Lecture 12: Effector Responses Against Infection (2)

Similar Activation of CD4+ and CD8+ T-Cells

• CD4+ and CD8+ T-cell subsets are generated in the thymus and seed peripheral tissues, especially lymph nodes.

• They wait there to be instructed by an antigen to be activatde and take on effector functions to kill pathogens.

• Both require an antigen and express a T-cell receptor (TCR), which provides specificity toward peptide-derived antigens from different pathogens.

• Initial antigen presentation alerts them to infection, but is insufficient for full activation – co-stimulation is required due to multiple controls and checkpoints.

Antigen Recognition in CD4+ T-Cells

• T-cells recognise antigens in the context of MHC class II.

  • Presented by phagocytes and antigen-presenting cells (APCs) like dendritic cells (DCs), which are professional APCs.

  • DCs present antigen on MHC II to stimulate T-cell proliferation and immune response generation.

Antigen Recognition in CD8+ T-Cells

• T-cells recognise antigens in the context of MHC class I.

  • MHC I is expressed on all cells, presenting both self-antigens and, during infection, pathogen-derived antigens.

  • DCs help to prime these cells through cross-presentation, loading external antigens onto MHC I.

Signal Required for T-Cell Activation

• Signal 1: Antigen Presentation

• Signal 2: Co-stimulation via receptor-ligand engagement.

  • T-cells express CD28.

  • Antigen-presenting cells (APCs) like dendritic cells upregulate CD80 and CD86 in response to activation (e.g. through PRRS recognising conserved patterns like LPS).

  • CD80/CD86 interact with CD28 through cell–cell contact, delivering the second signal that licenses T-cell activation.

• Signal 3: Cytokines

CD4+ and CD8+ T-Cell Function

• CD4+ and CD8+ T-cells function differently to provide the immune system with a range of responses for different pathogens.

• Activated CD4+ T-cells specialise into T helper subsets, which activate other immune cells downstream through cytokine production (soluble molecules).

• CD8+ T-cells are specialised killer cells in the adaptive immune response.

  • They have greater specificity and magnitude and kill infected cells directly by producing cytotoxic molecules.

Coordination of Immune Response by CD4+ and CD8+ T-Cells

• CD4+ and CD8+ T-cells communicate with each other to enhance immune responses.

• This communication helps them become better activated and produce growth factors that support clonal expansion.

• CD4 and CD8 responses occur simultaneously, and depending on the nature of the pathogen, multiple types of immune responses may be triggered.

CD4+ T-Cells

• Don’t generally kill pathogens or infected cells directly → instruct other cells to do the killing

• Function by helping other parts of the immune system

CD4+ T-Helper Cell Subsets

• Cells provide broad protection against various infections and challenges via a single, highly specific adaptive immune response that specialises into different Th subsets:

  • Th1

  • Th2

  • Th17 (produce IL-17)

  • TfH – help B-cells produce antibodies

  • Tregs – suppress immune responses to maintain balance, prevent inflammation and tissue damage

• Signal 3 (cytokine) is required to activate and imprint cells with their functional

• Dendritic cells (DCs) present the antigen, provide co-stimulation, and produce specific cytokines that prgramme the cells

  • Each subset requires a different soluble cytokine signal that determines its fate.

Cytokines

• Soluble signalling molecules e.g. IL-1, which act to regulate many different immune responses

• Cells function through the type of signalling molecule they produce

CD4+ Th1 Cells Regulation of Immune Response

• Cells contain transcription factors that control gene expression and drive cellular specialisation and their distinct immune functions.

• Th1 responses are typically directed against intracellular pathogens (e.g. viruses, some bacteria) that hide inside host cells to avoid immune detection → enter cytoplasm and reproduce

• T-cells recognise intracellular infections and provide signals to help infected cells kill the pathogen.

• In some cases, macrophages become infected, dysfunctional and inactive because microbes can evade immune pressures and survive inside them → microbes encode genes that help escape immune pressures and survive in cells

  • TB → Mycobacterium tuberculosis (Mtb) prevents the fusion of the phagosome with the lysosome, a critical step needed by the macrophages to degrade the microbe.

    • blocking phagosome maturation, using its own secreted proteins

• Th1 cells produce cytokines that act on macrophages to reactivate and enhance their pathogen-killing functions.

