Basics of Immunology - Lecture Course by M. V. Skok - Kyiv 2002

Chapter III. Immune Processes at the Organism Level

Lecture 13. General Immunological Phenomena

1. Anti-infectious Immunity.

a) response to bacterial infections.

Bacteria entering the Organism initially encounter the factors of nonspecific defense and pre-immune (natural) Antibodies. Antibodies opsonize bacteria and facilitate their phagocytosis by macrophages, as well as Complement The alternative pathway of complement activation. Activated macrophages produce IL-12, which promotes Th1 development and turns on the cellular branch of immunity. Interferon-γ and IL-2 produced by Th1 activate natural killer Cells and CTL precursors. Interferon promotes the expression of MHC II and Fc receptors on macrophages and dendritic cells, thereby enhancing humoral immunity.

Natural antibodies also form immune complexes with bacteria and their metabolic products, which bind to basophils, mast cells, and T cells, inducing The production of IL-4. This promotes Th2 development and the engagement of the humoral arm of immunity. Functions of antibodies in antibacterial immunity:

- binding and neutralization of toxins;

- complement activation;

- opsonization.

IgM and IgG are present in serum, whereas sIgA is found in mucous membranes, preventing bacterial adhesion to mucosal surfaces. IgE molecules, upon binding corresponding Antigens, interact with mast cells and promote the release of inflammatory mediators, which in turn facilitate the migration of lymphocytes to the site of inflammation. IgG capable of binding complement facilitates the destruction of bacteria via the classical pathway of complement activation.

Products of destroyed bacteria, as well as their toxins, are endocytosed by macrophages and dendritic cells and presented via MHC II. If the bacterium is an intracellular parasite, infected cells present it on their surface via MHC I. Thus, a bacterial infection engages both cellular and humoral arms of immunity. The predominance of one over the other depends on the type of bacterium and its METABOLISM within the host organism. If the bacterium does not parasitize intracellularly, nonspecific and humoral mechanisms are sufficient for its destruction. In cases of intracellular infection, a cellular response is indispensable.

б) відповідь на вірусні інфекції.

Viruses replicate inside infected cells; therefore, the primary means of combating an established viral infection is cellular immunity. Antibodies are effective at stages when the virus has not yet entered The Cell; they neutralize the virus and prevent its binding to cells, thereby mediating protective immunity. This principle underlies successful vaccinations against measles, mumps, and polio. Antibodies act as an auxiliary tool for the destruction of viral remnants during cell-mediated immunity and can also facilitate complement-dependent lysis of infected cells. Interferons α and β act directly on the virus, while interferon-γ activates both nonspecific defenses (macrophages, natural killer cells) and the cellular Immune Response, as well as directly inhibiting viral Replication within the cell. The destruction of infected cells is carried out by macrophages, natural killer cells, and cytotoxic T lymphocytes.

Immunization with a killed virus leads solely to the generation of humoral immunity, whereas recovery from a viral infection results in The formation of cytotoxic memory cells. A major challenge is the rapid mutation of viruses, which hinders The Development of effective Vaccines, such as against the Influenza virus.

в) відповідь на паразитарні інфекції

This is a distinct and quite complex topic in immunology. The fact is that parasitic agents (Protozoa, helminths) are much more complex than bacteria and viruses, both in their antigenic composition and metabolic characteristics. Many of them undergo several developmental stages, each characterized by a specific antigenic profile and particular localization (plasmodium malariae is a classic example). For this reason, developing vaccines against such infectious agents is an almost insurmountable task. However, this does not mean that the organism fails to mount an Immune Response to such infections. At certain stages, protective antibodies play a role by opsonizing parasites and promoting their phagocytosis and complement-dependent lysis. A special role is played by Class E IMMUNOGLOBULINS, which bind to eosinophils and stimulate the exocytosis of eosinophil granules, mediating extracellular lysis of parasites, particularly helminths. The cellular branch of the immune response is also activated.

