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Antigen Capture, Presentation & Recognition Dr Maliha Sumbul


These photomicrographs show phagocytosis of antibody-coated pneumococci Left: A neutrophil extends a pseudopod toward two pneumococci. Center: these bacteria have been engulfed (arrows), and the neutrophil is beginning to engulf four more pneumococci at the upper right. Right: Two pneumococci have escaped.


â–ş Antigen

presentation is a process in the body's immune system by which macrophages, dendritic cells and other cell types capture antigens and then enable their recognition by T-cells


â–ş The

basis of adaptive immunity lies in the capacity of immune cells to distinguish between the body's own cells and infectious pathogens


The host’s cells express “self” antigens that identify them as such.

These antigens are different from those in bacteria ("nonself" antigens) or in virally-infected host cells (“missingself”). THE MISSING – SELF HYPOTHESIS: the absence or altered expression of MHC class I molecules would render target cells susceptible to NK cell attack

The ability of the adaptive immune system to survey for infection requires specialized pathways of enabling recognition of pathogen-derived antigens by T cells


The First Molecular Basis of the "Missing Self" Hypothesis ► Francisco Borrego ► ►

1 I still remember the day my advisor, Dr. Rafael Solana, came to me with the article of Karlhofer et al. (1) in his hands and told me that that paper was the proof for the receptor inhibition model of the "missing self" hypothesis. I had just joined the Laboratory of Immunology in the University of Córdoba, in Spain, and my knowledge about NK cells was very basic. At that moment, I did not realize the whole significance of Yokoyama’s work, but as time went by, I fully understood the breakthrough that the manuscript represented. In 1990, Ljunggren and Kärre published in Immunology Today a very comprehensive review about the role of MHC class I Ags on NK cell recognition (2). According to the "missing self" hypothesis that they proposed, the absence or altered expression of MHC class I molecules would render target cells susceptible to NK cell attack. Klas Kärre adopted the concept of "missing self" while he was writing his Ph.D. thesis and found that it was easier to describe the common features of resistant target cells rather than susceptible ones (3). Four years earlier, in 1986, Kärre et al. published a seminal letter in Nature that showed in vivo that NK cells were able to reject tumor cells that have lost MHC class I expression (4). The finding that deficiency of MHC class I molecules constituted an alternative immune defense strategy was very provocative at the time. It was believed that NK cells worked like T cells by recognizing foreign Ags on the target cells, and it was very clear that NK cells were not MHC restricted. But what Kärre postulated was exactly the opposite in the sense that NK cells, like T cells, were strongly influenced by the expression of MHC class I molecules on the target cell (3). Following Kärre’s letter in Nature, many manuscripts were published about the role of MHC class I molecules in NK cell recognition. In vivo and in vitro experiments were reported both in the mouse and in human systems. In general, people in the field accepted that NK cell susceptibility was directly related to the absence of MHC class I expression on target cells

The Journal of Immunology, 2006, 177: 5759-5760. Copyright © 2006 by The American Association of Immunologists, Inc.


STEPS: ► Foreign protein or antigen is taken up by an antigenpresenting cell ► The antigen is processed and displayed on an MHC II molecule, which interacts with a T helper cell OR

Whole foreign proteins are bound by membrane antibodies and presented to B lymphocytes which process and present antigen on MHC II to a previously activated T helper cell, spurring the production of antigen-specific antibodies


Antigen receptors: ► B Lymphocytes: membrane bound antobodies can recognize wide variety of macromolecules – proteins, polysaccharides, lipids and nucleic acids – therefore, against many types of microbial cell wall and soluble antigens ►T

Lymphocytes: TCR – can only see peptide fragments of protein antigens and only when these peptides are presented by specialized peptide display molecules on host cells


Same microbe – different action ► Eg: ► Virus

– entered circulation and free in blood – combated by antibodies

► Virus

infected host cell – antibodies no longer effective – necessary to activate CTLs to kill infected cells and infection


MHC COMPLEX Responsive cell CD 8+

► ►

MHC class I molecules found on every nucleated cell of the body (and thus not on red blood cells).

