Immunology Test 3

Follicular Dendritic Cells (FDCs)

• have long processes that have complement receptors and Fc receptors

• Capture intact

• Antibody/Antigen/complement complexes for presentation to B-cells

Subcapsular Sinus Macrophages

• poorly phagocytic

• present intact antigens on complement receptors

3 Signals B-2 cells require to become activated

1. binding of B-cell receptor to an antigen

   • dimerization of receptors sends activating signals

2. binding of B cell co-receptor to complement factor C3d

   • CR1 cleaves C3b on pathogen surface to iC3b and C3d

   • CR2 binds C3d

   • CD19 enhances signaling

3. Linked antigen recognition by Tfh cell

T cell provides CD40L (binds B cell CD40)

• causes B-cell to proliferate (i.e., divide to produce many B-cell copies)

• promotes class switching and somatic hypermutation in the germinal center

T cell provides cytokines

required for initial B-cell proliferation, cytokines determine antibody class during class switching

When a B-cell receptor binds antigen,

it internalizes the antigen, breaks the proteins into peptide fragments, and presents peptides on MHC class II molecules

The requirement for T-cell help

serves as a mechanism of B-cell negative selection

• if a B-cell receptor binds self-antigen, there is unlikely to be a conjugate (partner) T-cell present

B cell forms a conjugate pair with a Tfh cell that recognizes a linked peptide

• CD40L and cytokines from the T cell drive B cell proliferation

• signals are delivered through a synapse

Activated B-cells differentiate into plasma cells in 2 waves

• first, the B-cell-T-cell conjugate pair moves to the medullary cords to form a primary focus

  • some B-cells differentiate to plasma cells secreting low affinity IgM antibody

• second, remaining B-cells move back to the lymphoid follicles to form a germinal center

  • germinal center B-cells undergo somatic hypermutation and class switching

  • results in high affinity, class switched antibodies

Dark zone of Germinal centers

• contains centroblasts

  • proliferating B-cells undergoing somatic hypermutation

Light Zone of Germinal Centers

• contains centrocytes sampling antigen presented by FDCs

  • centrocytes display the mutated B-cell receptor to see if it can bind antigen presented by FDCs

  • B-cells with the highest affinity receptors will bind and continue to receive survival signals from Tfh cells

  • selection occurs in increments (i.e., B-cells undergo multiple rounds of somatic hypermutation)

Following somatic hypermutation, B-cells with a low affinity receptor

undergo apoptosis

• cannot compete for the limited antigen presented by FDCs

Following somatic hypermutation, B-cells with a high affinity receptor

are signaled to survive

• the receptor binds antigen, and the B-cell continues to receive the CD40L and cytokines from a Tfh cell

• over time, B-cell undergoes affinity maturation

  • receptor affinity (i.e., binding strength) for antigen increases over time

Class Switching

• largely determined by cytokines

• takes place in germinal center

• requires interactions between CD40 (B cell) and CD40L (T cell)

  • individuals that lack CD40L have hyper IgM syndrome

  • T-cell types other than Tfh cells can also produce cytokines that influence this

B-cell responses to thymus-independent antigens

do not require T-cell help

• many common bacterial antigens are TI antigens

Two types of thymus-independent antigens

• TI-1 antigens

• TI-2 antigens

At low concentration, T-1 antigens

cause antigen specific B-cell responses

At high concentration, TI-1 antigens

cause polyclonal B-cell activation

• cause activation of most B cell through PRRs independent of B-cell receptor specificity

• examples: LPS and bacterial DNA which activate TLRs expressed by B cells

• gets an initial supply of antibody in circulation while the body waits for the slower thymus-depended response to become activated

• some of the resulting antibodies will bind to other antigens on the pathogen

• many of the resulting antibodies will be no use

TI-2 antigen responses

highly repetitive structures such as capsular polysaccharides

• the antigen cross-links many receptors to send a strong activating signal

• provides a quick antibody response to encapsulated bacteria as it can be difficult to get B-2 cells activated without an initial thymus independent response

• responses are made primarily by B-1 cells and marginal zone B cells of the spleen

IgM

• first antibody class to be produced during an immune response

• has relatively low affinity for antigen as it is produced prior to somatic hypermutation

• forms pentamers to increase overall binding strength

• other antibody classes are produced through class switching in the germinal center and have a higher affinity antigen binding site

IgG

most abundant antibody in blood serum

• 4 different subclasses with distinct functions make a good all-around antibody

• transported across the placenta

• after birth, maternal IgG declines during the first 6 months

IgG, monomeric IgA and IgM

protect extravascular tissues

• IgG and IgA are smaller than IgM and diffuse more easily into tissues

• IgG is the most abundant class in the blood

Dimeric IgA

protects mucosal surfaces

• most abundant antibody class in the body

• mucosal surfaces are expansive, and most pathogens enter through mucosal surfaces

