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

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

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

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

Phases of humoral immune response

Effector functions of antibodies Neutralization of microbes (incl.viruses) and their toxins Opsonization of microbec (binding to Fc receptors 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)

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)

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

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)

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

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

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

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

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

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- , activation of phagocytes, stimulation of production of opsonizing a complement binding antibodies; support defense against i.c. microbes

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

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

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

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

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)

Anti-viral defense a/ non-specific (innate) immune system - monocytes/macrophages, NK cells, interferons (IFN-  → activation of macrophages, IFN-  → antiviral activity, activation of NK cells) b/ specific (adaptive) immune system - T cells, antibodies (e.g. neutralization of viruses)

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

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)

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

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

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

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

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

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