Immune System & Hematopoiesis

• The immune system includes natural barriers, innate defenses, and adaptive defenses.
• Natural barriers include skin, mucus, ciliated epithelium, and gastric acid.
• Innate defenses are rapid, non-specific, and don't form memory.
• Adaptive defenses are slower, specific, and capable of forming memory.

Immune System Cells & Hematopoiesis

• Monocytes are immune cells with a kidney/pacman shaped nucleus.
• Neutrophils have a 3-5 lobed nucleus connected by a thin layer of chromatin.
• Eosinophils have a bi-lobed nucleus and more granules in the cytosol.
• Basophils have granules that cover the nucleus so that it's not visible.
• Small lymphocytes are the smallest cell; the nucleus takes up most of the cytoplasm.
• Granulocytes of myeloid origin include neutrophils, basophils, and eosinophils.
• Neutrophils function as phagocytic killers and are first responders.
• Basophils are involved in inflammation and allergic responses.
• Eosinophils combat worms and are involved in inflammation and allergic reactions.
• Mast cells are myeloid and function in local inflammatory and allergic reactions.
• Macrophages are myeloid; monocytes mature into DCs and macrophages.
• Macrophages are large phagocytes and first responders, stimulated by IFNγ.
• Dendritic cells reside in tissues and are responsible for processing and presenting foreign matter (APCs).
• Lymphocytes (T cells) are lymphoid.
• CD4+ T helper cells direct immune response and promote B cell differentiation.
• CD8+ T cytotoxic cells recognize and kill infected or abnormal cells.
• Lymphocytes (B cells) are lymphoid.
• B cells differentiate into plasma cells and form antibodies, providing humoral immunity.
• B cells mature in the bone marrow, while T cells require maturation in the Thymus
• Bursa Fabricius (Chicken butthole) is where B cells were discovered.
• The spleen is a major blood filter.
• Lymph nodes are major lymphatic fluid filters.

Outline of Innate and Adaptive Responses

• The process starts with the innate arm, progressing from a local basis to inflammatory response.
• Next is the acute-phase response, then the antigen-specific response, systemic response, and finally memory formation.

Initiation of Innate Response

• PAMPS (pathogen-associated molecular patterns) and DAMPS (damage-associated molecular patterns) are released.
• PAMPS and DAMPS promote iDCs and macrophages to release inflammatory cytokines like IL-1, IL-6, and TNFα.
• Once enough cytokines are present, the acute-phase arm is initiated.

Cytokines Produced

• Cytokines produced in the inflammatory response (endogenous pyrogen) include IL-1, IL-6, and TNFα.

Activation of Complement System

• The complement system activates the inflammatory responses
• Activation can occur through the classical pathway, alternative pathway, or lectin pathway
• The classical pathway requires an antibody to engage the microbe.
• The alternative pathway involves direct interaction with the microbe.
• The lectin pathway involves mannose-binding lectin.
• Chemotactic factors (C5a) attract phagocytes.
• Anaphylatoxins (C3a and C5a) stimulate mast cells to produce histamines and increase vascular permeability.
• Opsonins (C3b) promote phagocytosis.
• Direct Killers (C5b) act as the nucleation factor for the MAC.
• B-cell activation is mediated by C3d.
• C5b + C6 + C7 + C8 are inserted into the membrane to form the Membrane Attack Complex.
• Up to 15 C9s polymerize around the complex to form MAC.
• MAC induces cell lysis.

Chemotaxis & Leukocyte Migration of Neutrophils

• The process of chemotaxis & leukocyte migration of Neutrophils involves "slow, roll, attach, and extravasate".
• Chemokines establish a "runway" for phagocytic cells.
• Neutrophil integrins interact with endothelial selectins, resulting in rolling, margination, and diapedesis.
• Appendicitis is an example of the process.

