Innate Immunity and Adjuvants

Sukhbir Kaur

History of Immunology• Discovery of vaccination

by Edward Jenner in 1796.

• Based on observation cowpox infections protected the body against small pox.

• Discovery of antibodies and passive immunity by Shibasaburo Kitasato and Emil von Behring in 19th century.

• Paul Ehrlich - side chain theory of immunity.

• It predicted mechanism of antibody production from B-cells.

• Discovery of process of phagocytosis by Elie Metchnikoff (1895-1916).

• Ehrlich and Metchnikoff togather got Nobel Prize for Medicine for discovery of humoral(adaptive) immunity and innate immunity respectively.

Immune responses

Immune system

Adaptive immunity

Cell-mediated

T-cells

Humoral

B-cells

Innate immunity

Macrophages, Dendritic cells and

leucocytes.

Adaptive immunity

Main components: B-cells and T- cells which carry receptors known as BCR and TCR.Receptors have variable region and constant regions.Different gene segments encode variable regions.Each member of gene segment is randomly joined to other members. This leads to huge diversity of receptors.Receptors reacting with host components are eliminated.

Invasion by pathogen

Activation of T-cells and B-cells

Activated cytotoxic and helper T-cells

Antibody producing Plasma cells

Memory T- cells Memory B-cells.

Innate immunity

• Main components : Leucocytes ,Macrophages and Dendritic cells (collectively known as phagocytes).

• Not long lasting.• Immediate and temporary system of defence.• Discriminate self and foreign components.

This ability relies to greater extent on TLRs .• Activation of innate immunity is necessary for

induction of adaptive immunity.

Innate and adaptive immunity

Two theories of immune responses

Original theory

• Dendritic cells engulf pathogen, digest it into peptides and express these peptides on their surface.

• Migrate from site of infection to lymph nodes and present antigens to naïve T-cells with corresponding receptors .

• Pathogen is recognized in lymph nodes during T-cell activation.

New theory

• Activation of dendritic cells is needed to activate T-cells.

• Dendritic cells remove dead cells from host and self antigens are presented to T-cells but no activation of T cells as dendritic cells are not activated.

• Activation is mediated by TLRs.

Toll like receptors (TLRs)• Evolutionarily conserved between

insects and vertebrates.• Type I glycoprotein having :• extracellular domain characterized

by varying no. of leucine rich repeats(LRR) motifs.

• Cytoplasmic signaling domain, homologous to that of interleukin(IL)-1 receptors, known as Toll/IR-1 (TIR) domain.

• LRR domains consist of 19-25 tandem repeats of 24-29 amino acids and these are xLxxLxLxx acid residues.

• LRRs have horse shoe structure and are involved in ligand recognition

TLRs• The ligands for TLRs: a. lipids, b. protein andc. nucleic acids• These are potent

immune adjuvants that can trigger vigorous immune response so TLRs are called adjuvant receptors.

TLR Ligand Notes TLR1+TLR2 Bacterial lipoprotein(BLP)

(triacylated)Immune and non- immune cells

TLR2+TLR6 Mycoplasma macrophage activating lipopeptide-2 (MALP) (diacylated),

Immune and non-immune cells.

TLR3 Viral DNA and RNATLR4 Lipopolysacchrides(Gram

–ve bacteria)Most potent ligand

TLR5 flagellin Digestive tract, respiratoory tract, urinary tract, dendritic cells in mucosa

TLR7, TLR8 Imidazoquinolin, nucleoside guanosine or uridine rich ssRNA

TLR8 not active in mice

TLR9 Unmetylated CpG motifs Bacteria have unmethylated CpG

TLR Ligands and TLR4

TLR signaling pathways • Adaptor molecules which interacts with TIR domain

of TLRs to activate downstream signaling pathways:• MyD88: in all TLRs except TLR3. So, 2

pathways:MyD88 dependent and independent.• TRIF :used by TLR3 and TLR4.• TIRAP: used by TLR2 and TLR4 to recruit MyD88.• TRAM: bridge between TLR4 and TRIF.MyD88 NF-κB and MAP kinase activation

inflammatory cytokines. TRIF NF-κB andIRF3(interferon regulatory factor

3) activation type I IFN.

TRIF dependant signaling involves TANK-binding kinase 1(TBK 1) and IκB kinase, inducible (IKKi) acting asIRF3 kinase

RLR- cytoplasmic helicases• Retinoic acid-inducible gene I(RIG-I)- RNA helicase,

present in cytosol, detects viral infections.• Melanoma differentiation-associated gene 5

(MDA5) is homologous to RIG-I.• Both are found ubiquitously in all cells.• Possess two N-terminal caspase-recruitment

domains (CARDs) followed by helicase domain.• RIG-I 5’-triphosphate short or long dsRNAs.• MDA5 picornavirus family(has long dsRNA

of more than 2kbp.

RLR: signaling pathway• CARDs responsible signal

transduction which leads to NF-κB and IRF3/7 activation via adaptor molecule IPS-1 (IFN-β promoter stimulator 1)

• For IRF3/7 activation , TBK1 and IKKi are involved.

• For induction of type I IFN, STING (stimulator of interferon genes) also acts as a mediator. It is multi spanning membrane protein that associates with TBK.

NLRs • NLRs consist of: C-terminal

LRR domain, central NOD domain and N terminal effector domain which is involved in signaling.

• NOD1 and NOD2 –recognise components of peptidoglycan activate NF-κB and induce inflammatory response

• Other NLRs are involved in inflammosome formation and production of mature IL-1β and IL18 .

For inflammosome formation ,NODs activate Caspase-I which cleaves pro IL-1β to mature IL-1β .

4 types of inflammosomes: nucleotide-binding domain and leucine-rich repeat containing

family pyrin domain containing1 (NLRP1) nucleotide-binding domain and leucine-rich repeat containing

family ,CARDdomain containing4 (NLRC4), NLRP3 and absent in melanoma 2 (AIM2). NLRC4 detects flagellin and imidazoquinoline NLRP3 detect monosodium urate, abestos and cholesterol

crystals.

NLRs

CLRs C- type lectin receptor (CLR) recognize β-

Glucan which is cell wall component of fungi and yeast.

It consists of : Extracellular carbohydrate recognising

C- type lectin domain-CTLD cytoplasmic tyrosine containing domain Dectin-1 recognizes Candida, Aspergillus,

Pneumocystis and Coccidioides . Interaction of Dectin-1 with β-Glucan

triggers spleen tyrosine kinase(Syk)which activates NF- βB through CARD9,Bc110 and MALT1 as well as caspase-1.

CLRs

• Dectin -2 expressed on: tisue macrophages, dendritic cells and inflammatory monocytes.

• It has a classical sugar binding CTLD which recognizes high mannose structures in Ca2+ dependent manner.

• It recognizes: Cryptococcus neoformans, Candida albicans, Saccharomyces cerevisiae, Mycobacterium tuberculosis, Microsporumaudounii, Paracoccidioides brasiliensis and Histoplasma capsulatum.

Induction of adaptive immunity

Conclusion

• For development and modulation of innate immunity, innate immunity is necessary.

• PRRs-TLRs, RLRs, NLRs and CLRs recognize pathogens and inflammatory reactions which trigger adaptive immunity