EFFECTOR MECHANISMS

EFFECTOR MECHANISMS : 

EFFECTOR MECHANISMS Microbiology

CONTENTS : 

CONTENTS Effector Mechanisms of Cell-mediated Immunity-Eradication of Intracellular Microbes Effector mechanisms of CD4+ T lymphocytes Effector mechanisms of CD8+ T lymphocytes Effector Mechanisms of Humoral Immunity - The elimination of extracellular microbes and toxins Effector functions of antibody Antibody dependent cellular cytotoxicity Role of NK cells

Effector mechanisms of cell-mediated immunity : 

Effector mechanisms of cell-mediated immunity The effector phase of cell-mediated immunity is carried out by T lymphocytes Naive T cells are activated, proliferate and differentiated into effector cells and eliminate cell-associated microbes. The three types of effector T cells are- CD4+ TH1 & TH2 cells CD8+ CTLs CD4+ T cells activate phagocytes to destroy microbes residing in the vesicles of these phagocytes. CD8+ T cells kill any cell containing microbes or microbial proteins in the cytoplasm, thus eliminating the reservoir of infection.

Slide 5: 

Effector T cells generate from naïve T cells & stimulated by microbial antigens in lymph nodes & spleen. The differentiared effector T cells then migrate to the site of infection. Phagocytes at these sites that have ingested the micobes into intracellular vesicles display peptide fragments of microbial proteins attached to class II MHC molecules for recognition by CD4+ T cells. Peptide antigens derived from microbes living in the cytoplasm of infected cells are displayed by Class I MHC molecules for recognition by CD8+ effector T cells. Antigen recognition by effector T cells then activates these cells to pperform their task of eliminating microbes.

MIGRATION OF EFFECTOR T CELLS TO SITES OF INFECTION : 

MIGRATION OF EFFECTOR T CELLS TO SITES OF INFECTION Effector T cells migrate to sites of infection because these lymphocytes express high levels of adhesion molecules that bind to ligands that are expressed on endothelium or exposure to microbes & because chemoattractantcytokines are produced at infection site. Naïve T cells home to lymph nodes as a result of L-selectin binding to its ligand on high endothelial venules present only in lymph nodes. Activated T lymphocytes migrate out of the lymph nodes to sites of infection in peripheral tissyes mediated by E & P selectins & integrins LFA-1 & VLA-4.

Slide 9: 

In addition, chemokines that are produced in lymph nodes & sites of infection also participate in the recruitment of T cells to these sites. T cells that recognize microbial antigens in extra-vascular tissues are retained at these sites by integrin mediated adhesion to extracellular matrix. These antigen-specific T cells perform their effector function of eradiating the infection, whereas T cells that do not see antigen return via lymphaic vessels to the circulation.

EFFECTOR FUNCTION OF CD4+ T LYMPHOCYTES : 

EFFECTOR FUNCTION OF CD4+ T LYMPHOCYTES CD4+ T lymphocytes of the TH1 subset activate macrophages that have phagocytosed microbes, resulting in increased microbial activites of the phagocytes & killing of the ingested microbes. The ability of T cells to activate macrophages is dependent on antigen recognition, accounting for the specificity of the reaction. Effector T lymphocytes recognize the antigens of ingested microbes on macrophages. In response to this recognition, The T lymphocytes express CD40L, which engages CD40 on the macrophages, & the T cells secrete IFN-Ɣ which binds to IFN-Ɣ receptors on the macrophages.

Slide 15: 

This combination of signals activates the macrophages to produce microbicidal substances that kill the ingested microbes. Activated macrophages also secrete cytokines that induce inflammation (TNF, IL-2, Chemokines) & activate T cells (IL-12) & they express more MHC molecules & co-stimulators, which enhance T cell responses.

ELIMINATION OF MICROBES BY ACTIVATED MACROPHAGES : 

ELIMINATION OF MICROBES BY ACTIVATED MACROPHAGES Macrophages that encounter microbes secrete the cytokine IL-12 which stimulates naïve CD4+ cells to differentiate into IFN-Ɣ secreting TH1 cells & enhances IFN-Ɣ production. IFN-Ɣ activates the macrophages to kill ingested microbes. Activated macrophages catalyze the production of microbicidal substances in phagosomes & phagolysosomes. Major microbicidal substances produced in the lysosomes of macrophages are reactive oxygen intermediates(ROIs) nitric oxide(NO), & proteolytic enzymes which help in killing of microbes.

