MMI 188B Midterm 2

Similarities of B and T Cell Development

• receptor gene rearrangements provide diversity for both

• both originate in bone marrow

• both undergo negative selection to remove self-reactive clones

Differences in B and T Cell Development

• T cells mature in thymus, B cells mature in secondary lymphoid tissues

Self MHC

set of MHC Class I and II molecules encoded by an individuals own genes

• ensures T cells function effectively within the host’s genetic and immunological framework

T-Cell Precursor

common progenitor cell that gives rise to both alpha:beta and gamma:delta T-cell lineages within the thymus

T Cell Lineages

when developing, they differentiate into 2 different linages

• alpha:beta T cells (the majority)

• gamma:delta T cells (the minority)

Thymus

• primary lymphoid organ where thymocytes mature

• involutes as we get older

Thymocytes

developing immature T cells derived from bone marrow progenitors

Thymic Stroma

a network of epithelial cells that forms the structural framework supporting thymocyte development

Thymic Anlage

rudimentary thymic structure formed during embryogenesis from ectodermal cortical and endodermal medullary epithelial cells

Complete DiGeorge Syndrome

a genetic disorder in which the thymus fails to develop, resulting in a complete lack of T cells and severe immunodeficiency

Involution of the Thymus

the age-related degeneration of thymic tissue beginning about one year after birth, with fat gradually replacing thymocytes

Immunosenescence

the gradual decline of immune function with age, associated with reduced thymic activity and diminished production of new T cells

Thymic Cortex

• above the medulla

• contains many immature thymocytes

Thymic Medulla

• underneath the cortex

• many mature thymocytes

Hassall’s Corpuscles

• primary source of thymic stromal lymphopoietin (TSLP) which promotes T-reg production

  • plays a vital role in central tolerance, helping the body prevent autoimmune diseases by suppressing immune responses against self antigens

• involved in removal of apoptotic thymocytes (developing T cells that failed selection)

TSLP

• cytokine essential for activating myeloid dendritic cells within the thymus

• activate dendritic cells which induce the differentiation of developing T-cells into regulatory T-cells (T-regs)

Double-Negative Thymocytes (DN Thymocytes)

early thymic T-cell precursors that lack both CD4 and CD8 surface markers

Thymocyte Commitment to T-Cell Lineage

• this is done before the thymocyte rearranges their T-cell receptor genes

• involves changes in

  • gene expression

  • cell surface markers

Notch 1

• surface membrane-associated receptor on thymocyte that drives T-cell development

  • binds to its ligand on thymic epithelium and turns on the expression of genes essential for T-cell development that:

    • REMOVE REPRESSIVE TRANSCRIPTION FACTORS

    • RECRUIT CO-ACTIVATING TRANSCRIPTION FACTORS

Double-Positive Thymocytes (DP Thymocytes)

immature T cells in the thymus that express both CD4 and CD8 co-receptors on their surface

T-Cell Development from Progenitor to Double Positive T Cell (step-by-step)

1. T-cell precursors that enter the thymus express the hematopoietic stem-cell markers CD34 but none of the characteristic markers of mature T cells

2. As soon as they produce a complete receptor, gamma:delta cells can leave the thymus and travel to other tissues via the blood

3. Following beta-chain-rearrangement in the thymus, rearrangement of the alpha-, gamma-, and delta- chain genes resumes, and a productive alpha-chain gene rearrangement produces double-positive CD4 CD8 alpha:beta T cells

4. A minority of the thymocytes that resume rearrangement give rise to additional gamma:delta T cells

Result of Gene Rearrangement in Double-Negative Thymocytes

leads to assembly of either a gamma:delta receptor or a pre-T-cell receptor

Pre-T Alpha Chain

an invariant polypeptide that serves as a surrogate alpha chain in developing thymocytes

Pre-T-Cell Receptor (pre-TCR)

formed when a beta chain pairs with pT-alpha, creating a heterodimer that dimerizes further into a superdimer

Lymphocyte-Specific Protein Tyrosine Kinase (Lck)

a cytoplasmic tyrosine kinase that transduces signals from the pre-TCR through phosphorylation cascade

Pre-T Cell

a thymocyte that has successfully assembled a pre-TCR and received survival signals progresses to the pre-T cell stage

