Cell mediated immunity- a part of general immunology.

1. Cell mediated immunity
By –Dr Manaswini das
2nd yr Pg Microbiology
SCBMCH

2. INTRODUCTION-
• The term cell mediated immunity refers to the specific immune
response mediated by effector T-cells generated in response to the
antigen.
• Both activated helper T cells & cytotoxic T lymphocytes serve as
effector cells in CMI.
• It provides protection against intracellular microbes as well as tumor
cells.
• Although CMI is mainly T-cell mediated ( especially cytotoxic T
cells),many other effector cells such as NK
cells,macrophages,granulocytes are also components of CMI.

3. CMI &AMI are interdependent.
Cytokines released from T cells stimulate B cells to produce antibodies.
Many effector cells of CMI such as macrophages & NK cells use antibodies as receptors to recognize
the target cells for killing.
CMI also regulates the humoral immunity by releasing cytokines from activated Tcells that
stimulate the Bcells to transform into antibody secreting plasma cells.
Certain initial events are common to both CMI & AMI & they occur irrespective of the type of
immune response that will follow-
Antigen presentation to helper T cells.
Activation & differentiation of helper T cells into either TH 1 or TH 2 subsets.
Helper T cells are the central key that regulate the type of immune of response to occur.
Induction of TH1 cells secrete cytokines that stimulate CMI where as TH2 cells secrete certain
cytokines that in turn induce the B cells to produce antibodies.

4. Role of cell mediated immunity-
• CMI mediates the following immunological functions-
• Provides immunity against microbes residing in intracellular milieu –
• For obligate intracellular organisms- e.g- all viruses, some bacteria(mycobacterium,chlamydia&
Rickettsia) ,some parasites(plasmodium,leishmania,trypanosoma&cryptosporidium) & some fungi
(pneumocystis).
• For facultative intracellular organisms humoral immunity is active as long as the organism is
extracellular.once they come to intracellular milieu,CMI takes the leading role.e.g- include
bacteria like Listeria,salmonella & yersinia & fungi such as Histoplasma & cryptococcus.
• Mediates delayed hypersensitivity ( type-iv hypersensitivity) .
• Provides immunity against tumor cells & other damaged & altered cells.
• Transplantation immunity & graft- versus- host reaction.
• Pathogenesis of certain autoimmune diseases ( e.g- thyroiditis & encephalomyelitis)

5. Antigen presentation-
• For induction of immune responses ,recognition of Antigen by T-cells is essential.
• T cells cannot recognize the naive & free Antigens, but they do so only after the Ag is processed
into smaller antigenic peptides containing specific epitopes which are subsequently combined
with MHC molecules ( class l&ll ) & are presented on the host cell surface.
• Antigen presenting cell ( APCs)-
• Although Antigen presentation refers to presentation of Antigenic peptide to both TH cells & TC
cells by complexing with MHC-ll & MHC-I respectively,however APCs strict sense implies only to
those cells (e.g-dendritic cell & macrophage e.t.c) that present the Antigenic peptide along with
MHC class ll to TH cells.
• Since helper cell activation is necessary for virtually all immune responses ,the class ll MHC play a
pivotal role in controlling such responses.
• Infact unless otherwise stated the term ‘APC” usually refers to these specialized cells that bear
class ll MHC proteins.
• Cells presenting antigenic peptides along along with MHC class l molecules to TC cells are not
included under APCS. They are usually virus infected cells or tumor cells often referred to as
target cells as the activated TC cells cause lysis of these cells.

6. Class l MHC proteins are expressed by virtually all somatic cell types & used to present substances to CD8 T Cells
most of which are cytotoxic.Almost any cell can therefore presents Antigen to cytotoxic T cells & thus serve as a
target of a cytotoxic response.
Class ll MHC proteins are expressed only by macrophages & a few other cell types & are necessary for Antigen
presentation to CD4 T cells- subset that includes most helper cells.
Antigen presenting cells-
Professional APCs Nonprofessional APCs
Dendritic cells Fibroblasts(skin)
Macrophages Thymic epithelial cells
B cells Pancreatic beta cells
Vascular endothelial cells
Glial cells (brain)
Thyroid epithelial cells

