Evasion of Immunity IEvasion of Immunity IVertebrate and invertebrate immune systems.

Dr. Jo HamiltonParasitology BS

Introduction. Introduction. Successful parasites have evolved strategies for

survival & development in both invertebrate and vertebrate hosts.

The goal of a parasite is to propagate within the host and be transmitted to the next host.

The goal of the parasitised host is to cure or limit the infection.

During the next three lectures we will investigate strategies used by parasites to evade the host immune response.

In this session we will revisit the immune system of both vertebrates and invertebrates.

Objectives and learning outcomes. Objectives and learning outcomes.

By the end of this session students should be: Familiar with fundamental biology of vertebrate

& invertebrate immune systems. Familiar with the concept of innate and acquired

immunity in vertebrates. Recognise that there is only innate immunity in

invertebrates. Recognise the key players in both vertebrate and

invertebrate immune systems.

Immunity.Immunity. Resistance to infection is called immunity.

The term “immunity” is derived from the Greek word “immunis” meaning exempt.

There are two types of immunity in vertebrates.– Innate immunity – present from birth.– Acquired immunity – result of infection or

vaccination.

Invertebrates only posses innate immunity.

Innate immunity in vertebrates Innate immunity in vertebrates (also (also known as non-specific or natural immunity).known as non-specific or natural immunity).

Characteristics:Present from birth. Non-specific - acts on many organisms

and does not show specificity. Does not become more efficient on

subsequent exposure to same organisms.

Innate immunity in vertebrates.Innate immunity in vertebrates.

Non-specific Host Defences include:

Mechanical / physical barriers – skin, mucosal surfaces.

Prevention of stasis – peristalsis, flow of urine, upward movement of secretions in bronchial tree, coughing, vomiting.

Innate immunity in vertebrates.Innate immunity in vertebrates.

Chemical defences - Low pH of stomach contents, secretion of fatty acids in the skin.

Biological defence – complement, lysozyme, interferons, antimicrobial peptides, kinins, adhesion molecules, hormones, lactoferrin.

Cellular defence - e.g. phagocytes.

Innate immunity in vertebrates.Innate immunity in vertebrates.

Some of the key players in innate immunity to consider in more detail are:

Complement. Opsonization. Phagocytosis & the oxidative burst. Inflammation.

Innate immunity in vertebrates – Innate immunity in vertebrates – complement.complement.

Complement. - complex of 17 proteins present in normal serum.

2 pathways – classical & alternative.

Innate immunity in vertebrates – Innate immunity in vertebrates – classical complement pathway.classical complement pathway. Antigen-antibody complex forms, constant

region of antibody changes shape. Activates C1, acquires esterase activity. Activated C1 activates C2 & C4 which

activates C3, etc. Eventually, C8 & C9 activated forming

membrane attack complex (MAC) - pores in target cell membrane - lysis.

Innate immunity in vertebrates - Innate immunity in vertebrates - alternative complement pathway.alternative complement pathway.

Alternative pathway - C3 can interact directly with certain chemicals (teichoic acids, LPS) found in bacterial cell walls and activate the alternative pathway.

Innate immunity in vertebrates – Innate immunity in vertebrates – opsonization.opsonization. Opsonization - process of coating micro-

organisms with plasma proteins to make them more easily phagocytosable.

It is stimulated by complement bound to antibody-antigen targets.

Opsonization promotes adhesion between opsonized cell & macrophages. The opsonin binds to receptors on phagocyte membrane.

Opsonization and phagocytosis are more efficient in immune individuals.

Innate immunity in vertebrates– Innate immunity in vertebrates– cellular defence.cellular defence.Cellular defence involves:

Granulocytes (also known as the polymorphonuclear leukocytes e.g. eosinophils, basophils etc).

The reticulo endothelial system (e.g. macrophages, Kupffer cells of the liver and natural killer (NK cells).

Innate immunity in vertebrates – Innate immunity in vertebrates – White blood cells.White blood cells.Role of white blood cells in cellular defence. White blood cells (WBCs) are major

components of immune system.

Blood smear showing different blood cell types.

Innate immunity in vertebrates – Innate immunity in vertebrates – phagocytosis.phagocytosis. Certain WBCs highly mobile & carry out

phagocytosis.

