Antiviral Immunity

The Immune System:
The organs of the immune system are stationed throughout the body. They are generally referred to as the lymphoid organs because they are concerned with the growth, development, and deployment of lymphocytes, the white cells that are the key operatives of the immune system. 
Lymphoid organs include the bone marrow and the thymus as well as lymph nodes, spleen, tonsils and adenoids, the appendix, and clumps of lymphoid tissue in the small intestine known as Peyer's patches. The blood and lymphatic vessels that carry lymphocytes to and from the other structures can also be considered lymphoid organs. 
Cells destined to become immune cells, like all other blood cells, are produced in the bone marrow, the soft tissue in the hollow shafts of long bones (stem cells). The descendants of some so-called stem cells become lymphocytes, while others develop into a second major group of immune cells typified by the large, cell-and particle-devouring white cells known as phagocytes. 

The two major classes of lymphocytes are:

B cells complete their maturation in the bone marrow and become capable of being transformed into plasma cells upon contact with the antigen to produce the antibodies. 

T cells, on the other hand, migrate to the thymus, a multi-lobed organ that lies high behind the breastbone. There they multiply and mature into cells capable of producing immune response-that is, they become immunocompetent. In a process referred to as T cell "education," T cells in the thymus learn to distinguish self cells from non-self cells; T cells that would react against self antigens are normally eliminated. 

Upon exiting the bone marrow and thymus, some lymphocytes congregate in immune organs or lymph nodes. Others-both B and T cells-travel widely and continuously throughout the body. They use the blood circulation as well as a body-wide network of lymphatic vessels similar to blood vessels. 

The Lymphoid Organs

Laced along the lymphatic routes-with clusters in the neck, armpits, abdomen, and groin-are small, bean-shaped lymph nodes. Each lymph node contains specialized compartments that house platoons of B-lymphocytes, T lymphocytes, and other cells capable of enmeshing antigen and presenting it to T cells. Thus, the lymph node brings together the several components needed to spark an immune response. 
The spleen, too, provides a meeting ground for immune defenses. Like the lymph nodes, the spleen's lymphoid tissue is subdivided into compartments that specialize in different kinds of immune cells. Microorganisms carried by the blood into the spleen become trapped by the immune cells known as macrophages. (Although people can live without a spleen, persons whose spleens have been damaged by trauma or by disease such as sickle cell anemia are highly susceptible to infection). 

Clusters of lymphoid tissue are found in many parts of the body. They are common around the mucous membranes lining the respiratory and digestive tracts-areas that serve as gateways to the body. They include the tonsils and adenoids, the appendix, and Peyer's patches. 

The lymphatic vessels carry the lymph, which contains lymphocytes, macrophages, and foreign antigens. The vessels transport the mix to lymph nodes, where antigens can be filtered out and presented to immune cells. 
Additional lymphocytes reach the lymph nodes (and other immune tissues) through the bloodstream. An artery and a vein supply each node; lymphocytes enter the node by traversing the walls of the very small, specialized veins.

Lymphatic Vessels and Lymph Nodes

All lymphocytes exit lymph nodes in lymph via outgoing lymphatic vessels. At the base of the neck, large lymphatic vessels merge into the thoracic duct, which empties its contents into the bloodstream. 

Once in the bloodstream, the lymphocytes and other assorted immune cells are transported to tissues throughout the body. They patrol everywhere for foreign antigens, then gradually drift back into the lymphatic vessels, to begin the cycle all over again. 

Antiviral Immunity
Cold virus particles, once they slip into cells of the upper respiratory tract, start copying themselves. To defend itself, the immune system pumps out chemicals called cytokines. Two of these cytokines in particular contribute to the sore throat, sneezing, and runny nose of a cold. The lecture will describe the effector mechanisms, which the immune response uses to combat viral infections, and will then place these mechanisms in the context of acute infection with influenza virus.

Viruses are small, obligate intracellular parasites, which cause infection by invading cells of the body and multiplying within them. Within their life cycle they have a relatively short extracellular period, prior to infecting the cells, and a longer intracellular period during which they undergo replication. 
The immune system has mechanisms which can attack the virus in both these phases of its life cycle, and which involve both non-specific and specific effector mechanisms.

Non-Specific Mechanisms
Interferons:
Viral infection of cells directly stimulates the production of interferons (note that the "type 1" interferons which are produced non-specifically by many cell types in response to viral infection are quite distinct from the T cell cytokine gamma interferon which is produced by CD4+ and CD8+ T cells in response to antigenic stimulation). 

