by Academician Valery CHERESHNEV, Director, Institute of Ecology and Genetics of Microorganisms, Perm Research Center, RAS Urals Branch
Inflammations today account for 90 to 95 percent of pathologies in man. According to the latest data reported at the Congress of Immunologists in Stockholm in 2001, the list of these pathologies includes even atherosclerosis. This being so, decoding the mechanisms of inflammations turns out to be a fundamental task of biological research which has important applications in various fields.
Each one of us now and then falls victim to different inflammatory ailments, from a common nasal cold to cholecystitis, gastritis and more serious conditions, like pneumonia. Providing a general definition of this process is no simple matter, and according to present-day pathophysiology and general pathology: " Inflammation is a typological pathological process which has emerged in the process of evolution and which develops in response to local tissue lesions."
Today medical experts in various countries are investigating inflammatory process at the cellular-molecular level and this calls for "clearing up" relations between inflammations and immunity - the two body defenses against "invasions" by any alien agents. So, what is immunity? In the words of Academician Rem Petrov, this is "a method of organism's protection from living bodies and substances carrying signs of genetically alien information". That is inflammation is a local, and immunity is a global (of the whole body) mechanism of body defenses.
If we compare a complex multi-cellular organism to, let us say, a sovereign state, its immune system can be said to perform the functions of the Ministry of State Security and of the Border Police. It prevents the penetration of microbes and other parasites through biological barriers and "fights them back" if and when such incursions take place. In any case the battlefield is the focus of inflammation.
Apart from that the immune system keeps constant check on the "loyalty of individual citizens", doing away relentlessly with any and all infected, mutated and tumor cells, thus maintaining the genetic and phenotypic homeostasis of the organism.
This activity of the immune systems stems from the fact that an organism, consisting of multitudes of highly differentiated cells, and as
Effect of immune response factors upon effector functions of phagocytes in focus of inflammation.
Effect of immune response factors in focus of inflammation.
different from a human society, cannot afford a "pluralism of actions" of its integral "individuals". The supreme law for any one cell is the unconditional implementation of its genetically determined biological program which determines its own function and also the functions of various body organs and of the organism as a whole. And so it turns out that the immune system takes upon itself not only the functions of body defenses from external biological aggressors, such as microbes, parasitic helminths, unicells and all sorts of poisons of protein nature, but also the functions of a "censor" who checks on the proper observance of the general biological program by individual cells. And the "transgressors" are exterminated through the mechanisms of direct cytolysis (destruction of cells through their complete or partial dissolution) carried out by the killer-cells: cytotoxic T-lymphocytes (CTL) or with the help of immunocytes incorporating the genetic program of self-destruction of cells - apoptosis.
The "core" cells of the immune system are lymphocytes. Their main feature consists in the ability to identify a strictly defined molecular determinant whose total number exceeds 10 14 and, as a rule, is localized in the structure of various proteins.
Scientists distinguish between two main sub-populations of lymphocytes: В- lymphocytes, responsible for the production of antibodies or immunoglobulines (Ig), connecting antigens and T-lymphocytes (CTL) and T-helpers (specializing in regulatory functions, producing upon activation a broad spectrum of hormone- like substances of protein nature - cytokins, which mainly control the progress of an inflammation and the immune response).
So, what is the relationship between an inflammation and immunity? This problem was originally formulated in 1871 by the great Russian scientist Ilya Metchnikov (later Honorary Member of the St. Petersburg Academy and Nobel Laureate). He discovered in his experiments that leukocytes, like amoebas, digest various invasive bodies, most often microbic agents. As was commonly believed at that time, an accumulation of leukocytes in places of tissue lesions - was just a sign of some pathology in the organism and not its response to a lesion. But in Metchnikov's view such an "army" of cells was simply too much for signaling the presence of a pathology in the body. On the basis of his experiments the scientist arrived at a conclusion which was hailed by his contemporaries on the level of the doctrine of Hippocrates: leukocytes implement the defense response of the organism. And, as he later established, that was the responsibility of not some single cells, but of a whole system intended for body protection from an intruder agent. Metchnikov called them "gobble cells".
In the course of further studies the scientist came to the conclusion that with the growing "complexity" of living organisms (from amoeba and up to humans) the process of "gobbling" attains greater perfection. And while in the former the defensive reaction coincides with digestion (whatever gets into the organism is devoured), in living organisms of higher organization (having a developed system of blood circulation and a multitude of specialized tissues) these two processes are divided as a result of which their
response to an alien intrusion is more rapid. Thus in the larva of starfish the response comes in no less than 12 hours, while in man it arrives in a matter of minutes.
