Diseases of the immune system. The principles of immune diagnosis, immunotherapy, immunorehabilitation, immunization.

 

 

The relative difficulty of suppressing established immune responses is seen in animal models of autoimmunity, in which methods able to prevent the induction of autoimmune disease generally fail to halt established disease. Current treatments for immunological disorders are nearly all empirical in origin, using immunosuppressive drugs identified by screening large numbers of natural and synthetic compounds. The drugs currently used to suppress the immune system can be divided into three categories: first, powerful anti-inflammatory drugs of the corticosteroid family such as prednisone; second, cytotoxic drugs such as azathioprine and cyclophosphamide; and third, fungal and bacterial derivatives, such as cyclosporin A, FK506 (tacrolimus), and rapamycin (sirolimus), which inhibit signaling events within T lymphocytes. These drugs are all very broad in their actions and inhibit protective functions of the immune system as well as harmful ones. Opportunistic infection is therefore a common complication of immunosuppressive drug therapy. The ideal immunosuppressive agent would be one that targets the specific part of the adaptive immune response that causes tissue injury. Paradoxically, antibodies themselves, by virtue of their exquisite specificity, might offer the best possibility for the therapeutic inhibition of specific immune responses. We will also consider experimental approaches to controlling specific immune responses by manipulating the local cytokine environment or by manipulating antigen so as to divert the response from a pathogenic pathway to an innocuous one. We have discussed previously how the pathological responses that cause allergy, autoimmunity, or graft rejection can be prevented by innocuous, nonpathological T-cell responses.

Corticosteroids are powerful anti-inflammatory drugs that alter the transcription of many genes. Corticosteroid drugs are powerful anti-inflammatory agents that are used widely to suppress the harmful effects of immune responses of autoimmune or allergic origin, as well as those induced by graft rejection. Corticosteroids are pharmacological derivatives of members of the glucocorticoid family of steroid hormones; one of the most widely used is prednisone, which is a synthetic analogue of cortisol. Cortisol acts through intracellular receptors that are expressed in almost every cell of the  body. On binding hormone, these receptors regulate the transcription of specific genes, as illustrated. The expression of as many as 1% of genes in the genome may be regulated by glucocorticoids, which can either induce or, less commonly, suppress the transcription of responsive genes. The pharmacological effects of corticosteroid drugs result from exposure of the glucocorticoid receptors to supraphysiological concentrations of ligand. The abnormally high level of ligation of glucocorticoid receptors causes exaggerated glucocorticoid-mediated responses, which have both beneficial and toxic effects. Given the large number of genes regulated by corticosteroids and that different genes are regulated in different tissues, it is hardly surprising that the effects of steroid therapy are very complex. The beneficial effects are antiinflammatory and are summarized; however, there are also many adverse effects, including fluid retention, weight gain, diabetes, bone mineral loss, and thinning of the skin. The use of corticosteroids to control disease requires a careful balance between helping the patient by reducing the inflammatory manifestations of disease and avoiding harm from the toxic side-effects of the drug. For this reason, corticosteroids used in transplant recipients and to treat inflammatory autoimmune and allergic disease are often administered in combination with other drugs in an effort to keep the dose and toxic effects to a minimum. In autoimmunity and allograft rejection, corticosteroids are commonly combined with cytotoxic immunosuppressive drugs. Prednisone is a synthetic analogue of the natural adrenocorticosteroid cortisol. Introduction of the 1,2 double bond into the A ring increases anti-inflammatory potency approximately fourfold compared with cortisol, without modifying the sodium-retaining activity of the compound.

