MICR 201 Microbiology for Health Related Sciences

1. Lecture 10: Adaptive Immunity Edith Porter, M.D. MICR 201 Microbiology for Health Related Sciences

2. Lecture outline • Concept of immunity • Innate immunity • Adaptive immunity • Humoral and cellular adaptive immunity • Antigens and antibodies • B cells and humoral immunity • Effects of antigen-antibody binding • T cells and cellular immunity • Antigen presenting cells • Cytokines • Immunological memory

3. Overview of host defenses First Line of Defense Second Line of Defense • NK cells

4. Concept of immunity Innate immunity Adaptive immunity Acquired, available within days High specificity Memory In higher vertebrates • Functional at birth • Rapid responses: preformed or available within hours after infection • Limited specificity: pattern recognition via toll like receptors • Widely present in nature including in plants, invertebrates and vertebrates

5. Humoral and cellular immunity The thymus is located in mediastinum • Humoral immunity • Transferable with serum • Highly specific • Mediated by antibodies and lymphocytes who produce these antibodies • These types of lymphocyte mature in the bone marrow and are called B lymphocytes (B cells) • Cellular immunity • Mediated by lymphocytes that mature in the thymus and are called T lymphocytes (T cells) • T cells orchestrate the immune response

6. Lymphocytes http://www.aamdsglossary.co.uk/i/c/1_2_lymphocytes.jpg

7. Differentiation of B and T cells B for Bone marrow T for Thymus

8. The lymphatic system Primary Lymphatic tissue Secondary Lymphatic Tissue Antigen contact Spleen Lymph nodes Peyer’s patches Mucosa associated lymphatic tissue (MALT) • Lymphocyte formation and maturation • Bone marrow • Thymus

9. Main tasks of lymphocytes • Recognize foreign agents (antigen) • Lymphocytes carry specific antigen receptors on their surface • B-cell receptor, T-cell receptor • Block and eliminate foreign agents • Through antibodies • By activating host defense cells via cytokines • By destroying infected host cells that have been taken over by infectious agents

10. Antigen • Substances that causes the body to produce specific antibodies • Any molecule that can be recognized by and bound to an antibody (“antibody generating”) or a T cell • Typically proteins and carbohydrates • Epitop (or antigenic determinant) is part of the antigen and is the specific region with which an antibody interacts

11. Antigenic Determinants

12. Haptens • A molecule too small to stimulate antibody formation by itself • When combined with a larger carrier molecule it can initiate antibody production • Once antibodies are generated, hapten can be recognized by itself Example: Penicillin

13. Antibodies • Globulin proteins (immunoglobulins or Ig) • Made in response to an antigen • A bacterium or virus has many antigenic determinants against which antibodies can be made • Bi-functional • One portion binds specifically to particular structures called antigen • The other part interacts with host cells

14. Antibody structure • 2 heavy chains • 2 light chains • Connected with disulfide bridges • Variable regions in heavy and light chains: bivalent antigen binding sites, mediate specificity • Constant regions on heavy chain mediate effector function

15. Antibody classes • Each class shares the constant region of the antibody molecule but has many different variable regions • Each class interacts with different types of host cells • Differ in their effector function • 5 classes: • IgG • IgM • IgA • IgD • IgE

17. IgGantibodies • Monomer • 80% of serum antibodies • Fix and activate complement (classical pathway) • In blood, lymph, intestine • Cross placenta • Opsonin (enhance phagocytosis); neutralize toxins & viruses; protect fetus & newborn

18. IgMantibodies • Pentamer • 5-10% of serum antibodies • First Ig of an immune response • Fix and activate complement (classical pathway) • In blood, lymph, on B cells • Agglutinates microbes

19. IgAantibodies • Dimer • 10-15% of serum antibodies • In secretions (milk!!) • Protectionof mucosa • Mucosal pathogens like Haemophilus or Neisseria secrete IgA proteases

20. IgDantibodies • Monomer • 0.2% of serum antibodies • Mainly on B cells • Maturation sign

21. IgEantibodies • Monomer • ~0.002% of serum antibodies • Mainly on mast cells, basophils,and activated eosinophils • Allergic reactions; defense against parasitic worms

22. B cells • Bone marrow gives rise to B cells (B-lymphocytes) • Naïve but mature B cells migrate to secondary lymphatic tissue and become exposed to antigen • B cells recognizes epitopes with antigen specific B cell receptor • Each B-cell expresses a unique B cell receptor on its surface • B-cell receptor is actually the antibody produced by a particular B cell

23. Consequences of antigen recognition by B cells • Clonal selection and expansion • Increased antibody production • Plasma cell or memory cell development

24. Clonalselection and differentiation of B cells A clone originates from a single cell

25. Effectorfunction of antibodies • Begin after antigen-antibody complex has been formed • Aggluntination • Opsonization • Enhanced phagocytosis • Complement activation • Opsonization and enhanced opsonophagocytosis via c3b • Microbial lysis through C5b-C9n • Inflammation through C5a, C3a, C4a • Neutralization • Toxins • Viruses • Antibody dependent cytotoxicity • Eosinophils: secrete toxic granules onto helminths • NK cells: induce apoptosis of virus infected cells

