T CELL MEDIATED IMMUNITY (Cell Mediated Immunity)

CELL MEDIATED IMMUNITY (CMI) *Adaptive immune response based on antigen specific T cells *Antigen specific T cells *CD8 (cytotoxic) *CD4 (helper) *TH1 *TH2 *Transfer to naïve recipient (operational definition) *Not with antiserum *Can with lymphoid cells (syngeneic donor and recipient) *Mature naïve T cells must be activated by specific antigen in secondary lymphoid tissues

ACTIVATION OF NAÏVE T CELLS INTO EFFECTOR T CELLS *Purpose of secondary lymphoid tissues *Sites for initiation of adaptive response *Examples *Skin and soft tissues (SST) to regional lymph nodes *Blood stream to spleen *Respiratory mucosa to BALT and tonsils *Gastrointestinal mucosa to Peyer’s patches (GALT) *Dendritic cells important in SST infections *Immature *Phagocytic and migratory from infection sites *Mature *Interaction with T cells in 2 nd lymphoid tissues

ACTIVATION OF NAÏVE T CELLS INTO EFFECTOR T CELLS *Enter T cell zone of 2 nd lymphoid tissue and encounter *Dendritic cells *Macrophages *Presentation of antigen *Activation into effector T cells *Effector cells *Remain in 2 nd lymphoid tissue (CD4 TH2) *Travel to infection site (CD8 and CD4 TH1) *No encounter with specific antigen *Re-circulation to bloodstream

HOMING OF T LYMPHOCYTES *Homing *Movement of naïve T cells into secondary lymphoid tissue or effector T cells to infection site *Homing determined by *Chemokines *CCL19 and CCL21 *Cell adhesion molecules (CAM’s)

MOVEMENT OF T CELLS BY INTERACTION OF CAM’S *Interaction (contact) is mediated by *Cell adhesion molecules (CAM’s) *On surface of leukocytes and target cells *Work independently of antigen *Direct specific cell to cell contact *Classification based on *Structure *CD nomenclature

CLASSIFICATION OF CAM’S ON STRUCTURAL BASIS *Selectins *Surface of leukocytes and macrophages *L selectin *Initiates interaction with endothelial cells *Vascular addressins *Surface of endothelial cells *GlyCAM-1 and CD34 *Initiates interaction with leukocyte and macrophage selectins

CLASSIFICATION OF CAM’S ON STRUCTURAL BASIS *Integrins *Glycoproteins on surface of leukocytes and dendritic cells *Lymphocyte function associated antigen (LFA-1) *Initiates strong binding to *Immunoglobulin superfamily molecules *Intercellular adhesion molecules (ICAM’s) *Strong binding allows T cells to squeeze between endothelial cells

CLASSIFICATION OF CAM’S ON STRUCTURAL BASIS *Immunoglobulin superfamily molecules *Located on leukocytes, APC’s and endothelial cells *CD2, ICAM-1, ICAM -2 *Initiates interaction with integrins *Various roles in cell adhesion *Passage of T cells between endothelial cells *LFA-1 to ICAM’s *Interaction of T cells with APC’s *CD2 to LFA-3

ACTIVATION OF NAÏVE T CELLS REQUIRES CO-STIMULATION *Antigen presenting cells (APC’s) deliver both *Antigenic specific stimulation *Co-stimulation *Co-stimulation from professional APC’s *Dendritic cells, macrophages, B lymphocytes *Co-stimulatory molecule is B7 *B7 on APC’s binds to CD28 on T cells

CHARACTERISTICS OF PROFESSIONAL ANTIGEN PRESENTING CELLS *Mechanism of antigen uptake *Expression of MHC molecules *Expression of B7 *Antigens presented *Location in secondary lymphoid tissues *Location in body