ROLE OF TH2 CELLS : 

ROLE OF TH2 CELLS TH2 subset of CD4+ T lymphocytes stimulates eosinophil-rich inflammation. When differentiated TH2 cells recognize antigens, the cells produce cytokines IL-4 & IL-5. IL-4 stimulates the production of IgE antibody & IL-5 activates eosinophils. This reaction is important for defense against helminthic infections, because eosinophils bind to IgE-coated helminths & helminths are killed by the granule proteins of eosinophils.

EFFECTOR FUNCTIONS OF CD8+ CYTOLYTIC T LYMPHOCYTES : 

EFFECTOR FUNCTIONS OF CD8+ CYTOLYTIC T LYMPHOCYTES Differentiated CTLs recognize class I MHC-peptide complexes on the surface of infected cells by their TCR & by CD8 coreceptor. CTLs adhere tightly to cells, mainly by virtue of integrins on CTLs binding to their ligands on the infected cells. The antigen receptors and coreceptors of CTL cluster at the site of contact with the target cell. The CTLs are activated by antigen recognition & firm adhesion. Antigen recognition by effector CTLs results in the activation of signal transduction pathways that lead to the exocytosis of the contents of the CTL’s granules to the region of contact with the targets. CTLs kill target cells mainly as a result of their granule content creating pores in target cell membranes and introducing into the target cells substances that induce DNA fragmentation & apoptosis.

Two pathways of target cell apoptosis is stimulated by CTLs : 

Two pathways of target cell apoptosis is stimulated by CTLs Perforin-granzyme pathways The pore-forming protein of CTL granules is called perforin. When perforin is secreted from CTLs, it inserts into target cell membrane and is induced to polymerize by the high concentration of Ca2+ ions present in the extracellular environment. At the same time the CTLs secrete granule enzyme called granzymes, which enter target cells through the perforin pores or by binding to receptors on target cell membranes followed by endocytosis. Granzymes cleave and thereby activate enzymes called caspases that are present in the cytoplasm of the target cells, and the active caspases induce apotosis.

Slide 22: 

Fas pathway Activated CTLs also express a membrane protein called Fas ligand , which binds to a death-inducing receptor, called Fas(CD95), on target cells. Engagement of Fas activates caspases and induces target cell apoptosis. CD8+ T lymphocytes also secrete the cytokine IFN-Ɣ, which activates macrophages to destroy phagocytosed microbes and enhance the recruitment of additional leucocytes. Thus,CD8+ CLTs, like CD4+ helper cells, contribute to the elimination of microbes ingested by phagocytes.

EFFECTOR MECHANISMS OF HUMORAL IMMUNITY : 

EFFECTOR MECHANISMS OF HUMORAL IMMUNITY Humoral immunity is the type host defense that is mediated by secreted antibodies and is important for protection against extracellular microbes and their toxins. Antibodies prevent infections by blocking the ability of microbes to bind to & infect host cells. Antibodies also bind to microbial toxins and prevent them from damaging host cells. In addition, antibodies function to eliminate microbes, toxins, and infected cells from the body.

EFFECTOR FUNCTIONS OF ANTIBODY : 

EFFECTOR FUNCTIONS OF ANTIBODY Antibodies function distant from their sites of induction. Antibodies are produced after stimulation of lymphocytes by antigens in peripheral lymphoid organs. Some of the antigen-stimulated B lymphocytes differentiate into antibody-secreting cells which synthesize and secrete antibodies of different heavy chain classes (isotypes).

Slide 25: 

Antibodies use their antigen-binding (Fab) regions to bind to block the harmful effects of microbes & toxins, and they use their Fc regions to activate diverse effector mechanisms that eliminate these microbes & toxins. The effective binding of phagocytes & complement to antibodies occurs only after several Ig molecules recognize and become attached to a microbe or microbial antigen. Therefore, even the Fc-dependent functions of antibodies require antigen recognition by the Fab regions. This feature of antibodies ensures that they activate effector mechanisms only when they need to, that is, when they recognize their target antigens.

