Immunology and Virology - seconda parte

Characteristics of innate and acquired immunity

The main function of the immune system is self/non-self discrimination: our immune system is able to recognize what it’s self (our cells and our body) and what it’s non-self (everything is outside our body).

Immunology science studies

• Our protection from foreign macromolecules or invading organisms (viruses, bacteria, fungus, protozoa, parasites, …).
• Our responses to them.
• Our responses against our own aberrant cells (tumor cells), some of which are activated by our own immune system.
• Our wrong responses to our own proteins (and other molecules) in autoimmunity: sometimes our immune system can be wrong, and attacks our own cells/proteins.

We can consider our immune system as a “fighter” against:

• Infectious disease.
• Foreign substances (non-self macromolecules).
• Cancer cells (tumor cells).
• Self-molecules (autoimmune response).

A bit of history…

Immunology started with Edward Jenner, who discovered the first vaccine against smallpox (Vaiolo).

1. 1798: Vaccination treatise.
2. 1980: WHO announced the eradication of smallpox.
3. 1960: Immunology passes from an observational science to experimental science, so that we had technological advances (cell cultures in laboratory, monoclonal antibodies — production of antibodies, which recognize single proteins, to understand how they work and to fight them — immunochemistry, recombinant DNA, transgenic mice that had been modified with gene of other species, knockout mice where one gene is eliminated).

Major subdivisions of the immune system

The immune system is composed of two major subdivisions:

• Innate or non-specific immune system = it’s the first line of defence (500 million years ago), especially present on mammals and insects:
  • Cellular components.
  • Humoral components.

The innate immune system includes defences that are constitutively present and ready to be involved upon infection: it is the first system to act starting from the first contact with the pathogenic agent, at least in 12 hours; the most important cells are phagocytes (able to fight against pathogens).

• Adaptive or specific immune system = it’s the second line of defence (360 million years ago):
  • Cellular components.
  • Humoral components.

The adaptive immune system requires some time to react to an invading organism; the most important cells are lymphocytes; the time of response after the infection is from 1 up to 5 days.

All cells of the immune system have their origin in the bone marrow and they include:

• Stem myeloid cells (neutrophils, basophils, eosinophils, macrophages and dendritic cells).
• Stem lymphoid cells (B lymphocyte, T lymphocyte and Natural Killer).

Since pathogens show an intracellular replication (viruses and some bacteria and parasites) or extracellular replication (most bacteria, fungi and parasites), different components of the immune system have evolved to protect against these different types of pathogens. The immune system developed in parallel with pathogens, and so it’s able to defend itself in specific ways, acting different mechanisms.

Innate immunity

Components

1. Anatomical barriers:
   • Skin and mucous membranes: mechanical, chemical and biological.
2. Humoral components:
   • Complement, coagulation system, cytokines.
3. Cellular components:
   • Neutrophils, monocytes, macrophages, dendritic cells, NK cells, eosinophils.

Features

• Speed.
• Recognition of common molecules of microbial groups and molecules produced by damaged guest cells.

Anatomical barriers

Mechanical anatomical barriers

The epithelial surfaces form a physical barrier that is very impermeable to most infectious agents:

• The skin acts as our first line of defence against invading organisms.
• Movement due to cilia or peristalsis helps to keep air passages and the gastrointestinal tract free from microorganisms.
• The flushing action of tears and saliva helps prevent infection of the eyes and mouth.
• The mucus of the respiratory and gastrointestinal tract helps protect the lungs and digestive systems from infection.

Chemical anatomical barriers

• Fatty acids in sweat (produces a low pH and antimicrobial peptides) inhibit the growth of bacteria.
• Lysozyme and phospholipase in tears, saliva and nasal secretions can break down the cell wall of bacteria and destabilize bacterial membranes.
• Defensins (low molecular weight proteins) found in the lung and gastrointestinal tract have antimicrobial activity.
• Surfactants in the lung act as opsonins (substances that promote phagocytosis of particles by phagocytic cells).

