Immune Responses to Pathogens Part II

Immune Responses to Pathogens - Part II

• Adaptive immunity is more effective than innate immunity, it relies and depends on innate immunity
• Dendritic cells are part of the innate immune response that activates the adaptive immune response
• Dendritic cells present antigens (pathogen-derived peptides) to naive T cells
• A co-stimulatory signal is provided by T cells, both activate and initiate an adaptive response
• Immune responses include:
• Extracellular bacteria
• Intracellular bacteria
• Viruses

Intracellular Bacterial Infections

• Intracellular pathogens, like Mycobacterium tuberculosis and Listeria monocytogenes, invade and survive inside macrophages
• These pathogens evade innate immunity and replicate inside macrophages
• Released bacteria infect other macrophages when the original macrophage dies
• Intracellular infections are not affected by antibodies
• Cell-mediated immunity involving T cells and macrophages is needed for intracellular infections
• The goal is to super-activate macrophages to eliminate intracellular bacteria

Innate Immune Response

• Innate immunity responds in the same way to all pathogens, regardless of the type
• Phagocytosed bacteria can evade the destruction process, they do this by:
  • Surviving in acidic environments like M. tuberculosis
  • Escaping the phagosome, Listeria does this
  • Modifying the phagosome membrane, Salmonella, to prevent fusion of lysosomes
• Some phagosomes successfully fuse with lysosomes and degrade bacterial proteins
• Antigenic peptides from degraded bacteria are displayed on MHC class II proteins
• Macrophages express CD40 on their surface
• Infected macrophages die without signaling, and this allows bacterial to spread
• Macrophages cannot migrate to lymph nodes, T helper cells must travel to them

Recruiting Help

• At the infection site, dendritic cells phagocytose bacteria

• Dendritic cells migrate to the nearest lymph node

• Dendritic cells process the pathogen and present peptides on MHC class II proteins in the lymph node

• T helper cells are, at the lymph node, activated and migrate to the site of infection

• T helper cells secrete IFN-y (interferon-gamma) at the site of infection

Super Activation of Macrophages

• Macrophages need two signals to become super-activated:
• T helper cells secrete IFN-y and other cytokines
• CD40L (on T helper cells) binds to CD40 (on macrophages)
• Super-activated macrophages will:
• Increase lysosome-phagosome fusion
• Produce reactive nitrogen metabolites (NO), oxygen radicals, and proteases
• These toxic compounds kill intracellular and extracellular bacteria
• They can also damage any host tissues leading to inflammation
• T helper cells further drive inflammation by:
• Increasing blood flow to the area through cytokines
• Recruiting neutrophils and macrophages
• Enhancing bactericidal activity of immune cells

Summary (Immune Responses to Pathogens)

• Cell-mediated immunity is crucial for intracellular bacterial infections
• Innate response: macrophages phagocytose bacteria, but these often fail to kill them
• Some Bacteria are processed and their peptides are loaded onto MHC class II proteins
• Dendritic cells activate T helper cells in the lymph node
• Activated T helper cells migrate to the infection site
• T helper cells recognize infected macrophages and super-activate them
• Super-activated macrophages produce toxic substance that will kill bacteria but also damage the tissue
• A side effect of this is inflammation and tissue damage

Introduction (Virus Infections)

• Adaptive immunity is more effective than innate immunity, but only if the innate immune response is triggered
• Adaptive immunity is triggered by dendritic cells, part of the innate immune response
• Presenting pathogen-derived peptides to naive T cells
• Providing a co-stimulatory signal to fully activate naive T cells
• Viral infection process:
• Virus binds to a cell surface receptor
• Viral genome enters the host cell
• Virus uses host machinery to replicate and produce new virus particles
• New viruses infect neighboring to continue the cycle
• The immune response to viral infection involves:
• Antibody response (neutralizes virus, prevents cell entry)
• Cell-mediated response (cytotoxic T cells kill infected cells.)