WBCs chemotactically attracted to foci of disease or tissue damage.

Phagocytosis begins with engulfment of particulate matter (e.g. bacteria, clumps of virions, cell debris, etc.) into a phagosome.

Innate immunity in vertebrates – Innate immunity in vertebrates – phagocytosis cont’d.phagocytosis cont’d.

The phagosome fuses with lysosomes to form the phagolysosome.

Lysosomes contain number of enzymes including acid hydrolases, lysozyme, neutral proteases, myeloperoxidase, lactoferrin, & phospholipase A.

These enzymes can degrade biomolecules.

Innate immunity in vertebrates – Innate immunity in vertebrates – oxidative burst.oxidative burst.

Once engulfed, the white cell must kill the organisms by some means such as the “respiratory (or oxidative) burst".

Many pathogens and parasites succeed because they are able to avoid phagocytosis.

Innate immunity in vertebrates – Innate immunity in vertebrates – inflammation .inflammation .

Inflammation - (or inflammatory response) mechanism by which phagocytes and complement are recruited to site of tissue invasion.

Non-specific reaction to tissue damage. Cell damage initiates a complex series of steps leading to inflammation.

Innate immunity in vertebrates – Innate immunity in vertebrates – inflammation.inflammation. Inflammation involves:

• Vasodilation - swelling.• adhesion of leukocytes to endothelial cells of post-capillary venule, & emigration of phagocytes into tissues. • redness (blood flow). • pain (prostaglandins bind to nerve receptors).• heat (pyrogens).• Inflammation localised to area of infection / injury by release of substances from micro-organisms or chemical mediators released from cells in tissues, e.g. histamine from mast cells. • Once organisms are destroyed inflammation

settles down (resolves).

Acquired immunity (only in Acquired immunity (only in vertebrates).vertebrates).Also known as adaptive immunity / specific immunity.

Develops as response to an infection.

Called adaptive as immune system adapts itself to previously unseen molecules.

The induction of immunity by infection, or with a vaccine, is called active immunity.

Acquired immunity.Acquired immunity.

Induction of immunity by infection, or with vaccine, called active immunity.

Non-immune individual can be made immune by transferring serum or lymphocytes from immune individual. This is know as passive immunity and demonstrates that serum constituents (antibodies) and lymphocytes are involved in immunity.

Acquired immunity.Acquired immunity.

Characteristics of acquired immunity:

Immunological recognition.

Discrimination between self and non-self.

Immunological specificity.

Immunological memory.

Acquired immunity.Acquired immunity.

Immunity mediated by immune system, responds to infection by mounting immune response. An immune response must:

Recognise a micro-organism or parasite as foreign (non-self) as distinct from self.

Respond to the presence of a foreign organism by production of specific antibodies and specific lymphocytes.

Mediate the elimination of such organisms.

Acquired immunity.Acquired immunity.

There are two types of acquired immunity.

Cell-mediated immunity - this is immunity mediated by T-cells. T cells secrete lymphokines (e.g. interleukin-2) which interact with other cell types, and either activate or repress an immune response.

Humoral immunity - this is blood-specific immunity mediated by antibodies (Abs).

Acquired immunity – cell mediated Acquired immunity – cell mediated immunity.immunity.

Acquired immunity – cell mediated Acquired immunity – cell mediated immunity.immunity. Key cells involved in acquired immunity

response are lymphocytes. Two types lymphocyte develop in bone marrow

from common precursor. Each different response mediated by different

sets of lymphocytes. Following invasion by a foreign organism,

lymphocytes proliferate (i.e. divide) and differentiate (i.e. specialize).

Acquired immunity – cell mediated Acquired immunity – cell mediated immunity, B lymphocytes.immunity, B lymphocytes.

B lymphocytes (B cells): Found fixed in the lymph nodes, liver and

spleen. They are bone marrow-derived lymphocytes,

mature in Peyers’ Patches of the pancreas. During maturation, antigen-specified antibody is

displayed on the cell surface. If the cell is activated by an antigen, the B cells

excrete antibody.

Acquired immunity – cell mediated Acquired immunity – cell mediated immunity, T lymphocytes.immunity, T lymphocytes.