Biological anatomical barriers

The microbiota of the skin and in the gastrointestinal tract can prevent the colonization of pathogenic bacteria by secreting toxic substances, or by competing with pathogenic bacteria for nutrients or attachment to cell surfaces.

Humoral components

The humoral site is constituted by groups of proteins who constitute real complements.

Cellular components

There is a connection between the innate immunity and the acquired immunity: they work as an integrated system.

Acquired immunity

The humoral components are:

• Antibodies.
• Cytokines.

The cellular components are:

• Lymphocytes.

While in the innate immunity the response is always the same, in presence of different bacteria, here there are specific responses against each different pathogens.

Features

• Specificity and diversification in responses.
• Memory (the most effective response).

Properties

• Specificity: ensures that distinct antigens elicit specific responses.
• Diversity: enables immune system to respond to a large variety of antigens; acquired immunity responds to specific pathogens; lymphocytes are different, and we have specific receptors on them: they can recognize lot of different molecules!
• Memory: leads to enhanced responses to repeated exposures to the same antigens (the second time they meet the antigen, they are able to remember the first one and so they can respond quicker than the first time). We have memory cells that can survive long time in our body, mainly important to fight against pathogens without provoking the real disease another time.
• Clonal expansion: increases number of antigen-specific lymphocytes to keep pace with microbes (when a lymphocyte recognizes an antigen, it reproduces itself in order to fight it with a large number of lymphocytes).
• Specialization: generates responses that are optimal for defence against different types of microbes.
• Contraction and homeostasis: allows immune system to respond to newly encountered antigens.
• Nonreactivity to self: prevents injury to the host during responses to foreign antigens; some cells who are called “suppressive cells” are able to inactivate wrong lymphocytes.

Tolerance → autoimmune disorders

• Removal of lymphocytes that react with self antigens.
• Inactivation of self-reactive cells.
• Activation of suppressive cells.

Against extracellular pathogens, cellular immunity is different depending on the fight with:

• Viruses: are used cytotoxic lymphocytes, which act on infected cells.
• Bacteria: are used Helper lymphocytes, which help macrophages in killing bacteria.

After lymphocytes have acted and the pathogen has been eliminated, lot of cells die and survive only the memory cells.

Cellular components

• B lymphocytes are able to recognize specific microbes and produce specific antibodies, who act throughout neutralization, phagocytosis and complement activation.
• Helper T lymphocytes are able to help other cells with the activation of macrophages, and the proliferation and differentiation of T and B Lymphocytes. → inflammation.
• Cytotoxic T lymphocytes (CTL) are able to kill infected cells (where we can find the virus) in order to kill the virus.
• Regulatory T lymphocytes: they are suppressor cells, that can switch off the immune response.
• Natural Killer Cells (NK) are very important in killing infected cells but also tumor ones.

Accessory cell

Definition. Antigen-presenting cells (APCs) are a heterogeneous group of immune cells that mediate the cellular immune response by processing and presenting antigens for recognition by certain lymphocytes such as T cells.

• Dendritic cells: activation of T lymphocytes naïve, clonal expansion and differentiation of T lymphocytes effectors.
• Macrophages: activation of T lymphocytes effectors (which are already been activated) and macrophages (cell-mediate immunity).
• B lymphocytes: activation of T lymphocytes effectors; activation of B lymphocytes and production of antibodies (humoral immunity).

Cytokines

Are proteins secreted from different cell types (innate and acquired immune cells) that can mediate and regulate all aspects of innate and adaptive responses.

• Some cytokines can work on multiple cell types → pleiotropism.
• Some cytokines have the same function and effect → redundancy.

Antimicrobial immunity

Is the production of cytokines that confer resistance to the host cells (Interferons), but they are also important to activate KK cell and Cytotoxic cells (CTL).

