The adaptive immune response T cell effector functions

T Cell Effector Functions

Recap of T Cell Activation Signals

• Three signals are required for T cell activation:

  • Signal 1: Antigen recognition: The T cell receptor (TCR) on the T cell binds to the antigen presented by the major histocompatibility complex (MHC) on an antigen-presenting cell (APC).

  • Signal 2: Co-stimulation: Interaction between co-stimulatory molecules on the APC (e.g., CD80/86) and the T cell (e.g., CD28). This signal is crucial for T cell activation and prevents anergy.

  • Signal 3: Cytokines induce differentiation: Cytokines produced by APCs or other immune cells guide the differentiation of T cells into specific effector subsets (e.g., Th1, Th2, Th17). Examples include IL-12, IL-4, and IL-6.

• CD8 T cells require CD4 help, specifically IL-2 production, and increased CD80/86 expression by Antigen-Presenting Cells (APCs). CD4 T cells, particularly Th1 cells, produce IL-2, which is essential for the proliferation and survival of CD8 T cells. Additionally, CD4 T cells can enhance the expression of co-stimulatory molecules (CD80/86) on APCs, further promoting CD8 T cell activation.

T Cell Maturation

• T cell maturation occurs in stages:

  • Thymus: T cell precursors undergo positive and negative selection to ensure self-tolerance and MHC restriction.

  • Peripheral lymphoid organs: Naive T cells encounter antigens presented by APCs in lymph nodes and spleen, leading to activation and differentiation.

  • Peripheral infected organs: Effector T cells migrate to sites of infection to eliminate pathogens.

  • Lymphoid and infected organs (effector phase and antigen elimination): T cells carry out their effector functions, such as cytokine production and cytotoxicity, to clear the infection.

Learning Outcomes: T Cell Effector Functions

• Describe the functions and immune protection provided by Th subsets (Th1, Th2, Th17, Treg, and Tfh).

• Describe how directed T cell migration is achieved.

• Illustrate the effector functions of cytotoxic T (Tc) cells, including killing mechanisms.

• Understand the importance of homeostasis and T cell memory.

CD4 Th Cell Effector Functions

• CD4 Th cell effector function is largely governed by the cytokines they produce:

  • IFNγ → macrophage (mΦ)/ NK & CD8 T cells: IFNγ activates macrophages, enhancing their ability to kill intracellular pathogens. It also promotes the activation of NK cells and CD8 T cells, contributing to cell-mediated immunity.

  • IL-4/IL-5 → humoral response: IL-4 and IL-5 stimulate B cell activation, proliferation, and differentiation into antibody-secreting plasma cells. IL-4 promotes the production of IgE, while IL-5 enhances eosinophil activation.

  • IL-21 → ab production: IL-21 is a potent B cell growth and differentiation factor, promoting antibody production and isotype switching.

  • IL-10 and TGFβ → anti-inflammatory: IL-10 and TGFβ suppress immune responses by inhibiting the activation and function of immune cells, such as macrophages and T cells. These cytokines play a crucial role in maintaining immune homeostasis and preventing autoimmunity.

  • IL-17 → inflammation/ mucosal immunity: IL-17 induces the production of chemokines and cytokines that recruit neutrophils and other immune cells to sites of infection, particularly in mucosal tissues.

• Polarizing cytokine(s) (produced by APC/DC) influence effector cytokines (produced by CD4) and the type of response:

  • IL-12, IFNγ, IL-18: Pro-inflammatory, targets intracellular pathogens: These cytokines drive the differentiation of T cells into Th1 cells, which are essential for combating intracellular pathogens, such as viruses and bacteria. These cytokines also activate macrophages and NK cells.

  • IL-4: Anti-inflammatory, involved in peripheral tolerance: IL-4 promotes the differentiation of T cells into Th2 cells, which are involved in humoral immunity and allergic responses. IL-4 also contributes to peripheral tolerance by suppressing the activation of autoreactive T cells.

  • IL-6, IL-21: B cell activation, Ig isotype switch, and affinity maturation (all infections): These cytokines stimulate B cell activation, immunoglobulin isotype switching, and affinity maturation, leading to the production of high-affinity antibodies that can effectively neutralize pathogens.

  • IL-1, IL-6, TGFβ: Mast cell, eosinophil, and B cell activation & IgE production (helminths / toxins/ allergy): These cytokines promote the activation of mast cells, eosinophils, and B cells, leading to the production of IgE antibodies that are involved in allergic responses and the elimination of helminth parasites.

