Immunology - Immune Evasion

Question 1

Dendritic cells - general

Answer 1

responsible for the
initiation of adaptive immune responses. Paul
Langerhans first described DCs in human skin in 1868 but thought they were cutaneous nerve cells,
bone marrow (BM)-derived leukocytes are the most potent type of antigen-presenting cells (can
also be propagated in vitro from BM and blood using various combinations of growth factors such as
granulocyte macrophage-colony stimulating factor (GM-CSF) and Flt3 ligand), are specialised to
capture and process antigens converting proteins to peptides that are presented on major
histocompatibility complex (MHC) molecules recognised by T cells, heterogeneous (ie myeloid
and plasmacytoid DCs), although all DCs are capable of antigen uptake + processing and presentation to
naive T cells the DC subtypes have distinct markers and differ in location + migratory pathways + detailed
immunological function and dependence on infections or inflammatory stimuli for their generation + during
the development of an adaptive immune response, the phenotype and function of DCs play an extremely
important role in initiating tolerance, memory, and polarised T-helper 1 (Th1) + Th2 and Th17 differentiation

Question 2

DCs linking innate and adaptive immunity

Answer 2

Since DCs have numerous cytoplasmic processes they have a high surface area permitting intimate contact with a large number of surrounding cells (ie T cells, natural killer cells, neutrophils, epithelial cells etc., experimentally only one mature DC (mDC) is required to stimulate 100–3000 T cells), DC precursors migrate from the bone marrow through the blood stream to almost every non-lymphoid tissue where they reside in an immature state (iDC) continuously sampling their environment by endocytosis + macropinocytosis and phagocytosis, can extend their processes through the tight junctions of epithelia to increase capture of antigens even when there is no overt infection/inflammation, during pathogen invasion, resident iDCs detect intruders via pattern recognition receptor (ie TLRs) capture antigens and quickly leave the tissue -> crawl through the cells, cross the endothelium of lymphatic vessels and migrate to the draining lymph nodes (LN) in response to a number of chemokines such as CCL19 and CCL21, during their migration from the peripheral tissues DCs undergo phenotypical and functional maturation + stop capturing antigens while up-regulating the expression of co-stimulatory molecules such as CD80 and CD86 and the chemokine receptor CCR7 and secrete pro-inflammatory cytokines such as TNF-α and IL-12, after reaching the subcapsular sinus of the LN, DCs move to T-cell zones where the interdigitating DCs are actively involved in the presentation of antigens to T cells

Question 3

DC immunotherapy

Answer 3

Exploiting the immune-regulatory capacities of DCs holds great promise for the treatment of cancer + autoimmune diseases and the prevention of transplant rejection, manipulation of DCs could turn them into the most effective adjuvant to enhance the host’s immune defences, tumours have been shown to suppress DCs by secreting anti-inflammatory cytokines such as IL-10 and therefore conditioning the local DCs to form suppressive T cells -> in order to subvert these mechanisms DCs could be generated ex vivo + loaded with tumor antigens and re-injected to boost the host’s immunity against the tumour cells, DC vaccines generated in this way are generally safe with minimal side effects and have proven to be feasible and effective in some patients

Question 4

T cells

Answer 4

predominantly produced in the thymus, recognise foreign particles (antigen) by a surface expressed highly variable T cell receptor (TCR), two major types (helper T cell and cytotoxic T cell), helper T cells activate B cells, cytotoxic T cells kill virally infected cells and tumours

Question 5

T cell receptors, APC and MHC

Answer 5

cannot bind antigen directly, needs to have broken-down peptides of the antigen presented to it by an antigen presenting cell (APC), the molecules on the APC that present the antigen = major histocompatibility complexes (MHC), two types of MHC class I and class II, MHC class I presents to cytotoxic T cells, MHC class II presents to helper T cells

Question 6

Co-receptors

Answer 6

the binding of the TCR to the MHC molecule containing the antigen peptide is a little unstable and so co-receptors are required the CD4 co-receptor is expressed by helper T cells and the CD8 co-receptor by cytotoxic T cells, most T cells express either CD4 or CD8 some express both and some do not express either (“double negative” (DN)), most T cells are defined as CD4 or CD8 but some are classified into additional types such as invariant Natural Killer T cells (iNKT) and Mucosal Associated Invariant T cells (MAIT)

Question 7

T cell receptor chains

Answer 7

assist the transmission of the signal to the T cell, alpha + beta, majority of T cells are alpha-beta T cells but alpha-alpha T cells do exist, special group of T cells with gamma and delta chains instead of alpha and beta called gamma-delta (γδ) T cells, in order for the stimulus of antigen binding to the TCR to be relayed into the T cell the TCR is associated with the protein complex CD3 which is made up of four types of chains including two epsilon chains + two zeta chains + one delta and one gamma chain

