Testicular Immunology is the study of the immune system within the testis. It includes an investigation of the effects of infection, inflammation and immune factors on testicular function. Two unique characteristics of testicular immunology are evident: (1) the testis is described as an immunologically privileged site, where suppression of immune responses occurs; and, (2) some factors which normally lead to inflammation are present at high levels in the testis, where they regulate the development of sperm instead of promoting inflammation.
Immune cells of the human testis are not as well characterized as those from rodents, due to the rarity of normal human testes available for experiment. The majority of experiments have studied the rat testis due to its convenience: it is of relatively large size and is easily extracted from experimental animals.
Macrophages are directly involved in the fight against invading micro-organisms as well as being antigen-presenting cells which activate lymphocytes. Early studies demonstrated the presence of macrophages in the rat testis [5] Testicular macrophages are the largest population of immune cells in the rodent testis. [6] [7] Macrophages have also been found in the testes of humans, [8] guinea pigs, hamsters, [9] boars, [10] horses [11] and bulls. [12] They originate from blood monocytes which move into the testis then mature into macrophages. In the rat, testicular macrophages have been described as either “resident” or “newly arrived” from the blood supply. [13] [14] It is likely that most of the adult population of testicular macrophages in adult rats are a result of very rapid proliferation of early precursors that entered the testis during postnatal maturation [15]
Testicular macrophages can respond to infectious stimuli and become activated (undergo changes enabling the killing of the invading micro-organism), but do so to a lesser extent than other types of macrophages. [16] [17] An example is production of the inflammatory cytokines TNFα and IL-1β by activated rat testicular macrophages: these macrophages produce significantly less TNFα and IL-1β than activated rat peritoneal macrophages. [17] [18] Aside from responding to infectious stimuli, testicular macrophages are also involved in maintaining normal testis function. They have been shown to secrete 25-hydroxycholesterol, a sterol that can be converted to testosterone by Leydig cells. [19] Their presence is necessary for the normal development and function of the Leydig cells, [20] [21] [22] which are the testosterone-producing cells of the testis.
B-lymphocytes take part in the adaptive immune response and produce antibodies. These cells are not normally found in the testis, even during inflammatory conditions. The lack of B-lymphocytes in the testis is significant, since these are the antibody-producing cells of the immune system. Since anti-sperm antibodies can cause infertility, it is important that antibody-producing B-lymphocytes are kept separated from the testis.
T-lymphocytes (T-cells) are white blood cells which take part in cell-mediated immunity. They are often found within tissues where they can be activated by antigen-presenting cells upon infection. They are present in rat [23] [24] and human testes, [25] where they constitute approximately 10 to 20% of the immune cells present, as well as mouse [26] and ram [24] testes. Both cytotoxic T-cells and Helper T cells are found in the testes of rats. [27] Also present in the testes of rats and humans are natural killer cells [1] [27] and Natural killer T cells have been found in rats and mice.
Mast cells are regulators of immune responses, particularly those against parasites. They are also involved in the development of autoimmune diseases and allergies. Mast cells have been found in relatively low numbers in the testes of humans, rats, mice, dogs, cats, bulls, boars and deer. [28] [29] In the mammalian testis mast cells regulate testosterone production. [29] There are two lines of evidence that restriction of mast cell activation in the testis could be beneficial during treatment of inflammatory conditions; (1) In experimental models of testicular inflammation, mast cells were present in 10-fold greater numbers and showed signs of activation, [30] and (2) Treatment with drugs which stabilize mast cell activation has proved beneficial in treating some types of male infertility. [31] [32] [33]
Eosinophils directly fight parasitic infections and are involved in allergic reactions. They have been found in relatively low numbers in the rat, mouse, dog, cat, bull and deer testes. [28] Almost nothing is known about their significance or function in the testis.
