HDM TBL 7: Cellular Immunity Principles

Activated DC expresses what on its surface?

CCR7

• receptor for CCL21

which chemokine signal is secreated by stromal cells & high endothelial venules [HEV] located in draining lymph node?

CCL21

• attracts activated DCs to paracortex of lymph node

in lymph node, activated DC secretes what?

CCL19

• a chemoattractant for naive T cells

what process attaches T cell to endothelium during migration in HEV in lymph node?

• L-selection binidng to GlyCAM-1 & CD34

• LFA-1 activated by chemokine & binds tightly to ICAM-1

• → lymphocyte leaves blood & enters lymph node

*same as neutrophil recruitment in acute inflammatory response

CD4+ T cells only bind peptides presented by

MHC-II

CD8+ T cells only bind peptides presented by

MHC-I

what is the co-stimulatory signal btwn T cell & DC?

CD28 on T cell

&

B7 [CD80:CD86] on DC

activation of T cells requires?

1. antigen-specific signal

2. co-stimulatory signal

what is CD25?

high affinity IL-2 receptor

• binds IL-2, which generates signals needed for clonal proliferation of activated T cells [autocrine rxn]

role of cytokine IL-2

signals from T cell receptor, co-receptors, & co-stimulatory receptors → transcription of genes for both IL-2 & alpha-chain of IL-2 receptor, by the activated T cell

alpha-chain of IL-2 receptor

needed to produce high affinity IL-2 receptor [CD25]

*naive [resting] T cells do not have alpha-chain, so only express low affinity IL-2R → so they’re hypo-responsive to IL-2

anergy

state of immune unresponsiveness [T cell still viable tho]

T cells that attempt to activate w/o CD28:B7 co-stim become?

anergic

Basilixumab

monoclonal antibody that targets CD25 to block T cell proliferation

Daclizumab

monoclonal antibody that targets CD25 to block T cell proliferation

5 subtypes of effector T helper cells:

1. Th1

2. Th2

3. Th17

4. Tfh

5. Treg

what stimulates CD4+ T cells to differentiate into the 5 subtypes?

a 3rd signal = cytokines = “polarizing cytokines” produced by APC

which transcription factor directs differentiation of Th1 cells?

T-bet

which transcription factor directs differentiation of Th2 cells?

GATA3

which transcription factor directs differentiation of Th17 cells?

RORyT

which transcription factor directs differentiation of Tfh cells?

Bcl-6

which transcription factor directs differentiation of Treg cells?

FoxP3

deficiency in CD4+ T cells →

severe immunodeficiency: susceptibility to opportunistic infections & certain caners

*b/c CD4+ T cell subtypes contribute to both humoral & cell-mediated immunity

Th1 cells

• clear intracellular pathogens & cancer cells [major role]

  • via secretion of IGN-gamma & IL-2 [Th1 response]

    • enhance macrophage killing

    • activates macrophages for granuloma formation

    • enhance CD8+ kill thru IL-2, LTalpha, & TNF-alpha

• clear extracellular pathogens [contributing role]

  • secreted IFN-gamma promotes IgG secretion by activated B cells

    • IgG help w/ complement activation [classical pathway] & antigen opsonizaiton

  • secreted LTalpha & TNF-alpha activate neutrophils, enhancing their killing power

uncontrolled Th1 polarization →

chronic inflamm & autoimmune diseased

• Multiple Sclerosis [MS]

• Type 1 Diabetes

• Crohn Disease

*often involved increased levels of IFN-gamma & TNF-alpha, showing that Th1 cytokine excess is key driver

Etanercept

TNF inhibitor

• used to dampen overactivity from Th1 [also anti-IFN-gamma therapies]

Th2 cells

• clear extracellular pathogens, particularly helminth parasites & in allergic inflamm [Th2 response]

• contribute to antibody prod.

