Understanding the Complement System

Complement System

• The complement system comprises circulating and membrane-associated proteins.
• These proteins function in both innate and adaptive immunity.

Component Nomenclature

• Components are named C1 through C9, along with factors B, D, and P.
• Fragments of complement components are created through cleavage.
• These fragments are indicated by lowercase letters like C5a, C5b, C4a, C4b.

Complement Activation

• In innate immunity, the complement system is activated through the alternative and mannan-binding lectin pathways.
• In adaptive immunity, the complement system is activated through the classical pathway.

Classical Pathway

• Activated by antigen-antibody complexes (IgG, IgM).
• C1 (C1q, r, s) binds to the Ag-Ab complex, which leads to C1 activation.
• C1 acts as an enzyme, cleaving both C2 and C4.
• C4b + 2b form the C3 convertase C4b2b, which cleaves C3.
• C3b binding to C4b2b forms C4b2b3b.
• C4b2b3b is the C5 convertase that cleaves C5 into C5a and C5b, initiating the membrane attack complex.

Mannan-Binding Lectin (MBL) Pathway

• MBLs are serum proteins that bind to specific carbohydrates.
• MBL binding to mannose residues on glycoproteins activates this pathway.
• After MBL binds to mannose, it interacts with 2 MBL-activated serine proteases (MASP1 & MASP2).
• MASP activation leads to the activation of C2, C4, and C3, similarly to the classical pathway.

Alternative Pathway

• Initiated by cell-surface components of microbes recognized as foreign, like LPS.
• Spontaneous breakdown of C3 occurs, the most abundant serum complement component.
• C3b attaches to receptors on the surface of microbes.
• C3b binds to factor B.
• Factor B is cleaved by factor D, producing C3bBb, an unstable C3 convertase.
• C3bBb binds properdin factor (factor P) to produce stabilized C3 convertase.
• Additional C3b fragments are added to form C5 convertase C3bBb3b.
• C5 convertase cleaves C5 into C5a and C5b.
• C5b inserts into the cell membrane of microbes, initiating the membrane attack complex and cell lysis.

Terminal or Lytic Pathway

• Can be entered from the classical, MBL, or alternative pathways.
• Attachment of C5b to the bacterial membrane initiates the formation of the membrane attack complex (MAC) and cell lysis.
• C5b attaches to the cell membrane, followed by the addition of components C6, C7, and C8 to form C5b678.
• Subsequent addition of multiple molecules of C9 (poly C9) forms pores in the cell membrane of microbe resulting in lytic death.
• The MAC is C5b6789(n).
• C9 is homologous to "perforin" found in Tc and NK cell granules.

Functions of the Complement

• Opsonization and phagocytosis: C3b (or C4b) act as opsonins.
• Complement-mediated lysis: the MAC creates pores in cell membranes,induces osmotic lysis.
• Stimulation of inflammatory reactions: C5a, C3a, and C4a bind to receptors on neutrophils, they stimulate inflammatory reactions, eliminate microbes.
• C5a, C3a and C4a are chemoattractants to neutrophils.
• Providing stimuli for B cell activation and the humoral immune response.

Cytokines

• Proteins secreted by immune cells in response to microbes.

Role of Cytokines

• They stimulate growth and differentiation of lymphocytes
• They activate immune cells, eliminating microbes & Ag.
• They stimulate hematopoiesis.
• They are used in medicine as a therapeutic agent.

Nomenclature

• Monokines: from macrophage/monocyte.
• Lymphokines: from lymphocyte.
• Interleukins: from leucocytes & act on other leucocytes eg IL-1 & IL-2 & IL-3.
• Cytokines is the preferred name, produced by lymphocytes, monocytes or any other cell.

Classification of Cytokines

• Mediators and regulators of innate immunity, produced mainly by macrophages and NK cells.
• TNF-α activates neutrophils, resulting in inflammation.
• IL-1 activates neutrophils, resulting in inflammation.
• IL-12 activates T & NK cells.
• IFN-α and IFN-β have antiviral action, increasing expression of class I MHC in all cells.
• Chemokines mediate chemotaxis & migration of leukocytes into tissues.
• Mediators and regulators of adaptive immunity, produced by T lymphocytes.
• IL-2 leads to proliferation of T, NK, and B cells.
• IL-4 causes B cell isotype switch to IgE and mast cell proliferation.
• IL-5 results in B cell proliferation and activation of eosinophils.
• IFN-γ leads to macrophage activation and increased microbicidal functions.
• Stimulators of hematopoiesis: GM-CSF, IL-3 and IL-7.