Dendritic cells initiate adaptive immune responses. Relatively small amounts of dendritic cells have been found in the testes of humans, [34] rats [35] and mice. [36] [37] The functional role of dendritic cells in the testis is not well understood, although they have been shown to be involved in autoimmune orchitis during animal experiments. [28] [35] When autoimmune orchitis is induced in rats, the dendritic cell population of the testis greatly increases. [35] This is likely to contribute to testicular inflammation, considering the well-established role of dendritic cells in other types of autoimmune inflammation. [38]
Neutrophils are white blood cells which are present in the blood but not normally in tissues. They move out from the blood into tissues and organs upon infection or damage. They directly fight invading pathogens such as bacteria. Neutrophils are not found in the rodent testis under normal conditions but can enter from the blood supply upon infection or inflammatory stimulus. This has been demonstrated in the rat after injection with bacterial cell wall components to produce an immune reaction. [39] Neutrophils also enter the rat testis after treatment with hormones that increase the permeability of blood vessels. [40] In humans, neutrophils have been found in the testis when associated with some tumors. [41] In rat experiments, testicular torsion leads to neutrophil entry into the testis. [42] Neutrophil activity in the testis is an inflammatory response which needs to be tightly regulated by the body, since inflammation-induced damage to the testis can lead to infertility. [43] [44] It is assumed that the role of the immunosuppressive environment of the testis is to protect developing sperm from inflammation.
Sperm are immunogenic - that is they will cause an autoimmune reaction if transplanted from the testis into a different part of the body. This has been demonstrated in experiments using rats by Landsteiner (1899) and Metchinikoff (1900), [1] [29] mice [45] and guinea pigs. [46] The likely reason for this is that sperm first mature at puberty, after immune tolerance is established, therefore the body recognizes them as foreign and mounts an immune reaction against them. Therefore, mechanisms for their protection must exist in this organ to prevent any autoimmune reaction. The blood-testis barrier is likely to contribute to the survival of sperm. However, it is believed in the field of testicular immunology that the blood-testis barrier cannot account for all immune suppression in the testis, due to (1) its incompleteness at a region called the rete testis [29] and (2) the presence of immunogenic molecules outside the blood-testis barrier, on the surface of spermatogonia. [1] [29] Another mechanism which is likely to protect sperm is the suppression of immune responses in the testis. [17] [47] Both the suppression of immune responses and the increased survival of grafts in the testis have led to its recognition as an immunologically privileged site. Other immunologically privileged sites include the eye, brain and uterus. [48]
The two main features of immune privilege in the rat testis are;
It is also predicted that the high level of inflammatory cytokines in the testis contributes to immune privilege. [29]
The existence of immune privilege in the testes of rodents is well accepted, due to many experiments demonstrating prolonged, and sometimes indefinite, survival of tissue transplanted into the testis, [49] [50] or testicular tissue transplanted elsewhere. [51] [52] Evidence includes the tolerance of testicular grafts in mice and rats, as well as the increased survival of transplants of pancreatic insulin-producing cells in rats, when cells from the testes (Sertoli cells) are added to the transplanted material. [53] Complete spermatogenesis, forming functional pig or goat sperm, can be established by the grafting of pig or goat testicular tissue onto the backs of mice - however, immunodeficient mice needed to be used. [51]
The presence of immune-privilege in the human testis is controversial and insufficient evidence exists to either confirm or rule out this phenomenon.
Sperm are protected from autoimmune attack, which when it occurs in humans leads to infertility. [54] Local injury of seminiferous tubules caused by fine-needle biopsies in humans does not cause testicular inflammation (orchitis). [55] Furthermore, human testis cells tolerate early HIV infection with little response. [56]
In transplant experiments, primate testes fail to support grafts of monkey thyroid tissue. [57] Human testis tissue transplanted into the mouse elicited an immune response and was rejected, however, this immune response was not as extensive as that against other types of grafted tissue. [58]
How the testicular environment suppresses the immune response is only partially understood. Recent experiments have uncovered a number of biological processes that most likely contribute to immune privilege in the testes of rodents:
Since protection of developing sperm is so important to the survival of a species, it would not be surprising if more than one mechanism were in use.