Fate of Apoptotic Immature T Cells

ingested by macrophages in the thymic cortex

TCR Diversity

generated by gene rearrangement during T cell development

Beta-Chain TCR Gene Rearrangements

• beta-chain REARRANGES FIRST

• has V-D-J Segments

• Pre-T-Cell receptor contains a mature TCR-Beta Chain

• two attempts at recombination are possible for each chromosome (total of 4 attempts)

• nonproductive beta-chain rearrangements can be rescued by further rearrangement made possible by the two C-beta genes and their associated D-beta and J-beta segments

The Alpha:Beta Advantage

• commitment to alpha:beta lineage requires only one productive gene rearrangement

• commitment to the gamma:delta lineage requires a minimum of two productive rearrangements

Alpha-Chain TCR Gene Rearrangement

• rearranges after beta-chain

• only occurs in pre-T-Cells

• TCRalpha lacks D segments

• can undergo several sequential rearrangements

• rearrangement of the alpha-chain gene leads to the loss of the delta locus

  • delta-chain gene is eliminated

Double Negative T Cell Development Steps

• committed T-cell progenitor

• Rearrange beta, gamma, delta

• first checkpoint

  • pre-TCR receptor

• dividing pre-T cells

  • pTalpha receptor

Double Positive T Cell Development Steps

• rearrange alpha, gammam delta chain

  • CD8 + CD4 receptor

• second checkpoint

  • TCR

• single positive T cell

Positive Selection

ensures only thymocytes with T-cell receptors (TCRs) capable of recognizing and effectively interacting with self MHC molecules survive

• occurs in the thymus

• determines expression of CD4 or CD8

• does not occur to gamma:delta cells

MHC Restriction

cell recognizes antigen only when it is presented by the individual’s own MHC molecules

• does not occur to gamma:delta cells

Negative Selection

eliminates autoreactive T cells with TCRs that bind too strongly to self-peptide: self-MHC complexes

• ensures central tolerance, preventing self-reactive T cells from entering circulation and causing autoimmune disease

• removed in the thymus

• does not occur to gamma:delta cells

Thymoproteasome

• a specialized form of the proteasome found exclusively in cortical epithelial cells of the thymus

  • generates self-peptides optimized for positive selection of developing CD8+ T cells that recognize peptides presented by MHC Class I molecules

Single-Positive Thymocytes

express only one co-receptor, CD4 or CD8, following positive selection in the thymus

• CD4 single-positive thymocytes → precursors of helper T cells

• CD8 single-positive thymocytes → precursors of cytotoxic T cells

Steps and Results of Positive Selection

• Selects TCRs that bind to self MHC

  • called MHC restriction

  • selection made by cortical epithelial cells in the thymus

    • express both MHC I and MHC II

• Instructs expression of either CD4 or CD8 and other genes

  • interact with self MHC I → CD8

  • interact with self MHC II → CD4

  • other co-receptor is downregulated

• mature into single positive thymocytes

Steps and Results of Negative Selection

• dendritic cells present antigen

• removal of T cells with autoreactive TCRs is central tolerance

• autoimmune regulator (AIRE) induces expression of multiple tissue-specific genes in the thymus to allow negative selection

  • mutations in AIRE result in autoimmunity

• macrophages engulf dead T cells

Autoimmune Regulator (AIRE)

a transcription factor expressed by a specialized subset of medullary thymic epithelial cells (mTECs)

• extends negative selection to include tissue-specific antigens that are not normally expressed in the thymus

Autoimmune Polyendocrinopathy-Candidiasis-Ectodermal Dystrophy (APECED)

results from loss-of-function mutations in the AIRE gene

• without AIRE, tissue-specific antigens are not expressed in the thymus, so self-reactive T cells specific for those antigens escape negative selection

• these T cells enter the periphery and attack multiple organs and tissues, causing widespread autoimmune damage

Regulatory T Cells

specialized subset of CD4 T cells that help maintain self-tolerance and immune homeostasis

• recognize self antigens presented by MHC class II molecules but do not initiate immune responses

• suppress the activation and proliferation of self-reactive naive T cells, preventing autoimmune damage