 Corticosteroids are lipid-soluble molecules that enter cells by diffusing across the plasma membrane and bind to their receptors in the cytosol. Binding of corticosteroid to the receptor displaces a dimer of a heat-shock protein named Hsp90, exposing the DNA-binding region of the receptor. The steroid:receptor complex then enters the nucleus and binds to specific DNA sequences in the promoter regions of steroid-responsive genes. Corticosteroids exert their numerous effects by modulating the transcription of a wide variety of genes. Corticosteroids regulate the expression of many genes, with a net anti-inflammatory effect. First, they reduce the production of inflammatory mediators, including cytokines, prostaglandins, and nitric oxide. Second, they inhibit inflammatory cell migration to sites of inflammation by inhibiting the expression of adhesion molecules. Third, corticosteroids promote the death by apoptosis  of leukocytes and lymphocytes. Azathioprine was developed as a modification of the anti-cancer drug 6-mercaptopurine; by blocking the reactive thiol group, the metabolism of this drug is slowed down. It is slowly converted in vivo to 6- mercaptopurine, which is then metabolized to 6-thio-inosinic acid, which blocks the pathway of purine bio-synthesis. Cyclopho-sphamide was similarly developed as a stable pro-drug, which is activated enzymatically in the body to phosphoramide mustard, a powerful and unstable DNA-alkylating agent. Cytotoxic drugs cause immunosuppression by killing dividing cells and have serious side-effects. The two cytotoxic drugs most commonly used as immunosuppressants are azathioprine and cyclophosphamide Both interfere with DNA synthesis and have their major pharmacological action on dividing tissues. They were developed originally to treat cancer and, after observations that they were cytotoxic to dividing lymphocytes, were found to be immunosuppressive as well. The use of these compounds is limited by a range of toxic effects on tissues that have in common the property of continuous cell division. These effects include decreased immune function, as well as anemia, leukopenia, thrombocytopenia, damage to intestinal epithelium, hair loss, and fetal death or injury. As a result of their toxicity, these drugs are used at high doses only when the aim is to eliminate all dividing lymphocytes, and in these cases treated patients require subsequent bone marrow transplantation to restore their hematopoietic function. They are used at lower doses, and in combination with other drugs such as corticosteroids, to treat unwanted immune responses. Azathioprine is converted in vivo to a purine antagonist that interferes with the synthesis of nucleic acids and is toxic to dividing cells. It is metabolized to 6-thioinosinic acid, which competes with inosine monophosphate, thereby blocking the synthesis of adenosine monophosphate and guanosine monophosphate and thus inhibiting DNA synthesis. It is less toxic than cyclophosphamide, which is metabolized to phosphoramide mustard, which alkylates DNA. Cyclophosphamide is a member of the nitrogen mustard family of compounds, which were originally developed as chemical weapons. With this pedigree goes a range of highly toxic effects including inflammation of and hemorrhage from the bladder, known as hemorrhagic cystitis, and induction of bladder neoplasia. Cyclosporin A, FK506 (tacrolimus), and rapamycin (sirolimus) are powerful immunosuppressive agents that interfere with T-cell signaling. There are now relatively nontoxic alternatives to the cytotoxic class of drugs that can be used for immunosuppression in transplant patients.

 The systematic study of products from bacteria and fungi has led to the development of a large number of important medicines including the two immunosuppressive drugs cyclosporin A and FK506 or tacrolimus, which are now widely used to treat transplant recipients. Cyclosporin A is a cyclic decapeptide derived from a soil fungus from Norway, Tolypocladium inflatum. FK506, now known as tacrolimus, is a macrolide compound from the filamentous bacterium Streptomyces tsukabaensis found in Japan; macrolides are compounds that contain a manymembered lactone ring to which is attached one or more deoxy sugars. Another Streptomyces macrolide, called rapamycin or sirolimus, is being evaluated in clinical studies and is also likely to become important in the prevention of transplant rejection; rapamycin is derived from Streptomyces hygroscopicus, found on Easter Island ('Rapa ui' in Polynesian hence the name of the drug). All three compounds exert their pharmacological effects by binding to members of a family of intracellular proteins known as the immunophilins, forming complexes that interfere with signaling pathways important for the clonal expansion of lymphocytes. Cyclosporin A and tacrolimus block T-cell proliferation by inhibiting the phosphatase activity of a Ca2+-activated enzyme called calcineurin at nanomolar concentrations. Their mechanism of action, which we will discuss further in the next section, revealed a role for calcineurin in transmitting signals from the T-cell receptor to the nucleus. Both drugs reduce the expression of several cytokine genes that are normally induced on T-cell activation. These include interleukin (IL)-2, whose synthesis by T lymphocytes is an important growth signal for T cells. Cyclosporin A and tacrolimus inhibit T-cell proliferation in response to either specific antigens or allogeneic cells and are used extensively in medical practice to prevent the rejection of allogeneic organ grafts. Although the major immunosuppressive effects of both drugs are probably the result of inhibition of T-cell proliferation, they also act on other cells and have a large variety of other immunological effects, some of which might turn out to be important pharmacologically. Cyclosporin A and tacrolimus are effective, but they are not problem-free. First, as with the cytotoxic agents, they affect all immune responses indiscriminately. The only way of controlling their immunosuppressive action is by varying the dose; at the time of grafting, high doses are required but, once a graft is established, the dose can be decreased to allow useful protective immune responses while maintaining adequate suppression of the residual response to the grafted tissue. This is a difficult balance that is  not always achieved. Furthermore, although T cells are particularly sensitive to the actions of these drugs, their molecular targets are found in other cell types and therefore these drugs have effects on many other tissues.