г) вакцини

Vaccines are a means of preventing infection by establishing protective immunity, whereby an infectious agent, upon entering the body, encounters a pre-formed army of defenders.

The first vaccines, introduced back in Pasteur's time, were so-called Attenuated Vaccines. These involved introducing killed or weakened pathogens (or strains virulent to another animal species) into the organism, which often caused a mild form of the disease and conferred lasting immunity. However, such attempts were not always successful; for example, Salk's polio vaccine cost the health of thousands of American children because some batches were insufficiently attenuated. This risk is always present; furthermore, administering a whole pathogen with A large number of antigenic determinants often leads to undesirable side effects, including allergic reactions. Therefore, in developing new vaccines, the search naturally moved toward simplifying their antigenic composition. Initially, so-called Subunit Vaccines were proposed, containing not the whole pathogen, but only the specific fraction capable of inducing protective immunity.

Since the 1970s, with the intensive Study of the antigenic Structure of individual Proteins, the idea arose to create combined synthetic vaccines containing Peptides—antigenic determinants of various pathogens—to simultaneously induce immunity against multiple diseases. However, it turned out that short peptides containing only B epitopes generally do not provide lasting immunity. The question arose of combining a B epitope, a T-helper epitope, and a T-cytotoxic epitope in a single vaccine, as well as stimulating a response against multiple pathogen epitopes and enhancing the immune response through The addition of a natural adjuvant. In the 1970s and 1980s, these issues were addressed by synthesizing peptides artificially; as molecular biology Methods progressed, recombinant vaccines began to be utilized. Currently, the creation of such vaccines involves several stages:

1) Determination of the positions of the pathogen's B, T, and cytotoxic epitopes;

2) synthesis of oligonucleotides encoding these fragments;

3) insertion of these oligonucleotides into The genes of viral proteins successfully used for immunization (such as vaccinia virus or adenovirus) or into a bacterial protein Gene;

4) Induction of the synthesis of such Chimeric Proteins in bacteria to yield the required amount of antigenic material.

In this manner, for example, an experimental influenza vaccine was developed.

In recent years, so-called DNA vaccines have gained considerable popularity. It has been demonstrated that introducing unmethylated DNA encoding a specific protein epitope into the human or animal organism in the form of a plasmid successfully induces both humoral and cellular immunity against this epitope. Initial experiments investigating the metabolism of such administered DNA revealed that it is phagocytosed by macrophages and, following lysosomal proteolysis (which causes it no harm), is integrated into the macrophage genome. The macrophages begin synthesizing the target epitope as their own protein and presenting it via MHC I. Excess protein is secreted extracellularly and can be presented via MHC II. Consequently, both cellular and humoral immune responses are activated. The potential use of DNA vaccines is currently under intensive investigation, and such vaccines may well represent the future of immunization.

2. Allogeneic Body Reactions.

Beyond combating infectious agents, The Immune System maintains Homeostasis of the body's internal environment by eliminating mutated cells or foreign cells from other individuals of the same species (allogeneic cells) that have entered the body.

Experimentally, the allogeneic response can be observed using the mixed lymphocyte reaction model. When lymphocytes from two different mouse strains are co-cultured, their proliferation and intensive cytokine secretion are observed within 1 to 2 days. During this process, both MHC I and MHC II molecules bearing allogeneic peptides are mutually recognized. This reaction occurs because T lymphocytes of each strain—having undergone positive and negative Selection to recognize foreign peptides in the context of self-MHC—perceive the allogeneic MHC as an "altered self," meaning a self-molecule bearing a foreign peptide.

Under physiological conditions, the organism encounters altered self or allogeneic cells during tumor growth, Fertilization, and Pregnancy. An artificial, yet important and quite frequent, scenario is the transplantation of Organs and Tissues.