• suppress autoreactive T cells in the periphery (peripheral tolerance)

• create a backup line of defense against autoimmunity

• produce suppressor cytokines

  • IL-10, TGF-Beta

FoxP3

• transcription factor expressed by regulatory T cells

• mutation in this transcription factor causes autoimmunity

Location and Process of T Cell Development

• Proliferation and differentiation of double-negative CD3- thymocytes and double-positive CD3+ thymocytes

  • double-negative CD3- thymocytes in the subcapsular zone

• Positive Selection

  • double-positive CD3+ thymocytes in the thymic cortex

• Negative Selection

  • double-positive CD3+ thymocytes throughout cortex and especially at the cortico-medullary junction

• Entry to the Circulation

  • mature self-restricted, self tolerant, single positive CD4 or CD8 T cells leave the thymus in the blood venules

X-Linked Severe Combined Immunodeficiency (SCID)

a deficiency in T and B cells

SCID Thymus

• very small

• loss of lymphocytes

• no cortico-medullary differentiation

• Hassall’s corpsucles are absent

SCID Diagnosis

1. Enumeration of TRECs (T cell receptor excision circles, due to gene segment rearrangements)

2. Enumeration, phenotypic characterization, and functional analysis of circulating lymphocytes

3. Mutation analysis is used to provide definitive diagnosis

Defects That Result in SCID

1. Defects that impair lymphocyte survival

2. Defects in cytokine-mediated signals for lymphocyte maturation and proliferation (ex: mutation of IL2RG gene which encodes the common gamma chain, shared by IL-2R and other cytokine receptors)

3. Defects in machinery for somatic gene rearrangement that assembles the immuhnoglobulin and T-Cell Receptor genes during lymphocyte development, the so-called V(D)J recombination process

4. Mutations in genes that encode the CD3 molecules that participate in the formation of the CD3:TCR complex. This complex allows signaling through the pre-T-cell receptor

Basic 3 Roles of Naive T Cells

1. Meet specific antigen in secondary lymphoid tissu e

2. Undergo clonal expansion

3. Differentiate into effector T cells

Basic 3 Facts of Effector T Cells

1. Migrate to sites of infection

2. Antigen stimulates effector T cells to function

3. Two main kinds: CD8 T cells and CD4 T cells

Naive T Lymphocytes

• T cells that have not yet been stimulated by antigen

• Recirculate through peripheral lymphoid organs

  • visit lymph nodes

  • express L-selection that binds to ligands on High Endothelial Venules (HEV)

• Express TCR, but do NOT engage in effector functions (ex: cytokine production, cytolysis)

.

Effector T Lymphocyte

• T cells that have differentiated in response to antigen

• Perform effector functions required to eliminate pathogens

  • cytokine production

  • cytotoxins

  • cytolysis (cytotoxic T cells)

• Migrate to peripheral sites of infection or antigen entry

VLA-4

expressed by ONLY effector T cells that enables them to enter infected tissue

LFA-1, CD2, and CD44

• adhesion molecules secreted by effector T cells and naive T cells

• in effector T cells, they are made in larger quantities and it allows them to make stronger interactions with target cells

CD45O

secreted ONLY by effector T cells to increase their capacity to interact with target cells

Memory T Lymphocytes

• T cells that have differentiated in response to antigen and are

  • long-lived as clones

  • functionally silent

• capable of mounting enhanced responses to antigen challenge

• migrate to lymphoid tissues and peripheral sites of antigen entry

• survive without antigen recognition

• clones can remain for decades, replicating to maintain numbers

T-Cell Activation (T-Cell Priming)

first stage of the primary adaptive immune response in which a naive T cell encounters and responds to its specific antigen for the first time

• marks the transition from innate to adaptive immunity

• establishes a pool of antigen-specific effector and memory T cells, essential for both pathogen clearance and long-term immune protection

Activation of Naive T Cells by Antigen

• mature dendritic cell carries an antigen from infection site to secondary lymphoid tissue where it encounters a naive T cell

• naive T cell is primed encountering an antigen for the first time, initiating its adaptive immune response