Cyclosporin A and tacrolimus are both toxic to kidneys and other organs. Finally, treatment with these drugs is expensive because they are complex natural products that must be taken for prolonged periods. Thus there is room for improvement in these compounds, and better and less expensive analogues are being sought. Nevertheless, at present, they are the drugs of choice in clinical transplantation, and they are also being tested in a variety of autoimmune diseases, especially those that, like graft rejection, are mediated by Tcells. Immunosuppressive drugs are valuable probes of intracellular signaling pathways in lymphocytes. The mechanism of action of cyclosporin A and tacrolimus is now fairly well understood. Each binds to a different group of immunophilins: cyclosporin A to the cyclophilins, and tacrolimus to the FK-binding proteins (FKBP). These immunophilins are peptidyl-prolyl cis-trans isomerases but their isomerase activity does not seem to be relevant to the immunosuppressive activity of the drugs that bind them. Rather, the immunophilin:drug complexes bind and inhibit the Ca2+-activated serine/threonine phosphatase calcineurin. Calcineurin is activated in T cells when intracellular calcium ion levels rise after T-cell receptor binding; on activation it dephosphorylates the NFATc family of transcription factors in the cytoplasm, allowing them to migrate to the nucleus, where they form complexes with nuclear partners including the transcription factor AP-1, and induce transcription of genes including those for IL-2, CD40 ligand, and Fas ligand. This pathway is inhibited by cyclosporin A and tacrolimus, which thus inhibit the clonal expansion of activated T cells. Calcineurin is found in other cells besides T cells but at higher levels; T cells are therefore particularly susceptible to the inhibitory effects of these drugs. Rapamycin has a different mode of action from either cyclosporin A or tacrolimus. Like tacrolimus, it binds to the FKBP family of immunophilins. However, the rapamycin:immunophilin complex has no effect on calcineurin activity but, instead, blocks the signal transduction pathway triggered by ligation of the IL-2 receptor. Rapamycin also inhibits lymphocyte proliferation driven by IL-4 and IL-6, implying a common postreceptor pathway of signaling by these cytokines. The rapamycin:immunophilin complex acts by binding to a protein kinase named mTOR (mammalian target of rapamycin; also known as FRAP, RAFT1, and RAPT1).

This kinase  phosphorylates two downstream intracellular targets. The first is another kinase, p70 S6 kinase, which in turn regulates the translation of many proteins. The second is PHAS-1, a repressor of protein translation, which is inhibited by phosphorylation mediated by mTOR. Both PHAS-1 and p70 S6 kinase appear to mediate the effects of rapamycin in lymphocytes. Because rapamycin has different pharmacological activities from cyclosporin A and tacrolimus, trials are being undertaken to see if combination therapy involving rapamycin given together with either cyclosporin A or tacrolimus might provide more effective and safer treatment than the use of just one of these drugs. The rationale for such studies is that it may be possible to use lower amounts of each drug when used in combination, compared with the amounts required for treatment with a single agent. This might be a means of reducing unwanted side-effects. Signaling via T-cell receptor-associated tyrosine kinases leads to the activation and increased synthesis of the transcription factor AP-1 and other partner proteins, as well as increasing the concentration of Ca2+ in the cytoplasm (left panels). The Ca2+ binds to calcineurin and thereby activates it to dephosphorylate the cytoplasmic form of members of the family of nuclear factors of activated T cells (NFATc). Once dephosphorylated, the active NFATc family members migrate to the nucleus to form a complex with AP-1 and other partner proteins; the NFATc:AP-1 complexes can then induce the transcription of genes required for T-cell activation, including the IL-2 gene. When cyclosporin A (CsA) or tacrolimus are present, they form complexes with their immunophilin targets, cyclophilin (CyP) and FK-binding protein (FKBP), respectively (right panels). The complex of cyclophilin with cyclosporin A can bind to calcineurin and block its ability to activate NFATc family members. The complex of tacrolimus with FKBP binds to calcineurin at the same site, also blocking its activity. Antibodies against cell-surface molecules have been used to remove specific lymphocyte subsets or to inhibit cell function. Cytotoxic drugs kill all proliferating cells and therefore indiscriminately affect all types of activated lymphocyte and any other cell that is dividing. Cyclosporin A, tacrolimus, and rapamycin are more selective, but still inhibit most adaptive immune responses. In contrast, antibodies can interfere with immune responses in a nontoxic and much more specific manner.