Curiously, the testis contains factors such as cytokines, which are usually only produced upon infections and tissue damage. The cytokines interleukin-1α (IL-1α), IL-6 and Activin A are found in the testis, often at high levels. [62] [63] [64] [65] [66] In other tissues, these cytokine would promote inflammation, but here they control testis function. They regulate the development of sperm by controlling their cell division and survival. [67] [68] [69] [70] [71]
Other immune factors found in the testis include the enzyme inducible nitric oxide synthase (iNOS), and its product nitric oxide (NO), [72] [73] [74] transforming growth factor beta (TGFβ), [75] the enzyme cyclooxygenase-2 (COX-2) and its product prostaglandin E2, [76] and many others. Further research is required to define the functional roles of these immune factors in the testis.
Mumps is a viral disease which causes swelling of the salivary glands and testes. The mumps virus lives in the upper respiratory tract and spreads through direct contact with saliva. [77] Prior to widespread vaccination programs, it was a common childhood disease. Mumps is generally not serious in children, but in adults, where sperm have matured in the testis, it can cause more severe complications, such as infertility.
Gonorrhea is a sexually transmitted disease caused by the bacteria Niesseria gonorrhea which can lead to testicular pain and swelling. Gonorrhea also infects the female reproductive system around the cervix and uterus, and can grow in the mouth, throat, eyes and anus. [78] It can be effectively treated with antibiotics, however, if untreated, gonorrhea can cause infertility in men. Chlamydia is caused by the sexually transmitted bacteria Chlamydia trachomatis which infects the genitals. It more commonly affects women, and if untreated, can lead to pelvic inflammatory disease and infertility. [79] Serious symptoms in men are rare, but include swollen testicles and an unusual discharge from the penis. It is effectively treated with antibiotics.
Antisperm antibodies (ASA) have been considered as infertility cause in around 10–30% of infertile couples. [80] ASA production are directed against surface antigens on sperm, which can interfere with sperm motility and transport through the female reproductive tract, inhibiting capacitation and acrosome reaction, impaired fertilization, influence on the implantation process, and impaired growth and development of the embryo. Risk factors for the formation of antisperm antibodies in men include the breakdown of the blood‑testis barrier, trauma and surgery, orchitis, varicocele, infections, prostatitis, testicular cancer, failure of immunosuppression and unprotected receptive anal or oral sex with men. [80] [81]
Testicular torsion is a condition of physical twisting of the testis which results in cutting off the blood supply. It leads to damage that, if not treated within a few hours, causes the death of testicular tissue, and requires removal of the testis to prevent gangrene, and therefore can cause infertility. [82]
Orchitis is a condition of testicular pain involving swelling, inflammation and possibly infection. Orchitis can be caused by an autoimmune reaction (autoimmune orchitis) leading to a reduction in fertility. Autoimmune orchitis is rare in humans, compared to anti-sperm antibodies. [1] To study orchitis in the testis, autoimmune orchitis has been induced in the rodent testis. The disease starts with the appearance of testicular antibodies, then movement of macrophages and lymphocytes from the blood stream into the testis, breaking of the physical interactions between the developing sperm and Sertoli cells, entry of neutrophils or eosinophils, and finally death of the developing sperm, leading to infertility. [83] [84] [85]
Experiments in rats have examined, in fine detail, the course of testicular events during a bacterial infection. In the short term (3 hours) multiple inflammatory factors are produced and released by testicular macrophages. Examples are prostaglandin E2, [86] [76] inducible nitric oxide synthase (iNOS), [39] [87] TNFα [88] and IL-1β, although at lower levels than other tissues. [86] [47] Non-immune cells of the testis such as Sertoli cells and Leydig cells also able to respond to bacteria. [63] [89] During a bacterial infection, testosterone levels and the amount of testicular interstitial fluid are reduced. [39] Neutrophils enter the testis about 12 hours after infection. [39] Importantly, there is damage to the developing sperm, which start to die under severe infections. [39] [90] Despite all the data on the effects of bacteria on normal testis parameters, there is little experimental data regarding its effect on rodent fertility.
