|An Introduction to the Immune System|
It might be well to liken the immune system to a rifle where a target such as a bacterium is first sighted and then destroyed. Were the immune system more like a shotgun or even more dramatically like a stick of dynamite, it still would be possible to eliminate the target but at the cost of significant collateral damage. Thus the immune system must be able first to recognize a pathogen before it is able to effect its elimination. Immunologists have come to recognize two broad types of immune response. Both have the capacity to identify an appropriate target, however, they utilize different strategies for identifying their target.
The first is the innate immune response. This is the most evolutionarily ancient of the two systems being present in insects as well as mammals. This system is armed with a relatively limited repertoire of components capable of specific pathogen recognition. It has come to be known as the innate immune system because the recognition components are hard-wired into the system allowing immune cells to be innately prepared to perform their function at the moment of pathogen encounter. Recognition of pathogens and absence of recognition of self (a vital aspect of immune recognition that prevents self-attack) has developed over many millions of years through a gradual process of evolution. The limited repertoire that evolved comprises recognition elements that bind to the relatively limited number of markers found on pathogens that are never seen on self. See: Function of HLA Antigens . One of the important members of the innate immune system is the natural killer cell which shall be discussed below.
The second is the adaptive immune response. Unlike the innate immune response, the recognition components are not hard-wired into the system but are generated by a genetic mechanism that randomly reshuffles immune-related genes permitting the generation of a vast repertoire of recognition components. The repertoire so constituted is of sufficient size that at least one component can bind to any potential pathogen invading from the outside world. However, the enormous size of the repertoire necessitates cells representing each component must be few in number so that the total number of cells constituting the entire repertoire can be accommodated within the immune system. As a consequence there are too few pathogen-specific cells present at the moment of pathogen encounter. A period of immunologic latency follows, during which these pathogen-specific cells increase in number. The system is said to be adaptive because of this latency period. This system includes the B cells T cells.
Cells involved in the immune response in both the innate and adaptive systems are known as lymphocytes. Under the microscope all lymphocytes have the same appearance. However, we have come to recognize that there are many different types. See: Immunophenotype. We can distinguish them exploiting the expression of tell-tale protein markers found on their surfaces using a laboratory instrument known as a flow cytometer. Two types of lymphocytes are involved in adaptive immunity. One lymphocyte type is known as a B cell. They produce antibody. A vast repertoire of B cells is present, each producing a different antibody. B cells remain quiescent awaiting the arrival of a pathogen. When a pathogen arrives, the immune system selects a B cell producing an antibody capable of interacting with the pathogen and stimulating it to replicate to high numbers providing a capacity to generate adequate antibody to eliminate the pathogen. Another is known as the T cell. Like B cells, T cells comprise a vast repertoire. However, they recognize pathogens with receptors fixed to their surfaces. Like B cells, upon pathogen challenge, they are stimulated to expand so that they too can reach sufficient numbers to eliminate the pathogen.
Another type of lymphocyte is the natural killer cell. As mentioned above, it is a member of the innate immune system. It was named for a very interesting observation made at the time of its discovery. Unlike cells of the adaptive immune system, natural killer cells or NK cells do not require a latency period needed for expansion in their numbers like cells of the adaptive immune system in order to kill the pathogen. NK cells are naturally competent to effect immediate killing. In the years following their discovery, much has been learned about their function. We now know that they can also respond to a pathogen-infected cell by releasing cytokines (locally acting hormone-like molecules produced by immune cells) that can direct other cells to perform the killing. Researchers have found that these two functions reside in distinct and separable NK subtypes when found in the blood.
So far we have observed that there are very many T cells that are distinguishable by their targeting molecules found on their surfaces. However, T cells may also be distinguished by their function. Researchers studying one type of T cell known as the T helper cell found that these cells could be further divided functionally by the type of cytokine that they produce. Today we know that there are probably six or seven types of such cells. They have been named Th1, Th2, Th3, TR1, Th9, Th17 and T regulatory cells (Treg cells). See: T regulatory cells .
It became possible to assess the balance between these initially defined T helper cell subtypes present in the blood of an individual by laboratory testing. Lymphocytes are purified from blood and then stimulated in a test tube, causing them to produce cytokines. Using the flow cytometer, a laboratory instrument that is capable of analyzing cells one at a time, it became possible to quantify the relative numbers of cells producing cytokines representative of the T helper subtypes, Th1 and Th2 and then to calculate a ratio of the two subtypes. The resultant ratio became known as the Th1/Th2 ratio.
Reproductive immunologists have carried the study of these cell types to reproduction and, in particular, to the implantation of the early embyro into the lining of the uterus. A general theory emerged over the last ten years based on accumulated data proposing Th2 predominance as characteristic of successful pregnancy while Th1 predominance as characteristic of pregnancy failure (1) (2) (3) (4).The theory suggests that rebalancing of the Th1/Th2 ratio toward an increase in the relative number of Th2 cells might result in improved reproductive success. To select patients with a history of repeated reproductive failure who might benefit from such rebalancing, the patients peripheral blood needs to be assessed for its Th1 Th2 ratio. Those patients with an abnormally high ratio would be considered candidates for immune-based therapy directed toward restoring a more optimal ratio.