Interferon type 1 function

Type I interferons lead to the induction of an "antiviral state" in the cells, which is characterized by inhibition of both viral replication and cell proliferation, and also enhancement of the ability of natural killer cells to lyse virally infected cells. Indeed, the interferons may be among the most broadly active of all the immunologic and physiologic regulators.

Natural Killer Cells:
NK cells possess the ability to recognize and lyse virally infected cells and certain tumor cells. Whilst not showing antigen specificity, they clearly exhibit some degree of selectivity in targeting "abnormal" cells for lysis. 

• Trinchieri G.. Recognition of major histocompatibility complex class I antigens by natural killer cells. J. Exp. Med 1994. 180: 417-21 . 

• 11. Moretta L, Ciccone E, Mingari MC, Biassoni R, Moretta A.. Human natural killer cells: origin, clonality, specificity, and receptors. Adv. Immunol 1994. 55: 341-80 

The main advantage that NK cells have over antigen-specific lymphocytes in antiviral immunity is that there is no "lag" phase of clonal expansion for NK cells to be active as effectors, as there is with antigen-specific T and B lymphocytes. Thus NK cells may be effective early in the course of viral infection, and may limit the spread of infection during this early stage, while antigen-specific lymphocytes are being recruited and clonally expanded. 

Specific Mechanisms
Both humoral and cell mediated arms of the immune response play a role as specific effector mechanisms in antiviral immunity. 

T Cells and Lymphokines
T- cells contribute to the immune defenses in two major ways. 
 

1.		Regulatory T cells are vital to orchestrating the elaborate system. (B cells, for instance, cannot make antibody against most substances without T cell help). There are two types of regulatory cells:
		• Helper T cells, which are typically identifiable by the T 4 cell marker, and are essential for activating B cells and other T cells as well as natural killer cells and macrophages. • Suppressive T cells which release suppressive cytokines, such as TGF-b, IL-4 and IL-10 to down-regulate or suppress the immune response and keep it from going out of control by turning the helper cells off. (Borden EC. N Engl J Med 1992; 326: 1491-1492.)
2.		Cytotoxic T cells, (or "killer") T cells: They aggressively screen other cells for signs of infection and malignancy and secrete toxic molecules to kill any aberrant cells, thus ridding the body of cells that have been infected by viruses or transformed by cancer. They usually carry the T8 marker.

In normal individuals, T cells with specificity for self-antigens are either eliminated during differentiation or "suppressed" by regulatory mechanisms. In cell-mediated autoimmune disease, killer T cells reactive for a self-tissue antigen become active and destroy healthy tissues and organs. The ultimate result is loss of organ functionality. Examples of cell-mediated autoimmune diseases include: Type I diabetes, where the pancreas is attacked with resultant loss of insulin production; multiple sclerosis, where brain or spinal cord tissue is attacked with a resultant loss of central nervous system (CNS) function; and, rheumatoid arthritis, where the cartilage in the joints is attacked and the resultant inflammation leads to joint destruction.

T cells work primarily by secreting substances known as cytokines that include Lymphokines (which are also secreted by B cells) and their relatives, and the monokines produced by monocytes and macrophages. Both types are diverse and potent chemical messengers.

Cytokines
Cytokine function: A single cytokine may have many functions; conversely, several different cytokines may be able to produce the same effect.
 

• Binding to specific receptors on target cells, Lymphokines call into play many other cells and substances, including the elements of the inflammatory response. 

• They encourage cell growth, promote cell activation, direct cellular traffic, destroy target cells, and incite macrophages. 

Cytokines include the following types:
 