The potential of antibodies for removal of unwanted lymphocytes is demonstrated by antilymphocyte globulin, a preparation of immunoglobulin from horses immunized with human lymphocytes, which has been used for many years to treat acute graft rejection  episodes. Anti-lymphocyte globulin does not, however, discriminate between useful lymphocytes and those responsible for unwanted responses. Moreover, horse immunoglobulin is highly antigenic in humans and the large doses used in therapy are often followed by the development of serum sickness, caused by the formation of immune complexes of horse immunoglobulin and human anti-horse immunoglobulin antibodies. Nevertheless, anti-lymphocyte globulins are still in use to treat acute rejection and have stimulated the quest for monoclonal antibodies to achieve more specifically targeted effects. Immunosuppressive monoclonal antibodies act by one of two general mechanisms. Some monoclonal antibodies trigger the destruction of lymphocytes in vivo, and are referred to as depleting antibodies, whereas others are nondepleting and act by blocking the function of their target protein without killing the cell that bears it. IgG monoclonal antibodies that cause lymphocyte depletion target these cells to macrophages and NK cells, which bear Fc receptors and which respectively kill the lymphocytes by phagocytosis and antibody-dependent cytotoxicity. Many antibodies are being tested for their ability to inhibit allograft rejection and to modify the expression of autoimmune disease. We will discuss some of these examples after looking at the measures being taken to prepare monoclonal antibodies for therapy in humans. Antibodies can be engineered to reduce their immunogenicity in humans. The major impediment to therapy with monoclonal antibodies in humans is that these antibodies are most readily made by using mouse cells, and humans rapidly develop antibody responses to mouse antibodies. This not only blocks the actions of the mouse antibodies but leads to allergic reactions, and if treatment is continued can result in anaphylaxis. Once this has happened, future treatment with any mouse monoclonal antibody is ruled out. This problem can, in principle, be avoided by making antibodies that are not recognized as foreign by the human immune system, and three strategies are being explored for their construction. One approach is to clone human V regions into a phage display library and select for binding to human cells, as described in Appendix I In this way, monoclonal antibodies that are entirely human in origin can be obtained. Second, mice lacking endogenous immunoglobulin genes can be made transgenic for human immunoglobulin heavy- and light-chain loci by using yeast artificial chromosomes. B cells in these mice have receptors encoded by human immunoglobulin genes but are not tolerant to most human proteins. In these mice, it is possible to induce human monoclonal antibodies against epitopes on human cells or  proteins.

Finally, one can graft the complementarity-determining regions (CDRs) of a mouse monoclonal antibody, which form the antigen-binding loops, onto the framework of a human immunoglobulin molecule, a process known as humanization. Because antigen-binding specificity is determined by the structure of the CDRs (see Chapter 3), and because the overall frameworks of mouse and human antibodies are so similar, this approach produces a monoclonal antibody that is antigenically identical to human immunoglobulin but binds the same antigen as the mouse monoclonal antibody from which the CDR sequences were derived. These recombinant antibodies are far less immunogenic in humans than the parent mouse monoclonal antibodies, and thus they can be used for the treatment of humans with far less risk of anaphylaxis. Monoclonal antibodies can be used to inhibit allograft rejection. Antibodies specific for various physiological targets have been used in attempts to prevent the development of allograft rejection by inhibiting the development of harmful inflammatory and cytotoxic responses. One approach is to perfuse the organ before transplantation with antibodies that react with antigen-presenting cells and thus target them for destruction within the mononuclear phagocytic system. Depletion of antigen-presenting cells in the graft by this method is effective at preventing allograft rejection in animal models, although there is no convincing evidence that it is successful in humans. Antibodies have, however, been used to treat episodes of graft rejection in humans. Anti-CD3 antibodies are moderately effective as an adjunct to immunosuppressive drugs in the treatment of episodes of transplanted kidney rejection. A further approach to inhibiting allograft rejection is the blockade of the co-stimulatory signals needed to activate T cells that recognize donor antigens. In animal studies of graft rejection, a fusion protein made from CTLA-4 and the Fc portion of human immunoglobulin, which binds to both B7.1 and B7.2, has allowed the longterm survival of certain grafted tissues. Even more effective in a primate model of renal allograft rejection was the use of a humanized monoclonal antibody against the CD40 ligand (CD154), present on T cells. CD40 ligand binds to CD40, expressed on dendritic and endothelial cells, stimulating these cells to secrete cytokines such as IL-6, IL-8, and IL-12. The mechanism of the immunosuppressive effect of anti-CD40 ligand antibody is not known, but it is most likely to be a consequence of blocking the activation of dendritic cells by T helper cells recognizing donor antigens. Monoclonal antibodies against other targets have also had some success in preventing graft rejection in animals.