Th1 Activation of Macrophages

• Secretion of IFN-γ: Cytokine is vital for killing intracellular pathogens; it promotes macrophage production of reactive oxygen species (ROS) and noxious chemicals to enhance pathogen killing.

• CD40L-CD40 interaction: Direct contact between a Th1 cell and a macrophage engages the CD40 ligand on Th1 and the CD40 receptor on the macrophage, triggering downstream signalling and activation of immune effector functions.

• Innate and adaptive immunity interact to produce optimal immune responses.

Effects of macrophage activation by Th1 cells

• Upregulation of MHC II in response to IFN-γ → enhances antigen presentation to T-cells, increasing the activation of T-cells.

• Creates a positive feedback loop: more T-cell activation → more IFN-γ → stronger immune response. - ensures pathogen will be killed

• Activation occurs slowly, requiring prolonged contact between Th1 cells and macrophages.

• Sometimes referred to as Delayed-Type Hypersensitivity (DTH), T-cell and innate cell interactions drive long-lasting protective immunity.

How Do Th1 Cells Reinforce and Expand the Immune Response?

• Th1 cells produce cytokines (e.g. IFN-γ, GM-CSF) that act on the bone marrow, driving production of monocytes and macrophages from stem/progenitor cells.

• These cytokines enter the bloodstream and reach the bone marrow to increase macrophage-type cells, reinforcing the immune response.

• Th1 cells also produce IL-2, a growth factor needed by T-cells and NK cells for survival and proliferation.

  • → Promotes a feed-forward loop where T-cells help expand the immune response during infection.

• The immune system also uses this feedback to replenish immune cells lost due to cell death during infection.

CD8+ T-Cell

• Cytotoxic T-cells release cytokines like IFN-γ and TNF-α, which contribute to inflammation and antiviral responses.

• They also use physical cell contact mechanisms to induce cell death:
  → FasL (on T-cell) binds to Fas (on target cell) → triggers apoptosis (programmed cell death).

• This is a targeted and efficient way to eliminate infected cells.

CD4+ Th2 Cells Regulation of the Immune Response

• Cells evolved to mediate immunity against large helminth worms, which are still endemic in many warmer climates despite eradication efforts elsewhere.

• Helminth worms have been a strong evolutionary pressure on the immune system, leading to the development of specialised Th2 cells.

• Worms are large parasites, visible to the naked eye, and anchor into the intestine and gut, feeding on blood and stealing nutrients.
  → This can lead to malnutrition and growth abnormalities due to nutrient competition.

• Cellular responses are generally generated to deal with large extracellular pathogens (e.g. parasitic worms/helminths).
  → These pathogens are too big to be phagocytosed and can cause significant tissue damage if the immune response is not carefully regulated → pose a big threat

CD4+ Th2 Immune Response: Generation of IL-4

• Classically thought to help B-cells produce antibodies via IL-4, which helps B-cells produce IgE antibodies.

• Th2 cells use the CD40 ligand-CD40 pathway to communicate with other cells and provide different cytokines.

• IL-4 acts as a growth factor for B-cells.

• The germinal centre within lymph nodes selects B-cells and allows them to grow and proliferate.
  → IL-4 from Th2 cells causes B-cells to proliferate and differentiate into plasma cells (terminally differentiated B-cells specialised in secreting Ab) that produce IgE.

• Although strong evidence now suggests this is a function of Tfh cells, as they also produce IL-4.

CD4+ Th2 Cells: Activation of Granulocytes

• Activation of basophils, eosinophils, and mast cells via the type 2 immune response.

• Causes these cells to exit the bone marrow, enter tissues, and release their granules.

• Response mediated by Th2 cytokines, e.g.:

  • IL-4,

  • IL-5 (triggers eosinophil reaction),

  • IL-13.

• Signals from Th2 cells cause the production of more granulocytes.

• Granulocytes release noxious chemicals from granules → helps kill large parasites (e.g. worms), leading to parasite death.

Th2 Activation of Alternative Macrophages

• Th2 cells communicate with innate myeloid cells (e.g., macrophages) to induce a different pathway.

• Th2 cells produce IL-4 and IL-13, which act on macrophages to drive this alternative activation.

• These macrophages produce effector molecules such as:

  • TGF-β – a suppressive cytokine that promotes tissue repair.