1.		Interferons: It is one of the first cytokines to be discovered. It is produced by T cells and macrophages (as well as by cells outside the immune system), interferons are a family of proteins with antiviral properties. Interferon from immune cells, known as immune interferon or gamma interferon, activates macrophages.
2.		Tumor Necrosis Factor alpha TNFalpha: It is made by many different cells including neutrophils, lymphocytes, Natural Killer (NK) cells, astrocytes, endothelial cells and smooth muscle cells.  TNF alpha function:
		• TNFalpha initiates a cascade of cytokines, which mediate an inflammatory response.  • TNFalpha regulates the expression of many genes important for the host response to infection.
3.		Transforming Growth Factor-b (TGF-beta): It is found at highest concentration in platelets. It is found to stimulate macrophage secretion of various growth factors but in the same time inhibit activated macrophage production of reactive oxygen and reactive nitrogen metabolites.
4.		Macrophage Colony Stimulating Factor (M-CSF): It is produced by many cells including macrophages themselves and is important for the survival, proliferation and differentiation of monocytes, macrophages, osteoclasts and their precursors. M-CSF is an important cytokine for upregulation of Macrophage Scavenger Receptor activity.
5.		Interleukins: They are considered as messengers between leukocytes, or white cells. They were initially given descriptive names but, as their basic structure has been identified they are named as intereukins and they include the following types:
		• IL-1beta: It is a pro-inflammatory cytokine, which is secreted by macrophages activated by a number of stimuli including TNFalpha, bacterial endotoxin and IL-1beta itself. It exerts its effects on many different cell types locally at the site of production and systemically (at a distance) and attract different types of granulocytes and help their degranulation releasing their chemicals which causes the different disease symptoms.  • IL-2, originally known as T cell growth factor, or TCGF, is produced by antigen-activated T cells and promotes the rapid growth or differentiation of mature T cells and B cells.  • IL-3, is a T-cell derived member of the family of protein mediators known as colony-stimulating factors (CSF); one of its many functions is to nurture the development of immature precursor cells into a variety of mature blood cells. • IL-4, IL-5, and IL-6 help B cells grow and differentiate; IL-4 also affects T cells, macrophages, mast cells and granulocytes. IL-6 stimulates B-lymphocytes to produce antibodies and in concert with IL-1 causes T-cell activation. • IL-10: It is an immunoregulatory cytokine, which can exert a wide range of different effects on different cell types. It suppresses IL-2 and interferon gamma production by helper T-cells. It is also a potent modulator of monocyte/macrophage function. • IL-12: It stimulates growth of activated Natural killer cells, CD8+ and CD4+ T- cells. Activate the T helper cell response and increase the production of tumor necrosis factor (TNF) by macrophage cells. It also suppresses IL-4 induced IgE production. • IL-13: Interleukin 13 has very similar biological effects on macrophages to IL-4.

Humoral Immunity
(B-Cells "the antibody producing cells)

Antibody: 
Specific antibodies are important in and may protect against viral infections. The most effective type of antiviral antibody is "neutralizing" antibody - Each B cell is programmed to make one specific antibody. For example, one B cell will make an antibody that blocks a virus that causes the common cold, while another produces antibody that zeros in on a bacterium that causes pneumonia. 

When a B cell encounters its triggering antigen(along with collaborating T cells and accessory cells), it gives rise to many large cells called plasma cells. Every plasma cell is essentially a factory for producing antibody. They manufacture millions of identical antibody molecules and pours them into the bloodstream. 
A given antibody matches an antigen much as a key matches a lock. To some degree, however, the antibody interlocks with the antigen and thereby marks it for destruction. 

Antibodies belong to a family of large molecules known as immunoglobulins. Immunoglobulins are shaped to form a Y. 
Scientists have identified nine chemically distinct classes of human immunoglobulins (Ig)-four kinds of IgG and two kinds of IgA, plus IgM, IgE, and IgD. Each type plays a different role in the immune defense strategy. 
IgG:
It the major immunoglobulin in the blood, is also able to enter tissue spaces; it works efficiently to coat microorganisms, speeding their uptake by other cells in the immune system.
IgM:
It usually combines in star-shaped clusters, tends to remain in the bloodstream, where it is very effective in killing bacteria. 
IgA: 
It concentrates in body fluids-tears, saliva, and the secretions of the respiratory and gastrointestinal tracts-guarding the entrances to the body. 
IgE:
Under normal circumstances, it occurs only in trace amounts, probably evolved as a defense against parasites, but it is more familiar as the villain in allergic reactions (Allergy).
IgD: 
It is almost exclusively found inserted into the membranes of B cells, where it somehow regulates the cell's activation. 

Antibody which binds to the virus, usually to the viral envelope or capsid proteins, and which blocks the virus from binding and gaining entry to the host cell. The immune system responds to viral proteins encountered within most tissues of the body by generating helper T cells, which release inflammatory cytokines such as IL-2 and IFN-g. The response to viral proteins in mucosal tissue, in contrast, stimulates the induction of TGF-ß secreting cells and regulatory T cells which secrete IL-4 and IL-10. This cascade of events results in a suppressive regulatory response as well as induction of B cells that secrete IgA.

Virus specific antibodies may also act as opsonins in enhancing phagocytosis of virus particles - this effect may be further enhanced by complement activation by antibody-coated virus particles. In addition, in the case of some viral infections, viral proteins are expressed on the surface of the infected cell. These may act as targets for virus-specific antibodies, and may lead to complement-mediated lysis of the infected cell, or may direct a subset of natural killer cells to lyse the infected cell through a process known as antibody-directed cellular cytotoxicity (ADCC). At mucosal surfaces (such as the respiratory and gastrointestinal tracts), virus infection may induce the production of specific antibodies of the IgA isotype, which may be protective against infection at these surfaces. (This is the basis of immunisation with the current oral polio vaccine). 