 Of particular interest are certain nondepleting anti-CD4 antibodies: when given for a short time during primary exposure to grafted tissue, these lead to a state of tolerance in the recipient. This tolerant state is an example of the dominant immune suppression discussed in Section 13-27. It is long-lived and can be transferred to naive recipients by CD4 T cells producing cytokines typical of TH2 cells, although T cells producing other patterns of cytokines might also be involved. The presence of anti-CD4 antibody at the time of transplantation might favor the development of a nondamaging TH2 response, rather than an inflammatory TH1 response, because of a reduced strength of interaction between the graft cell antigens and responding naive T cells. In human bone marrow transplantation, depleting antibodies directed at mature T lymphocytes have proved particularly useful. Elimination of mature T lymphocytes from donor bone marrow before infusion into a recipient is very effective at reducing the incidence of graft-versus-host disease. In this disease, the T lymphocytes in the donor bone marrow recognize the recipient as foreign and mount a damaging alloreaction against the recipient, causing rashes, diarrhea, and pneumonia, which is often fatal. Mice grafted with tissue from a genetically different mouse reject that graft. Having been primed to respond to the antigens in the graft, they then reject a subsequent graft of identical tissue more rapidly (left panels). Mice injected with anti-CD4 antibody alone can recover immune competence when the antibody disappears from the circulation, as shown by a normal primary rejection of graft tissue (center panels). However, when tissue is grafted and anti-CD4 antibody is administered at the same time, the primary rejection response is markedly inhibited (right panels). An identical graft made later in the absence of anti-CD4 antibody is not rejected, showing that the animal has become tolerant to the graft antigen. This tolerance can be transferred with T cells to naive recipients (not shown). Antibodies can be used to alleviate and suppress autoimmune disease. Autoimmune disease is detected only once the autoimmune response has caused tissue damage or has disturbed specific physiological functions. There are three main approaches to treatment. First, anti-inflammatory therapy can reduce tissue injury caused by an inflammatory autoimmune response; second, immunosuppressive therapy can be aimed at reducing the autoimmune response; and third, treatment can be directed specifically at compensating for the result of the damage.

For example, diabetes, which is induced by autoimmune attack on pancreatic β cells, is treated by insulin replacement therapy. Anti-inflammatory therapy for autoimmune  disease includes the use of anti-cytokine antibodies; anti-TNF-α antibodies induce striking temporary remissions in rheumatoid arthritis. Antibodies can also be used to block cell migration to sites of inflammation; for example, anti-CD18 antibodies prevent leukocytes adhering tightly to vascular endothelium and reduce inflammation in animal models of disease. The ultimate goal of immunotherapy for autoimmune disease is specific intervention to restore tolerance to the relevant autoantigens. Two experimental approaches are under investigation. The first aims at blocking the specific response to autoantigen. One way to attempt this is to identify the clonally restricted T-cell receptors or immunoglobulin carried by the lymphocytes that cause disease, and to target these with antibodies directed against idiotypic determinants on the relevant antigen receptor. Another way is to identify particular MHC class I or class II molecules responsible for presenting peptides from autoantigens and to inhibit their antigen-presenting function selectively with antibodies or blocking peptides. This approach has been successful in some animal models of autoimmunity, for example experimental autoimmune encephalomyelitis (EAE), in which it seems that a limited number of clones of T cells, responding to a single peptide, might cause disease. However, autoimmune disease in humans and most animal models is driven by a polyclonal response to autoantigens by T and B lymphocytes. For this reason, immunotherapy based on the identification of specific receptors carried by pathogenic lymphocytes is unlikely to succeed. Immunotherapy based on the identification of the particular MHC molecules that drive an autoimmune response is more likely to be effective, but such therapy would also inhibit some protective immune responses. The clinical course of 24 patients was followed for 4 weeks after treatment with either a placebo or a monoclonal antibody against TNF-α at a dose of 10 mg kg-1. The antibody therapy was associated with a reduction in both subjective and objective parameters of disease activity (as measured by pain score and swollen-joint count, respectively) and in the systemic inflammatory acute-phase response, measured as a fall in the concentration of the acute-phase reactant C-reactive protein. In mice with experimental autoimmune encephalomyelitis (EAE), macrophages process myelin basic protein (MBP) and present MBP peptides to TH1 lymphocytes in conjunction with co-stimulatory signals. Activated TH1 cells secrete cytokines, which activate macrophages. The activated macrophages can, in turn, injure the oligodendrocytes.