7. Antigen presentation-

8. Antigen processing pathways-
• For induction of immune response (both CMI & AMI) antigens must be presented to Th cells.
• In addition for CMI induction antigen presentation to TC cells is essential.
• Two well defined pathways are are used by immune system for this purpose-
• Cytosolic pathway-
• Here the endogenous (intracellular) antigens such as viral antigens & tumor antigens are
processed & presented along with MHC class l molecules to CD8 T cells.
• Endocytic pathway-
• In this pathway , the exogenous antigens (extracellular microbes & their products e.g-toxins ) are
processed & complexed with MHC classll molecules & presented to TH cells . The cells involved in
endocytic pathway include the APCs such as macrophages,dendritic cells & Bcells.
property Cytosolic pathway Endocytic pathway
Antigen processed endogenous exogenous
Antigen is complexed with MHC-l molecules MHC-ll molecules
Antigen is presented to Tc cells Th cells

10. Endocytic pathway cytosolic pathway

12. Helper Tcells activation & differentiation-
• Helper T cell activation &differentiation is the central event that regulates both CMI( via cytotoxic T cells) & AMI(by
antibody secreting plasma cells.)
• Signal generation-
• Activation of TH cells requires generation of 3 specific signals-
• 1) Antigen specific signal – it involves binding of antigenic peptide present in the groove of MHC-ll onAPCsto TCR
present on the surface of TH cells.CD4 molecules of TH cells also interact with β2 domain of MHC-ll.
• 2)Costimulatory signal- it invoves binding of CD28 molecule on TH cells to B7 molecules on APCs.
• 3)Cytokine signal- APCs (macrophages)secrete IL-1 which acts on TH cells.
• Signal transduction-following induction of signal , its transmission is essential for TH cell activation.signal
transduction is initiated at CD4 molecule which interacts with CD3 complex , & inturn transmit the signal leading
to activation of TH cells.
• Differentiation of helper T cells-Activated TH cells secrete increased amount of IL-2 as well as IL-2 receptor ( IL-2R
or CD25)
• IL-2 binds to its receptor on the same TH cells & also on other TH cells to proliferate & differentiate.
• TH cells get activated & become lymphoblast cells which subsequently differentiate into memory & effector TH
cells.

15. IL-1 released by APC
On APC- it Increases the
surface expression of class-ll
MHC molecules.
TH cells to promote IL-2
secretion &IL-2 receptor
expression.thus potentiates
TH proliferative response.
autocrine paracrine
IL-2 secreted by
TH cells
autocrine paracrine
Proliferative
response of
activated TH cells .
Activating cytotoxic
cells
Macrophages can secrete IL-1 ,(TNF &IL-6 ) that can mimic with IL-1 activity.
Even if a T cell has both signals from APC ,it will not proliferate in absence of IL-2 or its IL-2receptor.

16. T-cell mediated immune response-
• An important component of any T-cell immune response is a rapid expansion in the numbers of antigen
specific T-cells.this expansion is mediated largely by IL-2 produced by helper T cells.
• Because high affinity receptors for IL-2 are induced on antigen recognition & are not expressed by resting T
cells,only activated T cells respond to IL-2.
• Effector functions of naïve T cells is limited.but can evove dramatically following initial activation.immune
response therefore entails both qualitative & quantitative changes in the responding T cells.
• The great majority of newly expanded T cells die probably as a result of apoptosis. One important negative
regulator is CTLA-4, a T cell surface molecule whose expression is induced by activation.
• CTLA-4 shares considerable sequence similarities with CD28. & like CD-28 binds B7.1& B7.2.unlike CD28
however ,however CTLA-4 appears to inhibit rather than promote T cell responses.
• Mice genetically lacking CTLA-4 die with polyclonal proliferation of T lymphoblasts , indicating CTLA-4 plays a
critical role in maintaining T lymphocyte homeostasis.
• T cell population is heterogenous w.r.t both functional capabilities & cell surface phenotypes . T cells are
divided into helper cells which promote cell mediated , antibody responses & cytotoxic cells kill Antigen
bearing target cells.

17. • Helper T cells usually express CD4+ & cytotoxic T cells CD 8+. it is important that expression of CD4 & CD8 really
correlates with MHC restriction. Thus some CD4+ T cells have cytolytic activity & certain CD8+ T cells function as
helper cells.
• Effector TH cells-
• They are derived either from the naïve TH cells or preexisting memory TH cells following antigenic stimulus.they are
short lived (few days to weeks). They further differentiated into either TH1 or TH2 subsets.this differentiation is very
crucial as they secrete distinct cytokines that further mediate specific functions.
• Cytokines secreted by TH1 cell stimulate cytotoxic T cells & induce CMI ,while cytokines secreted by TH2 cell
stimulates B cell producing different classes of antibodies( humoral immune responses)
• IL-12 secreted by macrophage plays an important role in the differentiation of TH cells. It promotes TH1 subset
proliferation.
• Helper T cells provide signals both for CMI & AMI.when activated ,helper T cells produce soluble lymphokines that can
regulate the activities of T cell ,B cells, monocyte macrophages & other cells of the immune system.T cell help for B
cell differentiation also occurs through direct contact between the 2 cell types ,which results in direct stimulation of
receptors B cells &TH derived lymphokines.

