Immune response

Describes the basic properties and mechanisms of T cells and B cells in maintaining Immune Response against foreign antigens or infections and covers the UG and PG portion of immunology.

1. Immune Response- CMI & AMI Dr. Suprakash Das Assist. Prof.

2. Introduction Immune response refers to the highly coordinated reaction of the cells of the immune system and their products. It has 2 arms  Humoral or Antibody mediated immune response (AMI)  It provides protection to the host by secreting antibodies that can bind to the microbial antigens that are circulating or present on the cell surface and neutralize them. No role against intracellular antigens . Cell mediate immunity (CMI)  Crucial role in providing protection against intracellular microbes as well as tumors. Although, CMI is mainly T-cell mediated but other components like NK cells, macrophages, granulocytes are also important.

3. Difference between CMI & HMI

4. Introduction CMI and AMI are interdependent- They don’t work independently but are highly dependent on each other. Cytokines released from T cells stimulate B-cells to produce antibodies. Many effector cells of CMI like Macrophages and NK cells use antibodies to recognize the target cells for killing. CMI regulates HMI by regulating transformation of B-cells into antibody secreting plasma cells. There are certain initial events that takes places before induction of either CMI or AMI. These events occur irrespective of the type of immune response that will follow.

5. Antigen presentation to Helper T-cells Activation and Differentiation of Helper T-cells into T H 1 or T H 2 subsets T H 1 cells  secret Cytokines CMI T H 2 cells  secret Cytokines  HMI TH1 Cytokines and their functions  IL2 Promotes activation of T H and T C cells. Activates NK cells to become LAK cells (Lymphokine Activated killer cells) INF- Activates the resting macrophages to activated Macrophages Activates B cells to produce IgG. Delayed type of Hypersensitivity. Inhibits T H 2 cell proliferation. TNF- Enhances phagocytic activity of Macrophages.   TH2 Cytokines and their functions  IL-4 Inhibits T H 1 cells differentiation. Stimulates B cells to produce IgE and IgG-1 &4. IL-5 Enhances the proliferation of Eosinophils. IL-4&5- protection against Helminthic Infections and mediate Allergic Reactions. IL-6 B-cell proliferation and Antibody production IL-10 Inhibits T H 1 differentiation.

6. Interactions between the innate and acquired immune systems in response to bacterial infection of the skin. In response to bacteria that have breached the epithelial barrier  Keratinocytes synthesize anti-microbial peptides, chemokines, and cytokines. These factors lead to activation of the dermal endothelium, inducing the migration of innate leukocytes and memory T cells into the skin and additionally guiding these cells via chemotactic gradients. These factors and bacterial antigens activate innate phagocytes to kill ingested organisms and activate dendritic cells to migrate to the skin-homing lymph nodes.

7. Interactions between the innate and acquired immune systems in response to bacterial infection of the skin. In the lymph nodes, dendritic cells present bacterial antigens to naı¨ve and central memory T cells , leading to stimulation of pathogen-specific cells. Effector CD8 cells exit the lymph node , home to inflamed skin and kill pathogens. Helper CD4 T cells provide help to B cells , inducing the production of antibodies that directly neutralize pathogens and lead to additional targeting of innate responses. Antibody-directed phagocytosis by innate cells leads to enhanced antigen presentation, further enhancing acquired responses.

8. Cluster of differentiation in helper T cell The helper T cells can be Th1, Th2, Th9, Th17, or Th22 cells , depending upon the pathogen and types of cytokines present in the cellular microenvironment. They are responsible for responding to both intracellular as well as extracellular pathogens and have also been found to be involved in various diseases. CD4  T helper (Th) cells play a central role in orchestrating adaptive immune responses to invading pathogens through their ability to differentiate into specialized effector subsets. Part of this customized response requires the development of T follicular helper ( Tfh ) cells , which provide help to B cells for the generation of Germinal Centers (GCs) and long-term protective humoral responses.