Antibodies against MHC class II molecules block this process by  blocking the interaction between TH1 cells and antigen-presenting macrophages. Modulation of the pattern of cytokine expression by T lymphocytes can inhibit autoimmune disease. The second approach to immunotherapy for autoimmune disease is to try to turn a pathological autoimmune response into an innocuous one. This approach is being pursued experimentally because, as we learned in Chapter 13, tolerance to tissue antigens does not always depend on the absence of a T-cell response; instead, it can be actively maintained by T cells secreting cytokines that suppress the development of a harmful, inflammatory T-cell response. As the pattern of cytokines expressed by T lymphocytes is critical in determining the perpetuation and expression of autoimmune disease, the manipulation of cytokine expression offers a way of controlling it. There are various techniques, collectively known as immune modulation, that can affect cytokine expression by T lymphocytes. These involve manipulating the cytokine environment in which T-cell activation takes place, or manipulating the way antigen is presented, as these factors have been observed to influence the differentiation and cytokine-secreting function of the activated T cells. As discussed in earlier chapters, CD4 T lymphocytes can be subdivided into two major subsets, the TH1 cells, which secrete interferon (IFN)-γ, and the TH2 cells, which secrete IL-4, IL-5, IL-10, and transforming growth factor (TGF)- β. In many cases, autoimmune disease is associated with the activation of TH1 cells, which activate macrophages and drive an inflammatory immune response. In animal models of experimentally induced autoimmune disease, such as EAE, the relative activation of the TH1 and TH2 subsets of T lymphocytes can be manipulated to give either a TH1 response and disease, or a TH2 response that confers protection against disease.

 The preferential activation of TH1 or TH2 cells can be achieved by direct manipulation of the cytokine environment or by administering antigen by particular routes, for example by feeding. Recent evidence shows that patterns of cytokines secreted by T lymphocytes are very complicated and that the TH1 and TH2 subdivision of T lymphocytes is a considerable oversimplification. For example, CD4 T cells have been identified that develop in a cytokine environment rich in IL-10, and in turn secrete high levels of IL-10 and little IL-2 and IL-4. This pattern of cytokine secretion has bystander effects on other T cells and suppresses antigen-induced activation of other CD4 T lymphocytes. These cells have been provisionally designated Tr1 cells (T regulatory cells 1). Another subset of T cells with immunosuppressive bystander effects secretes TGF-β as the dominant cytokine and has  been designated TH3. Such cells might be predominantly of mucosal origin and activated by the mucosal presentation of antigen. A further subset of T cells also seems to be implicated in immunoregulation. These are the NK1.1+ CD4 T cells, so named because they bear the receptor NK1.1, which is usually found on NK cells. NK1.1+ T cells, which we discussed, recognize antigens, including lipid antigens, presented by the class I-like molecule CD1 (and respond by secreting IL-4. Thus, when stimulated, the NK1.1+ T cells can act to promote TH2 responses. Although there is no direct evidence that NK1.1+ T cells are involved in immunomodulation in humans, in mice that suffer spontaneous autoimmune disease this population of cells is either missing or decreased. Furthermore, transfer of NK1.1+ T cells into such mice prevents the onset of the autoimmune disease. Immune modulation aims to alter the balance between different subsets of responding T cells such that helpful responses are promoted and damaging responses are suppressed. As a therapy for autoimmunity it has the advantage that one might not need to know the precise nature of the autoantigen stimulating the autoimmune disease. This is because the administration of cytokines or antigen to modulate the immune response causes changes in the pattern of cytokine expression that have bystander effects on lymphocytes with the presumed autoreactive receptors. However, the drawback of this approach is the unpredictability of the results. In murine models of diseases such as diabetes and EAE, most of the results suggest that a TH2 response can protect against TH1-mediated disease, but there is evidence that TH2 lymphocytes can also contribute to the pathology of these diseases. An additional problem is the difficulty of modulating established immune responses.

 

 

Vaccination

 

Vaccination is the administration of antigenic material (a vaccine) to stimulate an individual's immune system to develop adaptive immunity to a pathogen. Vaccines can prevent or ameliorate morbidity from infection. The effectiveness of vaccination has been widely studied and verified; for example, the influenza vaccine,[1] the HPV vaccine, and the chicken pox vaccine. Vaccination is the most effective method of preventing infectious diseases; widespread immunity due to vaccination is largely responsible for the worldwide eradication of smallpox and the restriction of diseases such as polio, measles, and tetanus from much of the world.