PHAGOCYTOSIS INDUCES CO- STIMULATORY ACTIVITY IN MACROPHAGES *Resting macrophages express *No B7 *Low level MHC II *Degradation of bacteria in phagolysosome release molecules which stimulate expression of B7 and MHC II *Lipopolysaccharide *Macrophages most commonly present antigens to CD4 T cells *Listeria monocytogenes avoids MHC II presentation

PHAGOCYTOSIS INDUCES MATURATION OF DENDRITIC CELLS *Immature dendritic cells present in skin and soft tissues *High level of phagocytosis and no co-stimulatory activity *Phagocytosis induces *Migration to lymphoid tissue *Maturation to mature dendritic cells *Interdigitating reticular cells *Expression of *B7, MHC I, MHC II, CCL18 and DC-SIGN *Dendritic cells stimulate CD4 and CD8 T cell responses

ENDOCYTOSIS INDUCES B7 EXPRESSION IN B CELLS *Immunoglobulin receptors on B cells selective bind soluble protein antigen from extracellular environment *Lipopolysaccharide *Antigen / Immunoglobulin complex is internalized by *Receptor mediated endocytosis *Complexes delivered to endocytic vesicles *Degraded into peptides and bound to MHC II *Expression of B7 *B cells present antigen to CD4 T cells

ANTIGEN ACTIVATION OF T CELL RECEPTOR COMPLEX INITIATES INTRACELLULAR SIGNALING PATHWAYS *Antigen binding transmits signal via CD3 and zeta proteins *Transmitted signal activates *Receptor associated kinases (Fyn) *Fyn leads to phosphorylation of ITAM’s *Immunoreceptor tyrosine based activation motifs (ITAM’s) *Cytoplasmic tails of CD3 and zeta proteins

ANTIGEN ACTIVATION OF T CELL RECEPTOR COMPLEX INITIATES INTRACELLULAR SIGNALING PATHWAYS *ZAP-70 (cytoplasmic tyrosine kinase) binds to phosphorylated ITAM’s of zeta chain *Co-receptors (CD4 and CD8), with associated tyrosine kinase (Lck), bind to MHC molecules *Co-receptor binding to MHC allows Lck to phosphorylate and activate ZAP-70

PROLIFERATION AND DIFFERENTIATION OF ACTIVATED T CELLS BY INTERLEUKIN-2 *Naïve T cells express low affinity IL-2 receptor consisting of beta and gamma chains only *Activation of naïve T cells induces *Synthesis and secretion of IL-2 *Synthesis of IL-2 receptor alpha chain *Alpha chain combines with beta and gamma chains to make high affinity IL-2 receptor *IL-2 binds to receptor and initiates T cell proliferation

ANTIGEN RECOGNITION BY NAÏVE T CELLS IN ABSENCE OF CO- STIMULATION *Naïve T cell only activated by professional APC carrying specific peptide:MHC complex and co-stimulatory molecule *T cell beomes anergic when it encounters APC carrying specific peptide:MHC complex without co-stimulatory molecule *No effect on T cell which encounters APC carrying no specific peptide:MHC complex but has co-stimulatory molecule

EFFECTOR OPTIONS OF CD8 AND CD4 T CELLS FOLLOWING ANTIGEN ACTIVATION *CD8 committed to becoming cytotoxic effector cells *CD4 T cells can differentiate along two pathways *TH1 (help with CMI) *TH2 (help with humoral immune response) *Mechanisms of differentiation not well understood *Most immune responses involve both TH1 and TH2

CD4 T CELL RESPONSE TO MYCOBACTERIUM LEPRAE *Mycobacterium leprae is an intracellular pathogen, agent of leprosy and directs either TH1 or TH2 response *Most effective immune response is mediated by TH1 cells *Immune response mediated by TH1 cells results in *Tuberculoid leprosy *Immune response mediated by TH2 cells results in *Lepromatous leprosy

Figure 6-21

ACTIVATION OF NAÏVE CD8 T CELLS TO CYTOTOXIC EFFECTOR CELLS *Activation of naïve CD8 cells requires strong co- stimulation *Dendritic cells provide strong co-stimulation *Express high levels of B7 *APC with sub-optimal co-stimulation require CD4 T cell help *Naïve CD8 and CD4 cells must recognize specific antigen on on same APC