• transferred through breast milk to an infant to protect the infant’s mucosal surfaces

IgE

• binds to mast cells underlying surface and surrounding blood vessels

• binds to Fcε receptors on mast cells and awaits antigen binding

• when on mast cells binds antigen, the mast cell secretes inflammatory mediators

  • promote bodily functions that aim to flush out the parasite, e.g., vomiting diarrhea, increased mucus production

  • recruit other cells

Infants

start to make their own IgM before birth and IgG and IgA at around 3 months

High affinity neutralizing antibodies

prevent attachment of microbes, toxins, and venoms to cell surface (and therefore, prevent entry into cells)

Monomeric IgA

can also activate the classical complement pathway, but dimeric IgA does not

IgM and IgG activate the classical complement pathway

bind to the pathogen surface and recruit C1, which activates the classical complement pathway

• they do not interact with C1 when not bound to the pathogen surface

Parasites

are too large to injest

Binding of an eosinophil

to an IgE coated parasite triggers exocytosis of granules

Natural Killer (NK) cells

• have a receptor for IgG

• recognize host cells coated with IgG

  • kills infected cells

  • complements the function of CD8 T cells

IgG antibody functions

• most abundant class in the blood

• protects extravascular tissues and is transferred across the placenta

• good all-around antibody class

  • opsonization, neutralization, activation of classical complement pathway, ADCC

  • doesn’t play a major role in defense against parasites

IgA Antibody Functions

• most abundant antibody class in the body; dimeric IgA is the primary class that protects mucosal surfaces

• overall, its primary function is neutralization

IgE

defense against helminth parasites— works with mast cells, eosinophils and basophils

IgM Antibody Functions

• first antibody class to be secreted

• relatively low affinity binding site and functions as a bulky pentamer

• effective at activating the classical complement pathway

• contributes to neutralization and protection of mucosal surfaces

IgD Antibody Function

binds to basophils in the respiratory tract and functions in defense against respiratory pathogens

X-linked agammaglobulinemia (XLA)

• affects ~1 in 200,000

• defective B cell receptor signaling during B-cell development results in no functional B-2 cells

• role of B cells: produce antibodies

• symptoms: susceptible to infections with extracellular bacteria and fungi, and some viruses recurrent infections can lead to tissue and organ damage

• treatments: antibodies to treat infections

Dendritic Cells

• obtain peripheral tissue antigens at the infection site and travel in the lymphatics to the draining lymph node

• present antigen to T-cells in the T-cell areas of secondary lymphoid tissues

Inflammation

increases drainage of fluid into lymphatic vessels

Naïve T-cells

have not yet encountered their antigen

Effector T-cells

• T-cells that have been activated and can carry out their function

• if a naïve T-cell receptor binds the antigen/MHC complex presented by a dendritic cell, it becomes activated to produce this

Secondary Lymphoid Tissues

are linked through lymphatic vessels

T-cells

can move from tissue-to-tissue sampling antigen presented by dendritic cells

During an infection,

antigen specific T-cells are trapped in the draining secondary lymphoid tissue very efficiently (within a couple of days)

Immature dendritic cells

• constantly sample antigens of all kinds in the environment

• they become mature (activated) when:

  • a pathogen PAMP binds to a dendritic cell PRR

Following activation, dendritic cells

• synthesize the co-stimulatory molecule B7

• increase antigen processing and presentation on MHC molecules

• upregulate the chemokine receptor CCR7 (binds chemokines produced by secondary lymphoid tissues)

T-cell Signal 1

the T-cell receptor complex (including CD4 or CD8) binds to peptide: MHC complex presented by dendritic cell

T-cell Signal 2

CD28 on T-cell binds to the costimulatory molecule B7 on mature dendritic cell

T-cell differentiation and proliferation

requires a third signal

• cytokines

  • IL-2 for proliferation

  • differentiation to one of the CD4 T-cell subsets is determined by cytokines

Dendritic cells that present a self-peptide

are not typically mature, so they do not express the co-stimulatory B7 molecule and therefore do not activate T-cells (this provides a mechanism of negative selection in the periphery)

CD8 T-cells

often have a higher threshold for activation

For some intracellular pathogens,

activation of the T-cell receptor and CD28 co-stimulatory receptor is sufficient to activate the T-cell.

For many intracellular infections,

CD8 T-cells require additional help from a CD4 T-cells activated by the same pathogen.