19. Enhances growth of mast cells&eosinophils.
& IL13

20. • TH1 CELL-
• IFN-γ activates macrophages by inducing NO
synthase that increase microbicidal activity
• .IFN-γ acts on activated B cells to induce IgG1 –an
isotype that binds strongly to all 3 classes
macrophage Fcγ receptors & so functions as an
extremely potent opsonin.The overall effect is to
potentiate both engulfment & killing by
phagocytes.
TH2 CELLS-
• This type of defense action is effective against
large multicellular parasites such as helminths
,which can killed extracellularly by eosinophils but
are too large to be engulfed by macrophages.
• Macrophages play a little role inTH2 mediated
immune responses ,in part because IL-10 inhibits
IFNγ production & IL-4 selectively favours
production of (IgE & IgG4) that are not recognized
by macrophage Fc receptors.
• TH1 derived IFNγ inhibits the development of TH2
cells & TH2 derived IL-4 intereferes with the TH1
cell development.so,one subtype can
predominates over other due to these positive &
negative feedbacks.

21. • Applications-
• Leishmania major invades macrophages stimulating them to produce IL-12 thus
promoting TH1 development kills the parasite, while TH2 shows vigorous antibody response
but no macrophage response,parasite kills the host.
• Patients with a TH1 dominated response develop less aggressive ,tuberculoid form of the
disese due to contained by a brisk macrophage response ,while TH2 dominated response
develop disseminated ,lepromatous form of leprosy.
• Auto immune disorders associated with the destruction of host tissues such as diabetes
mellitus,multiple sclerosis or IBD predominantly invove TH1 responses .By contrast allergic
disorders such as seasonal rhinitis, asthma & contact dermatitis in which IgE, mast cells &
eosinophils play a predominant role dominated by TH2 cells.
• In spite of that, Someday it may be possible to treat or prevent these disorders by selectively
influencing the development or functions of individual TH subtypes.for e.g- IL-12 administered
at the time of vaccination has been found to enhance protective TH1 reactions against certain
pathogen in animals.
• .

22. Cytolytic T cells-
• Tc lymphocytes function to eradicate cells ,such as virus infected host cells & tumor cells..most TC cells express CD8
rather than CD4 & hence recognize antigens in association with class l rather than classll MHC proteins.
• THE first signal alone induces high affinity IL-2 receptors on the TC cell .the second signal is furnished by IL-2 secreted
from a nearby activated TH lymphocytes.
• On receiving both signals ,the activated TC cells acquires cytotoxic activity enabling to kill the cell by releasing
specific toxins on to the target cell & in others the TC cells induces the target cell to commit suicide by apoptosis.
• The activated TC cell also proliferates ,giving rise to additional TC cells with the same Ag specificity.
• CTLs are important in response to certain intracellular bacterial pathogens including Listeria , mycobacteria &
allograft rejection & may play a role in immune surveillance against malignancy.

24. Activation of CTL
• Generation of activated CTL from naïve TC cells
requires induction of atleast 3 signals. Antigen
specific signal- induced by binding of TCR-CD3
complex of naïve TC cells to MHC-l peptide
complex of target cells.CD8 of TC cells also
interacts with ἁ3 domain of MHC-l.
• Costimulatory signal-CD28 of naïve TC cells
interacts with B7 molecules on target cells.
• Third signals-IL-2 (secreted by TH1 cells ) acts on
high affinity IL-2 receptors on TC cells.
• Following induction ,the transmission of signal
occurs similar to TH cells

25. Effector cells of CMI Antigen specificity
cytotoxicT cells specific
NK cells Non specific
Cells performing ADCC(NK
cells,macrophages,neutrophils &eosinophils)
Nonspecific
• Killing by cytotoxic granules- The CTL granules contain perforin (cytolysin) & granzymes a family of related
serine proteases .
• The perforin form 10 to 20 nm pores in plasma membrane of the target.,but not sufficient to kill the target
molecules ,which have the ability to repair membranes & there by avoid osmotic lysis.
• Rather the pore are means of delivering granzymes into the target.& it is the granzymes that induce death of the
targets by triggering apoptosis.

26. Killing by the Fas ligand-Fas pathway-
• In addition to killing by cytolytic
granules,Ag recognition stimulates CTLs
to express Fas ligand ,a member of TNF
family.the interaction of Fas ligand with
Fas ( a cell surface molecule related to
TNF receptors)induces apoptosis in the
Fas expressing cell.
• The Fas death pathway also used by CD4+
TH1 cells which donot express cytolytic
granules.
• Human & mice with mutations that
interfere with Fas expression or function
develop a clinical disorder characterized
by massive accumulation of T cells & by
autoimmunity.