  • secrete IL-4, IL-5, IL13

• drive type I hypersensitivity response

  • prod of IL-4, IL-5, IL-13 → promotes IgE class switching, eosinophil activation, & mucus prod. AKA hallmarks of allergic inflammation

IL-4 & IL-5

drive eosinophil, basophil, mast cell responses

together contribute to allergic inflamm [Type 1 hypersensitivity]

IL-13

stim. prod of mucus of goblet cells

IL-4

drives prod. of IgG & IgE

IgG

helps w/ opsonization

prod. by IL-4

IgE

functions in eosinophil activation → helping w/ killing of helminth parasites

prod. by IL-4

uncontrolled Th2 polarization →

atopy

• chronic Th2 cytokines increase → inflammation, itch, airway hyperresponsiveness, & tissue damage

atopy

genetic tendency to develop allergic disorders

Ex’s:

• allergic rhinitis [hay fever]

• allergic asthma

• atopic dermatitis [eczema]

• food allergies

dupilumab

an anti-IL-4 monoclonal antibody used to block IL-4R alpha subunit, shared by both IL-4 & IL-13 receptors

by reducing this shared pathway, Dupilumab helps reduce symptoms of atopic dermatitis, asthma, & other allergic disorders

Th17 cells

• eliminate extracellular pathogens, esp. fungi & bacteria particularly at epithelial barriers [skin, lungs, GI tract]

  • secrete IL-17 & IL-22

IL-17

activates neutrophils & activates stromal cells/fibroblasts to release G-CSF to increase neutrophil prod. & recruitment

IL-22

promotes epithelial cell division, tissue repair, & healing

IL-17 & IL-22

activate epithelial cells to release antimicrobial peptides [AMPs] thus enhances barrier function

people who lack IL-17 or its receptor →

susceptible to chronic infections w/ Candida albicans [becomes opportunistic when immune defenses down]

uncontrolled Th17 polarization →

autoimmunity & autoinflammatory diseases

• IBD

• psoriasis

• ankylosing spondylitis

persistent IL-17 signaling drives chronic inflamm., tissue damage, & disease progression

Secukinumab

monoclonal antibody that neutralizes IL-17A

FDA-approved for treating plaque psoriasis, psoriatic arthritis, & ankylosing sponydlitis

Tfh cells

AKA Follicular T helper cells

• activate naive B cells to initiate antibody response to infection

  • secrete IL-21

    • received via IL21R on B cell

• co-stim via CD40-CD40L interaction needed for these to occur

  • Tfh provides co-stim signal termed thymus or T-cell dependent activation of B cells

IL-21

promote Ig affinity maturation, generation of memory B cells, & Ig class switching

• essential for optimal humoral immunity

Treg cells

• mediates peripheral tolerance & supression of:

  • effector T cells

  • DC cells

  • primary immune response after pathogen controlled

• secretes IL-10 & TGF-beta

• express CTLA-4

• express IL-2R [CD25]

CTLA-4

inhibits APC function by stripping away B7

IL-2R in Treg cells

[CD25]

acts as IL-2 sink by outcompeting for IL-2

CTLs

Cytotoxic T Lymphocytes AKA CD8+ [Cytotoxic] T cells

• killers that target & kill host cells infected w/ intracellular pathogens, viruses, or are neoplastic [major role]

• killing is v. specific

  • causes minimal collateral damage to normal host cells due to focused delivery of cytotoxic chemicals

• critical in inhibiting spread of infection to healthy cells

• can target all types of host cells

• contraction of T & B cell populations after antigen eliminated [other role]

  • via FasL-Fas during contraction phase of immune response

3 mechanisms CD8+ T cells use to induce apoptosis

1. Perforin-granzyme

2. Fas-FasL binding

3. Cytokines [mainly TNF-alpha]

Perforin-granzyme

a mechanism CD8+ T cells use to induce apoptosis

• perforin released by CD8+ T cell forms perforin complex [pore] on target cell membrane

• granzymes enter thru pore & activate apoptotic enzymes [caspases] which → apoptosis of target cell

Fas-FasL binding

a mechanism CD8+ T cells use to induce apoptosis

• CD8+ T cell expresses FasL on surface

  • → binds to cell surface molecule Fas expressed on target cell → activates apoptosis

Cytokines

a mechanism CD8+ T cells use to induce apoptosis

• TNF-alpha & LT induce apoptosis

LT

lethal toxin AKA lymphotoxin

2 subpopulations of Memory T cells

1. Central memory T cells

2. Effector memory T cells

central memory T cells

• remain in secondary immune organs

• express CCR7 that helps retain these cells in secondary immune organs

• contribute to activation of B cells upon re-encountering same antigen

effector memory T cells

• migrate to other tissues

• express CCR5 that tracks them to diff. tissues

• quickly differentiate into effector cells upon re-encountering same antigen

impaired immunity after age 55 b/c?

memory T cells survive for decase, but rely on continuous replication thru signals delivered via cytokines

as we age, memory T cells senesce due to telomere shortening on repeated divisions