Effector CD8 T cells

• T-cells form a conjugate (cognate) pair with their target cell

• cytokines and cytotoxins are delivered through a synapse

  • focuses delivery at site of contact

• the molecule perforin produces a pore in the target cell membrane

• granzymes (proteases) are delivered through the pore and activate apoptosis

  • target cell undergoes apoptosis

Following activation, a naïve CD4 T cell

differentiates into one of several subsets depending on the cytokines it receives

ILC2 cells

act during the innate immune response and promote the type 2 response

ILC1 cells

act during the innate immune response and promote the type 1 response

ILC3 cells

act during the innate immune response and promote the type 3 response

Effector CD8 T cells (cytotoxic T cells)

destroy cells infected with an intracellular pathogen

• an infected cell will display pathogen antigens on MHC class I molecules

• cytotoxic T-cells will kill any cells that display the same antigen that activated the T-cell

• the T-cell no longer requires a co-stimulatory signal after it has been activated

CD4 T-cell differentiation

is determined by the cytokines it receives at the time of activation

• different CD4 T-cell subsets are defined by the different cytokines they produce

Linked Recognition

the B-cell and T-cell receptors recognize different epitopes within the same antigen

Tfh cells activate

naïve B cells and are required for the formation of germinal centers

Activation of naïve B-cells

requires binding of CD40 to CD40L and cytokines

T-helper 1 cells

• help combat intracellular infections

• upregulate mactophages (i.e., increase killing ability via phagocytosis)

  • increase ability of macrophages to destroy the material in their intracellular vesicles

  • important in the response to intracellular bacteria and viruses

• this and macrophage form a conjugate pair

• macrophages require 2 signals for upregulation

  • cytokine IFN-γ

  • CD40L, which binds CD40 on the macrophages

The granuloma

is a chronic interaction between macrophages and T-cells which walls off the pathogen and prevents dissemination through the body

T helper 2 cells

• enhance response to helminths (worms) by eosinophils and basophil, mast cells, and IgE

• promote bodily functions aimed to flush out the parasite

  • increased mucus production, diarrhea, vomiting, smooth muscle contraction, etc.

T helper 17 cells

• enhance the response to extracellular bacteria and fungi by:

  • promoting the production and recruitment of additional neutrophils

  • stimulating mucosal epithelial (barrier) cells to increase the rate of turnover and production of antimicrobial molecules (inhibits colonization of mucosal surfaces)

• discovered in 2007

Natural T-regulatory cells

• produced in the thymus during T-cell development

• some T-cells that are negatively selected become Tregs

• recap: play important role in inhibiting the activation of potentially self-reactive T-cells

Induced (or adaptive) Tregs

• differentiate from naïve CD4 T-cells upon activation in the presence of anti-inflammatory cytokines

• mainly produced in the mucosal immune system in response to harmless antigens (e.g., non-microbial antigens or antigens from commensal microbes)

γ:δ T-cells

• maintain tissue integrity

  • recognize conserved antigens that diminish stressed cells from healthy cells

  • kill infected/damaged cells and promote tissue repair

  • account for most T cells in tissues

• do not interact with MHC molecules

  • do not need the CD4/CD8 co-receptors

• can be thought of as innate-like cells

  • respond quickly to stress/infections like an innate response

  • show less receptor diversity than α:β cells

Severe combined immunodeficiency (SCID)

• affects 1 in 100,000

• also known as “bubble boy disease”

• no functional T-cells

• several causes: IL-2R mutation, RAG protein mutation, abnormal thymic development

Mucosal Surfaces

• protect the gastrointestinal tract, respiratory tract, urogenital tract, and glands

• thin, permeable, expansive

• most pathogens enter the body here

• are populated by commensal microbes

Mucus

composed of mucins

Mucins

• glycosylated polypeptides that are cross-linked to form large, extended molecules

• lubricates mucosal surfaces

• protects mucosal surfaces

  • physically impedes microorganisms

  • retains antibody and anti-microbial molecules

  • turns-over every couple of days

• produced by goblet cells in the mucosal epithelium

Secondary Lymphoid tissues of the small intestine

include the GALT (Peyer’s pathches, isolated lymphoid follicles and appendix) and mesenteric LNs

Microfold Cells (M cells)

transport microbial antigens from gut lumen to underlying lymphoid tissue

Paneth cells

in the epithelium secrete antimicrobial substances

Non-mucosal tissues

• only have occasional contact with microbes

  • an immune response is only generated in the presence of an infection, i.e., not proactive

  • when the barrier is breached, tissue macrophages produce an inflammatory response to attract more leukocytes to the area to clear the infection

  • inflammation also causes damage to host tissues

Mucosal tissues

• have continuous contact with microbes

  • mucosal immune system proactively makes adaptive immune responses against antigens in the gut lumen, so it is prepared for a breach in the barrier