Question 8

CD8+ (cytotoxic) T cells - general

Answer 8

generated in the thymus and express the T-cell receptor, express a dimeric CD8 co-receptor usually composed of one CD8α and one CD8β chain, CD8+ T cells recognise peptides presented by MHC Class I molecules found on all nucleated cells, CD8 heterodimer binds to a conserved portion (the α3 region) of MHC Class I during T cell/antigen presenting cell interactions

Question 9

CD8+ (cytotoxic) T cells - mechanisms (1)

Answer 9

aka CTLs, immune defence against intracellular pathogens and tumour surveillance, three major mechanisms to kill infected or malignant cells, first = secretion of cytokines (primarily TNF-α and IFN-γ), which have anti-tumour and anti-viral microbial effects

Question 10

CD8+ (cytotoxic) T cells - mechanisms (2)

Answer 10

production and release of cytotoxic granules, these granules are also found in NK cells, granules contain two families of proteins (perforin and granzymes), perforin forms a pore in the membrane of the target cell similar to the membrane attack complex of complement, the pore by perforins allows the granzymes also contained in the cytotoxic granules to enter the infected or malignant cell, granzymes are serine proteases which cleave the proteins inside the cell shutting down the production of viral proteins and ultimately resulting in apoptosis of the target cell, the cytotoxic granules are released only in the direction of the target cell aligned along the immune synapse to avoid non-specific bystander damage to healthy surrounding tissue, CD8+ T cells are able to release their granules kill an infected cell then move to a new target and kill again (often referred to as serial killing)

Question 11

CD8+ (cytotoxic) T cells - mechanisms (3)

Answer 11

via Fas/FasL interactions, activated CD8+ T cells express FasL on the cell surface which binds to its receptor Fas on the surface of the target cell, causes the Fas molecules on the surface of the target cell to trimerise which pulls together signalling molecules that activate the caspase cascade which also results in apoptosis of the target cell, because CD8+ T cells can express both molecules Fas/FasL interactions are a mechanism by which CD8+ T cells can kill each other (fratricide) to eliminate immune effector cells during the contraction phase at the end of an immune response

Question 12

Th1/Th2 cells

Answer 12

• Th1-polarised cells are responsible for control of intracellular pathogens such as viruses and some bacteria, IL-12 and IFN-γ are important cytokines involved in Th1 responses, the intracellular transcription factors T-bet and STAT-4 are essential for Th1 cell differentiation and function
• Th2 polarised cells are important in the defence against large extracellular organisms such as helminths, utilise cytokines such as IL-4, IL-5 and IL-13 that promote eosinophilia + mastocytosis and goblet cell hyperplasia, Gata-3 and STAT-6 are essential for Th2 cell differentiation and function

Question 13

Allergy/Autoimmunity

Answer 13

If the Th1/Th2 balance is disturbed there can be severe consequences, asthma and allergy are Th2-driven, some autoimmune diseases such as type 1 diabetes and multiple sclerosis are Th1-driven

Question 14

Th17 cells

Answer 14

recently discovered T helper cell subset, characterised by its production of IL-17, IL-23 promotes the expansion of these cells, have been linked to several inflammatory conditions such as arthritis and IBD

Question 15

Treg cells

Answer 15

Regulatory T cells, a subpopulation of cells that maintain homeostasis and tolerance within the immune system, subsets include inducible Tregs + CD25+CD45RBlo Tregs etc.

Question 16

HCMV-associated disease

Answer 16

Human cytomegalovirus is a beta-herpesvirus that causes lifelong infection in humans, prevalence of 55-100% within the human population depending on different socioeconomic and geographical factors, primary HCMV infection is generally asymptomatic in healthy hosts but can cause severe and sometimes fatal disease in immunocompromised individuals and neonates, the leading infectious cause of congenital abnormalities in the Western world affecting 1-2.5% of all live births, HCMV intrauterine infection and can cause low birth weight + hearing loss + visual impairment + microcephaly + hepatosplenomegaly + varying degrees of mental retardation, also causes serious disease in organ transplant recipients and AIDS patients either after primary infection or reactivation of latent infection, in immunocompromised individuals infection is often controlled by early treatment with antivirals but problems exist such as toxicity + the emergence of antiviral-resistant HCMV strains and the availability of antiviral drugs in developing countries

Question 17

The immune response to cytomegalovirus

Answer 17

Extracellular and intracellular pathogen recognition receptors such as Toll-like Receptors (TLRs) recognise components of the virion which triggers the innate immune response to the virus -> production of inflammatory cytokines, such as type 1 interferons (IFN) + tumour necrosis factor alpha (TNF-α) and interleukin-6 (IL-6) -> recruit and activate phagocytic cells such as dendritic cells which can engulf HCMV-infected cells, natural killer (NK) cells are also recruited to initial sites of infection and eliminate HCMV-infected cells by the release of cytotoxic proteins which destroy target cells, data from the murine cytomegalovirus (MCMV) model of infection suggests that neutrophils contribute to early control of virus infection, the adaptive immune response contributes to the long-term control of HCMV, HCMV infection results in an extremely large cytotoxic CD8+ T cell response that appears to be crucial for the control of primary HCMV infection and reactivation from latency, HCMV-specific IFN-γ-secreting CD4+ T cells are associated with decreased virus shedding into urine thus limiting its spread to new hosts. B cells also play a role in controlling HCMV infection and may also help prevent its transmission to new hosts as maternal antibodies can confer protection to foetuses in utero