ACTIVATION OF NAÏVE CD8 T CELLS TO CYTOTOXIC EFFECTOR CELLS *Mechanisms of CD4 help *CD4 effector T cells secrete cytokines stimulating APC to increase level of B7 *Naïve CD4 T cells secrete interleukin-2 which stimlates CD8 cells *Requirement for stronger co-stimulation of CD8 cells means activation only when evidence of infection certain

PERFORMANCE OF EFFECTOR T CELL FUNCTIONS *Classification of molecules which perform functions *Cytokines *Proteins made by cells and affect behavior of cells *Autocrine action *Paracrine action *Produced by all effector T cells *Cytotoxins *Proteins which kill target cells *Produced by cytotoxic CD8 cells

CYTOTOXINS AND CYTOKINES OF T CELLS *T cells are distinguished by *Cytokines and cytotoxins produced and the effects on immune response *CD4 T cells produce and act primarily through cytokines *Macrophage stimulating (TH1) *B cell activating (TH2) *CD8 T cells produce and act primarily through cytotoxins *Perforin and Granzymes

SELECTIVE KILLING OF INFECTED CELLS BY CD8 CYTOTOXIC T CELLS *Cytotoxic CD8 kill by inducing apoptosis *Cells do not lyse or disintegrate but shrivel and shrink *Pathways of Apoptosis *Formation of transmembrane pores allowing proteolytic enzymes to enter *Perforin and granzymes cytotoxins *Induction of apoptosis signal following cell-cell binding *Fas ligand (CD8 T cell) to Fas receptor (Target cell)

TH1 T CELLS AND MACROPHAGE ACTIVATION *Macrophage activation *Enhancement of macrophage function against intracellular pathogens by TH1 cells *Phagosome fused more efficiently with lysosome *Important with Mycobacteria *Activation of macrophages requires 2 signals provided by TH1 cells *Interferon-gamma *CD40 ligand

INTRACELLULAR PATHOGENS AND GRANULOMA FORMATION *Mycobacteria can resist killing by activated macrophages resulting in formation of granulomas *Granulomas *Localized inflammatory response characterized by *Central core of infected macrophages surrounded by activated T cell *Central core of granuloma *Macrophages fused into multinucleated giant cells surrounded by large single macrophages (epithelioid cells) *In tuberculosis, centers of large granulomas display cheese- like appearance *Caseation necrosis

TH2 T CELLS AND ACTIVATION OF B LYMPHOCYTES *CD4 TH2 cells activate B cells which recognize same antigen *Cognate interaction *Activation takes place in secondary lymphoid tissue *Mechanism *TH2 cell receptor binds to peptide:MHC II complex on B cell *TH2 cell synthesizes *CD40 ligand which binds to CD40 receptor on B cell *Interleukin-4, interleukin-5, interleukin-6 *Stimulate proliferation and differentiation of B cells

KNOWLEDGE OF COGNATE INTERACTION IMPROVES VACCINE DESIGN AND EFFICACY *Vaccination of infants and young children against Haemophilus influenzae type b *Haemophilus influenzae type b *Agent of invasive disease in children < 5 years *Meningitis and epiglottitis *Prior to vaccine availability *Morbidity of 1 in 200 children *Mortality of 5% *Approximately 70% of cases in children < 18 months *Major virulence factor is type b capsular polysaccharide

KNOWLEDGE OF COGNATE INTERACTION IMPROVES VACCINE DESIGN AND EFFICACY *1980 vaccines consisted of *Purified type b capsular polysaccharide *Protects children > 18 months *1990 vaccines consisted of *Purified type b capsular polysaccharide conjugated to protein *ActHIB (Sanofi Pasteur) *Tetanus toxoid *HibTITER (Wyeth) *Diphtheria CRM 197 protein *Protects children > 2 months