T lymphocytes (T-cells): Found in lymph nodes, liver, spleen, also freely

circulating in the blood. Matures in thymus. They have cell surface receptor

of a pre-determined specificity. These cells regulate cellular immunity. Two main T cell types: helper T cells (Th cells have

the CD4+ receptor) & suppressor / cytotoxic T cells (Tc cells display the CD8+ receptor).

Acquired immunity – cell mediated Acquired immunity – cell mediated immunity, macrophages.immunity, macrophages.

A third important cell type are macrophages.

These cells play essential role in processing & presenting immunogens to lymphocytes.

Also important effector cells (i.e. they carry out destruction of foreign material e.g. phagocytosis).

Carry receptors for antibody molecules which allows them to attach to antibody-antigen complexes before phagocytosing them.

Acquired immunity – generation of Acquired immunity – generation of immune response. immune response.

In order for an immune response to be activated, an object must first be recognised as foreign.

An immunogen is any molecule that stimulates an immune response. In general, proteins are the best immunogens, followed by carbohydrates and then nucleic acids. Lipids are very poor.

An antigen is any molecule that is capable of generating an antibody response (antigen = antibody generating).

Acquired immunity – generation of Acquired immunity – generation of immune response. immune response. Upon an initial infection, it takes about 4-7 days to

generate an immune response. After seven days get primary immune response.

Initially, IgM produced but B cells differentiate further into IgG producing cells. After about three weeks primary immune response turned off.

During this initial period Ab producing cells and memory B cells are formed.

When same agent encountered by host again, body recognises it, stimulates the memory cells to secrete Abs. This is called the secondary immune response.

Memory can last for few weeks or can last for years.

Acquired immunity – generation of Acquired immunity – generation of immune response. immune response. There are three types of effector immune response. Humoral (blood) - antibody response mediated by B

cells & regulated by T cells.

Cell-mediated (cellular) - delayed-type hypersensitivity and cytotoxicity mediated by CD4+ and CD8+ T cells.

Tolerance - non-specific response mediated by T cells. Healthy individuals tolerant to own tissues, sometimes immune response fails to recognise self giving rise to autoimmune diseases or transplant rejection in transplantation surgery.

Humoral immunity – antibodies. Humoral immunity – antibodies. Large glycoproteins released by B cells. Antibodies (Abs)

specifically interact with antigens. Body can produce millions of antibody specificities genetically as the B cells mature. There are five classes of Ab:

IgM – largest & first Ab to be made antibody response. IgM can mediate neutralisation, fix complement, agglutinate and immobilise antigens.

IgG - this is the main serum Ab. This is synthesized during the secondary immune response. Able to do all Ab mediated functions.

Humoral immunity – antibodies Humoral immunity – antibodies cont’d. cont’d. IgA - is mucosal antibody. Sometimes called secretory Ab

as mucosal cells secrete them when mucosal pathogens begin to establish colonies.

IgD - is receptor antibody found on the surface of immunocompetent cells. This functions in the afferent response.

IgE - binds to the surface of mast cells causing degranulation of the cell and release of histamine into circulation. This ab is involved with allergies.

Humoral immunity – antibodies Humoral immunity – antibodies cont’d. cont’d. Abs are important for us in five ways. neutralisation - an Ab molecule covers up sites on

toxic molecule or virus.

opsonization - this is Ab-mediated phagocytosis. Macrophages have antibody receptor sites on surface, able to bind to antigen-antibody complexes before phagocytosing them.

complement fixation - a complicated system that reacts to antigen/antibody complexes (see also complement notes in innate immunity).

Humoral immunity – antibodies Humoral immunity – antibodies cont’d. cont’d. agglutination/precipitation - Abs cross-link

antigens into large complexes making them easier to phagocytose & destroy.

immobilization - Abs bind to flagella etc. & prevent organisms from escaping macrophage death.

Cellular immunity – Th and Tc Cellular immunity – Th and Tc cells.cells. Often directed against intracellular parasites & cancer.

Infected cells killed by macrophages under directions of CD4+ Th cells. Cytotoxic T cells (CD8+ directed) also participate by releasing toxic components which kill the cell.