Question 18

Immune evasion by HCMV

Answer 18

dedicates a large proportion of its genome to down-regulating NK cell activity, has acquired a viral homologue of IL-10 (an immune modulatory cytokine that has been demonstrated to suppress anti-cytomegalovirus immunity), also targets the activation of the adaptive immune response (down-regulate the expression of major histocompatibility complex (MHC) class I and II molecules on cells such as antigen presenting cells which prevents the presentation of HCMV-derived peptides bound to MHC molecules to unstimulated T cells (a process required for the activation of T cells)), has evolved proteins which prevent apoptosis of infected cells promoting its dissemination within the host

Question 19

immune responses to viruses - Via cytotoxic cells

Answer 19

virus invades the cells of its host in order to survive and replicate, class I major histocompatibility complex proteins (or MHC class I) to display pieces of protein from inside the cell upon the cell surface (ie of viruses), Cytotoxic T cells have specialised proteins on their surface that help them to recognise virally-infected cells (T cell receptors (TCRs)), each cytotoxic T cell has a TCR that can specifically recognise a particular antigenic peptide bound to an MHC molecule, if the T cell receptor detects a peptide from a virus it warns its T cell of an infection -> the T cell releases cytotoxic factors to kill the infected cell and therefore prevent survival of the invading virus, viruses are highly adaptable and have developed ways to avoid detection by T cells (some viruses stop MHC molecules from getting to the cell surface to display viral peptides), another immune cell specialises in killing cells that have a reduced number of MHC class I molecules on their surface (natural killer cell or NK cell for short), when the NK cell finds a cell displaying fewer than normal MHC molecules it releases toxic substances in a similar way to cytotoxic T cells which kill the virally-infected cell, cytotoxic cells are armed with preformed mediators, cytotoxic factors are stored inside compartments called granules in both cytotoxic T cells and NK cells until contact with an infected cell triggers their release, one of these mediators is perforin a protein that can make pores in cell membranes that allow entry of other factors into a target cell to facilitate destruction of the cell, granzymes are also stored in and released from the granules and they enter target cells through the holes made by perforin, once inside the target cell they initiate apoptosis, another released cytotoxic factor is granulysin which directly attacks the outer membrane of the target cell, destroying it by lysis, cytotoxic cells also newly synthesise and release cytokines after making contact with infected cells including interferon-gamma and tumour necrosis factor-alpha that transfer a signal from the T cell to the infected or other neighbouring cells to enhance the killing mechanisms

Question 20

immune responses to viruses - Via cytotoxic cells

Answer 20

Virally infected cells produce and release interferons which prevent replication of viruses by directly interfering with their ability to replicate within an infected cell + act as signalling molecules which makes neighbouring cells increase the numbers of MHC class I molecules on their surface, so that T cells surveying the area can identify and eliminate the viral infection

Question 21

immune responses to viruses - Via antibodies

Answer 21

before they get the chance to infect a cell, antibodies are proteins that specifically recognise invading pathogens and bind to them, the antibodies neutralise the virus meaning that it is no longer capable of infecting the host cell, many antibodies can cause agglutination to make an easier target for immune cells, activation of phagocytes (a virus-bound antibody binds to Fc receptors on the surface of phagocytic cells and triggers phagocytosis), activation of the complement system opsonises and promotes phagocytosis of viruses and can damage the phospholipid bilayer present on some types of virus

Question 22

Natural killer cells

Answer 22

lymphocytes in the same family as T and B cells coming from a common progenitor, as cells of the innate immune system NK cells are classified as group I Innate Lymphocytes (ILCs), kill virally infected cells and detect + control early signs of cancer without any priming or prior activation, also found in the placenta, secrete cytokines such as IFNγ and TNFα which act on other immune cells like Macrophage and Dendritic cells to enhance the immune response, Activating receptors recognise molecules that are expressed on the surface of cancer cells and infected 
cells, Inhibitory receptors act as a check on NK cell killing (most normal healthy cells express MHC I receptors which mark these cells as ‘self’, inhibitory receptors recognise cognate MHC I, cancer cells and infected cells often lose their MHC I), releases cytotoxic granules containing perforin and granzymes which leads to target cell lysis, the genes for both MHC I and the NK cell inhibitory receptors which recognise them vary a lot between individuals, NK cell varieties also change with age and are affected by chronic viral infections such as cytomegalovirus (CMV), strong inhibition when healthy normal cells exoress enough MHC I molecules to induce a strong inhibitory signal in NK cells, reduced inhibition = tumour cells or cells infected by viruses often downregulate MHC class I molecules which means activation, strong activation = transformed or infected cells sometimes increase the expression of molecules that are recognized by activating NK cell receptors (activating ligands)