Virus Infections

• Viruses must infect cells to be able to replicate
• Cycle of viral infection:
• Virus binds a cell surface receptor
• Viral genome enters the host cell
• Virus uses the host machinery to replicate and assemble new virus particles
• New viruses spread to other cells, continuing the infection
• Immune response:
• Antibodies neutralize viruses and prevent infection
• Cytotoxic T lymphocytes (CTLs) kill infected cells to stop viral replication

Innate Responses to Virus Infections

• Pattern Recognition Receptors (PRRs) detect viruses.
• Toll-like receptors (TLRs) on endosomes does this by:
• Detecting double-stranded RNA (viral-specific)
• Uncapped single-stranded RNA
• Cytoplasmic DNA (unusual in host cells)
• Secretion of Type I Interferons (IFN-α/β) activates as a cascade
• IFN-a/β bind to receptors on uninfected cells, leading to an anti-viral state
• Uninfected cells shut down protein synthesis, preventing viral replication
• Slows the virus, while adaptive immunity is required for elimination

Antibody Responses

• Antibodies produced by cells, neutralize viruses by preventing them from host cells
• Antibodies sources:
• Natural infection
• Vaccination
• The process of antibody production is the same between extracellular bacterial pathogens and virus infections

Cell Mediated Immunity

Activation of T Cells

• Viral proteins in infected cells are processed:
• Cytoplasmic viral proteins are presented via MHC class I
• Dendritic cells endocytose viruses and present via MHC class II
• T cell activation includes:
• TCR must recognize the MHC-peptide complex
• Co-stimulation via CD28-B7 interaction
• Cytokines are needed for full activation

Cross-Presentation in Dendritic Cells

• Dendritic cells can present exogenous viral peptides on both MHC class I and II
• This allows the activation of both:
• CD4+ T helper cells
• CD8+ cytotoxic T cells (CTLs)
• This method generates effective T cell responses to viruses that don't infect DCs

Cytokines & CTL Activation

• Dendritic cells provide:
• IL-12 & IL-18 to instruct T helper cells to produce IFN-y
• IL-12 & IL-18 to CD8+ T cells to stimulate perforin & granzyme production for effector CTLs
• Activated T helper cells produce IL-2, facilitating CTL proliferation as they are unable to produce sufficient levels of IL-2
• Effector CTLs (killer T cells) require just MHC-peptide recognition and no co-stimulation

CTL Killing Virus - Infected Cells

• Effector CTLs patrol for infected cells displaying MHC class I + viral peptide
• the Killing process
• CTL binds the cell infected via TCR-MHC class I interaction
• After binding, perforin creates pores in the membrane of the target cell
• Granzymes will use perform pores to entre and trigger the apoptosis sequence
• CTL detaches to find another infected cell

Key Features of CTL Killing

• Requires direct cell-to-cell contact
• Antigen-specific -> only infected cells displaying viral peptides are killed
• Serial Killing -> CTLs kill multiple target cells
• After viral clearage:
• Most effector CTLs undergo apoptosis
• Some CD4+ and CD8+ T cells become memory cells

Summary (Viral Infections)

• Both antibody and cell-mediated responses combat viral infections
• The antibody response: Neutralizes virus and prevents infection of new cells
• Cell-mediated response:
• Dendritic cells cross present viral peptides presented on MHC class 1 and 2, and this activates both Helper T cells and CTLs
• CTLs identify and kill infected cells by using TCR-MHC class I interaction
• Killing is both antigen-specific and calls for perforin and Granzyme

Introduction (Auto Immune Diseases)

• Clonal deletion eliminates most self-reactive T and B cells during development in the Thymus and the Bone Marrow
• Failure of clonal deletion can cause autoimmune diseases, where the immune system attacks the body's tissues
• Not fully understood how Self-reactive lymphocytes escape
• Autoimmune disseases are categorized on involvement of
• Anti-bodies attacking self components
• T cells that mediating tissue destruction

Antibody-Mediated Autoimmune Disease: Myasthenia Gravis

• This is caused by the production of anti-bodies against acetylocholine receptors on muscle cells

• Mechanism:

• Anti-bodies bind and block acetylocholine receptors and therefore prevent neurotransmiter signaling

• Endocytosis and degradation of receptors leads to fewer receptors available

• Reduced neuromuscular junction efficiency results in muscle weakness or paralysis issues