Most of them start with inflammation:

• Recruitment of lymphocytes.
• Recruitment of plasmatic proteins (complement).
• Production of cytokines.

Active and passive immunity

• Active immunity = This refers to immunity produced by the body following exposure to antigens.
  • Naturally acquired active immunity → Exposure to various pathogens leads to sub-clinical or clinical infections which result in a protective immune response against these pathogens.
  • Artificially acquired active immunity → Immunization may be achieved by administering live or dead pathogens or their components. Vaccines used for active immunization consist of live (attenuated) organisms, killed whole organisms, microbial components or secreted toxins (which have been detoxified).
• Passive immunity =
  • Naturally acquired passive immunity → Immunity is transferred from mother to fetus through placental transfer of IgG or colostral transfer of IgA.
  • Artificially acquired passive immunity → Immunity is often artificially transferred by injection with gamma-globulins from other individuals or gamma-globulin from an immune animal.

Examination questions

• Characteristics of innate immunity:
  1. Self-not self antigen.
  2. Response times.
  3. Mechanical, chemical, biological, anatomical barriers (defensines, cathelicidin).
  4. Humoral components (complement, pentrassin, ficolin, collectin).
  5. Cellular components.
• Characteristics of acquired immunity:
  1. Self-not self antigens.
  2. Response times.
  3. Humor and cellular components.

Innate immunity

Cells of the innate immune system

Most of those cells are phagocytes:

• Neutrophils.
• Macrophages.
• Monocytes.
• Dendritic cells.

which are able to phagocytose parasites, bacteria, viruses, ...

There are also not specific killer cells:

• NK cells (Natural Killer Cells), are very good in fighting tumor cells.
• LAK cells.
• K cells.
• Eosinophils.

When we perform the blood analysis, we can observe the presence/absence of an infection and the type of it: for example in the viral infection grows the number of lymphocytes, while during a bacterial infection, we can observe the increasing of neutrophils number.

All those cells derive from a Multipotent Progenitor Hematopoietic Stem Cell, which is present in the bone marrow, and which can differentiate during hematopoiesis in:

• Common myeloid progenitor → Myeloid line originates neutrophils, basophils, eosinophils and monocytes.
• Common lymphoid progenitor → Lymphoid line originates erythrocytes, platelets, B and T lymphocytes.

Neutrophils

Structure: 12-15 mm in diameter.

They represent the 70% of leukocytes.

They are also called “Polymorphonuclear cells” (PMN), as they present a characteristic multi-lobed nucleus (particular shape, composed by more than one lobe); they are recruited to the site of infection to phagocytose microorganisms and kill them intracellularly.

PMN contribute to tissue damage that occurs during inflammation: when they work in the site of infection, they are able to destroy and damage a part of the tissue itself.

They are also recognized for the presence of specific granules inside the cytoplasm, which can be:

1. Primary granules = azurophilic granules, abundant in young PMN, contain cationic proteins and defensins that can kill bacteria, proteolytic enzymes like elastase, and cathepsin G to break down proteins, lysozyme to break down bacterial cell walls, and characteristically, myeloperoxidase, which is involved in the generation of bacteriocidal compounds.
2. Secondary granules: granule found in mature PMNs; these contain lysozyme, NADPH oxidase components, which are involved in the generation of toxic oxygen radicals, and characteristically lactoferrin, an iron chelating protein and B12-binding protein.

PMN derive from bone marrow (1011 neutrophils each day), and they have a half-life of 6-8 hours, after which they die for apoptosis, and are eliminated by macrophages residing in the liver and spleen.

Neutrophils are the first cells called in the site of infection, as they are quick in eliminating the pathogenic agent, but contemporary they damage the tissue on which they work!

Monocyte-macrophages

Structure: 10-15 mm in diameter.

They are characterized by a bi-lobate nucleus (with two lobes reniform), and CD14 membrane marker.