• after encountering a specific antigen, they

  • proliferate

  • differentiate

  • leave lymph node in efferent lymph and enter the circulation

Immature Dendritic Cell

• functions as sentinels that capture, uptake, and process antigens from pathogens through phagocytosis, macropinocytosis, and receptor-mediated endocytosis

• located in tissues

  • known as Langerhans cells in skin

Dendritic Cells Changing Function and Morphology

• in lymphatic circulation, they stop phagocytosis so MHC II moves out of the endocytic vesicles onto the cell surface

• when reaching T-cell area of lymph node, the maturing cell halts uptake and processing of antigens and focuses on presenting the to T cells

Micropinocytosis

a receptor-mediated endocytic process used by dendritic cells to capture small volumes of extracellular fluid containing bacteria or viral particles

• a method for dendritic cells to internalize pathogens

Macropinocytosis

a NON-receptor-mediated process that captures larger volumes of extracellular fluid, including pathogens not recognized by endocytic receptors

• a method for dendritic cells to internalize pathogens

Dendritic Cell MHC Class II Processing and Presentation

used in

• receptor-mediated endocytosis of bacteria

• macropinocytosis of bacteria or viruses

then they’re presented to CD4 T cells

Dendritic Cell MHC Class I Processing and Presentation

used in

• viral infection

  • viral proteins from viral DNA get broken down

• cross-presentation of exogenous viral antigens

then they’re presented to CD8 T cells

T-Cell Area (T-Cell Zone)

outer cortex region of the lymph node where naive T cells and antigen-bearing dendritic cells interact

• primary site of T cell activation during adaptive immune responses

Homing

the process by which naive T cells exit the bloodstream and enter the T-cell zone of lymph nodes

L-Selectin

adhesion molecule on naive T cells ONLY that binds to vascular addressing on HEV surfaces

Vascular Addressins

mediate the initial tethering and rolling of T cells along HEV walls during homing

• include CD34 and GlyCAM-1

Integrins

• activated upon TCR and CD28 signaling

• help stabilize TCR:MHC interactions and cell-cell contact

• help activated T cells leave the vasculature to go to sites of inflammation

ICAM-3

binds to DC-SIGN and promotes initial stable contact between naive T cells and dendritic cells for antigen scanning

CD2 and LFA-3

an adhesion molecule on the T cell surface and its ligand on dendritic cells and other antigen-presenting cells

Cognate Pair (or Conjugate Pair)

a long-lived complex between a mature dendritic cell and an antigen-specific naive T cell

S1P

a lipid molecule that creates a chemotactic gradient within the lymph node

Functions of T Cell Integrins like LFA-1

• Adhesion of T cells to APCs

  • T cell activaiton

• Binding of T cells to endothelium

  • T cell migration

• Ligands for integrins are induced on APCs and endothelial cells by inflammatory cytokines and microbes

  • these ligands promote T cell responses to infectious agents

• Affinity of integrins for their ligands is increased by

  • chemokines (rapid response during inflammation)

  • TCR ligation (amplifies response to antigens on APCs)

LFA-1 (Lymphocyte Function-Associated Antigen 1)

• integrin adhesion molecule that is expressed by all leukocytes

• binds ICAM-1, ICAM-2, ICAM-3

• promotes adhesion and cytoskeletal reorganization during T cell activation

• promotes stable TCR:MHC interaction

Transient Adhesive Interaction Between a Naive T Cell and a Dendritic Cell

naive T cell binds to dendritic cell cell with low affinity LFA-1-ICAM-1 interactions, but through recognition of a specific antigen, a signal is sent to LFA-1 that induces a conformational change which increases affinity and prolongs cell-cell contact

Co-Stimulatory Signal

the second signal required for activation, survival, and proliferation of naive T cells, in addition to antigen recognition by the T-cell receptor (TCR) and co-receptor (CD4 or CD8)

Co-Stimulatory Receptor

binds to a corresponding co-stimulator on an antigen-presenting cell to deliver the co-stimulatory signal

• CD28 and B7 molecule

CD28

• co-stimulators receptor present on naive T cells

  • the only B7 receptor on naive T cells

• expressed as a homodimer on

  • 90% of CD4+ T cells

  • 50% of CD8+ T cells

• binds B7 expressed on APCs (including B cells)