In 1891 Metchnikov called his "gobble cells" phagocytes (Gk. phagein - to eat). Later on his phagocytic concept was called "cell immunity theory" and it remains valid to this day although its author could not have anticipated its diverse implications. This being so, let us take a look at the achievements of the biological science, especially in immunology, since the days of Metchnikov.
INFLAMMATIONS AND IMMUNE REACTIVITY
As has been said before, inflammation is a universal and genetically programmed body response to lesions of various nature. It consists in a concentration of phagocytes and other body defenses in the zone of lesion for the purpose of liquidation of any biologically aggressive matter therein and for the restoration of the structure and functions of the affected tissues.
But for the phagocytes to perform their functions in relation to microbes, they themselves require help on the part of soluble opsonins (protein stimulators of phagocytosis) and regulatory support from T-helpers. Even prior to contact with a phagocyte - to be more exact, with a macrophage (MP) - a pathogen is covered over with separate protein factors of the complement system (set of immune proteins), acting with low selectivity with respect to many microbe species, and also what we call antigen-specific antibodies of class G and M (IgG and IgM). And as for the former, they can attack microbes directly - through the formation of what is called a membrane-attacking complex (MAC) which damages bacterial wall membranes.
But the main effect of the complement consists in activating phagocytes and "targeting" them at the objects of phagocytosis. For that phagocytes possess complement-binding CR receptors. In their turn, antibodies fixed on the surface of a microbe also "mark" microbes since phagocytes possess what are called Fc-receptors (FcR) connecting outwardly oriented Fc-fragments of antibodies, which are non-specific to antigens. Apart from the complement, this process can also involve other humoral defense factors, like the C-reactive proteins.
As different from non-specific factors, for the production of antigen-specific lymphocyte-effectors (one of the forms of leucocytes) and antibodies there must be a preliminary contact of the lymphoid tissue with antigens and an interval (of several days) for the clonal proliferation of cells to take place. And in the beginning an antigen in lymphoidal organs is introduced to T-lymphocytes by antigen-presenting cells (A-cells) which are more often either macrophages or similar dendrite cells of the stromal (performing the function of support structure) micro-surrounding of lymphocytes. In the process of an immune response memory cells are produced which do not "engage" an antigen at once, but can survive for a long time even after the antigen disappears from the organism. These cells enhance the immune response while substantially reducing the time of its development in the conditions of repeated advents of antigens into the organism. This phenomenon is used by medics for preventive vaccinations.
Pathogenic factors, including those of antigenic nature, are distinguished by great diversity. And this being so, the body responses must also be different. Thus the progress of an inflammation and immune response involves a whole range of cells possessing functional features and this determines the different "scenarios" of the development of immune and inflammatory reactivity. This, however, depends in many respects on the availability of differentiated T- helpers, beginning from their little-differentiated precursors. In the beginning T- helpers-0 are produced from the latter which can then become differentiated in two alternative directions-Tx1 or Tx2. Each is capable of secreting strictly definited spectra of cytokins on a competitive basis. And these have different effects on the production by B-lymphocytes of different isotypes of antibodies, possessing different functions, and impact different types of cells in Tx activation directly in the focus of inflammation. As a rule, an inflammation is developing locally, but it also involves (in different degrees) practically all of the body systems and, above all, the immune and neuroendocrinal ones.
Directly involved in an inflammation are microvessels responding to lesions (especially post-capillary venulae, stromal cells) of the affected organ, leukocytes migrating into the focus of inflammation and also factors of the complement system and many other plasma proteins.
Inflammation involves a range of some well-known external manifestations as well as microstructural changes. The former include pain, edema, hyperemia, local or systemic rise of temperature, dynamic changes of the structure and functions of the affected organ. The latter include a exudative-vascular reaction, migration into the focus of inflammation of leukocytes with the formation of cellular infiltrates, and at the final stage - of fibroblasts and other cells participating in the process of reparatory restoration or sclerosing of the damaged tissues.