During the course of a viral infection, antibody is most effective at an early stage, before the virus has gained entry to its target cell. In this respect, antibody is relatively ineffective in primary viral infections, due mainly to the lag phase in antibody production. 

Disorders of the Immune System Allergy
The most common types of allergic reactions-hay fever, some kinds of asthma, and hives-are produced when the immune system response to a false alarm. In a susceptible person, a normally harmless substance-grass pollen or house dust, for example-is perceived as a threat and is attacked. 

Such allergic reactions are related to the antibody known as immunoglobulin E. Like other antibodies, each IgE antibody is specific; one reacts against oak pollen, another against ragweed. The role of IgE in the natural order is not known, although some scientists suspect that it developed as a defense against infection by parasitic worms. 

The first time an allergy-prone person is exposed to an allergen, he or she makes large amounts of the corresponding IgE antibody. These IgE molecules attach to the surfaces of mast cells (in tissue) or basophils (in the circulation). Mast cells are plentiful in the lungs, skin, tongue, and linings of the nose and When an IgE antibody siting on a mast cell or basophil encounters its specific allergen, the IgE antibody signals the mast cell or basophil to release the powerful chemicals stored within its granules. These chemicals include histamine, heparin, and substances that activate blood platelets and attract secondary cells such as eosinophils and neutrophils. The activated mast cell or basophil also synthesizes new mediators, including prostaglandins and leukotrienes, on the spot. 

It is such chemical mediators that cause the symptoms of allergy, including wheezing, sneezing, runny eyes and itching. They can also produce anaphylactic shock, a life-threatening allergic reaction characterized by swelling of body tissues, including the throat, and a sudden fall in blood pressure. 

Phagocytes, Granulocytes, and Their Relatives
Phagocytes (literally, "cell eaters") are large white cells that can engulf and digest marauding microorganisms and other antigenic particles. Some phagocytes also have the ability to present antigen to lymphocytes. 

Important phagocytes are monocytes and macrophages. Monocytes circulate in the blood, and then migrate into tissues where they develop into macrophages ("big eaters"). Macrophages are seeded throughout body tissues in a variety of guises. Specialized macrophages include alveolar macrophages in the lungs, mesangial phagocytes in the kidneys, microglial cells in the brain, and Kupffer cells in the liver.

Peripheral blood film showing one monocytes in the center 


Different types of white cells

Macrophages:
They are versatile cells that play many roles. 
As scavengers, they rid the body of worn-out cells and other debris. Foremost among the cells that "present" antigen to T cells, having first digested and processed it, macrophages play a crucial role in initiating the immune response. 

As secretory cells, monocytes and macrophages are vital to the regulation of immune responses and the development of inflammation; they churn out an amazing array of powerful chemical substances (monokines) including enzymes, complement proteins, and regulatory factors such as interleukin-1. At the same time, they carry receptors for Lymphokines that allow them to be "activated" into single-minded pursuit of microbes and tumor cells. 

Macrophages are not the only cells to present antigen to lymphocytes. Other antigen-presenting cells include B cells, as noted above, and dendritic cells, irregularly shaped white blood cells found in the spleen and other lymphoid organs. Dendritic cells typically have long threadlike tentacles that enmesh lymphocytes and antigens. Langerhans cells are dendritic cells that travel about in the skin, picking up antigen and transporting it to nearby lymph nodes. Many other types of body cells, properly stimulated, can also be recruited to present antigens to lymphocytes. 

Neutrophils:
Another critical phagocyte is the neutrophil. Neutrophils are not only phagocytes but also granulocytes: they contain granules filled with potent chemicals. These chemicals, in addition to destroying microorganisms, play a key role in acute inflammatory reactions. 

Also known as polymorphonuclear leukocytes or polymorphs (because their nuclei come in "many shapes"), granulocytes include eosinophils and basophils as well as neutrophils. (The cells are named for the way they stain in the laboratory: eosinophils, for instance, have an affinity for acidic dyes such as eosin.) The phagocytic neutrophil uses its prepackaged chemicals to degrade the microbes it ingests; eosinophils and basophils typically "degranulate," releasing their chemicals to work on cells or microbes in their surroundings.

Mast Cells:
The mast cell is a non-circulating counterpart of the basophil. Located in the lungs, skin, tongue, and linings of the nose and intestinal tract, the mast cell is responsible for the symptoms of allergy. 

Platelets:
Another related structure is the blood platelet. Platelets, too, contain granules. In addition to promoting blood clotting and wound repair, platelets release substances that activate components of the immune system. 