Antigen-Presenting Cells

• Macrophages, Neutrophils, and Dendritic cells are Antigen-Presenting cells

Phagocytosis

• In phagocytosis, microorganisms in membrane-enclosed vesicles are delivered to a lysosome by membrane fusion.
• Attachment occurs via TLRs, LPS receptors, and PAMP receptors → Internalization → Digestion → antigen presentation via MHC II or MHC I to T cells.
• Oxygen-independent digestion involves lysozymes, phospholipase A2, ribonuclease, desoxyribonuclease, and proteases.
• Oxygen-dependent digestion involves reactive oxygen species and reactive nitrogen species.

Natural Killer (NK) Cells

• Natural Killer Cells are employed if the pathogen is viral
• NK cells exhibit antibody-dependent cell-mediated cytotoxicity.
• NK cells contain perforin and granzymes that initiate apoptosis
• They secrete IFNγ, which stimulates macrophages.
• Promotes a shift toward TH1 response.

Initiation of Acute-Phase Response

• The acute-phase response is activated by IL-1, IL-6, TNFα, tissue injury, prostaglandins, leukotrienes, and interferons.
• Cytokines such as IL-1, IL-6, and TNFα which are part of the inflammatory response from the innate arm.
• Tissue injury from PAMPS (exogenous pathogen) and DAMPS (endogenous pathogen).
• Prostaglandins and Leukotrienes stimulate cyclooxygenases, leading to pain signaling.

Acute-phase Response

• The acute-phase promotes changes that support host defenses, including fever, anorexia, sleepiness, metabolic changes, complement components, coagulation proteins, and C-reactive proteins.
• C-reactive proteins are produced in the liver and serve as a biomarker of inflammation.
• C-reactive proteins complex with bacterial/fungal polysaccharides (opsonization).
• CRP activates the complement pathway.

Overall Effects of Acute-Phase Response

• Vasodilation
• Increased vascular permeability
• Recruitment of neutrophils to the site of infection
• C-protein release

Cardinal signs of inflammation

• Heat
• Pain
• Redness
• Swelling

Inflammatory Cascade

• Activation of extracellular kallikrein leads to bradykinin production, resulting in vasodilation, histamine release, prostaglandin production, and pain sensation.

Bridging of the Innate arm and Adaptive arm

• Immature DCs (iDCs) circulate and secrete cytokines when they sense something foreign.
• Immature DCs perform phagocytosis, digest pathogens, carry antigen debris to lymph nodes, and activate naive T-cells (TH0).

Cell-mediated Response

• Cell-mediated response combats intracellular pathogens through TH0 → TH1, requiring IL-12 and IFNγ.

Humoral Response

• Humoral response combats extracellular pathogens through TH0 → TH2, requiring IL-4.
• Antigen-presenting cells present antigens to T cells through MHC I or MHC II.

MHC I

• MHC I is present on all nucleated cells.
• MHC I presents endogenous antigens.
• Proteins inside the cell are degraded by proteasomes.
• Peptide residues are moved to the ER by TAP.
• Peptides are loaded onto MHC I molecules.
• The complex is transported through the Golgi and goes to the cell surface.
• MHC I displays antigens to CD8 + cytotoxic T cells.

MHC II

• MHC II is present only on antigen-presenting cells (APCs).
• MHC II presents exogenous antigens.
• MHC IIs are processed in the ER and move toward the cell surface, fusing with antigens from phagocytic cells.
• Antigens load onto MHC II, and the complex goes to the cell surface.
• MHC II displays antigens to CD4 + helper T cells.

Adaptive Arm Antigen-specific Immune Response - T cells

• Humoral response involves CD4 TH2 T cells (initiated by IL-4) that produce IL-4 and IL-10.
• Humoral response enhances B cell differentiation into plasma cells and antibodies and enhances IgG production.
• Cell-mediated response involves CD4 TH17 T cells (initiated by IL-12 and IFNγ) that produce IL-17 and TNFα.
• Cell-mediated response activates epithelial cells and neutrophils and promotes the production of antimicrobial peptides.
• CD4 TH1 T cells promote and reinforce the inflammatory response, activate macrophages, and are essential for combating intracellular bacteria.