Updated CD4 Th Subsets:

• TH1 cell

  • Polarizing cytokines: IFN-γ, IL-18, IL-12, IL-6, TNF-α

  • Master transcriptional regulator: T-Bet

  • Effector cytokines: IFN-γ, TNF

  • Effector functions: Combats intracellular pathogens, activates macrophages

  • Role in disease: Tissue inflmmation, Autoimmunity

• TH2 cell

  • Polarizing cytokines: IL-4

  • Master transcriptional regulator: GATA3

  • Effector cytokines: IL-4, IL-5, IL-13

  • Effector functions: Combat extracellular pathogens, particularly helminth worms, activates eosinophils

  • Role in disease: Allergy

• TH17 cell

  • Polarizing cytokines: IL-6, IL-23, IL-21, IL-1, IL-6 + TGF-B

  • Master transcriptional regulator: RORYt

  • Effector cytokines: IL-17A, IL-17F, IL-22

  • Effector functions: Combats extracellular pathogens (bacteria, fungi) in barrier tissues

  • Role in disease: Autoimmunity, Tissue inflammation, Inflammatory skin disease

• TFH cell

  • Polarizing cytokines: IL-12, IL-6, TGF-B

  • Master transcriptional regulator: Bcl-6

  • Effector cytokines: IL-4, IL-21

  • Effector functions: Regulates affinity maturation of germinal center B cells

  • Role in disease: extracellular pathogens, including food antigens, toxins

• Peripheral TREG cell

  • Polarizing cytokines: TGF-B

  • Master transcriptional regulator: FoxP3

  • Effector cytokines: IL-10,TGF-B

  • Effector functions: Suppresses immune responses

  • Role in disease: inhibits antitumor response.

Effector Mechanisms of Th1 Cells

• Th1 mediated immunity clears intracellular microbes.

  • Macrophage activation: Th1 cells produce IFN-γ, which activates macrophages, enhancing their ability to phagocytose and kill intracellular pathogens. Activated macrophages also produce cytokines, such as TNF-α and IL-12, which further amplify the immune response.

  • Increase IgG3 and IgG2a production by B cells, leading to opsonization and phagocytosis: Th1 cells promote the production of IgG3 and IgG2a antibodies by B cells. These antibody subtypes are efficient at opsonizing pathogens, making them more susceptible to phagocytosis by macrophages and neutrophils.

  • Neutrophil activation: Th1 cells can activate neutrophils, enhancing their ability to kill extracellular pathogens through the production of reactive oxygen species and the release of antimicrobial peptides.

Effector Mechanisms of Th2 Cells

• Th2 cells eliminate helminthic parasites.

  • Promote antibody production, which neutralizes microbes and toxins: Th2 cells stimulate B cell activation and differentiation into antibody-secreting plasma cells. The antibodies produced can neutralize microbes and toxins, preventing them from infecting cells.

  • IgE production: Th2 cells promote the production of IgE antibodies, which bind to mast cells and basophils. Upon encountering allergens or helminth parasites, IgE-bound mast cells and basophils release inflammatory mediators, leading to allergic reactions and the expulsion of parasites.

  • Eosinophil activation: Th2 cells produce IL-5, which activates eosinophils. Eosinophils release toxic granules that kill helminth parasites.

  • Suppress MΦ: Th2 cells can suppress macrophage activation, which is important for preventing excessive inflammation in response to helminth infections.

Effector Mechanisms of Th17 Cells

• Th17 cells produce IL-17.

• IL-17 induces chemokines (immune cell attractants) which recruit neutrophils and monocytes, increasing inflammation and protecting against bacteria. IL-17 also stimulates the production of antimicrobial peptides by epithelial cells, further enhancing the barrier function.

• Th17 also maintain epithelial barrier function. They stimulate epithelial cells to produce tight junction proteins, which strengthen the barrier and prevent pathogen invasion.

• Th17 are important for mucosal (e.g. gut) immunity. They play a crucial role in protecting mucosal surfaces from infection by bacteria and fungi.

• Eliminate extracellular bacteria and fungi.

Effector Mechanisms of Tfh Cells

• Tfh cells activate B cells to proliferate and differentiate into antibody-producing plasma cells. This interaction occurs in the germinal centers of lymph nodes and spleen.

• They develop in concert with Th1, Th2, or Th17 cells. This allows Tfh cells to tailor the antibody response to the specific type of pathogen.