9. Cluster of differentiation in helper T cell CD4 T helper cells are the primary orchestrators of the adaptive immune response, mediating a variety of cellular and humoral responses against pathogens and cancer. Although they lack any capacity to directly kill or engulf pathogens, they are powerful activators of effector cells such as macrophages, cytotoxic T cells, and B cells. On the other hand, Regulatory T cells (Tregs) are potent suppressors of the immune response important in limiting the immune reaction. Recent advances have led to the discovery of a diverse set of T helper subsets, each with unique functions.

10. CMI & HMI Works in Highly Co-ordinated system

11. Antigen Presentation  Antigen Presenting Cells (APCs) For induction of immune responses, recognition of antigens by T-cells is essential. T cells can’t recognize the native and free antigens, but can do so only after antigen is processed into smaller antigenic peptides containing specific epitopes which are subsequently combined with MHC molecules (Class -1 /2) and presented on the host cell surface. Antigen presentation means the presentation of antigenic peptide to both T H (Helper T cell) and T C (Cytotoxic T cells) by complexing with MHC class 2 and class 1 respectively. However, APCs in strict sense refers to only only those cells ( e.g , Dendritic cells, Macrophages etc.) that present antigenic peptides to T H cells. Virus infected cells or Tumor cells presents antigenic peptides to T C cells leading to their activation and they aren’t considered as APCs rather termed as “Target cells” . Professional APCs- Dendritic Cells, Macrophages, B-cells. Non-Professional APCs- Fibroblasts (Skin), Endothelial cells (vessels), Glial cells (Brain).

12. Antigen Presentation  Antigen Presenting Pathways For induction of immune response the the antigens must be presented to T H cells and also to T C cells. Two well defined pathways are used for this purpose  Cytosolic Pathway- Here, the endogenous (i.e. Intracellular ) antigens such as Viral or tumor antigens are processed and presented along MHC Class 1 molecules to CD8 T cells. Endocytic pathway- The exogenous antigens ( extracellular microbes and their products like toxins) are processed and complexed with MHC class 2 molecules and presented to T H Cells. Professional APCs are mostly involved in this pathway.

13. Cell Mediated Immune Response (CMI) CMI refers to the destruction of cells carrying intracellular microbes and other abnormal cells, such as tumor cells by various specific and non-specific cells of immune system, of which the most important is the cytotoxic T cells. Role of CMI- For obligate intracellular organisms like Mycobacteria, Chlamydia, Rickettsia, plasmodium, leishmania etc. CMI remains the only effective immune response. For facultative intracellular organisms like Listeria, Salmonella, Yersinia and Fungi such as Histoplasma and Cryptococcus, CMI becomes the leading immune response once the organism goes intracellular from extracellular. Provides immunity against tumor and damaged or altered cells. Mediates Type-4 Hypersensitivity.

14. Cell Mediated Immune Response (CMI)- Effector Cells CMI can be mediated by both Antigen specific and non-specific effector cells. They perform their function by direct killing of the target cells. Antigen specific- Cytotoxic T cells (Most important) Non-specific – NK Cells, Macrophages, Neutrophils, Eosinophils. CMI is interdependent on HMI as non specific effector cells use Antibodies for their killing (ADCC- Antibody Dependent Cell Mediated Cytotoxicity).

15. T cell stimulation and polarization require four signals from dendritic cells

16. Signal one  determines antigen specificity and consists of interaction of the T cell receptor (TCR) with peptides loaded onto dendritic cell major histocompatibility (MHC) molecules. Signal two  consists of co- signaling and can be either positive, leading to cell activation (co-stimulation) or negative, leading to no response (co-inhibition) . Co- signaling molecules, including CD80 and CD86, are upregulated on dendritic cells after binding of pathogen-associated molecular patterns (PAMP) to their cognate receptors.

17. Signal three  involves the polarization of CD4 T cells into either Th1, Th2, or regulatory T cells. Immature dendritic cells are polarized by the binding of type 1, type 2, or regulatory PAMP and differentiate into mature dendritic cells that induce the formation of Th1, Th2, or T regulatory T cells, respectively. In general, viral-associated PAMP give rise to Th1 responses, and PAMP from parasitic organisms favour Th2 responses.