B cells

• Immunoglobulin Formation
  • IgA - mucosal immunity
  • IgD - Naive B cell antigen receptor
  • IgE - defense against helminth parasites, immediate hypersensitivity (allergy) -*IgG - opsonization, complement activation, antibody-dependent mediated cytotoxicity, neonatal immunity -- Innate recycling system, has largest half life
  • IgM - First antibody release in any infection, naive B cell antigen receptor, complement activation
• Transcytosis: movement of an antibody from the basolateral side to the apical side

Antiviral Responses

• The goal of the immune response in a viral infection is to eliminate both the virus and infected host cells.

Innate Response

• Interferons are the first antiviral response
  • Interferons stimulate cell-mediated immunity, enhancing recognition of virally infected cells and activating effector cells
  • Interferons activate an "antiviral state" in target cells and adjacent cells, enhancing T-cell recognition of the infected cell
  • Interferons cause systemic symptoms like malaise, myalgia, chills, and fever
  • There are two categories of interferons

Interferons

• (Type I) Interferon α is produced by B-cells, epithelial cells, monocytes, macrophages, iDCs, DCs. Activates the JAK-STAT pathway and increases expression of MHC I.
• (Type I) Interferon β is produced by Fibroblasts. Activates the JAK-STAT pathway and increases expression of MHC I. - (Type II) Interferon γ is produced by Activated T-cells, NK cells. Macrophage activating factor and increases expression of MHC II on macrophages.

Natural Killer Cells

• Antibody-dependent cell-mediated cytotoxicity
• Contains performance and granzymes Initiates apoptosis
• Secretes IFN - stimulates macrophages
• Pushes toward TH1

Adaptive Arm

• 1. CD4 TH1
• 2. CD8 cytotoxic T-cells

Secondary Viral Challenge

Bacterial Immunopathogenesis

• Inflammatory/acute-phase responses can initiate significant tissue and systemic damage:
• • Antibodies that share determinants with human proteins --- Rheumatic fever: antibodies are made against bacteria but we have human heart proteins that look a lot like these antibodies, which leads to cross-reactivity
• • Overproduction of IL-1, IL-6, TNFα --- Chronic overproduction → granuloma formation (dense infiltration of leukocytes) --- Systemic production → Sepsis / shock
• • Non-specific activation of CD4+ T cells by superantigens --- Cytokine storm → Shock ---- Pyrexia, systemic vasodilation, hypotension, systemic leukocyte adhesion to endothelial tissues, reduced heart contractility, diffuse capillary damage in the lung, coagulation pathways activated leading to DICs

Bacterial Evasion of Protective Responses

• Mechanisms Include
  • Inhibition of phagocytosis and intracellular killing in phagocyte
  • Inactivation of complement function
  • Cleavage of IgA
  • Intracellular growth (avoid antibodies)
  • Change in bacterial antigenic appearance

Viral Immunopathogenesis

• RSV
• Varicella Zoster Virus: Dermatome
• • VZV creates chicken pox and then is later reactivated into shingles
• • Symptoms of many viral diseases are the consequence of cytokine action and immune responses: --- Cellular damage --- Flu-like symptoms --- Immune complex disease
• • Adults experience shingles more dramatically than children experience chicken pox due to stronger immune response

Viral Evasion of Immune Responses

• Mechanisms
• • Evade detection (e.g., staying intracellular, downregulating MHCI)
• • Preventing activation (e.g., ebola prevents STAT1 from getting into the nucleus)
• • Encode protein that suppress the immune system

Viral Evasion of Immune Responses

• Epstein Barr Virus (EBV)
• • Specialized protein EBNA1 (expressed by latent EBV infected cells)
• • Mechanism of evasion Inhibits proteasome degradation, thereby inhibiting MHCI from recognizing it
• Kaposi's Sarcoma Associated Herpes Virus (KSAHV)
• • Specialized protein LANA 1
• • Mechanism of evasion Inhibits proteasome degradation, thereby inhibiting MHCI from recognizing it
• Herpes SImplex Virus
• • Specialized protein ICP47
• • Mechanism of evasion Interacts directly with TAP to prevent peptide binding to MHC I
• Human Cytomegalovirus
• • Specialized protein US6
• • Mechanism of evasion Inhibits ATP binding by TAP