  • when bound, it activates a T cell to divide

B7 Molecule

co-stimulator expressed on professional antigen-presenting cells

CTLA4

• acts as a negative regulator, suppressing further T-cell activation and proliferation when bound to B7 to maintain immune balance and prevent overreaction

• binds B7 20x stronger which makes it act as a brake on CD28

  • CD28 would otherwise allow the T cell to divide

Signals for T Cell Activation

• Antigen

  • determines specificity of response*

  • mediated by TCR:MHC+peptide interactions

• Co-stimulators

  • surface molecules expressed by APCs and T cells

  • provide amplification*

  • determine type of response*

• Cytokines

  • growth and differentiation factors for T cells

  • produce by APCs and T cells themselves

  • provide amplification*

  • determine type of response*

CD28:B7

• necessary for effective co-stimulation of antigen-specific naive T cells

• induce activation of PI-3 kinase and the NFkappaB signal transduction pathway

  • NFkappaB is a transcriptional response element

  
  

• promote cell survival through upregulation of Bcl-xL

• increase production of IL-2

• promote IL-4 production and T cell differentiation

Bcl-xL (B-cell lymphoma-extra large)

• transmembrane molecule in mitochondria

• anti-apoptotic protein that prevents release of mitochondrial contents like cytochrome c

  • leads to caspase activation and apoptosis

Activation of Naive T Cell (Two Signals)

• requires BOTH an antigen-specific signal AND a co-stimulator signal

  • B7 co-stimulators signal delivered by the APC

• CD28 acts as an accelerator for the naive T cell that binds B7 in APC

• CTLA4 acts as a break on CD28 on activated T cells, binding B7 20 times stronger than CD28

T-Cell Synapse

a specialized region of contact between a T cell and an antigen-presenting cell (such as a dendritic cell) where ligand-receptor interactions occur

• facilitates communication and signal integration during T-cell activation

ITAMs

short amino acid sequences located in the cytoplasmic tails of CD3 proteins and the zeta chain of the TCR complex

Summary of Intracellular Signaling Pathways

• Initiated by T-Cell Receptor Complex + CD4 or CD8 co-receptor + CD28 co-simulatory receptor

• When naive T cell interacts w dendritic cell presenting its antigen, the ZAP70 kinase activates, initiating a series of reactions

• NFkappaB, NFAT, and AP-1 change the pattern of gene expression

• cell division, proliferation, and differentiation to effector T cells

T-Cell Signaling Defects

• abnormalities in TCR signaling pathway that impair activation, proliferation, or effector function of T cells

• can result in immunodeficiency disorders

• Severe Combined Immunodeficiency (SCID)

  • results in a complete failure of T cell activation and severe impairment of adaptive immunity

• Distal TCR Signaling Defects

  • cause partial or variable immune dysfunction depending on which pathway is disrupted

IL-2

cytokine crucial for T-cell proliferation and differentiation, originally called T-cell growth factor

• acts as a central regulator of adaptive immune response, promoting clonal expansion of antigen-activated T cells

Autocrine Action

when a cytokine acts on the same cell that secreted it

Paracrine Action

when a cytokine acts on nearby but different cell types

IL-2 Receptor on Naive T Cells

express the low-affinity receptor, a heterodimer of the beta chain and the gamma chain

IL-2 Receptor on Activated T cells

express the high-affinity receptor, a heterodimer comprising an alpha-chain, a beta-chain, and a gamma-chain

How IL-2 Affects Activated T Cells

• IL-2 is synthesized and secreted by the activated T cell

• IL-2 binds to the IL-2 receptor on the same T cell

• co-stimulation through CD28 increases the synthesis of IL-2 by 100-fold in the T cell

• T cell proliferates and differentiates from signaling from IL-2

• some immunosuppressive drugs inhibit cytokine IL-2 or the IL-2 receptor to block T cell activation and differentiation

Antigen Recognition Without Co-Stimulation

may result in

• no response (ignorance)

• apoptosis

• anergy (functional inactivation)

T-Cell Anergy

an irreversible, tolerance-inducing mechanism that prevents activation of self-reactive T cells in the absence of co-stimulation

Adjuvants

substances grant enhance the immunogenicity of an antigen by stimulating the innate immune system