Neutralization of microbes and Microbial toxins : 

Neutralization of microbes and Microbial toxins Antibodies bind to & block, or neutralize, the infectivity of microbes & the interactions of microbial toxins with host cells. Most microbes use molecules in their envelops or cell walls to bind to & gain entry into host cells. Antibodies may attach to these microbial envelope or cell wall molecules & prevent the microbes from infecting & colonizing the host.

Slide 29: 

Microbes that infect cell may damage these cells, are released, and go on to infect other neighboring cells. Antibodies may find the microbes during their trasmit from cell to cell & thus limit the spread of infection. If an infectious microbe does colonize the host, its harmful effects may be caused by endotoxins or exotoxins , which often bind to specific receptors on host cells & thus mediate their effects. Antibodies against toxins prevent binding of the toxins to host cells and thus block harmful effect of the toxins.

OPSONISATION AND PHAGOCYTOSIS : 

OPSONISATION AND PHAGOCYTOSIS Antibodies coat microbes & promote their ingestion by phagocytes. The process of coating particles for subsequent phagocytosis is called opsonization. When several antibody molecules bind to a microbe , an array of Fc regions is formed projecting away from the microbe. If the antibody belong to certain isotypes , their Fc regions of Ɣ chains, called FcƔRI (CD64), which is expressed on neutrophils & macrophages.

Slide 33: 

As a result, the phagocyte extends its plasma membrane around the opsonized microbe & ingests the microbe into a vesicle called a phagosome, which fuses with lysosomes. The binding of antibody Fc tails to FcƔRI contains a signaling chain that triggers numerous biochemical pathways in the phagocytes. The activated neutrophil or macrophage produces, in its lysosomes, large amount of reactive oxygen intermediates, nitric oxide, & proteolytic enzymes, all of which combine to destroy the ingested microbe.

ANTIBODY- DEPENDENT CELLULAR CYTOTOXICITY : 

ANTIBODY- DEPENDENT CELLULAR CYTOTOXICITY Definition: A number of cells that have cytotoxic potential express membrane receptors for the Fc region of the antibody molecule. When antibody is specifically bound to a target cell, these receptor –bearing cells can bind to the antibody Fc region, and thus to the target cells & subsequently cause lysis the target cell. Although these cytotoxic cells are nonspecific for antigen, the specificity of the antibody directs them to specific target cells. This type of cytotoxicity is reffered to as ADCC.

Slide 35: 

Among the cells that can mediate ADCC are NK cells, macrophages, monocytes, neutrophils, & eosinopils. Target-cell killing by ADCC does not involve complement-mediated lysis, but appears to involve a number of different cytotoxic mechanisms. When macrophages, neutrophils, or eosinophils bind to a target cell by way of Fc receptor, they become more active metabolically; as a result, the lytic enzymes in their cytoplasmic lysozymes or granules increase. Release of these lytic enzymes the site of the Fc-mediated contact may result in damage to the target cell.

Slide 36: 

In addition, activated monocytes, macrophages, and NK cells have been shown to secrete TNF , which have a cytotoxic effect on the bound target cell. Since both NK cells & eosinophils contain perforin in cytoplasmic granules, their target-cell killing also may involve perforin-mediated membrane damage similar to the mechanism described for CTL-mediated cytotoxicity.

Slide 37: 

A special type of ADCC, mediated by eosinophils, play a role in defense against helminthic infections. Most helminths are too large to be phagocytosed, and they have thick integuments that make them resistant to many of the microbicidal substances produced by neutrophils & macrophages. The IgE opsonizes worms, and eosinophils, which express a high affinity IgE-specific Fc receptor called FcεRI, bind to the opsonised worms. The bound eosinophils are activated to release their granules, which contain proteins that are toxic to helminths.

EFFECTOR MECHANISM OF NK CELLS : 

EFFECTOR MECHANISM OF NK CELLS NK cells appear to kill tumor cells & virus infected cells by a process similar to CTLs. Cytoplasm of NK cells contains numerous granules containing perforin & granzymes. After a NK cell adheres to a target cell, degranulation occurs with release of perforin & granzymes at the junction of the interacting cells. Kill the target cell by apoptosis.

REFERENCES : 

REFERENCES

Slide 41: 

THANK YOU