T cell tolerance

state of hyporesponsiveness to self-antigens

maintained by central & peripheral mechanisms that attenuate or eliminate responses to self-antigen

central tolerance

• tolerance mech. that acts during thymic development

• main mech: negative selection

  • clonal deletion of thymocytes w/ TCRs that bind w/ high affinity to self-antigens

  • mediated by AIRE [transcription factor]

    • induces expression of tissue-specific proteins in thymus that participate in neg. selection

peripheral tolerance

• tolerance mech. that act in secondary lymphoid tissues to eliminate or silence autoreactive T cells that escaped central tolerance

main mechanisms of peripheral tolerance [4]

1. immune-privileged sites [immune sequestration]

2. apoptosis

   1. via Fas-FasL or PD1-PD1L

3. anergy

   1. lack co-stim

4. suppression by regulatory cells

   1. TGF-beta & IL-10

immune-privileged sites [immune sequestration]

• sites where tissues are anatomically sequestered via blood barriers or molecularly shielded by FasL tissues

• includes: CNS, testes, eye, gravid uterus [uterus w/ fetus]

• self-antigens at these sites are physically separated from immune system, therefore don’t activate autoreactive T cells

  • + local expression of FasL promotes apoptosis of infiltrating lymphocytes

Attenuation of T cell Responses mechanisms

1. Fas-FasL interaction

2. PD1-PD1L interaction

3. CTLA-4-B7 interaction

4. T regulatory cells

   1. secrete IL-10 & TGF-beta

Fas-FasL interaction

• Fas belongs to TNF-alpha receptor family & signals for apoptosis when it binds its soluble ligand FasL

• activated T cells express Fas → undergo apoptosis when engagedby FasL expresPD1-PD1Ksed on same cell [suicide] or a nearby T cell [fractricide]

PD1-PD1L interaction

• activated T cells express PD1, which binds to PD1L on APC [or cancer cells] → inactivates effector T cells & induces apoptosis of effector T cells

• presents problem in cancer

  • as PD1L is broadly expressed on many cancers → apoptosis of cancer-specific cytotoxic T cells

Nivolumab

monoclonal antibodies that block PD1L

[cancer treatment]

Pembrolizumab

monoclonal antibodies that block PD1L

[cancer treatment]

CTLA-4-B7 interaction

• attenuates T cell activation indirectly by preventing signaling thru CD28

• activated T cells express CTLA-4 which binds w/ higher affinity to B7 molecules [compared to CD28] thereby outcompeting CD28

  • can even strip B7 off APC

    • T cells become anergic

• Cbl-b mediated ubiquitination of TCR signaling proteins

Immune checkpoint proteins

• PD1

• CTLA-4

*checkpoint inhibitors used in cancer/tumor treatment

Abatecept

[Orencia]

• soluble CTLA-4 fusion proteins used to block problematic T cell activation in auto-immune conditions

• for rheumatoid arthritis

• also used to minimize transplantation rejection rxns

Cbl-b mediated ubiquitination of TCR signaling proteins

• in absence of CD28 costim., Cbl-b ubiquitinates signaling proteins downstream of TCR & CD3, suppressing T-cell activation & promoting anergy

severe combined immunodeficiency [SCID]

• primary immune deficiency caused by mutaitons in T cell signaling components [adenosine deaminase (ADA), IL-7Ralpha or JAK3, ZAP-70]

  • → absent or severely reduced T cells

  • B & NK cells variably present depending on gene mutation

• AKA “bubbly boy/baby disease” [David Vetter]

• increased susceptibility to viral, fungal, bacterial, & opportunisitic infections

  • multiple classes of pathogens

• presents in early infancy w/ recurrent severe infections, chronic diarrhea, & failure to thrive

• common opportunisitic pathogens:

  • pneumocystis jirovecii

  • candida spp

  • CMV

• → defects in cell-mediated & humoral immunity b/c T cells required for optimal B cell activation

• Treatments

  • HSC transplant

  • gene therapy: ADA gene transfer using retrovirus vector

DiGeorge syndrome [q22 deletion syndrome]

• genetic disorder caused by deletion of 30-40 genes in middle of chromosome 22 at location 22q11.2

• 90% of cases occur due to new mutation, 10% inherited

• mostly due to deletion of gene coding for TBX1

  • transcriptional factor req. for organogenesis of 3rd embryonic pouch to form thymus & parathyroid glands