During fertilization, allogeneic cells (spermatozoa) enter the female reproductive tract and are potentially vulnerable to recognition as foreign. Evidently, nature has devised special protective mechanisms against such recognition (otherwise, sexual reproduction would be impossible). The female genital tract is impermeable to CTLs, and mucosal secretions exhibit immunosuppressive activity. However, these barriers are not absolute; many cases of Infertility are caused by anti-sperm antibodies in the female body, which bind to spermatozoa, impede their motility, and prevent fertilization. These are typically immunoglobulin A (IgA) class antibodies, although serum anti-sperm IgG is also observed. The presence of anti-sperm antibodies poses a significant challenge for those wishing to conceive, while conversely offering hope for those seeking Contraception. Consequently, the induction of such a response underlies THE CONCEPT OF developing an "anti-pregnancy vaccine."

Once fertilization is successfully achieved, the embryo begins to develop within the female body, carrying Genetic information from both mother and father; thus, to the mother, it is to some extent an allogeneic graft. It remains incompletely understood which protective mechanisms prevent the maternal immune system from mounting a response against the allogeneic fetus. It is clear that the Placenta serves as a physical barrier preventing the passage of CTLs and certain antibody classes. The placenta does not express MHC class I and II antigens. Evidently, specific regulatory mechanisms operate during pregnancy to prevent immune recognition of the fetus. This protection is not absolute: The phenomenon of Rhesus incompatibility is well known, wherein a Rhesus-negative mother reacts to fetal Rhesus antigens by producing specific antibodies. Conversely, maternal antibodies acquired by the fetus initially across the placenta (IgG) and subsequently through colostrum (IgA) constitute the newborn's first line of immune defense.

Tumor growth shares certain analogies with the state of pregnancy. Specifically, the development of many tumors is accompanied by the appearance in the Blood of proteins characteristic of embryonic development, which are also detectable in pregnant women. These include alpha-fetoprotein, carcinoembryonic antigen, and trophoblast-specific globulin. Evidently, these proteins are generally characteristic of intensive growth processes. Furthermore, it is hypothesized that tumor growth involves cellular dedifferentiation, causing cells to revert to a pseudo-embryonic state.

The discovery of carcinoembryonic antigens raised high hopes for overcoming Cancer through immunological means and developing a corresponding vaccine. These expectations were further stimulated by the identification of oncogenes—genes activated during tumor progression. Initially, it was hoped that their gene products might serve as specific tumor antigens. However, as knowledge regarding The Nature of cancer advanced, the Prospects of developing such a vaccine became increasingly elusive. In particular, it was established that oncogenes encode normal growth factors or their receptors, meaning malignant transformation is not invariably accompanied by the appearance of neoantigens. Moreover, tumor cells were shown to alter their expression profile of histocompatibility proteins, which likewise hinders their immune recognition.

Although hopes for a universal cancer vaccine proved unfounded, numerous studies have been conducted to stimulate the host's endogenous immune response once a tumor has begun to develop. Specifically, attempts were made to immunize patients with their own tumor cells (i.e., administering them alongside adjuvants). At one time, great expectations were placed on the activation of CTLs using interleukin-2 (IL-2). This involved isolating patient leukocytes, culturing them with IL-2, and reinfusing them into the organism in the hope of achieving activation. However, in many cases, such total activation also stimulated suppressor cell activity. A more modern approach involves transfecting isolated tumor cells with genes for IL-2 and the IL-2 receptor to render the cells sensitive to this cytokine. Patients have also been administered IFN to stimulate The activity of natural killer cells, which are considered primary effectors in destroying potential tumor cells. While all these approaches yielded specific results in certain cases, a universal immunological Treatment for malignancies has not been established. Currently, with the advent of novel paradigms in molecular biology, hopes for overcoming oncological diseases have shifted to other areas. Notably, these include attempts to induce apoptosis in tumor cells by manipulating p53 and Bcl-2 proteins, as well as utilizing vascular endothelial growth factor (VEGF) antagonists to inhibit tumor angiogenesis. It is well established that malignant tumors can only expand if their growth is supported by angiogenesis, which supplies nutrients to the neoplastic cells. By blocking vessel growth, tumor expansion can essentially be halted. At present, this approach is considered promising for the treatment of solid tumors.

