Defense against pathogens, possibilities of therapeutic affecting of the immune system

1. Defense against pathogens, possibilities of therapeutic affecting of the immune system Martin Liška

2. Extracellular microorganisms • Typically bacteria or parasites • For defense against extracellular microbes and their toxins, specific humoral immune response is important

3. Humoral immune response • Recognition of antigen by specific Ig, bound i cell membrane of naive B lymphocyte • The binding of antigen cross-links Ig receptors of specific B cells and then biochemical signal is delivered to the inside B cell; a breakdown product of the complement protein C3 provides necessary „second signal“ • Clonal expansion of B cell and secretion of low levels of IgM

4. Humoral immune response • Protein antigens activate CD4+ T helper cells after presentation of specific antigen • T helper cells exprime CD40L on their surface and secrete cytokines →proliferation and differentiation of antigen-specific B cells, isotype switching, affinity maturation

5. Phases of humoral immune response

6. Effector functions of antibodies • Neutralization of microbes (incl.viruses) and their toxins • Opsonization of microbec (binding to Fcreceptors on phagocytes; at the same time, stimulation of microbicidal activities of phagocytes) • ADCC (Antibody-dependent cell-mediated cytotoxicity) – IgG opsonized microb is destroyed by NK cells after its binding to IC • Activation of the complement system (classical pathway)

7. Defense against extracellular pathogens (bacteria and unicellular parasites) a/ non-specific (innate) immune system - monocytes/macrophages, neutrophils, complement system, acute phase proteins (e.g.CRP) b/ specific (adaptive) immune system - antibodies (opsonization, neutralization)

8. Defense against multicellular parasites • Production of IgE→ coating and opsonization of parasites • Activation of eosinophils - they recognize Fc regions of the bound IgE, then they are activated and release their granule contents, which kill the parasites • Th2-lymphocytes support this type of immune response

9. Intracellular microorganisms • Initially: non-specific immune response (ingestion by phagocytes) • Some microorganisms are able to survive inside phagocytes (e.g.some bacteria, fungi, unicellular parasites, viruses) – they survive inside phagosomes or enter the cytoplasm and multiply in this compartment • The elimination of these microorganisms is the main function of T cells (specific cell-mediated response)

10. Processing and presentation of antigen • Professional antigen-presenting cells: macrophages, dendritic cells, B lymphocytes (they express constitutionally class II MHC) a/ exogenous antigens – e.g.bacterial, parasitic, viral (if they are ingested in IC or during the processing of infected cells) • hydrolysed in endosomes to linear peptides → presentation on the cell surface together with class II MHC to CD4+ T lymphocytes

11. Processing and presentation of antigen b/ endogenous antigens – e.g.autoantigens, foreign antigens from i.c.parasites or tumorous antigens • hydrolysed to peptides → transportation into ER → in Golgi complex they are associated with class I MHC → presentation on the cell surface to CD8+ T lymphocytes

12. T cell-mediated immune response • Presentation of peptides to naive T lymphocytes in peripheral lymphoid organs → recognition of antigen by naive T lymphocytes • At the same time, T lymphocytes receive additional signals from microbe or from innate immune reactions → production of cytokines → clonal expansion → differentiation → effector & memory cells → effector cells die after elimination of infection

13. T cell-mediated immune response • TCR (T cell receptor) – T cell antigen-specific receptor - TCR recognizes (together with co-receptors - CD4 or CD8) the complex of antigen and MHC • a signal is delivered into the cell through molecules associated with TCR and co-receptors (CD4 or CD8) after antigen recognition

14. T cell-mediated immune response • APC exposed to microbes or to cytokines produced as part of innate immune reactions to microbes express costimulators that are recognized by receptors on T cells and delivered necessary „second signals“ for T cell activation • Activated macrophages kill ingested bacteria by reactive oxygen intermediates, NO and lysosomal enzymes

15. T cell-mediated immune response • Naive CD4+ T (helper) cells→ effector cells (activation of macrophages for killing of ingested microbes, activation of B cells for production of antibodie, activation of other cells) - Th1 lymphocytes: production of IFN-g, activation of phagocytes, stimulation of production of opsonizing a complement binding antibodies; support defense against i.c.microbes

16. T cell-mediated immune response - Th2 lymphocytes: production of IL-4 and IL-5, stimulation of IgE production, activation of eosinophils (= defense against multicellular parasites); they suppress defense reactions against i.c.microbes

17. T cell-mediated immune response • Naive CD8+ T (cytotoxic) lymphocytes → effector cells (killing of target cells, activation of macrophages) • differentiation into CTL – their function is killing of cells producing cytoplasmic microbial antigens • Killing the cells mainly by induction of DNA fragmentation and induction of apoptosis

18. Mechanisms of resistance of intracellular microbes to cell-mediated immune response • Inhibiting phagolysosome fusion • Escaping from the vesicles of phagocytes • Inhibiting the assembly of class I MHC-peptide complexes • Production of inhibitory cytokines • Production of decoy cytokine receptors

19. Defense against intracellular pathogens (bacteria and unicellular parasites) • Intracellular bacteria (Mycobacteria, Listeria monocytogenes, Legionella pneumophila), parasites (Cryptococcus neoformans, Plasmodium falciparum), fungi (Leishmania, Trypanosoma cruzei) • Specific immune response is necessary

20. Anti-viral defense • Viruses may bind to receptors on a wide variety of cells and are able to infect and replicate in the cytoplasm of these cells, which do not possess intrinsic mechanisms for destroying the viruses • Some viruses can integrate viral DNA into host genome and viral proteins are produced in the infected cells (e.g. Retroviruses)

21. Possibilities of therapeutic affecting of the immune system • Immunomodulation = therapeutic approach to modulation of affected immune function • Based on their effect, we can distinguish following immunomodulators: • immunostimulants • immunosuppressives • immunomodulators with the complex effect

22. Immunostimulants • They stimulate the immune system • Methisoprinol (Isoprinosine) – used for the treatment of severe or recurrent viral infections • Cytokines – IL-2 (anti-tumour therapy), colony-stimulating factors (e.g. treatment of severe granulocytopenia)

23. Immunosuppressives • They are used for the treatment of autoimmune or allergic diseases, in transplantology • Corticosteroids – anti-inflammatory, immunosuppressive effect • Cytostatics, antimetabolites (cyclofosfamide, methotrexate, cyclosporine A) – lymfocytotoxic effect • Monoclonal antibodies – antiCD3

24. Immunomodulator with the complex effect • Bacterial lysates (Broncho-Vaxom, Ribomunyl) • Transferfactor • Antihistamines – esp. 3rd generation

25. Substitution • Typically substitution therapy with intravenous Ig (IVIG) in primary or secondary hypo- or agamaglobulinemia • Life-long substitution • IVIG 200-300 mg/kg every 3 weeks

26. Active and passive immunization a/ active immunization • immunization with vaccines made from killed or attenuated microorganisms or their products • the immunity is long-lasting • both B cell and T cell based immunity are activated • injective, oral administration • prophylactic procedure

27. Active immunization • DTP (diphtheria, pertussis, tetanus) • H.influenzae • N.meningitidis • Pneumococci • BCG • MMR (measles, mumps, rubella) • Poliomyelitis • Hepatitis A,B

28. Active and passive immunization b/ passive immunization • provides humoral temporary (approximately 3 weeks) • prophylactic or therapeutic