Monocyte-macrophages function in phagocytosis, intracellular (phagocyted pathogens) and extracellular (non-phagocyted pathogens) killing of microorganisms.

There are two kinds of phenotypes of macrophages, with different functions:

• M1: macrophages which act as antigen-presenting cells to stimulate T cells (they present on their surface just a little piece of a broken bacteria, so that T-Lymphocytes can recognize them).
• M2: macrophages which can contribute to tissue repair (the tissue firstly damaged by neutrophils).

They derive from Myeloid Line of the blood, where monocytes are formed: they are summoned in tissues, where they become macrophages and then go through:

• Differentiation in:
  • Microglia (CNS).
  • Kupffer cells (liver).
  • Alveolar macrophages (lung).
  • Osteoclasts (bone).
• Activation in macrophages (in tissues in general) by a pathogen.

The plasmatic membrane of those cells is very big and extended, in order to link and phagocyte a large number of bacteria (many bacteria in only one macrophage!). → Alveolar macrophages.

Not specific killer cells

• NK cells.
• LAK cells.
• K cells.
• Eosinophils.

Natural Killer (NK) and Lymphokine Activated Killer (LAK) cells can non-specifically (without specificity) kill virus infected and tumor cells (however, they work very well); these cells are also important in tumor surveillance (control of tumor cells).

Basophils

They are non-specific cells which represent less than 1% of blood leukocytes and are not normally present in tissues, but only in presence of an infection.

Structurally and functionally are similar to mast cells.

Eosinophils

They contain enzymes (basic proteins) that damage the parasitic cell wall; they are present in the mucous membrane, and are especially involved in parasite defence, throughout the production of acid proteins which destroy the parasite barrier.

Dendritic cells APC (Antigen Presenting Cells)

Structure: have characteristic dendrites.

These cells are present in two different stages/phenotypes in our body, on the base of the receptor expressed on their:

1. Immature in the tissue level (capture of antigen).
2. Mature in the lymph nodes (antigen presentation to T lymphocytes).

They are also called sentinel cells, as they present antigen to naïve T lymphocytes: after the antigen capture, they are activated and matured, and then they are able to leave the tissue and reach the near lymph nodes (by mean of lymphatic vessels), and during this migration they change phenotype. When they are in the lymph node, they present the antigen to the T cells of the acquired specific immune response (to start the second part of the immunity).

Dendritic cells

Dendritic cells can derive from:

• Fetal hematopoietic organs → Langerhans cell of the skin.
• Bone Marrow → Common dendritic cell precursor of:
  • Classical dendritic cell. Conventional dendritic cell (previously called Myeloid dendritic cell) (cDC or mDC) secrete Interleukin 12 (IL-12), Interleukin 6 (IL-6), TNF, chemokines, all involved in the inflammation.
  • Plasmacytoid dendritic cell. Are able to produce large amounts of Type I Interferons (IFN) upon viral infections, thus constituting important mediators in antiviral immunity (they can induce an anti-viral state which prevent virus to proliferate once inside the cell).
  • Inflammatory dendritic cell.

Mast cells

Are important in the skin, especially in the mucous epithelium near blood vessels.

They present granules containing cytokines, histamine and other mediators (for the inflammation).

They express the receptor for IgE and IgG, as they are involved in the allergies (in fact, they produce histamine, which is on the base of allergic symptoms).

Innate immunity functions

The innate immunity system has three different functions:

1. First response against pathogens (restricts growth and/or eliminates them).
2. It recognizes damages or dead self cells by eliminating them, starting the repair process (in which two macrophages play an important role).
3. It stimulates the specific immune response.

Cells of innate immunity recognize microbes and self damaged cells, in particular:

• Pathogen-Associated Molecular Patterns (PAMPs).
• Microbial-Associated Molecular Patterns (MAMPs).
• Damaged-Associated Molecular Patterns (DAMPs).

PAMPs

• Viruses: Glycoproteins are on the surface of the cell, while re