27. NATURAL KILLER CELLS-Nk cells derive from a BM precursor &also found in the lung interstitum,intestinal mucosa &
liver. It s development does not require thymus .those who lack T cells & B cells may have normal NK cells & converse is
also true.
They are rare in thymus,lymphnodes,& are not usually found in thoracic duct lymph.
They are large granular lymphocytes constitute 10-15% of peripheral blood lymphocytes & 3-4% of splenic blood
lymphocytes. they are derived from a separate lymphoid lineage distinct from T lymphocytes. .NK cells are cytotoxic but
antigen nonspecific.
They are part of innate immunity ,act as first line of defense & donot require prior contact with Ag. (hence the term
(“NATURAL KILLING). NK cells act against virus infected host cells & tumor cells initially till the TC cells are activated&
take over the functions & they use perforins & granzymes to kill target cells.
NK CELL MARKERS-they lack the T cell markers such as CD3,CD4 or CD8 molecules (hence called null cells).instead
possess specific surface markers such as CD16&CD56.they have no MHC restriction.,are part of innate immunity & donot
require prior contact with Ag& donot differentiate into memory cells.
NK CELLS Thus function as a bridge between the innate &aquired immune system.

30. Mechanism of NK cell mediated cytotoxicity-
• NK cells are not MHC restricted.they directly recognize certain ligands e.g-glycoproteins present on the surface of
altered host cells like virus infected cells or tumor cells.however ,such ligands are also present on normal cells.still
Nkcells are capable of distinguishing normal host cells from altered cells mediated by 2 types of receptors present on
NK cell surface (theory of opposing signal models.)
• Activation receptors- (e.g-NKR-P1,CD16) when these receptors are engaged with ligands present on the target cells,
NK cells become activated.
• Inhibitory receptors-(such as C- type lectin inhibitory receptors)-they recognize a part of MHC-l molecule (HLA-
E)which is present on the surface of all normal nucleated cells.
• Binding of inhibitory receptors to MHC-l molecules,generates an inhibitory signal that suppresses NK cells even if they
are bound to the activation receptors.this is because inhibitory signals are the dominant signal & overrides the
activation signal.
• However in virus infected cells & tumor cells the MHC-l expression is remarkably reduced.in such cases there would
not be any inhibitory signal ,hence binding of activation receptors to its ligand leads to activation of NK cells.
• The enzymes are constitutively expressed in NK cells (i.e they are cytotoxic all the time,even without exposure to the
Ag.).
• NK cells also respond to IL-12 produced by macrophages & secrete IFN-γ which in turn activates the macrophages &
kill the microbes.
• It also mediate their function via ADCC.
• LY-49A & killer cell inhibitory molecule(KIR ) are cell surface molecules found on mouse NK cells & human
respectively interact with MHC-l & inhibit natural killing.

31. Antibody depedent cell mediated cytotoxicity-
• A no of nonspecific cytotoxic cells express receptors (FCR) on their surface that can bind to Fc region of any
Immunoglobulin.following contact with target cell coated an antibody ,these Fc r bearing cells can bind to Fc
portion of antibody coated on target cells & subsequently cause lysis of the target cell.
• Although these cytotoxic cells are nonspecific for the Ag ,the specificity of Ab directs them towards the
specific target cell.this type of cytotoxicity is antibody depedant cell mediate cytotoxicity.
• ADCC exhibited by NK cells,macrophages,monocytes,neutrophils & eosinophils.they release various cytotoxic
factors like perforins,granzymes ,lytic enzymes,free radicals,TNF e.t.c .however there is no complement
depedent cytolytic activity.
• NK cells secrete perforins,granzymes, neutrophils release lytic enzymes,eosinophils release lytic enzymes &
perforins & macrophages produce lytic enzymes & TNF.

35. STRUCTURE OF THE T –CELL ANTIGEN RECEPTOR

36. TCR ἁ &ß genes & the GENERATION of TCR DIVERSITY

37. I
Interaction of TCR with superantigens-

38. STAGES OF THYMOCYTE DEVELOPMENT

39. POSITIVE & NEGATIVE SELECTION OF THYMOCYTES-

41. T-CELL ACTIVATION- SIGNAL TRANSDUCTION BY THE TCR

43. CD45: A TYROSINE PHOSPHATASE REQUIRED FOR TCR SIGNALLING

44. COSTIMULATION BY CD28