The active agent of a vaccine may be intact but inactivated (non-infective) or attenuated (with reduced infectivity) forms of the causative pathogens, or purified components of the pathogen that have been found to be highly immunogenic (e.g., outer coat proteins of a virus). Toxoids are produced for immunization against toxin-based diseases, such as the modification of tetanospasmin toxin of tetanus to remove its toxic effect but retain its immunogenic effect.

In common speech, 'vaccination' and 'immunization' have a similar meaning. This distinguishes it from inoculation, which uses unweakened live pathogens, although in common usage either is used to refer to an immunization. Vaccination efforts have been

 met with some controversy since their inception, on scientific, ethical, political, medical safety, and religious grounds. In rare cases, vaccinations can injure people and, in the United States, they may receive compensation for those injuries under the National Vaccine Injury Compensation Program. Early success and compulsion brought widespread acceptance, and mass vaccination campaigns have greatly reduced the incidence of many diseases in numerous geographic regions.

Generically, the process of artificial induction of immunity, in an effort to protect against infectious disease, works by 'priming' the immune system with an 'immunogen'. Stimulating immune responses with an infectious agent is known as immunization. Vaccination includes various ways of administering immunogens.

Some vaccines are administered after the patient already has contracted a disease. Vaccinia given after exposure to smallpox, within the first three days, is reported to attenuate the disease considerably, and vaccination up to a week after exposure likely offers some protection from disease or may modify the severity of disease.[13] The first rabies immunization was given by Louis Pasteur to a child after he was bitten by a rabid dog. Subsequent to this, it has been found that, in people with uncompromised immune systems, four doses of rabies vaccine over 14 days, wound care, and treatment of the bite with rabies immune globulin, commenced as soon as possible after exposure, is effective in preventing the development of rabies in humans.[ Other examples include experimental AIDS, cancer and Alzheimer's disease vaccines. Such immunizations aim to trigger an immune response more rapidly and with less harm than natural infection.

 

Most vaccines are given by hypodermic injection as they are not absorbed reliably through the intestines. Live attenuated polio, some typhoid and some cholera vaccines are given orally to produce immunity in the bowel.

 

Adjuvants and preservatives

 

Vaccines typically contain one or more adjuvants, used to boost the immune response. Tetanus toxoid, for instance, is usually adsorbed onto alum. This presents the antigen in such a way as to produce a greater action than the simple aqueous tetanus toxoid. People who get an excessive reaction to adsorbed tetanus toxoid may be given the simple vaccine when time for a booster occurs.

 In the preparation for the 1990 Gulf campaign, Pertussis vaccine (not acellular) was used as an adjuvant for Anthrax vaccine. This produces a more rapid immune response than giving only the Anthrax, which is of some benefit if exposure might be imminent.

Vaccines may also contain preservatives to prevent contamination with bacteria or fungi. Until recent years, the preservative thiomersal was used in many vaccines that did not contain live virus. As of 2005, the only childhood vaccine in the U.S. that contains thiomersal in greater than trace amounts is the influenza vaccine,[2] which is currently recommended only for children with certain risk factors.[15] Single-dose Influenza vaccines supplied in the UK do not list Thiomersal (its UK name) in the ingredients. Preservatives may be used at various stages of production of vaccines, and the most sophisticated methods of measurement might detect traces of them in the finished product, as they may in the environment and population as a whole.

 

Vaccination versus inoculation

 

Many times these words are used interchangeably, as if they were synonyms. In fact, they are different things. As doctor Byron Plant explains: "Vaccination is the more commonly used term, which actually consists of a "safe" injection of a sample taken from a cow suffering from cowpox... Inoculation, a practice probably as old as the disease itself, is the injection of the variola virus taken from a pustule or scab of a smallpox sufferer into the superficial layers of the skin, commonly on the upper arm of the subject. Often inoculation was done "arm to arm" or less effectively "scab to arm"...

Vaccination began in the 18th century with the work of Edward Jenner.

 

 

Types

Vaccines work by presenting a foreign antigen to the immune system to evoke an immune response, but there are several ways to do this. Four main types are currently in clinical use:

1. An inactivated vaccine consists of virus or bacteria that are grown in culture and then killed using a method such as heat or formaldehyde. Although the virus or bacteria particles are destroyed and cannot replicate, the virus capsid

 proteins or bacterial wall are intact enough to be recognized and remembered by the immune system and evoke a response. When manufactured correctly, the vaccine is not infectious, but improper inactivation can result in intact and infectious particles. Since the properly produced vaccine does not reproduce, booster shots are required periodically to reinforce the immune response.