18. Signal four  leads to spatial imprinting of T cells , leading to the acquisition of homing receptors that induce selective recirculation through the tissue in which antigen was first encountered. Dendritic cells from the intestine uniquely produce retinoic acid, inducing T cells to upregulate the T cell gut-homing receptors a4b7 and CCR9 and suppress expression of the skin-homing receptor cutaneous lymphocyte antigen (CLA). Signals that polarize T cells to migrate to other sites, including the skin, brain, and pulmonary epithelia, have yet to be identified.

19. Natural Killer (NK) Cells NK cells are large granular lymphocytes that constitutes 10-15% of peripheral blood lymphocytes, derived from a separate lymphocyte linage. They are cytotoxic but non-specific. They are part of innate immunity and don’t require prior contact with antigen. NK cells act against the virus infected cells and tumor cells till the Tc cells are activated and take over the function. NK cells lack the T cell markers like CD3/4/8 (hence also called Null cells ), instead they have CD 16& 56. NK Cells don’t differentiate into memory cells. Mechanism of NK cells mediated Cytotoxicity- NK cells directly recognize certain ligands (Glycoproteins) present on the altered cell surface without the help of MHC molecules. This is mediated by 2 types of receptor present on NK cell surface ( Theory of Opposing Signal Model )

20. Natural Killer (NK) Cells Activation receptors- (NKR-P1, CD 16) : When these receptors are engaged with ligands present on target cells, NK cells become activated. Inhibitory Receptors- (C-type lectin inhibitory receptors) - They recognize a part of MHC 1 molecules (HLA-E) which is present on all nucleated cells. Binding of inhibitory receptor to MHC 1 molecules generates an inhibitory signal that suppresses NK cells even if they are bound to activation receptors. However, in target cells, the MHC 1 expression is remarkably reduced. In such cases there would not be any inhibitory signal  NK Cell activation. Target Cell destruction- Mechanism of target cell lysis by NK cells and T cells are similar i.e. by secreting Perforins and Granzymes . Perforins form pores on the target cell through which granzymes enter and lyse the target cells.

21. NK Cell mediated Cytotoxicity

22. Two ways to kill. Granule-mediated cytotoxicity is initiated by the targeted release of lytic granules toward a locally attached target cell. Granzymes can then enter the target cell by perforin-pores in the plasma membrane (left) or by endocytosis and perforin-aided escape from endosomes (right). Granzymes can then induce caspase activation, mitochondrial dysfunction, or caspase-independent apoptosis. (B)Death receptor-mediated cytotoxicity is induced by surface expression of FasL or TRAIL , which can engage and activate their respective receptor. This results in caspase activation, mitochondrial dysfunction, and apoptosis.

23. Antibody dependent cell mediated cytotoxicity (ADCC)- A number of non-specific cytotoxic cells express receptors ( FcR ) on their surface that can bind to Fc region of any immunoglobulin. Following contact with a target cell coated with an antibody, these FcR bearing cells can bind to Fc portion of the antibody coated on the target cells, and subsequently cause lysis of the target cells. Although these cells are non-specific for antigen, the specificity of the antibody directs them towards the specific target cells. This type of cytotoxicity is referred to as ADCC. ADCC is shown by NK Cells, Macrophages, Neutrophils and Eosinophils . They release various cytotoxic factors into the target cells like perforins, Granzymes, Lytic enzymes, free radicals, TNF etc.

24. Humoral/Antibody Mediated Immune Response (HMI/AMI) AMI occurs through the following 3 sequential steps  Activation of B-cells following contact with microbial antigen (B cells act as APCs) Proliferation and differentiation of B cells into effector cells (antibody producing plasma cells) and Memory cells. Effector FunctionProduction of Ab. N eutralization O psonization C omplement activation