More recently, information has emerged that has shown that the theory does not resolve certain questions regarding reproductive success and failure. For example, it is now well recognized that the process of implantation of the early embryo into the uterine lining involves several Th1-type cytokines. Further, events that take place at the site of implantation do not involve either Th1 or Th2 lymphocytes but rather involve members of the innate immune system. The best characterized of these lymphocytes is the NK or natural killer cell. The NK cell is of particular interest today because we have come to recognize that NK cells resident in the uterine lining act in very different ways than those that circulate in the peripheral blood. The so-called decidual NK cell (decidua is the pregnancy-transformed lining of the uterus) is a cell that might be regarded as a hybrid of the two types of NK cells that circulate in the blood because it combines the two functions of blood NK cells: killing and cytokine release. In the uterine lining, the decidual NK cell, while fully-armed for killing activity becomes active only to the extent that it releases cytokines that support the growth and transformation of the uterine lining for the support of the placenta. The inherent killing power of the decidual NK cell appears to be reserved until danger is recognized, such as infection, whereupon it assumes the role of killer cell, acting to eliminate infected cellular material.
Because of its dual function, the decidual NK might be regarded as distinct from the two types of NK cells circulating in blood. Decidual NK cells serve a particularly important role in supporting the growth of the placenta producing a collection of different cytokines that direct blood vessel growth and transformation. Under normal conditions, the killing capacity that is inherent within decidual NK cells remains latent. Investigators have pointed to these distinctions, suggesting that laboratory study of blood NK cells is, theoretically, unlikely to shed light upon the activity of decidual NK cells. However, in medicine, theory alone is insufficient to make predictions. Publications cited below demonstrate high NK activity in blood NK cells correlates with reproductive failure.
Two therapies used by physicians practicing reproductive immunology are intravenously administered immunoglobulin (a biologic preparation of the antibody-containing fraction of serum collected from many different donors) commonly known as IVIG and Humira. Humira is one of a new class of drugs that bind and render inactive one of the most potent cytokines released by Th1 cells known as TNFÎ±. These drugs were selected because they were reasoned to be useful in patients with high levels of NK cell activity or Th1 predominance in the peripheral blood.
IVIG therapy has been used for a variety of immunologic conditions. The first demonstration that such preparations might be useful in treating conditions that involve an excessive immune response was made in 1981. Subsequently the number of autoimmune and inflammatory diseases responding to IVIG therapy has been greatly expanded. A recent literature review appearing in the journal Trends in Immunology describes such uses in the general field of clinical immunology. The review cites literature indicating that IVIG therapy in part acts by diminishing TNFÎ±.
The development of Humira represents one of the great achievements in translational medicine (the translation of basic science discoveries into clinical practice). It was recognized that a number of conditions characterized by over-activity of the immune system exhibit, amongst other features, excess quantities of the Th1 cytokines. Basic scientists reasoned that an agent that could bind and inactivate the most potent of these cytokines, TNFÎ±, might mitigate the damage effected by excess Th1 activity. Humira was developed as an antibody that could be administered to patients reducing or eliminating excess levels of the cyctokine. Its success in clinical practice has been no less than phenomenal. It is now used in a variety of conditions, including rheumatoid arthritis, psoriasis, inflammatory bowel diseases, to mention just a few.
A number of reproductive immunologists reasoned that these two drugs might be useful in patients suffering recurrent reproductive failure. A second literature review on IVIG appears in the journal Clinical Chemistry and Laboratory Medicine, that describes its specific use in reproductive failure.
We ourselves published the first two papers demonstrating the success of the Humira in patients suffering recurrent pregnancy failure who had elevated ratios of the Th1/Th2 ratio as demonstrated in lymphocytes taken from the blood:
Paper 1: Winger EE, Reed JL: Treatment with tumor necrosis factor inhibitors and intravenous immunoglobulin improves live birth rates in women with recurrent spontaneous abortion. Am J Reprod Immunol 2008; 60:8â€“16.
It has become clear that the immune system must balance the needs of activation of the immune system in response to challenges with the ever-present danger of attacking oneself, or, in the case of pregnancy, the embryo. See study: LINK
As noted above, cells may mature into Th1 and Th2 cells responding to immunologic challenges in the different ways that have been described. It is now clear that T cells may also mature into cells that perform a very different role. They suppress immune responses. Immune responses must be contained, particularly after a challenge has been dealt with, it is necessary that the immune response be attenuated so as to limit collateral damage. Moreover, it is now recognized that such cells are essential to the prevention of immune attack on the fetus.
In addition to the immunotherapies described above, sub-anti-coagulant doses of heparin are also commonly used to prevent spontaneous abortion. It is believed that heparin acts as an anti-inflammatory agent in a manner analogous to Humira. See study: LINK
In summary, the field of reproductive immunology is a recognized academic and clinical field within the broader field of immunology. Translation of the concepts developed within the field has been practiced by a respected minority of clinicians practicing reproductive medicine. Their success has received considerable attention both within the field and by the public. The published data support the predictive value of immunologic testing.