BENG 186A Lecture 11: Immune Response

What is innate v. adaptive immunity?

• Innate

  • Rapid response cells, neutrophils, macrophages, dendritic cells, mast cells associated with allergic reactions, and natural killer cells

  • They respond without prior exposure

• Adaptive immunity

  • B and T cell lymphocytes. Highly specific antigen receptors. B cells produce antibodies and T cells coordinate responses

  • Long lived memory

What is innate immunity?

• Predetermined

• Physical barriers such as skin and mucous membranes

• Physiological barriers such as body temperature, gastric pH and enzymatic activity

• Phagocytic cells such as macrophages and neutrophils

• Natural killer cells target infected or abnormal host cells

• Circulating proteins such as opsonins and complement, and these help to enhance pathogen recognition

• Cytokines help to coordinate and amplify immune signaling

What is innate immunity recongnition?

• Based on pathogen associated molecular patterns

  • 20 different types of molecules are recognized

  • Recognized by tall-like receptors or patter recognition receptors

  • Examples” Unmethylated CpG DNA, double stranded RNA, and bacteria cell walls

  • Enables a broad detection of infection without requiring prior exposure

What are innate immunity effectors?

• Intracellular pathogens

  • Reduce viral replication

  • Host cell killed by natural killer cells

  • Cellular suicide, apoptosis

• Extracellular pathogens

  • Direct killing by natural killer cells

  • Phagocytosis - macrophages and neutrophils

  • Control infection before adaptive immunity is fully activated

What is adaptive immunity?

• Specific, can recongnize 10^9-10^11 different antigens

• Memory - faster, stronger response upon re-exposure

• B-cells make antibodies and target extracellular pathogens and toxins

• T-cells are the main mechanism to remove intracellular pathogens

  • Th - helper T cells produce cytokines

  • Tc - cytotoxic T cells kill cells

What is an antigen?

• A molecular species that is recognizable by the adaptive immune system

• Often found in vaccines - focus the immune response on defined target antigen rather than the whole pathogen

What is B-cell recongnition?

• Antibodies

  • Around 150kDa protein made up of 4 protein domains, two heavy and two light chains

  • Produced by B cells

  • Two antigen binding sites on each antibody, both identical

  • Each effector function is mediated by fc region which is the bottom part of the antibody that can interact with other immune components

  • Can be secreted, or on B cell membranes in which case they are basically B cell receptors

What is B cell specificity?

• The specificity removes self reactive cells to prevent autoimmunity

• B cell development in bone marrow

• Thymus is associated with T cell development

• MHC is major histocompatibility complex and it presents antigens

• B cells are exposed to self antigens, if they have affinity those cells are eliminated

• When removal of self antigen specific B cells goes awry, self reactive B cells can persist and cause autoimmune disorders

What is antibody response to biomaterials?

• Mechanisms that biomaterials can create a humoral antibody mediated response

• Biomaterial itself is an antigen → directly stimulate antibody production

• Biomaterial combined with native proteins to create antigens

• Biomaterial products such as leached agents and phagocytosed biomaterial is broken down

What is the humoral response? What does the production of antibodies depend on?

• Antigen

• Antigen concentration, higher concentration leads to stronger or more sustained response

• General state of immune system affects how effectively B cells are activated

• Presence of sensitizing agents which are adjuvants, and they help to enhance the response by increasing immune activation and improve antigen presentation

What are B cell effector regions? What is the antibody Fc region?

• For the fc region isotype, IgM secreted by native B cells and souble form appears as a pentamer

• IgD is on the surface of B cells it is not secreted

What is effector B cell class switching?

• IgG most abundant and key for systemic immunity

• Igf has a role in hypersensitivity and allergy and engages mast cells

• IgA is found in tears, saliva and milk to protect mucosal surfaces

What is fc region?

Affects how each isotype directs how antibodies interact with other immune cells and the downstream effects

What is antibody function for non cell mediated fucntions?

• Non cell mediated functions neutralize or eliminate pathogens by binding antigens

• Agglutination is aggregation of large particulate material by antibodies to block function and crosslink large particles

• Precipitation is antibody-antigen complex that leads to precipitation of antigen that forms an insoluble complex that is cleared from circulation

• Neutralization is when antibodies bind to block function

• Lysis is when antibodies bind to and lead to lysis of the cell membrane

What is cell mediated antibody function?

• Opsonins tag antigens for phagocytosis in antibody dependent cell mediated phagocytosis

• Antibody dependent cellular cytotoxicity is when antibody binding leads to recognition by natural killer cells and cell specific killing, and this links antibody specificity to cellular effector functions

What are antibodies raised against many antigens?

• Conventional wisdom: polyethylene glycol will help material hide from the immune system by reducing protein adsorption and masking surface recognition

• Property of PEG that helps it avoid immune system is hydrophilicity as well as flexible polymer chains which create a hydration layer that limits immune cell and protein binding and decreases antigen recognition

• Antibodies specific for PEG have been found, especially after repeated exposure and this means that PEG becomes an antigenic target

What is uricase?

• Gout

• Uric acid crystals build up and cause painful joint inflammation

• Uricase is an enzyme that can catalyze uric acid to allantoin which is more soluble and easily excreted product

• Uricase is not naturally expressed in humans

• Uricase can reduce uric acid levels and treat gout, and it can also be immunogenic and trigger unwanted immune response

What is pegloticase?

• Krystexxa is polyethylene glycol modified uricase

• 10 strands of 10kDa PEG modified onto uricase to reduce the immunogenicity to shield from immune recognition and prolong circulation time

• Around 40 percent of patients developed anti PEG antibodies whether delivered subcutaneously or intravenously

• Mild to moderate adverse reactions, however severely increased the clearance of drug for those who developed antibodies

What are the lessons learned about PEG?

• PEG has little to no immunogeniticity which means it is not recognized by immune system

• PEG protein conjugates had significant antibody response, especially if the protein is non human → protein component remains immunogenic

• More PEG modification such as larger number of chains per protein led to reduced antibody recognition and better shielding of antigenic sites

What is T cell recognition?

• T cells recognize antigen fragments presented on major histocompatibility complex

• Antigen fragments created through proteolytic processing

• T cell recognizes MHC/antigen through a T cell receptor which leads to a highly targeted immune response

What are T cells? What are the CD4+ helper T cells and the CD8+ cytotoxic T cells?

• CD4+ helper T cells secrete specific cytokines related to the immune response for example to enhance phagocytosis or stimulate specific immune cells

• CD8+ cytotoxic T cell directly eliminate infected or abnormal host cells and they release perforin and granzymes from granules to directly kill cells

• Perforin forms pores in target cells membranes and allows granzymes to enter and trigger programmed cell death and limit damage to surrounding tissue

What is donor compatibility?

• MHC mismatches between recipient and donor can cause organ rejection

• Set of alleles MHC is expressed from is called MHC haplotype

• 8-10 HLA (human leukocyte antigen) marker, each with hundreds of possible variations that need to match which is challenging

• 25% change of HLA match with a sibling

What are donor-host relationships?

• Autograft is the same individual, fully compatible and not rejected

• Allograft is same species and a risk of immune rejection due to differences in MHC (HLA) molecules

• Xenograft is between different species and this can be animal to human and can generate a strong immune response due to large evolutionary differences in antigenic markers