  • REL-α – also involved in tissue repair.

• Important in infections like large helminth worms that damage the intestine — the immune system needs to kill the worm while also beginning tissue repair.

CD4+ Th17 Cells: Regulation of the Immune Response

• Each T-cell subset is specialised for specific pathogens and works to recruit different innate and granulocyte populations into tissues.

• Th17 cells are specialised for extracellular bacteria (e.g., E. coli in the gut) and fungi (e.g., Candida albicans – thrush or mouth infections).

• Cytokines produced by Th17 cells:

  • IL-17:

    • Induces chemokine expression from epithelial and fibroblast cells → recruits immune cells

    • Recruits neutrophils into infected tissues.

  • IL-22:

    • Kills extracellular pathogens.

    • Promotes production of antimicrobial peptides that destroy bacteria and fungi (e.g., by punching holes in them).

Activation of CD8+ T-Cells to Become Cytotoxic T Lymphocytes (CTLs)

• CD8+ T-cells recognise infected cells via MHCI, typically presented by dendritic cells (DCs) that have phagocytosed or absorbed pathogen proteins.

• IL-2 (growth factor) is required for their expansion and proliferation to find and kill infected cells.

• MHCI is critical for CTLs to identify target cells.

• After initial activation, CTLs no longer require co-stimulation to kill.

• CTLs locate infected cells, localise it and release cytotoxic granules to kill them – this is the main mechanism of CD8+ T-cell-mediated killing.

How Do CTL Kill Infectected Cells

• CTL loosely binds to the target (infected cell/APC).

• Antigen recognition by CTL via MHCI presentation.

  • The CTL reorganises its cytoskeleton to direct granules to the site of contact.

  • This allows granules to cluster at the point of contact and be delivered precisely, avoiding collateral damage to nearby healthy cells.

• CTL release cytotoxic granules across the narrow cell-cell contact interface:

  • Enables specific, quick killing (within minutes).

  • Induces apoptosis (programmed cell death) in the infected cell.

Apoptosis

• Programmed form of cell death → Controlled death that is less inflammatory than necrosis (intracellular contents released driving inflammation and immune response

• Induced via cytotoxic granules and causes the condensation of the nucleus and membrane blebbing

CTL Cytotoxic Granule Contents

• Granzymes

  • serine proteases that induce apoptosis in target cell

• Granulysin

  • antimicrobial and can induce apoptosis

• Perforin

  • helps deliver granule contents to target cell – released first and perforates the target cell, producing holes in the membrane to allow the entry of the granules

CD8+ Cytotoxic T Cell Target Recognition

• CD8+ T cells recognise antigen presented via MHC Class I molecules, which are expressed on all nucleated cells in the body.

• This allows them to detect intracellular infections in any nucleated cell.

• Killing is highly selective — CD8+ T cells only kill infected cells, not healthy ones.

CD8+ T-Cell Mediated Killing

• Killing infected cells stops viral spread to other cells.

• CD8+ T cells only recognise their specific (cognate) antigen and will only kill infected cells, leaving neighbouring healthy cells untouched.

• Secretion of cytotoxic granules is highly directed, targeting only the bound cell for destruction.

CD8+ CTLs Regulating the Immune Response

• CD8+ T cells produce cytokines, including IFN-γ, which:

  • Activates macrophages

  • Upregulates MHC I on infected cells (enhancing antigen presentation) → feedforward loop

  • Induces interferon-responsive anti-viral genes in target cells e.g. IFR1, boosting antiviral immunity

• CD8+ T cells also secrete TNF-α and LT-α, which further help activate macrophages.

• These actions form a feedforward loop, amplifying the immune response.

How Do CD4+ Th1 and CD8+ T Cells Work Together Against Intracellular Pathogens?

• Th1 and CD8+ T cells act in concert to mount an effective immune response.

• Both can help activate macrophages to kill intracellular pathogens.

• Th1 cells produce IL-2, a growth factor that supports the proliferation of nearby CD8+ T cells.

• This creates a complementary and synergistic effect, strengthening immune response.

• Though they have distinct roles, their actions reinforce each other within infected tissues.

Activation and Functions of CD8+ T-cells (CTLs)

• CD8+ T-cells get activated to cytotoxic T-cells (CTLs).