According to the dynamics of development of an acute inflammation not complicated by the development of an infection, one can single out several successive stages which can be clearly identified in experiments on animals. The first is tissue alteration, or lesion. It initiates a reaction of the endothelium of post-capillary venules and the system of hemostasis which provokes within minutes the development of an exudative-vascular reaction. At the second stage the impact of the microbial antigen promotes a migration and
the subsequent activation of polymorphic-nuclear leukocytes, mostly neutrophils: onset in 25-40 min., maximum in 3-6 hours. This takes place with the inclusion into the process of the complement system, immunoglobulins, many acute-phase proteins and certain other serum factors. The effect of these mechanisms is directed at the elimination of the antigen. At the peak of the neutrophil-dependent phase there begins the migration of mononuclears - monocytes (a form of leukocytes) and lymphocytes. The former are differentiated at the focus of inflammation into macrophages, and after about 24 hours mononuclears become the predominant cellular elements of the infiltrate. This stage ends with the final sterilization of the focus of inflammation, its "purification" from the products of tissue decay. Also developing at that stage are the reparative processes, which predominate at the final stage.
The migration of fibroblasts into the focus of inflammation starts in 1-3 days from the moment of alteration, and in another 2-3 days there occurs an active formation by them of collagen fibers and other components of the extracellular matrix. In the final analysis all of these processes lead to a complete regeneration, or cicatrization (scarring) of the affected tissue.
The length and degree of the different stages of the process of inflammation depend on the nature of lesion and the attending conditions, including the development of immunodeficit.
Despite the universal nature of the underlying mechanisms of inflammation in every individual case, the process is unique by its manifestations. Its individual peculiarities depend on its localization in different organs, the etiology, phenotypic and genetic properties of the invader macroorganism, the ratio of the duration and intensity of separate stages and particular mechanisms at its basis.
By the degree of involvement into the process of different antiinflammatory mechanisms it can be divided into two alternative variants: exudative- destructive, or purulent inflammation, and a productive, or prolife rating-cellular one. Of decisive importance in the former case are neurophils, possessing a strong phlogogenic potential, and also, functionally connected with them, a complement system and immunoglobulins (especially of G-class, or more exactly their main subclass IgGI). In the latter case the purulent reaction is far less pronounced and the prevailing cellular element of the infiltrate are mononuclears (monocytate-macrophages and lymphocytes), and in some cases (when tissue responds to helminths or their larvae) - eosinophils.
The development of exudative-destructive inflammation is associated, as a rule, with an aggression of pyogenic bacteria which are rapidly proliferating in the extracellular medium. In its turn a predominant form of response to the obligatory (commonly occurring) infection with intracellular pathogenes is the development of what we call productive, or proliferating-cellular inflammation with the priority involvement of "in-flammatory" macrophages, the functionally cooperating with them T-lymphocytes and normal killer-cells. And the latter can attack the specifically modified cells without a preliminary immune response. The involvement into the process of inflammation of many types of cells, subcellular elements and organic systems predetermines the formation of complex mechanisms of regulation of the inflammatory and immune reactivity both at the local and all-body levels.
According to its duration an inflammatory process can be acute (up to one month), subacute (from 3 to 6 months) and chronic. In the latter case there occurs conservation of the alteration mechanism in, let us say, the form of prolonged infection of the damaged tissue. And chronic inflammation in its turn can undergo recidivation, or assume torpid or progressing forms.
If the process of inflammation is prolonged, there frequently occurs a transformation of its different manifestations in time and localization. Thus with the aggravation of a flaccid inflammation the process continues in what we call exudative-destructive direction, and in the structure of the newly formed abscess there appear layers with unlike morphologo-functional characteristics.
What we call the classical versions of inflammation are essentially local processes whose biological essence boils down to a concentration of the vital resources and the defense factors of the organism in the tissue lesion zone. This function is implemented by "launching" of a stress program by the neuroendocrinal system and also by changes in the regenerative potentials of bone marrow and lymphoid organs, synthesis of acute phase proteins (APP) in the liver, etc.
The systemic reaction to local lesions not only provides for the priority delivery to the focus of inflammation of the necessary cell and humoral factors, but also promotes the neutralization in the bloodstream of the infected contaminants, toxic products of tissue decay and other biologically aggressive factors.
As has been said already, the immune system is responsible for the maintenance of the genetic homeostasis of the organism. According to the concept formulated by Academician Rem Petrov, the immune system, like the organs of sense, can be described as a scanner which checks on the incoming information-it checks on biological objects inside the body for any "alien presence". And when such "alien" antigens are detected, the system memorizes and analyzes them and responds to their entry with supplying to the focus of inflammation of antigen-specific Ig and T-lymphocytes.