• Normal - nerve signals release acetylcholine, which binds receptors on muscle cells and sodium influx casuses muscles to contract

• Myasthenia gravis:

• Autoantibodies reduce receptor numbers

• Weakened muscle occurs due to impaired neurotransmission

• Autoimmune can diseases can attack:

• Specific organs/tissues (Myasthenia Gravis)

• Multiple self-components (Lupus)

  • Lupus:
    • Antigen anti-body complexes from precipitates
    • Can cause kidney blockage and organ domage
    • Commonly treated with steriod hormones to supress immune response

Cell Mediated Auto immune Disease: Type 1 Diabetes Mellitus T1DM

• Self-reactive T cells attack insulin-producing beta cells in the pancreas
• Genetic and Enviormental Triggers
• Genetic predisposed individuals exposed to enviormental antigens develop an agressive automimmune response
• Mechanism
• Antigen precenting cells in partreatic isotets process enviormental antigens
• CD4+ and CD8+ T cell are activated and infiltrate the islets, causing chronic inflamation
• T cell-mediated Beta-cell destruction
• CD4+ T cells release pro-inflamatory cytokines and active macrophages
• CD8+ T cells directly include opoptosis of beta cells
• Anti-bodies against islet cells are persent in early disease stages but there role in destruction in unclear
• Outcome:
• Leads to a loss of insulin production

Summary (Auto Immune Dieases)

• Autoimunity occurs when the immunesystem attacks normal tissues/organs, leading to domage of disfunction
• Types of autoimune disease:
• Anti-body mediated:
• Antibodies block tissue/organ function
• Complement activation may lead to tissue destruction
• Cell-mediated:
• T cells secretr cytokins, cassuing tissue damage
• Macrophage activation leads to furthur destruction of tissues/organs

Introduction (Graft Rejection)

• Transplantation involves removing an organ, tissues, or hematopioetic steem celss from a donor and placing it into a recipient
• Types of transplants include:
• Autograft: A transplant within the same person (using veins from the leg in heart bypass surgery as an example)
• Allograft: A transplant from one individual to another
• MHC mismatch and immune response:
• Donor tissue has different MHC alleles from the recipient, so makes them non-self antigents
• T cells may strongly bind forgn MHC proteins and trigger an immune response, known as Host.vs-Graft disease
• Graft-vs-Host Disease
• Occurs when donor immune cells will hematopoid stem cell transplants are done
• Recipient will recognizerz the recipients MHC as forign, and attack the hosts tissues

Organ Rejection "Host-vs-Graft" Disease

• Occurs when the recipients t cells arracted to the transplanted organ and tissues Two mechamzsism of T cell aviation
• Indirect allorecognition
• Recipiet APCs take up and processs Doner MHC Proteins
• T cells recogniz these peptides as foring and Mount and immunesponce
• Simar to pathogen specic immunity but directed against the graft
• Direct alloerecognition
  • Donor APCs Transfered with the grafs migrate and interact with recepiton T cells
  • Reeipientt cells recogniz the doner MHC as forign and react strongly, even without pepride perantation
  • Con lead to at cross reaction which t cells mistakenly recorgmize forign mHC as selfmHC bound to forign peptides
  • immune response to the greft reactions

Immune Response in Graft Rejection

• Activated T helper cells:
  • Infiltrate the graft and recruit/activate macrophages
  • Macrophages release inflammatory mediators & proteases, damaging the graft and also a delay type bypersensitivity response
  • Macrophage Secrete IL-2 to activate cytokic T linphoztyes CtLS
• B cell activation:
• Reognize foreign MHC proteins and differentiate into plasma cells
• Plasma cells produce antibodies aainst doner cells
• Compiement activation lead too membrane attack complex Mac formation and destruction of donor cells Cytotoxix T cells CTLs
• Recogniz and kull transplanted cells expressing Foreign MHC class in One that the major cars or greft reflection