Complement
The complement system is made up of a series of about 25 proteins that work to "complement" the activity of antibodies in destroying bacteria, either by facilitating phagocytosis or by puncturing the bacterial cell membrane. Complement also helps to rid the body of antigen-antibody complexes. In carrying out these tasks, it induces an inflammatory response. 

Complement proteins circulate in the blood in an inactive form. When the first of the complement substances is triggered-usually by antibody interlocked with an antigen-it sets in motion ripple effect. As each component is activated in turn, it acts upon the next in a precise sequence of carefully regulated steps known as the "complement cascade." 

In the so-called "classical" pathway of complement activation, a series of proteins gives rise to a complex enzyme capable of cleaving a key protein, C3. In the "alternative" pathway-which can be triggered by suitable targets in the absence of antibody-C3 interacts with a different set of factors and enzymes. But both pathways end in creation of a unit known as the membrane attack complex. Inserted in the wall of the target cell, the membrane attack complex constitutes a channel that allows fluids and molecules to flow in and out. The target cell rapidly swells and bursts.

Complement cascade

One byproduct causes mast cells and basophils to release their contents, producing the redness, warmth, and swelling of the inflammatory response. Another stimulates and attracts neutrophils. Yet another, C3b, opsonizes or coats target cells so as to make them more palatable to phagocytes, which carry a special receptor for C3b. 
The C3b fragment also appears to play a major role in the body's control of immune complexes. By opsonizing antigen-antibody complexes, C3b helps prevent the formation of large and insoluble (and thus potentially damaging) immune aggregates. Moreover, receptors for C3b are also present on red blood cells, which appear to use the receptors to pick up complement-coated immune complexes and deliver them to the Kupffer cells in the liver. 

Cytokines are small proteins which allow cells of the immune system to communicate with one another via cytokine receptors expressed at the cell surface.
Macrophages secrete the following cytokines under different conditions:

Viral Immune Evasion
Why we ever get infected even in the absence of immune suppression?

EVASION OF THE NON-SPECIFIC IMMUNE RESPONSE

The non-specific immune response is so termed because, unlike 'specific' immune mediators such as B and T lymphocytes, it doesn't require prior exposure and amplification to be effective. Some of the mediators of the non-specific response are macrophages and monocytes, NK cells, and inflammatory mediators such as cytokines. Cytokines, mostly produced by T helper cells, are most critical in the acute phase of the immune response; interleukin-1 (IL-1), interleukin-2 (IL-2), interferon-g (IFN-g), and tumour necrosis factor (TNF) induce inflammation, recruit and stimulate other immune components, and generally induce an inhospitable environment for parasites (2). 2. Bendtzen K. 1994. Cytokines and natural regulators of cytokines. Immunol. Lett. 43: 111-23 

Perhaps the most spectacular asset of some viruses is their ability to block cytokines (3). 3. McFadden G, Graham K, Ellison K, Barry M, Macen J, Schreiber M, Mossman K, Nash P, Lalani A, Everett H. 1995. Interruption of cytokine networks by poxviruses: lessons from myxoma virus. J. Leukoc. Biol. 57: 731-8 

Since NK cells are probably important in the early phase of clearing infections (12).
12. Brutkiewicz RR, Welsh RM. 1995. Major histocompatibility complex class I antigens and the control of viral infections by natural killer cells. J. Virol. 69: 3967-71 
one would expect that some viruses target these cells. Some NK effects are also mediated by cytokines, and in particular IFN-g (13). 13. Biron CA. 1994. Cytokines in the generation of immune responses to, and resolution of, virus infection. Cur. Opin. Immunol. 6: 530-8 

EVASION OF THE SPECIFIC IMMUNE RESPONSE

Antibodies and complement affect viruses at two points in their replication cycles: during their extracellular phase antibodies can bind and neutralize the virus directly, and during the viral intracellular phase antibody and complement can interact with exposed (membrane-associated) viral proteins, leading to antibody-dependent cell-mediated cytotoxicity (ADCC) or complement-mediated cytolysis.
Every virus capable of infecting vertebrates has some means of dealing with the immune response. These methods range from the very rapid replication that may allow some viruses to complete a replication cycle before the specific immune response has a chance to develop - to the profound, such as the near-total ablation of the immune system in late-stage HIV infection. In several instances, viruses block the responses by interfering with certain components of the immune system. 

It is conceivable that antiviral therapy could be directed against these mechanisms. Conversely, it is also possible that the viral products could give rise to therapeutic products; Also viral anti-inflammatory agents, perhaps, could precisely target specific cytokines.