ENDOGENOUS PATHWAY

Responsive cell CD 4+

MHC ( major histocompatibility co ) Class II molecules

found only on a few specialized cell types, including macrophages, dendritic cells and B cells, all of which are professional antigen-presenting cells (APCs) EXOGENOUS PATHWAY


Professional APCs â–ş Ability

of these cells to both display antigens for T cells and provide the additional signals needed to activate naĂŻve T cells


â–ş The

human leukocyte antigen system (HLA) is the name of the major histocompatibility complex (MHC) in humans

(genes reside on chromosome 6)


The major HLA antigens are essential elements for immune function. Different classes have different functions:

HLA class I antigens present peptides from inside the cell (including viral peptides if present). Foreign antigens attract killer T-cells (also called CD8 positive- or cytotoxic T-cells) that destroy cells Endogenous pathway

► ►

HLA class II antigens present antigens from outside of the cell to T-lymphocytes. These particular antigens stimulate T-helper cells to multiply, and these T-helper cells then stimulate antibody-producing B-cells to produce Antibodies to that specific antigen. Self-antigens are suppressed by suppressor T-cells Exogenous pathway

HLA class III antigens encode components of the complement system


HLA have other roles: ► They are important in disease defense ►

They may be the cause of organ transplant rejections

► They

may protect against or fail to protect (if down regulated by an infection) cancers

They may mediate autoimmune disease (examples: type I diabetes, coeliac disease)

► Also,

in reproduction, may be involved in mate selection


HLA and autoimmune diseases HLA allele HLA-B27

Diseases with increased risk Relative risk Ankylosing spondylitis

12 14

HLA-DR3

HLA-DR4

Acute anterior uveitis

15

Autoimmune hepatitis

14

Primary Sjรถgren syndrome

10

Diabetes mellitus type 1

5

Rheumatoid arthritis

4

Diabetes mellitus type 1

6

HLA-DR3 and-DR4 combined Diabetes mellitus type 1

15

HLA-B47

15

21-hydroxylase deficiency


â–ş

Helper T cells (CD4+) serve as managers, directing the immune response They secrete chemicals called lymphokines that

stimulate cytotoxic T cells and B cells to grow and divide, attract neutrophils, and enhance the ability of macrophages to engulf and destroy microbes


Antigen recognition â–ş Unlike

B cells, T cells fail to recognize antigens in the absence of antigen presentation, with the important exception of the superantigens

â–ş The

T cell receptor is restricted to recognizing antigenic peptides only when bound to appropriate molecules of the major histocompatibility complex (MHC), also known in humans as Human leukocyte antigen (HLA))


Superantigens (SAgs) ►a

class of antigens which cause non-specific activation of T-cells resulting in massive cytokine release.

► SAgs

can be produced by pathogenic microbes (including viruses, mycoplasma, and bacteria) as a defense mechanism against the immune system

Compared to a normal antigen-induced T-cell response where 0.001- 0.0001% of the body’s Tcells are activated, these SAgs are capable of activating up to 20% of the body’s T-cells


►An ►

epitope is a portion of a molecule to which an antibody binds. Epitopes can be composed of sugars, lipids or amino acids

The large number of activated T-cells generates a massive immune response which is not specific to any particular epitope on the SAg thus undermining one of the fundamental strengths of the adaptive immune system, that is, its ability to target antigens with high specificity

More importantly, the large number of activated T-cells secrete large amounts of cytokines (the most important of which is TNF-alpha) TNF-alpha is particularly important as a part of the body's inflammatory response, and in normal circumstances (where it is released locally in low levels) helps the immune system defeat pathogens. However when it is systemically released in the blood and in high levels (due to mass T-cell activation resulting from the SAg binding), it can cause severe and life-threatening symptoms, including shock and multiple organ failure.


► Diseases

associated with superantigen production ► Toxic Shock Syndrome ► Kawasaki Disease ► Eczema ► Psoriasis ► Rheumatoid arthritis ► Diabetes mellitus ► Scarlet fever


â–ş Most

cells are capable of presenting antigens and activating the adaptive response. Some cells, however, are specially equipped to acquire and present antigen, and to prime naive T cells.