26. Effector functions of antibodies

27. Anti-helminthiccytotoxicity of eosinophils

28. Natural killer cells • Large granular lymphocyte-like cells • Part of first line of defense (innate immunity) • Activated by interferons (produced by virus infected cells) and other cytokines • Target altered host cells • Virus infected • Infected with intracellular organism • Tumor cells • Induce cell suicide (apoptosis) • Cells covered with antibodies (antibody dependent cytotoxicity) • Direct sensing of altered cells

29. The kiss of death Tumor Cell • After NK cell have recognized their target they release their large granules containing • Pore-forming toxins • Enzymes that induce suicide of target cell NK-Cell t0 t60’

30. T-cell mediated immunity • After differentiating in the thymus, T cells migrate to lymphoid tissue • T cells become activated effector T cells when stimulated by an antigen • T cells respond to digested antigens via T-cell receptor • T cells recognize antigen only when presented by other cells on special molecules • Major histocompatibility complex (MHC) • Some effector T cells become memory cells

31. T-Ly T-cell antigens • Short contiguous amino acid (aa) sequence • Processed antigens • Antigen must have been unfolded and degraded • Primary aa structure • Only when bound to a specialized antigen presenting molecule (MHC) MHC APC

32. MHCmolecules • Major HistocompatibilityComplex • Same as HLA (human leukocyte antigen) • Determine compatibility of donor and recipient in transplantation • Every individual as a unique set of MHC molecules • Within an individual all cells are equipped with the same set • Have a peptide binding groove onto which antigen can be loaded • MHCI: peptides newly synthesized and degraded in cytoplasma (endogenous) • MHC II: peptide fragments generated in phagolysosome (exogenous)

33. Classification of T cells • Depends on surface molecules on T cells that determine the interaction with MHC molecules and their type of response • Cytokine release • Sending trigger to target cell to commit cell suicide • T helper cells • Cytotoxic T cells (“T killer cells”) • Regulatory T cells

34. T-helper cells • Express the surface molecule CD4 • Recognizes exogenous digested antigen presented on MHC type II molecule • Interact with antigen presenting cells • Macrophages • Dendritic cells • B-cells • Respond with secretion of cytokines and activate immune cells

35. Antigen presenting cells • Express MHC II • Highly specialized in uptake of foreign antigen, degradation and presentation to T helper cells via MHC II • Macrophages and dendritic cells • Take up antigen via phagocytosis • B cells • Bind antigen with surface antibody and internalize the complex

36. The CD4 : MHC II Interaction TH Cell Cytokines TCR CD4 Digested Ag MHC II Microbe Ag Presenting Cell

37. T helper cells in action

38. T helper cell subclasses • TH1 • Secrete the cytokine IFNg • Activates macrophages • Promotes IgG antibody production in B cells • TH2 • Secrete the cytokine IL4 • Promotes IgE production in B cells • Pro-allergic

39. Cytotoxic T cells • Express the surface molecule CD8 • Recognizes endogenous antigen presented on MHC type I molecule • Can interact with any nucleated cell • Respond with secretion of perforin and granzyme • Kill target cells via apoptosis in a highly specific manner

40. The CD8 : MHC I Interaction CTL TCR CD8 Endogenous Ag MHC I Endogenous Ag Any Nucleated Cell

41. CTL mediated cytotoxicity

42. Regulatory T cells • Treg and TH3 • Differentiate from T helper cells • Turn off immune response when Ag no longer present • Use inhibitory cytokines (IL10)

43. Summary for cell mediated immunity

44. Principals cells in the adaptive immune response

45. Summary for cytokines

46. Superantigens • Activate simultaneously up to 20% of all TH cells • Cause an intense immune response due to release of cytokines from host cells (“cytokine storm”) • Fever, nausea, vomiting, diarrhea, sunburn-like rash, shock, death • Examples : Toxic Shock Syndrome Toxin

47. Immunological memory • Once lymphocytes have encountered their specific antigen they undergo clonal expansion • Some of these cells develop further into memory cells • Can circulate for many years • Upon re-contact with the same antigen they quickly proliferate and resume effector function • B cells: antibody production • T cells: cytokine production (TH, Treg) and cytotoxicity (CTL)

48. Example: Primary and secondary immune responses to an antigen • IgM is always the first antibody • IgG follows IgM • IgG level does not go back to baseline • Re-exposure to the same antigen will lead to an augmented and accelerated immune response with higher residual antibody levels

49. Types of adaptive immunity

50. Important to remember • Key players in adaptive immunity • Antibodies • B-cells • T-cells • Lock- key principle: Ag-Ab • B cells make antibodies • 5 Types of antibodies : IgM (first), IgG (placenta), IgD (maturation), IgA (mucosa), IgE (allergies) • Antibodies can agglutinate, activate complement, promote phagocytosis, neutralize and initiate cell lysis by NKcells • T cells recognize digested antigen when presented to them on MHC molecules • Main effector T-cells • Helper T-cells: strengthen defense cells • Cytotoxic T-cells: kill infected cells • Regulatory T cells: down regulate immune response • Cytokines serve cell-to-cell communication