• CTLs induce apoptosis (cell death) in infected cells by directing the release of cytotoxic granules.

• Killing is highly specific, preventing collateral damage to healthy cells.

• In addition to killing, CTLs produce cytokines like IFNγ, TNFα, and LTα to promote other immune responses.

Role of Tfh’s in Antibody Production By B-Cells

• Tfh cells help B-cells class switch and produce different antibody isotypes.

• This leads to the formation of specialised plasma cells that:

  • Produce large amounts of antibody.

  • Are long-lived, contributing to effective immunity.

• This process is a key mechanism adopted by many vaccines.

Antibody Effector Mechanisms

• NOT DIRECTLY TOXIC

• They generally FACILITATE the destruction of antigens by ENHANCING innate immune mechanisms –antibody has a specific region to recognise a specific antigen – can recognise conserved structures from a pathogen

  • 1. NEUTRALISATION

  • 2. AGGLUTINATION

  • 3. OPSINISATION – covering a bacteria or pathogen in an antibody can assist other aspects of the immune system

  • 4. ANTIBODY-DEPENDENT CELL-MEDIATED CYTOTOXICITY (ADCC)

  • 5. ACTIVATION OF COMPLEMENT

  • 6. SPECIALISED RESPONSES

Ab Effector Function: Neutralisation

• Antibodies bind directly to pathogens or toxins to prevent attachment and entry into host cells, physically blocking their function.

• Many bacteria produce toxins (e.g., cholera) that bind to host cell receptors and cause damage. Antibodies recognise and block the toxin, preventing this interaction.

  • act to neutralise the toxin

• Antibodies can bind to viral spike proteins (e.g., COVID and the ACE2 receptor), blocking the virus from entering host cells via receptors.

• Antibodies can also block molecules or secretion systems used by bacteria to inject toxins into cells

Antibody Effector Function: Agglutination

• The clumping of particles (e.g., bacteria) by antibodies.

• In the presence of bacteria, antibodies bind to the bacteria and form a complex mesh/lattice that clumps the bacteria together.

• This large antibody-bacteria complex is easier for phagocytes to engulf and clear from the body.

• Additionally, this agglutinated complex can be removed via natural body processes:

  • Intestinal contractility (helping to expel bacteria)

  • Airway clearance (e.g., triggering a cough reflex to expel bacteria).

• This can involve the complement system, creating a complex network of antibodies and complement proteins to stick pathogens together.

Antibody Effector Function: Opsinonation

• Ab can help bind complement factors to the surface of microbes and ‘prepare the table’ for macrophages.

• Macrophages have an Fc receptor that recognises conserved regions of Ab (Fc region) and will bind and bring the bacteria to their surface of the phagocyte to be engulfed

Antibody Effector Function: Antibody-Dependent Cell-Mediated Cytotoxicity

• antibody binds to an infected cell or a large extracellular pathogen (e.g., parasites) that is too large to be phagocytosed.

• The antibody acts as a bridge between the pathogen and immune cells via Fc receptors.

  • Immune cells involved: NK cells, macrophages, eosinophils, and neutrophils.

• Degranulation occurs where these immune cells release toxic granules that mediate the death of the infected cell or pathogen.

• If a pathogen is covered in antibody, signals through Fc receptors lead to a stronger immune response by enhancing granulocyte degranulation.

• For example, parasitic larvae covered in antibodies attract eosinophils that localise to the pathogen and release toxic granules to kill it.

Antibody Effector Function: Complement

• Triggers activation of the complement cascade – kills cells by physically lysing cells

Antibody Effector Function: Specialised Response

• Synergism between adaptive and innate immune responses occurs via the production of specialised antibodies, like IgE.

• Th2 cells and Tfh cells help drive the production of the IgE isotype.

• Mast cells are covered with the FcεR1 receptor, which specifically recognises IgE.

• When IgE binds to the FcεR1 receptor, mast cells degranulate, releasing histamine, toxic granules, lipid mediators, and cytokines.

• These mediators:

  • Kill the parasite.

  • Initiate an inflammatory response by:

    • Increasing vasodilation.

    • Increasing vascular permeability.

    • Leading to the accumulation of immune cells at the infection site.

  • This process is a feed-forward loop that helps kill the pathogen.