25. A] HMI- Activation of B-cells Antigens that activate B cells fall into 2 categories  Most of the antigens are Thymus Dependent (TD) : They activate B cells indirectly via activation of T cells. Activated T H cells Cytokines Activation of B cells. B. Thymus independent antigens (TI) : Antigens like bacterial capsule are not processed by APC and directly activate B cells without the help of T cell cytokines. Activation of B-cells The first and foremost step that occurs is the recognition of TD antigen by B cell membrane immunoglobulin ( mIg ) receptor  receptor mediated endocytosis of antigen. Then the antigen is processed into smaller antigenic peptides that are presented in complex with MHC-2 to activate TH cells (Endocytic pathway) This leads to induction of 3 signals 

26. A] HMI- Activation of B-cells Signal-1 :  It is inducted by cross linkage of IgM on B cell membrane with the microbial antigen. Signal-2  It is an additional signal provided by binding of CD 40 on B cells with CD40L on activated TH cells. Signal-3  It is usually Cytokine stimulus due to T H cell produced cytokines. Signal Transduction Following induction of signal, its transmission is essential for B cell activation and it is initiated by B Cell Receptor (BCR).

27. B] HMI- Proliferation and Differentiation of B-cells Naïve B cells released from Bone Marrow  B cell areas of peripheral lymphoid organs (Cortex of lymph node and marginal zone of spleen.) Primary Lymphoid follicles . Antigenic exposure Activation of naïve B cells Proliferation. Primary lymphoid follicles  Secondary lymphoid follicles Germinal Centers Dark Zone Light Zone

28. B] HMI- Proliferation and Differentiation of B-cells Events in the Dark Zone of Germinal Center  Activated B cells (differentiate) larger dividing cells –( Centroblasts ) transforms into nondividing cells, Centrocytes . Centroblasts express membrane Ig by a process k/a Somatic Hypermutation . This results in alteration of membrane Ig affinity by which it binds to corresponding antigen. Because somatic mutations occur randomly, they generate membrane Ig with both High & Low affinity. Centrocytes with low affinity undergo Apoptosis Phagocytosed by special Tingible body Macrophages. Centrocytes with high affinity Survive and migrate to light zone. The process of enhancement of Ig for Ag binding is called Affinity maturation .

29. B] HMI- Proliferation and Differentiation of B-cells Events in the light zone of Germinal Center  Binding of centrocytes to follicular dendritic cells -> The centrocytes with high affinity Ig undergo maturation by binding to a special type of dendritic cells ( Follicular Dendritic Cells- FDC ). Then the mature dendritic cells undergo class switch over. Class Switch Over- Early in the immune response IgM is the predominant immunoglobulin secreted by the B cells. But as the maturation progresses the same B cells undergo a phenomenon called CLASS SWITCHING. Binding of cytokines produced by T H cells induces Class Switch Over. Different cytokines induce production of different classes of Ig by switching mechanism.

30. B] HMI- Proliferation and Differentiation of B-cells Cytokines Ig Class Produced INF-𝛾 IgG2a/ IgG3 IL-5 + TGF β IgA/ IgG2b IL-4 IgE /G1/G4 IL-2,4,5 IgM IL-4,5,6 +INF-𝛾 IgG

31. B] HMI- Proliferation and Differentiation of B-cells Differentiation of centrocytes into Plasma cells and Memory Cells  After class switching , the selected centrocytes undergo further differentiation into effector cells (Plasma cells) and Memory cells in the light zone of germinal center . Plasma cells are large Ab secreting cells; produce secretory Ig enormously, but do not have membrane Ig or MHC class-2 molecules. Memory B cells bear high affinity Ig of all classes as compared to naïve B cells that only bear low affinity IgM/ IgD membrane Ig. They are long lived cells which responds to secondary antigenic stimulus.

32. C] Effector Functions of AMI Antibodies secreted from plasma cells perform a number of biological functions. Promotes Opsonization- FcRs , present on phagocytes surface recognize the Ab coated microbes  Binding Enhanced phagocytosis. Transcytosis- Movement of Ab via epithelial cells from their basolateral side to apical side. Mediates mucosal immunity- This is due to transcytosis of IgA mostly to gut lumen that neutralizes microbes at local mucosal sites. Activates complement mediated inflammation and cytolysis Promotes ADCC.

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