Cytokine Therapy

• Interferon α is used clinically in viral hepatitis HCV & in melanoma treatment.
• Interferon β is used to treat multiple sclerosis.
• IL-2 is in clinical trials for tumor treatment.
• GM-CSF is used in cancer patients to promote BM recovery and correct neutropenia.
• TNF and IL-1 antagonists treat rheumatoid arthritis.

Immunomodulation

• Adjustment of the immune response to a desired level, as in immunopotentiation or immunosuppression.

Immunopotentiation

• Enhancement of the immune response by increasing its rate and prolonging its duration.
• It is achieved through the administration of another substance called an adjuvant.

Adjuvants

• Agents that stimulate the immune system.
• They increase the response to a vaccine, without having any specific antigenic effect.
• Inorganic adjuvants include aluminium salts.
• Organic adjuvants include squalene.
• Oil-based adjuvants:
  • Complete freund's adjuvant (CFA): water-in-oil emulsion containing killed Mycobacteria.
  • Incomplete freund's adjuvant: water-in-oil emulsion without Mycobacterium.
• Virosomes: A virosome is a phospholipid bilayer vesicle containing hepatitis A and influenza antigens.
• Cytokines for example IL-12.

Mechanisms of Action of Adjuvants

• Prolong retention of the immunogen.
• Increase the size of the immunogen, promoting phagocytosis and presentation by macrophages.
• Stimulate the influx of macrophages and other immune cells to the injection site.
• Increase local cytokine production.

Immunosuppression

• Suppression of the immune system.

Indications

• Hypersensitivity responses.
• Autoimmune disease.
• After transplantation to prevent rejection.

Induction

• Drugs.
• Radiation.
• Anticancer drugs.

Methods of Immunosuppression in Clinical Use

• Cyclosporine and tacrolimus (FK-506):
  • Fungal macrolides.
  • Inhibit T cell activation by blocking T cell cytokine production (IL-2).
  • Commonly used immunosuppressive drugs for prevention of graft rejection.
  • Tacrolimus is less nephrotoxic than cyclosporin.
• Azathioprine and mycophenolate mofetil:
  • Antiproliferative drugs, inhibit synthesis of purines for cell division.
  • Block lymphocytes proliferation.
• Corticosteroids:
  • Anti-inflammatory drugs.
  • Inhibit the secretion of proinflammatory cytokines (IL-1, IL-3, IL-4, IL-5, IL-8, TNF-α).
  • Decrease the migration of inflammatory cells.
• Anti-CD3 monoclonal antibody:
  • Depletes T cells by binding to CD3 molecules.
• Anti-IL-2 receptor antibody:
  • Inhibits T cell proliferation by blocking IL-2 binding to its receptor.

Prototypical Immune Response Sequence

• Triggered when an Ag enters the body and encounters APCs.
• APCs capture a minute amount of the Ag by phagocytosis.
• 2-:Antigen processing: APCs process the Ag to very small fragments (peptides).
• Antigen presentation: APCs present peptides plus class II MHC molecules to T helper cells.
• Binding of peptide-MHC complex to TCRs → activation of helper T lymphocytes.
• Secretion of cytokines by the activated TH cells.
• IL-2 acts on the producing TH cells themselves, leading to their proliferation and activation.
• IFN-γ activates macrophages & phagocytosis.
• Activation of B lymphocytes:
  • While TH cells are being activated, some B cells recognize and bind antigen through their BCRs.
  • B cell activation needs 2 signals:
    • The first signal is binding of the Ag to BCR
    • The second one is provided by helper factors (cytokines) secreted by TH cells, e.g., IL-2, IL-4, IL-5 ( “T cell help”).
• B cell proliferation and differentiation: activated B lymphocytes can differentiate into either memory B lymphocytes or plasma cells, which secrete antibodies.
• Activation of cytotoxic T cells:
  • Tc lymphocytes recognize Ag on the surface of target cell (e.g.virus-infected cell) which express class I MHC molecules.
  • Tc cell activation also requires IL-2 & IFN-γ from a nearby activated Th cell to kill the target cell.