane of the sperm avoids sperm damage by preventing the attack by self-reacting lymphocytes, e.g., CD 35 (C3b-C4b receptor), CD 46 (membrane cofactor protein), CD 55 (decay accelerating factor) and CD 59 (membrane attack complex indicator). However, in the presence of ASA, these complement inhibitors are blocked, resulting in infertility.
Diagnostic tests for immunological infertility
- Kibrick or gelatin agglutination test (GAT)
- Franklin Dukes test or tray slide agglutination test (TSAT)
- Microtray agglutination test (MAT)
- Isojima or sperm immobilizing test (SIT)
- Mixed antiglobulin reaction test (MAR)
- Immunobead test (IMB)
- SCMC-test: IgG and IgA anti-sperm antibodies could be demonstrated on the spermatozoa or in the cervical mucus.
- Postcoital test (PCT) screens for sperm antibodies in cervical mucus and serum.
Several strategies can be employed to improve the deleterious effects of ASA mediated infertility. Among these, three methods are most common:
- Decreasing ASA production by corticosteroids (to reduce seminal ASA titre, e.g. cyclosporine A).
- Removing ASA already bound to sperm, through immunodepletion, sperm washing and IgA protease treatment. This cleaves the Fc portion of IgA thereby enabling sperm penetration.
- Using assisted reproductive technologies (ART)
In general, the hormonal environment during pregnancy shifts the cytokine profile away from cell-mediated immunity (Th1), favouring Th2 cytokine profile. Thus, optimum reproductive immunological environment promotes fetal health and development during pregnancy thereby reducing unexplained infertility cases.