Testicular inflammation can be a symptom of the following diseases: Coxsackie A virus, [91] [92] varicella (chicken pox) [91] [93] human immunodeficiency virus (HIV), [94] dengue fever, [95] Epstein Barr virus-associated infectious mononucleosis, [91] [96] syphilis, [97] leprosy, [98] tuberculosis. [99]
The immune system is a network of biological systems that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to parasitic worms, as well as cancer cells and objects such as wood splinters, distinguishing them from the organism's own healthy tissue. Many species have two major subsystems of the immune system. The innate immune system provides a preconfigured response to broad groups of situations and stimuli. The adaptive immune system provides a tailored response to each stimulus by learning to recognize molecules it has previously encountered. Both use molecules and cells to perform their functions.
A testicle or testis is the male reproductive gland or gonad in all bilaterians, including humans. It is homologous to the female ovary. The functions of the testes are to produce both sperm and androgens, primarily testosterone. Testosterone release is controlled by the anterior pituitary luteinizing hormone, whereas sperm production is controlled both by the anterior pituitary follicle-stimulating hormone and gonadal testosterone.
Macrophages are a type of white blood cell of the innate immune system that engulf and digest pathogens, such as cancer cells, microbes, cellular debris, and foreign substances, which do not have proteins that are specific to healthy body cells on their surface. This process is called phagocytosis, which acts to defend the host against infection and injury.
Testicular cancer is cancer that develops in the testicles, a part of the male reproductive system. Symptoms may include a lump in the testicle or swelling or pain in the scrotum. Treatment may result in infertility.
The epididymis is an elongated tubular structure attached to the posterior side of each one of the two male reproductive glands, the testicles. It is a single, narrow, tightly coiled tube in adult humans, 6 to 7 centimetres in length; uncoiled the tube would be approximately 6 m long. Uncoiled the tube would be approximately 6 m long. It connects the testicle to the vas deferens in the male reproductive system. The epididymis serves as an interconnection between the multiple efferent ducts at the rear of a testicle (proximally), and the vas deferens (distally). Its primary function is the storage, maturation and transport of sperm cells.
Spermatogenesis is the process by which haploid spermatozoa develop from germ cells in the seminiferous tubules of the testis. This process starts with the mitotic division of the stem cells located close to the basement membrane of the tubules. These cells are called spermatogonial stem cells. The mitotic division of these produces two types of cells. Type A cells replenish the stem cells, and type B cells differentiate into primary spermatocytes. The primary spermatocyte divides meiotically into two secondary spermatocytes; each secondary spermatocyte divides into two equal haploid spermatids by Meiosis II. The spermatids are transformed into spermatozoa (sperm) by the process of spermiogenesis. These develop into mature spermatozoa, also known as sperm cells. Thus, the primary spermatocyte gives rise to two cells, the secondary spermatocytes, and the two secondary spermatocytes by their subdivision produce four spermatozoa and four haploid cells.
Sertoli cells are a type of sustentacular "nurse" cell found in human testes which contribute to the process of spermatogenesis as a structural component of the seminiferous tubules. They are activated by follicle-stimulating hormone (FSH) secreted by the adenohypophysis and express FSH receptor on their membranes.
Testicular atrophy is a medical condition in which one or both testicles diminish in size and may be accompanied by reduced testicular function. Testicular atrophy is not related to the temporary shrinkage of the surrounding scrotum, which might occur in response to cold temperature.