• They help B cells to generate different Ig isotypes (class switch) and undergo affinity maturation in the germinal center. Isotype switching allows B cells to produce antibodies with different effector functions, while affinity maturation increases the affinity of the antibodies for their target antigen.

• The Ig isotype selected depends on the most appropriate innate immune effector interaction. For example, IgG antibodies are effective at neutralizing toxins, while IgE antibodies are important for eliminating helminth parasites.

• Tfh cells contribute to the eradication of most classes of pathogens, unlike Th1, Th2, and Th17 cells. They coordinate the antibody response to a wide range of pathogens.

• First BCR bound ag is internalized, degraded, and presented in MHCII to Tfh. This allows Tfh cells to recognize and interact with B cells that have encountered their specific antigen.

Effector Mechanisms of Tregs

• Tregs inhibit effector responses in 3 ways:

  1. Tregs produce anti-inflammatory cytokines (IL-10 and TGF-b), which can affect all tissues. IL-10 and TGF-β suppress the activation and function of immune cells, such as macrophages and T cells, thereby reducing inflammation.

  2. Tregs can provide CTLA-4, which engages CD80/86, inducing anergy. CTLA-4 is an inhibitory receptor that binds to CD80/86 on APCs, preventing them from delivering co-stimulatory signals to T cells. This leads to T cell anergy, a state of unresponsiveness.

  3. Tregs can "rip-off" and endocytose CD80/86, blocking signal 2, leading to anergy. By removing CD80/86 from the surface of APCs, Tregs prevent these molecules from interacting with CD28 on T cells, thereby blocking co-stimulation and inducing anergy.

Summary of CD4 Th Effector Function

• Different subtypes of Th cells have different functions, largely depending on the cytokines they make.

• Key Th cells are: Th1, Th2, Th17, Tfh, and Treg.

• Some Th cells carry out their function in the periphery, activating other immune cells like mφ, PMN, Eosinophils.

• Some Th cells have their effector functions in the lymph node (B cell activation).

• Cytokines produced by each:

  • Th1: IFNγ

  • Th2: IL-4, IL-5

  • Treg: IL-10, TGFβ

  • Th17: IL-17

  • TfH: IL-21

T Cell Migration

• T cells respond to pathogens only on direct contact with pathogen-derived antigen; therefore, they must migrate to sites where antigen is found.

• Analogy: Imagine a few thousand balloons (T cells) detecting tiny structures (antigens) that arise suddenly anywhere on Earth, recognizable only by direct contact. An intricate guidance system must be at work.

• T cells in circulation are directed to the right part of the body by the interaction between molecules on the T cells and on endothelial cells lining blood vessels. These molecules include adhesion molecules, such as selectins and integrins, and chemokine receptors.

• Cytokines (produced in the innate immune response) upregulate adhesion molecule expression on endothelium to initiate migration. This allows T cells to bind to and roll along the endothelium.

• Chemokines are a type of cytokine that attract cells to particular sites (e.g., the lymph nodes or infected tissue). They bind to chemokine receptors on T cells, triggering intracellular signaling pathways that lead to cell migration.

• CD8 (Cytotoxic) T cells and some Th cells carry out their function in the periphery (killing or activating other immune cells like mφ, PMN, Eosinophils).

• Some Th cells have their effector functions in the lymph node (e.g., B cell activation), directed by chemokine gradients.

T Cell Migration - Recap

• Naïve entry to lymph node (HEV)

• Effector entry into inflamed tissue

• E- and P-selectin

• CXCL10

• LFA-1 and VLA-4

• CXCL10

• Process:

  • T cells bind loosely, roll, and then enter tissue by diapedesis/ extravasation. A similar mechanism underlies migration of granulocytes and monocytes into tissue.

  • Lose adhesion

  • Activation of integrins on T cell

  • Stable arrest

  • Extravasation/ migration

Chemokine Gradient

• The production of chemokines at the infection site creates a chemokine gradient that attracts cells into the tissue. A similar mechanism underlies migration of granulocytes and monocytes into tissue.

T Cell Migration

• Naïve and effector T cells need to migrate to different tissues. Naive T cells migrate to secondary lymphoid organs, such as lymph nodes and spleen, while effector T cells migrate to sites of infection.

• Upon activation and differentiation, T cells alter their adhesion molecule expression. This allows them to migrate to different tissues.

• Adhesion molecule expression is partly determined by the APC cell, which helps direct the T cell to the right tissue (e.g., gut). For example, APCs in the gut express adhesion molecules that promote the migration of T cells to the gut mucosa.