Cells involved in cellular immunity must be able to recognise self, especially as many of their targets are cells infected by agents that are within them. This means killing ones own cells in an effort to rid the infection. Self recognition is mediated by the Major Histocompatibility Complex antigens (MHC antigens). All our cells display these MHC antigens in specific patterns on the cell surface.

Cellular immunity – Major Cellular immunity – Major Histocompatibility Complex.Histocompatibility Complex. Macrophages must process the antigen & then display

pieces of the antigen on its cell surface. They then present this antigen to T cells, which recognize the antigen as being foreign as well as recognising the MHC antigens. If the T cell “sees” both antigen and MHC it becomes activated: if it “sees” only the MHC antigen nothing happens.

When macrophages display antigen plus Class I MHC they stimulate CD8+ cells (i.e. they make cytotoxic T cells) when they displayed antigen plus Class II MHC they stimulate CD4+ cells (i.e. helper T cells).

Invertebrate immune system.Invertebrate immune system.Comparison of vertebrate & invertebrate immunity.

Vertebrates Invertebrates

Innate Immunity Innate Immunity(e.g. antimicrobial peptides) (e.g. antimicrobial

peptides) Acquired immunity ---------------------

Phagocytic cells Phagocytic cells(Macrophages neutrophils etc) (Haemaocytes)----------------------------------- Melanization----------------------------------- Phenoloxidase cascadesCytokines Macrokines Immune competent tissues Immune competent tissues

N.B. Invertebrate immune system comprises only innate system; it is non-specific and has no memory component. Vertebrate immune system both innate and acquired components.

Invertebrate immunity.Invertebrate immunity.

The invertebrate immune system is comprised of two branches:

The humoral response (N.B. this is not antibody mediated) is concerned with soluble components such as antimicrobial peptides (AMPs), agglutinins (lectins) and macrokines (these are similar to cytokines).

The cellular response includes phagocytosis (haemocytes), encapsulation and nodulation.

Invertebrate immunity – humoral Invertebrate immunity – humoral response.response. Antimicrobial peptides. Wide range including

defensins, cecropins, andropins, ceratotoxins, drosomycin & penaeidins etc. Their action leads to lysis of invading organism e.g. bacteria & protozoa.

Macrokines. There is growing evidence of these cytokine-like molecules. Haemolymph preparations have been shown to stimulate vertebrate immune effector cells (e.g. macrophages).

Agglutinins (lectins). Agglutinate invading organisms making them easier to phagocytose.

Invertebrate immunity – cellular Invertebrate immunity – cellular response.response.

Phagocytosis - Haemocytes (amoebocytes) front line of invertebrate cellular. Foreign (non-self) invaders are taken into a phagocytotic vacuole where proteolytic enzymes & free oxygen radicals destroy the pathogen (in a similar way to vertebrate macrophages). Bacteria and yeast (<10microns) can be phagocytosed.

Invertebrate immunity – cellular Invertebrate immunity – cellular response cont’d.response cont’d. Encapsulation - If invader too large for

phagocytosis (e.g. the egg of a parasitic wasp), encapsulation might ensue. Invader is compacted under layer of haemocytes. This is accompanied by melanization. The melanized capsule adheres to host tissues but is walled off from the host. Phenoloxidases mediate melanization reaction but also have other tasks including wound healing, cuticle pigmentation & sclerotisation.

Invertebrate immunity – cellular Invertebrate immunity – cellular response cont’d.response cont’d.

Nodulation - Microaggregates of haemocytes & bacteria encased in haemocytes are melanised & removed from circulation.

Phagocytosis, encapsulation and nodulation mediated by eicosanoids (prostaglandins, leukotrienes).

Invertebrate immunity – Invertebrate immunity – mechanical / physical barriers.mechanical / physical barriers.

In addition to the cellular & humoral defences, invertebrates also have mechanical or physical defences.

These include the cuticle, epithelia and in the case of insects the peritrophic membrane.

Summary.Summary.

By the end of this session you should be: Familiar with fundamental biology of vertebrate

& invertebrate immune systems. Familiar with the concept of innate and acquired

immunity in vertebrates. Recoginise that invertebrates have only innate

immune system. Recognise the key players in both vertebrate and

invertebrate immune systems.

Next session.Next session.

We will:

Describe immunity to particular parasites.

Explore the strategies that parasites use to evade the hosts’ immune system.