Minimizing Graft Rejection

• Tissue typing: Identifies Hla and HMC Al leles to find amatching Doner
• Best Doner I - Dentical twin for genetic match almost always accepterd Closera letive higher likeiihood of shared MHC allegies
• Immunosuppressive drigs: Cyclosporin A surpresses T celk acitvation Increases infection risk weekened to kill virus Infected cells
• Other methods
• Improved sugury techniques Using healthier and less damaged organs

Summary (Graft Rejection)

• Successful transplantation depends on the recipient's immune system accepting the organ, accepting your tissue
• Matching MHC alleles minimizes rejection risk
• Strategies to improve greft acceptance
• T celk suppression immunosupressive drugs
• Use of healthier less damaged organs
• Better su rgicaltechniques

Introduction (Antibodies as Tools in Research)

• Antibodies have various uses in clinical, research, and diagnostic settings

• They can be produced by:

• Amuniving animals; rabbits, nice, goats

• Hybridoma cells, grown in tissue culture

• Perified anti-bodies are used for

  • Detection of antigens

• Quantification of antigens

Diagnostic Uses of Antibodies

• Medical applications of antibodies:
• Pathogen detection: identifies bacteria, viruses, toxins in patient samples
• Serological testing detects antibiotics in the blood as Corona test Hormone level measurement: thyroid hormones, pregnancy tests
• Blood and tissue analysis; used for blood typing and immune cell enumeration
• Therapeutic use: traetments for cancers, crons diese, psoriasis

Polyclonal Antisera

• Production:

• Amunization of animals, rabbits goats with angent

• Sream liquid portion of blood after clothing contains multiple antibodys recognizing different Eppies

• Key characteristics

• Multiple b cells respond to different Eppies

• Plasma cells to produce varied anti bodies and results in a polyclonal antearam

• Limited Supply and lack of Standardization

• Limitations:

  • Not useful for distingueshing mninor antigetic differences e

• G Mutant V us wild Typerus

• Monochlonan anti-Bodies so below arprelerable

• speciticity

Monoclonal Anti-Bodies

• Production Process:

• Mouse immunization with an antigen strong seconday immunesponce

• B cells activated porliferate differentic into plasma cells

• Spleen removal fusion of plasma cells with myeloma cells cancerious

• Hybridoma Formation: Red tains and Martelli from My elon

• Red tains anitbody production from plasma cells Seleition of Hybridmas producing thed sired and with ELISA Expansion perification of monoclonal antbodies Storige Hybridmas can be frozin forlong term use

• Bat medium seleviton Hybridmas servivi

• unfiusred myeloma cells de duetolack HGPRT Un fused Plazema celle de naturally due toss tortlite

EnzymeLinked Amun sorbent Assay ELISA

• Highly sensitive method for detecting anti bodies so antigen
• types of ELISA and atbody detection and antigen detection Eliza play coated with artiggen Hybridmaculture medium added

Wizard has to remove sound anti bodies

• secondary enzyme linked the anti body added find is primary anti body enzyme sub straight election color change indicates anti body presence automated Eliza readers determine concentration antigen detection
• primary tibody code is the Eliza plate theorem sample containing antigen added - in line secondary ibody added and is antigen clor change pontifies antigen concentration via standard curve
• High sensitivity can detect picogram amounts of antigen

Immunofluorescence

• uses fluorescent - abele de bodies to detect Proteins on the cells - procedure
• fluorescent art to body binzs to antigen bearing cell wizard has to remove sound anti bodies light excited Sion fluorescent emissions dectec
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Summary (Antibodies)

• antibody applications pathogen detection blood and tissue timing hormone measuring polytomnal anitcera recognise multiple eppipis not standard supply monoclonal anti bodies specific for a single episope produced usig Hybrdoma technology elisa detects bodies all antigens highly sentitive is immunorfleoresence ses fluorescent anitsed by moicroscopy or FACS cell seortig and immune profiting

Adaptive immunity relies on innate immunity, with dendritic cells activating T cells by presenting antigens. This collaboration initiates an adaptive immune response effective against extracellular bacteria, intracellular bacteria, and viruses. Intracellular bacterial infections require T cell and macrophage-mediated immunity due to the pathogens' ability to evade antibodies and replicate within macrophages.