T Cell Migration Summary

• T cell migration happens in 4 steps: rolling, activation by chemokines, adhesion, and extravasation.

• This is mediated by selectins (rolling) and integrin (firm adhesion) and chemokines and integrin activation.

• T cell adhesion molecule expression changes dependent on cell differentiation (naïve vs effector) and the location of the infection.

Effector Mechanisms of CD8+ T Cells (CTLs; cytotoxic T lymphocytes)

• CD8+ cells are usually cytotoxic in nature. They kill target cells that are infected with viruses or other intracellular pathogens.

• Killing is antigen-specific: The target cell must bear the same antigen that activated the naïve CTL presented in MHC-I on its surface. This ensures that CTLs only kill infected cells.

• CTL must contact the target cell. Adjacent cells lacking the target antigen in MHC-I are not affected. This prevents CTLs from killing healthy cells.

• CTL remain intact after they kill target cells. Each CTL is capable of killing many target cells in sequence. This allows CTLs to efficiently clear infections.

• "Selective (ag-specific) serial killers"

Mechanisms of CD8+ T Cell Killing

• Two main mechanisms:

  1. Granzyme/perforin granules

  2. Fas – Fas Ligand

• Both result in apoptosis (programmed cell death) by inducing caspase activation. This prevents the release of cellular contents that could damage surrounding tissues.

• Apoptosis vs. Necrosis:

  • Programmed (controlled) cell death.

  • Premature (uncontrolled) cell death.

  • Uncontrolled release of cell material induces inflammation.

Granzyme and Perforin Mediated Death

• Focusing of granules at target cell. When a CTL binds to a target cell, it releases granzymes and perforin from its granules. Perforin creates pores in the target cell membrane, allowing granzymes to enter the cell. Granzymes then activate caspases, leading to apoptosis.

Summary of T Cell Effector Function

• Once activated in the lymph node, many T effector cells leave and carry out their effector functions in tissues.

• Effector cells are attracted to the site of infection by chemokines and adhesion molecules.

• CD8 (Tc) cells kill target cells that present the same Ag/MHCI on their surface by Fas-FasL or granzyme/perforin.

Unconventional T Cells

• Thought to be somewhere between innate and adaptive immunity.

  • γδ T cells

  • iNKT cells

  • MAIT (Mucosal-associated invariant T cells)

• γδ T cells

  • T cells TCR: γ:δ

  • Antigen: Non-peptide, lipids

  • MHC: Independent (like- PRR)/ MHC like proteins

  • Location: Skin, mucosal lymphoid tissue, blood

  • Main target: Bacterial infections, pathogenic toxins, stress markers

  • Function: Cytotoxicity (perforin/granzyme) and cytokines (IFNγ, TNFα and IL-17). Can phagocytose

• iNKT cells

  • T cells TCR: iα:β

  • Antigen: Microbial lipids/ glycolipids

  • MHC: CD1

  • Location: Mucosal surfaces

  • Main target: Infected/ transformed cells/ bacterial infections

  • Function: Cytotoxcity (mainly Fas:FasL) and cytokine (e.g. IL-2/TNFα)

• MAIT

  • T cells TCR: Semi-iα:β

  • Antigen: Microbial-derived vitamin B2 metabolites

  • MHC: MR1 (MHC class I related)

  • Location: Mucosal surfaces

  • Main target: bacteria, mycobacteria and yeasts

  • Function: Cytotoxcity and cytokine (e.g. IL-17, IL-18, IL-12, IFN- I and IFNy)

• T cells

  • T cells TCR: α:β

  • Antigen: peptide

  • MHC: MHCI/II

  • Location: Lymph, peripheral tissue, blood

  • Main target: Infected/ transformed cells

  • Function: Cytotoxcity (perforin /granzyme and Fas:FasL) and determining immune response (cytokines)

Unconventional T Cells and ILC

• Thought to be somewhere between innate and adaptive immunity.

• Tissue resident.

• Mucosal surfaces.

• First responders.

Further Reading

• Kuby 8th Edition – Chapters 8, 10, 12, and 14.

• Male, Brostoff, Roth, Roitt 8th Edition – Section 2: Chapters 9&10

• "Basic Immunology” Abbas, Lichtman and Pillai.

• “Janeway’s Immunobiology” Murphy, Travers and Walport.

• British Society for Immunology (BSI) Bitesize; https://www.immunology.org/public-information/bitesized-immunology