â–ş Dendritic

cells, B cells, and macrophages play a major role in the innate response, and also act as professional antigen presenting cells (APC). These professional APCs are equipped with special immunostimulatory receptors that allow for enhanced activation of T cells


â–ş Several

different types of T cell can be activated by professional APCs, and each type of T cell is specially equipped to deal with different pathogens, whether the pathogen is bacterial, viral or a toxin

â–ş The

type of T cell activated, and therefore the type of response generated, depends, in part, on the context in which the antigen was first encountered by the APC


Intracellular antigens: MHC Class I ►

Intracellular antigens are mainly produced by viruses replicating within a host cell, though antigens here can also derive from cytoplasmic bacteria or the host cell's own proteins

The host cell digests cytoplasmic proteins by a specialized enzyme complex, the proteasome into small peptides

A specialized carrier, the Transporter associated with Antigen Processing (TAP) complex moves the peptide into the endoplasmic reticulum, allowing the antigenic peptide to be coupled to an MHC Class I molecule and transported to the cell surface

MHC Class I molecules present antigen to CD8+ cytotoxic T cells


With the exception of some cell types (such as erythrocytes), Class I MHC is expressed by almost all host cells

Cytotoxic T cells (also known as TC, killer T cell, or cytotoxic T-lymphocyte (CTL)) are a population of T cells which are specialized for inducing the death of other cells

Recognition of antigenic peptides through Class I by CTLs leads to the killing of the target cell, which is infected by virus, intracytoplasmic bacterium, or are otherwise damaged or dysfunctional


Extracellular antigens: MHC Class II â–ş Dendritic

cells (DCs) phagocytose exogenous pathogens, such as bacteria, parasites or toxins in the tissues and then migrate, via chemotactic signals, to T cell enriched lymph nodes

â–ş During

migration, DCs undergo a process of maturation in which they lose phagocytic capacity and develop an increased ability to communicate with T-cells in the lymph nodes


► The

DC uses lysosome-associated enzymes to digest pathogen-associated proteins into smaller peptides

► In

the lymph node, the DC will display these antigenic peptides on its surface by coupling them to MHC Class II molecules

► This

MHC:antigen complex is then recognized by T cells passing through the lymph node

► Exogenous

antigens are usually displayed on MHC Class II molecules, which interact with CD4+ helper T cells


► CD4+

lymphocytes, or TH, are immune response mediators, and play an important role in establishing and maximizing the capabilities of the adaptive immune response

► Expression

of Class II is more restricted than Class

I. ► High levels of Class II are found on dendritic cells, but can also be observed on activated macrophages and B cells.


Dendritic cells (DCs) ►

Immune cells that form part of the mammalian immune system

Their main function is to process antigen material and present it on the surface to other cells of the immune system, thus functioning as antigen-presenting cells

Dendritic cells are present in small quantities in tissues that are in contact with the external environment, mainly the skin (where there is a specialized dendritic cell type called Langerhans cells) and the inner lining of the nose, lungs, stomach and intestines. They can also be found in an immature state in the blood.

Once activated, they migrate to the lymphoid tissues where they interact with T cells and B cells to initiate and shape the adaptive immune response

At certain development stages they grow branched projections, the dendrites, that give the cell its name. However, these do not have any special relation with neurons, which also possess similar appendages.


Basic model of how dendritic cells interact with CD4(+) T-cells


B-cells have the ability to capture and present antigen. Because this happens through the B-cell receptor, this is “specific� capture and presentation This allows the efficient presentation of antigen to CD4(+) T cells by B-cells [B cells already have the ability to make antibody to that antigen]


► Cytotoxic T Lymphocytes

(CTL’s, CD8(+) Tcells) can be activated by two pathways. ► In both pathways the CTL must recognize antigen which has been processed and presented on MHC class I molecules. ► In

the first pathway, CD4(+) T-cells (TH1 cells) provide help in launching the CTL on its way

► In

the second pathway, dendritic cells provide the help

(The DIAGRAM indicates that the help is different – this probably prevents the dendritic cell from being killed by the CTL!!??)


a) How CD8(+) T-cells get activated

1

2

b) How they kill target cells


► After

activation, the CTL is now capable of killing target cells – that is, cells that are expressing the activating antigen in the context of MHC class I. ► There are two killing mechanisms: ► The secretion of enzymes onto the target cell which induce permeability changes and apoptosis – perforin and granzymes ►

The interaction of Fas on the CTL with FasL on the target cell activates caspases within the target cell and results in apoptosis.


QUESTIONS ?


microbiology