Testicular sperm extraction (TESE) is a surgical procedure in which a small portion of tissue is removed from the testicle and any viable sperm cells from that tissue are extracted for use in further procedures, most commonly intracytoplasmic sperm injection (ICSI) as part of in vitro fertilisation (IVF). TESE is often recommended to patients who cannot produce sperm by ejaculation due to azoospermia.
A sperm granuloma is a lump of leaked sperm that appears along the vasa deferentia or epididymides in vasectomized individuals. While the majority of sperm granulomas are present along the vas deferens, the rest of them form at the epididymis. Sperm granulomas range in size, from one millimeter to one centimeter. They consist of a central mass of degenerating sperm surrounded by tissue containing blood vessels and immune system cells. Sperm granulomas may also have a yellow, white, or cream colored center when cut open. While some sperm granulomas can be painful, most of them are painless and asymptomatic. Sperm granulomas can appear as a result of surgery, trauma, or an infection. They can appear as early as four days after surgery and fully formed ones can appear as late as 208 days later.
Certain sites of the mammalian body have immune privilege, meaning they are able to tolerate the introduction of antigens without eliciting an inflammatory immune response. Tissue grafts are normally recognised as foreign antigens by the body and attacked by the immune system. However, in immune privileged sites, tissue grafts can survive for extended periods of time without rejection occurring. Immunologically privileged sites include:
Sertoli cell-only syndrome (SCOS), also known as germ cell aplasia, is defined by azoospermia where the testicular seminiferous tubules are lined solely with sertoli cells. Sertoli cells contribute to the formation of the blood-testis barrier and aid in sperm generation. These cells respond to follicle-stimulating hormone, which is secreted by the hypothalamus and aids in spermatogenesis.
OX-2 membrane glycoprotein, also named CD200 is a human protein encoded by the CD200 gene. CD200 gene is in human located on chromosome 3 in proximity to genes encoding other B7 proteins CD80/CD86. In mice CD200 gene is on chromosome 16.
An autoimmune disease is a condition that results from an anomalous response of the adaptive immune system, wherein it mistakenly targets and attacks healthy, functioning parts of the body as if they were foreign organisms. It is estimated that there are more than 80 recognized autoimmune diseases, with recent scientific evidence suggesting the existence of potentially more than 100 distinct conditions. Nearly any body part can be involved.
Reproductive immunology refers to a field of medicine that studies interactions between the immune system and components related to the reproductive system, such as maternal immune tolerance towards the fetus, or immunological interactions across the blood-testis barrier. The concept has been used by fertility clinics to explain fertility problems, recurrent miscarriages and pregnancy complications observed when this state of immunological tolerance is not successfully achieved. Immunological therapy is a method for treating many cases of previously "unexplained infertility" or recurrent miscarriage.
Immune tolerance in pregnancy or maternal immune tolerance is the immune tolerance shown towards the fetus and placenta during pregnancy. This tolerance counters the immune response that would normally result in the rejection of something foreign in the body, as can happen in cases of spontaneous abortion. It is studied within the field of reproductive immunology.
Chemorepulsion is the directional movement of a cell away from a substance. Of the two directional varieties of chemotaxis, chemoattraction has been studied to a much greater extent. Only recently have the key components of the chemorepulsive pathway been elucidated. The exact mechanism is still being investigated, and its constituents are currently being explored as likely candidates for immunotherapies.
A spermatogonial stem cell (SSC), also known as a type A spermatogonium, is a spermatogonium that does not differentiate into a spermatocyte, a precursor of sperm cells. Instead, they continue dividing into other spermatogonia or remain dormant to maintain a reserve of spermatogonia. Type B spermatogonia, on the other hand, differentiate into spermatocytes, which in turn undergo meiosis to eventually form mature sperm cells.
Antisperm antibodies (ASA) are antibodies produced against sperm antigens.
In vitro spermatogenesis is the process of creating male gametes (spermatozoa) outside of the body in a culture system. The process could be useful for fertility preservation, infertility treatment and may further develop the understanding of spermatogenesis at the cellular and molecular level.
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