2.                                  In an attenuated vaccine, live virus or bacteria with very low virulence are administered. They will replicate, but locally or very slowly. Since they do reproduce and continue to present antigen to the immune system beyond the initial vaccination, boosters may be required less often. These vaccines may be produced by passaging, for example, adapting a virus into different host cell cultures, such as in animals, or at suboptimal temperatures, allowing selection of less virulent strains, or by mutagenesis or targeted deletions in genes required for virulence. There is a small risk of reversion to virulence, which is smaller in vaccines with deletions. Attenuated vaccines also cannot be used by immunocompromised individuals. Reversions of virulence were described for a few attenuated viruses of chickens (infectious bursal disease virus, avian infectious bronchitis virus, avian infectious laryngotracheitis virus [3], avian metapneumovirus

3.                                  Virus-like particle vaccines consist of viral protein(s) derived from the structural proteins of a virus. These proteins can self-assemble into particles that resemble the virus from which they were derived but lack viral nucleic acid, meaning that they are not infectious. Because of their highly repetitive, multivalent structure, virus-like particles are typically more immunogenic than subunit vaccines (described below). The human papillomavirus and Hepatitis B virus vaccines are two virus-like particle-based vaccines currently in clinical use.

4.                                  A subunit vaccine presents an antigen to the immune system without introducing viral particles, whole or otherwise. One method of production involves isolation of a specific protein from a virus or bacterium (such as a bacterial toxin) and administering this by itself. A weakness of this technique is that isolated proteins may have a different three-dimensional structure than the protein in its normal context, and will induce antibodies that may not recognize the infectious organism. In addition, subunit vaccines often elicit weaker antibody responses than the other classes of vaccines.

 A number of other vaccine strategies are under experimental investigation. These include DNA vaccination and recombinant viral vectors.

 

 

Routes of administration

 

A vaccine administration may be oral, by

 

injection (intramuscular, intradermal, subcutaneous),

 

by puncture,

 

transdermal or

 

intranasal.

 

 

Vaccine

 

A vaccine is a biological preparation that improves immunity to a particular disease. A vaccine typically contains an agent that resembles a disease-causing microorganism, and is often made from weakened or killed forms of the microbe, its toxins or one of its surface proteins. The agent stimulates the body's immune system to recognize the agent as foreign, destroy it, and "remember" it, so that the immune system can more easily recognize and destroy any of these microorganisms that it later encounters.

 Vaccines may be prophylactic (example: to prevent or ameliorate the effects of a future infection by any natural or "wild" pathogen), or therapeutic (e.g. vaccines against cancer are also being investigated; see cancer vaccine).

 

The term vaccine derives from Edward Jenner's 1796 use of cow pox (Latin variola vaccinia, adapted from the Latin vaccīn-us, from vacca, cow), to inoculate humans, providing them protection against smallpox.

 

"With the exception of safe water, no other modality, not even antibiotics, has had such a major effect on mortality reduction and population growth."

 

Effectiveness

 

Vaccines do not guarantee complete protection from a disease. Sometimes, this is because the host's immune system simply does not respond adequately or at all. This may be due to a lowered immunity in general (diabetes, steroid use, HIV infection, age) or because the host's immune system does not have a B cell capable of generating antibodies to that antigen.

Even if the host develops antibodies, the human immune system is not perfect and in any case the immune system might still not be able to defeat the infection immediately. In this case, the infection will be less severe and heal faster.

Adjuvants are typically used to boost immune response. Most often aluminium adjuvants are used, but adjuvants like squalene are also used in some vaccines and more vaccines with squalene and phosphate adjuvants are being tested. Larger doses are used in some cases for older people (50–75 years and up), whose immune response to a given vaccine is not as strong.

The efficacy or performance of the vaccine is dependent on a number of factors:

 

                       the disease itself (for some diseases vaccination performs better than for other diseases)

 

                       the strain of vaccine (some vaccinations are for different strains of the disease)

 

                       whether one kept to the timetable for the vaccinations (due to  Vaccination schedule)

 

                       some individuals are "non-responders" to certain vaccines, meaning that they do not generate antibodies even after being vaccinated correctly

                       other factors such as ethnicity, age, or genetic predisposition.

 When a vaccinated individual does develop the disease vaccinated against, the disease is likely to be milder than without vaccination

 

The following are important considerations in the effectiveness of a vaccination program:

 

1.                      careful modelling to anticipate the impact that an immunization campaign will have on the epidemiology of the disease in the medium to long term

 

2.                      ongoing surveillance for the relevant disease following introduction of a new vaccine and

 

3.               maintaining high immunization rates, even when a disease has become rare.