Endosomal TLR-3 and Viral Recognition

Endosomal TLR-3

• TLR-3 is an endosomal pattern recognition receptor present in immune cells.
• It specifically recognizes double-stranded RNA (dsRNA), commonly associated with RNA viral infections.
• Activation of TLR-3 leads to an antiviral immune response when dsRNA is detected during viral replication.

Complement System Overview

• Comprises a group of proteins in blood that enhance antibody and phagocytic immune responses.
• Main functions include opsonization (coating pathogens for phagocytosis), lysis (forming membrane attack complex to kill pathogens), and promoting inflammation via anaphylatoxins (C3a, C5a).

Complement's Role with Intracellular Bacteria

• Opsonization helps prepare extracellular bacteria for phagocytosis before host cell entry.
• Once bacteria enter host cells, the complement system's direct impact is limited, as it cannot access these intracellular spaces.

T Cells (T Lymphocytes)

• Key players in adaptive immunity, maturing in the thymus gland and eliminating infected/cancerous cells.
• Types include Helper T Cells (CD4+), Cytotoxic T Cells (CD8+), and Memory T Cells.

Helper T Cells (CD4+ T Cells)

• Secrete cytokines to assist other immune responses.
• Activate B cells for antibodies, stimulate cytotoxic T cells, and coordinate immune activity.
• Subtypes include:
  • Th1: Targets intracellular pathogens like viruses.
  • Th2: Aids in responses to extracellular parasites and allergens.
  • Th17: Involved in fungi response and autoimmune diseases.
  • Regulatory T Cells (Tregs): Suppress immune responses to prevent autoimmunity.

Cytotoxic T Cells (CD8+ T Cells)

• Function to directly kill infected or malignant cells.
• Identify infected cells through MHC I antigen presentation and induce apoptosis via perforin and granzymes.

Memory T Cells

• Provide long-term immunity and a quicker response upon re-exposure to antigens.

B Cells (B Lymphocytes)

• Mature in bone marrow, primarily responsible for antibody production.
• Functions include:
  • Producing specific antibodies that neutralize pathogens or mark them for destruction.
  • Presenting antigens on MHC II to helper T cells, enhancing the immune response.

Mechanisms of Antigen Presentation

• MHC Class II molecules present exogenous antigens to CD4+ T cells after processing by antigen-presenting cells (APCs).
• MHC I molecules present intracellular antigens, crucial for activating CD8+ T cells.

Cross-Presentation

• Allows dendritic cells to present extracellular antigens on MHC I, crucial for activating CD8+ T cells against non-directly infected cells.
• Mechanisms include:
  • Cytosolic Pathway: Antigens processed in the cytosol, then loaded onto MHC I.
  • Vacuolar Pathway: Antigens processed within endosomes, with direct loading onto MHC I.

Clinical Relevance of Cross-Presentation

• Important for vaccine development and cancer immunotherapies targeting CD8+ T cells.
• Mismanaged cross-presentation may lead to autoimmune diseases.

Cytoplasmic and Endosomal DAMPs

• DAMPs signify stress or damage in cells, recognized by the immune system.
• Primarily related to tissue damage rather than direct viral recognition.

Extracellular TLR-2

• TLR-2 is located on innate immune cell surfaces, recognizing bacterial components and some viral proteins.
• Primarily associated with bacterial infections rather than viral.

Cytoplasmic NOD-like Receptors (NLRs)

• Recognize bacterial components in the cytoplasm, activating inflammatory responses.
• Less involved in viral recognition compared to bacterial infections.

Complement System and Intracellular Bacteria

• Operates mainly against extracellular pathogens; limited effectiveness against intracellular bacteria.
• Cell-mediated immunity, especially CD8+ T cells and macrophages, is crucial for managing intracellular infections.

Summary of Immune Response

• The complement system aids in the initial response but relies more on cell-mediated immunity after pathogens enter cells.
• B cells, through humoral immunity, actively produce antibodies to neutralize extracellular pathogens and toxins.

Explore the crucial role of TLR-3 in immune response as a pattern recognition receptor located in endosomes. This quiz covers the function of TLR-3 in recognizing double-stranded RNA (dsRNA) during viral infections, emphasizing its importance in the immune defense mechanism. Test your knowledge on this vital component of the immune system!