    • absent of hypoplastic thymus

    • decreased T cell numbers

    • impaired cell-mediated immunity

    • hypoplastic/absent parathyroid glands → hypocalcemia & tetany

    • cardiac defects

    • facial features: low set ears, micrognathia, hypertelorism [wide set eyes], short philtrum

• increased susceptibility to viral, fungal, & opportunistic infections due to impaired T cell immunity

Mnemonic for DiGeorge syndrome

CATCH-22

Cardiac defects

Abnormal facies

Thymic aplasia

Cleft palate

Hypocalcemia

22q11.2 deletion

Acquired immune deficiency syndrome [AIDS]

• secondary [or acquired] immune deficiency caused by HIV infection

• progressive decrease in number of CD4+ T cells

  • below 200/mm³ = AIDS

• HIV enters CD4+ T cells by binding to surface marker CD4 [receptor] & CCR5 & CXCR4 [coreceptors]

• increased susceptibility to infections caused by opportunistic pathogens

• increased risk of certain cancers

• reactivation of latent infections normally controlled by T cell immunity

  • Tb, varicella-zoster virus, cytomegalovirus

• treatment

  • managed w/ antiretroviral therapy

receptor for EBV & HHV8 entry

CD21

receptors for HCV entry into B cells

CD81

Perforin deficiency AKA Familial Hemaphagocytic lymphohistiocytosis

• genetic disorder of cytotoxic lymphocyte function

• triggered by viral infections [commonly EBV]

• caused by mutastions in perforin [PRF1] or related genes

  • → decreased functions of CTLs & NK cells

• persistent antigen stim → excessive cytokine prod

• uncontrolled T cell activation

• excess IFN-gamma

• cytokine storm [IL-1, IL-6, IL-8, TNF-alpha]

• hemaphagocytosis in bone marrow, liver, spleen

  • activated macrophages engulfing RBCs, WBCs, & platelets

• high fever, hepatosplenomegaly, lymphandenopathy, cytopenia

• severe hyperinflammation from failure of CTLs & NK cells to eliminate infected cells

• Lab findings

  • hyperferritinemia [RBCs being eaten up]

  • hypertriglyceridemia

  • elevated soluble IL-2 receptor [sCD25]

• Treatment

  • immunosuppressive drugs

  • etoposide-based protocol

  • HSC transplant [curative]

autoimmune lymphoproliferative syndrome [ALPS]

• caused by mutations in Fas or FasL [apoptosis pathway]

  • prevents normal eliminated of activated lymphocytes after an immune response

    • accumulation of double negative T cells

      • lymphadenopathy & hepatosplenomegaly

      • autoimmune cytopenias

      • multi-organ autoimmunity

• increased risk of developing hematological malignancies

• doesn’t lead to an immune deficiency

CAR-T Cell Therapy

genetically engineered Chimeric Antigen Receptors [CARs]

• synthetic receptors genetically introduced into a pt’s T cells to enable recognition & killing of cancer cells

  • recognixed antigens on tumor cells independently of MHC presentation

Yescarta

a CD19-targeted CAR-T cell therapy approved for treatment of certain B cell non-Hodgkin lymphomas

Prednisone

corticosteroid that inhibits inflammation

immunosuppressant used in transplantation

Mycophenolate mofetil

inhibits T cell proliferation

immunosuppressant used in transplantation

Tacrolimus, Cyclosporin

inhibits T cell activation

immunosuppressant used in transplantation

Basiliximab, Daclizumab

anti-CD25

T cell depleting antibodies

Belatecept

CTLA-4 Fc fusion protein

immunosuppressant

Blocking PD-1

anti-PD1 and anti-PD1L mAbs block apoptotic signals delivered by PD-1 to cancer-specific T cells → so those T cells continue to survive

*pose signif. risk of autoimmunity due to loss of attenuation of T cell response

Nivolumab

anti-PD1 mAb

pembrolizumab

anti-PD1 mAb

durvalumab

anti-PD1L mAb

avelumab

anti-PD1L mAb

atezolizumab

anti-PD1L mAb

blocking CTLA-4

• monoclonal antibodies that normally inhibit T cell activation by competing w/ CD28 for B7 binding, reducing co-stim

• blocking CTLA-4 restores T cell co-stim via B7-CD28 binding, which enhances T cell activation against tumors/cancers

*pose signif. risk of autoimmunity due to loss of attenuation of T cell response

Ipilimumab [Yervoy]

FDA-approved for melanoma treatment

blocks CTLA-4