  • mucosal immune responses to innocuous antigens are non-inflammatory

  • if barrier is breached, infection can be cleared quickly with minimal inflammation

Mucosal tissue macrophages

• perform phagocytosis, but do not produce an inflammatory response

• don’t perform inflammatory functions associated with systemic tissue macrophages

  • monocytes lose their inflammatory potential when they arrive in the lamina propria from blood

  • do not secrete inflammatory cytokines

  • do not express T cell co-stimulatory molecules in order to become activated

Gut dendritic cells

• promote tolerance to food antigens (no adaptive immune response)

• move to mesenteric lymph nodes and active Tregs

  • they present antigen and secrete anti-inflammatory cytokines, which promotes differentiation of T-cells to the Treg subset

Oral Tolerance

• tolerance of non-microbial protein antigens taken in through the oral route

• systemic and mucosal immune systems become unresponsive

Mucosal dendritic cells in the secondary lymphoid tissue

• promote an anti-inflammatory adaptive immune response

  • not the same as oral tolerance to food antigens

  • proactive, anti-inflammatory immune response

The anti-inflammatory response involves:

• activation of B cells to produce plasma cells secreting IgA

• activation of Tregs (keep other T cells in check)

• activation of some inflammatory T cell subsets (kept in check by the Tregs)

  • they are already ready and waiting if the barrier is breached

Dimeric IgA

• the primary antibody class that protects mucosal surfaces

• the primary function is neutralization (a non-inflammatory function)

  • inflammatory immune cells and molecules are not present in the lumen of the gut

• IgM contributes to the protection of the mucosal surfaces (but it is a lower affinity antibody)

Antigen presentation by dendritic cells

can be anti-inflammatory or inflammatory depending on the cytokines present in the environment

In the absence of a barrier breach

dendritic cells promote an anti-inflammatory response to microbial antigens

Anti-inflammatory cytokines in the microenvironment

promote Treg differentiation and IgA production for a pro-active, anti-inflammatory response

Breach of the barrier

by a pathogen or opportunistic commensal results in an inflammatory response

• Tregs take off the brakes and allow inflammatory subsets to function to produce inflammatory immune response

Comprised epithelial cells

secrete inflammatory cytokines

• produces an inflammatory cytokine environment in the underlying tissue

• if a pro-active immune response has already been generated, the infection may be dealt with promptly with minimal inflammation

Celiac Disease

pro-inflammatory responses are generated against gluten

Crohn’s Disease

pro-inflammatory responses against commensal bacteria

After primary infection is cleared

• relatively high antibody levels are sustained for a couple of months

• low level of antibody is maintained in circulation indefinitely by long-lived plasma cells

Memory Response (secondary adaptive immune response)

• the immune response against second and subsequent exposure to an antigen

• quicker and more effective (often clearing infection without symptoms)

Characteristic of memory B cell response

• protective memory is provided by low-level antigen specific antibody produced by long-lived plasma cells

• reactive memory is provided by memory B cells

• more antigen specific when compared to the initial number of antigen specific naïve cells

• memory B cells have already undergone class switching and somatic hypermutation

  • they are activated earlier because it takes less antigen

  • they immediately produce class-switched high affinity antibodies

• memory B cells are preferentially activated over new naïve B cells that would produce a lower affinity antibody

With subsequent exposure to antigen:

• antibody concentration increases

  • more memory cells in secondary response compared to naïve cells in primary response

  • memory cells activated quicker than naïve cells

• the affinity of antibody for antigen continues to rise

  • memory B cells undergo further somatic hypermutation and affinity maturation

Memory T cell characteristics

• effector memory T cells circulate in peripheral tissues and are already differentiated and ready to act (provide protective memory)

• central memory T cells circulate in secondary lymphoid tissues

  • undifferentiated, but more readily activated then naïve T cells (less dependent on co-stimuation

  • start producing cytokines earlier than newly activated naïve T cells (within 12-24 hrs)

Primary Response

• small number of pathogen-specific cells respond at the start

• delay before pathogen-specific antibodies are produced

• non-isotype-switched antibody having a mixture of affinities for the pathogen is produced at the start

• high threshold of activation

• delay before effector T cells are generated and are able to enter infected tissues

• innate immunity works alone until an adaptive response is generated

Secondary Response

• large number of pathogen-specific cells respond immediately

• pathogen-specific antibodies already present

• antibodies are isotype-switched and have high affinity for the pathogen

• lower threshold of activation

• effector T cells are present and can enter infected tissue immediately

• close cooperation between innate and adaptive immunity from the start