Summing this up, the immune system, like the central nervous system, represents one more mechanism
Stages of acute inflammation.
which supplements (on the basis of acquired experience) the genetically determined program of conduct of an organism. But the analytic activity of the immune system occurs outside the framework of our conscience.
When lesions occur, the two systems work in cooperation, stimulating the development of the process of adaptation which mobilizes the body resources for the elimination of the causal factor and its impacts. And like the central nervous system, the immune system forms its own morphologo-functional dominant centering upon the antigen-specific clones of T- and B-lymphocytes.
The regulatory functions of the immune system cover all the vital organs, but its closest and multiple links are with the neuroendocrinal system. They are integrated into a common immunoneuroendocrinal supersystem. In cases of acute inflammations the main connecting link between various regulatory systems are certain "remote-action" cytokins (such as IL-1 or IL-6), factor of tumor necrosis, etc.
The immunocytes influence the nervous system not only through cytokins but also through a number of hormones (including most of the tropic hormones of the pituitary body), endorphins, neuromediators and other capable of producing activated lymphocytes. Nearly all hormonal and many neuromediators generated by peripheral nerves possess immunotropic effects. A number of antiinflammatory cytokins are produced in small amounts by neutrons and macroglia cells and macroglia directly in the central nervous system. Their production is "triggered" not only by a substantial tissue lesion, but also by a strong psychogenic trauma.
EVOLUTIONARY ASPECTS OF IMMUNE SYSTEM FORMATION
The immune system is the central antiinflammatory, or antiphlogistic, mechanism and also the most vulnerable system in the conditions of immunodeficit. It should be pointed out that the immune system of the mammals is the result of a long process of evolution. Even the primitive invertebrates possessed specialized phagocytes - amoeba-like cells identifying the objects of phagocytosis with the help of contact receptors. These cells, with the participation of opsonins (protein stimulators of phagocytosis) are capable of simultaneous binding with a phagocyte on the one hand and a microbe on the other.
The basic mechanisms of the process of inflammation are, above all, factors of the system ofpaleoimmunity: phagocytes of blood and tissues, endothelium of post-capillary venules, systems of complement and homeostasis, acute phase (such as CRP) and antibiotics-like proteins and other antigen-nonspecific protective organisms.
In the invertebrates such mechanisms can effectively implement the immune protection of the organism. The system of paleoimmunity in higher animals is incapable of solving this problem all by itself. This concerns, above all, counteracting the rapidly evolving pathogenic microflora. Therefore the appearance of the antigen-specific system of neoimmunity in the vertebrates can be regarded as the outcome of a crisis of relations between the macro- and microorganisms in the period of the Cambrian evolutionary explosion (circa 350 mln years ago). The formation of lymphoid organs came as an adequate reply to this challenge which ensured the survival of higher organisms.
The introduction into the genome ofmacroorganisms of viral recombinases made it possible to "reshuffle" the genetic segments of variable genes Ig- and T- cell receptor and thus produce and clonally seal a great multitude of varieties of these genes, which had not been originally encoded in the zygote. As has been recently established, the theoretically possible number of antigen-specific clones of lymphocytes amounts to about 10 18 versions. As a matter of fact this makes it possible for the immune system to identify practically any antigen.
The system of neoimmunity can be regarded as a superstructure over the system of paleoimmunity. This is warranted because, first, antibodies and immunocompetent effector cells were formed as a result of evolutionary metamorphoses of these or other factors of paleoimmunity and, secondly, they always impact as a "booster" link the focus of inflammation in conjunction with the basic mechanisms of paleoimmunity which depend upon them. In the process of evolution there occurred a cooperation of the antigen-specific and antigen-nonspecific mechanisms.
Because of that the regulatory mechanisms became increasingly complicated - the mechanisms which provide for the mutual links between the immune and other, above all neuro-endocrinal, systems of the organism. The formation of what we call a common complex of immunoneuroendocrinal regulation came as the summit of development of the bioinformation systems.
In humans, the effectiveness of combating inflammation depends on the degree of cooperation of the antigen-specific factors of neoimmunity and the evolutionary more ancient, and less specific mechanisms of paleoimmunity both-in the focus of inflammation and in the organism as a whole.