Immunological approaches remain crucial for the Diagnosis of malignant transformations and for specific Chemotherapy strategies:

1) immunological methods for detecting carcinoembryonic antigens in blood and on cells are employed to diagnose cancerous and precancerous states;

2) Monoclonal Antibodies against these antigens, conjugated with radioactive isotopes of copper, boron, or technetium, are utilized both for tumor localization (via radiography) and for targeted radiotherapy (destroying tumor cells via radioactive irradiation);

3) these same antibodies, or their fragments, conjugated with plant or Bacterial toxins (such as ricin or the diphtheria toxin subunit), represent potential weaponry for eliminating transformed cells.

An artificial means of stimulating an allogeneic immune response is the transplantation of organs and tissues within the same species. The most widespread type of transplantation is blood transfusion. The antigen system recognized in this context was discovered by Karl Landsteiner. These are polysaccharide antigens expressed on erythrocytes. While numerous Blood Group Antigens are now known, the ABO blood group antigens and the Rhesus factor remain the principal determinants for recognition. Successful blood transfusion fundamentally requires matching donor-recipient pairs compatible for these antigens. Furthermore, it is preferable to transfuse packed red Blood Cells rather than whole blood to avoid recognizing antigens expressed on other blood cellular elements.

It should be noted that transfused blood functions within the recipient's organism only temporarily until endogenous blood reserves are replenished. In contrast, the transplantation of other organs and tissues entails the graft replacing the patient's own organ or tissue for an extended period—ideally, for the remainder of their life. In such transplantations, the primary antigens recognized are Major Histocompatibility Complex (MHC) class I antigens (in complex with allogeneic peptides). A distinction is made among:

- autotransplantation, where tissue is grafted within the same organism, such as transplanting one's own Skin to burn sites;

- isotransplantation, performed between genetically identical twins or animals of the same inbred strain;

- allotransplantation, which involves transplantation between different individuals of the same species; and

- hetero- (or xenotransplantation), representing interspecies organ or tissue grafts.

Clearly, graft rejection occurs exclusively in the latter two scenarios. The rejection mechanism is predominantly cellular, accompanied by infiltration of the graft by recipient immune cells. At various stages of rejection, virtually all known immune effector mechanisms participate: macrophage- and natural killer-mediated lysis, antibody-dependent and complement-dependent lysis, and the destruction of graft cells by CTLs.

Despite the inherent risk of rejection, thousands of Kidney, Liver, lung, and Heart transplantations are performed worldwide today. To prevent rejection, meticulous matching of donor-recipient pairs is paramount. This requires thorough HLA typing of both individuals. However, the probability of 100% histocompatibility in allogeneic transplantation is exceedingly low, meaning the risk of rejection persists. The success of transplantation relies on the judicious administration of immunosuppressants. Early transplant practice utilized cytostatic drugs—such as methotrexate, cyclophosphamide, and azathioprine—which inhibited nucleic acid synthesis and thereby slowed the proliferation of immune cells. Naturally, these agents, also employed in oncology, exhibit numerous adverse side effects and exert a detrimental impact on the recipient's organism. Steroid medications (such as prednisolone) are also employed; as noted in previous lectures, they are toxic to many immune cells and prevent their activation. A revolutionary milestone in transplantology was the Introduction of cyclosporine A, an 11-member cyclic peptide isolated from Fungi. Cyclosporine A acts as a calcineurin inhibitor, selectively affecting cells activated by graft antigens while leaving memory cells—also present in the body—unharmed.

Certain privileged sites exhibiting low levels of MHC class I expression do not undergo rejection. These include the cornea, Cartilage, and bone. Bone Marrow transplantations are typically performed in individuals with compromised immune systems, thereby reducing the risk of rejection.

It must be emphasized that if a graft contains immunocompetent cells and rejection is suppressed, the graft may mount an immune attack against the recipient; this phenomenon is designated as "graft-versus-host disease." This condition is characterized by fever, anemia, weight loss, skin rashes, and diarrhea, and in extreme cases, it can prove fatal to the recipient.