Lecture 2Innate immunity

•General features of innate immunity

•Example of an inducible innate immune mechanism

interferon

•Example of a preformed innate immune mechanism

complement

Reading: Parham chapters 1 and 7.15 to 8.10

Example: Complement (C’)

Example: Interferon

Innate defense is both preformed and inducible

See Fig 8.38 Parham

Figure 8.1

Defensins (epithelium)

Figure 8.6

Progression of Immunity

At least three cell types reside within or beneath the epithelium and induce inflammation in response to trauma or microbial products: Macrophages, Mast Cells, and Langerhan’s cells (a skin dendritic cell)

Figure 8.5

Features of innate immunity

Cellular components: •Phagocytic cells, granulocytes: release toxins, release cytokines, some can move rapidly •High precursor frequency of reactive cells•Multiplicity of receptors for conserved aspects of

microbes

Soluble components:•Blood plasma proteins that recognize (tag) microbes•Plasma proteins that destroy microbes •Proteins that sequester molecules needed for microbial

nutrition

Components Principle FunctionsBarriersEpithelial layers Prevent entryDefensins and Cryptidins Microbial killing

Circulating and Tissue Effector CellsNeutrophils Early phagocytosis and killing of microbesMast Cells Release of inflammatory granulesMacrophages Efficient phagocytosis and killing of microbes: cytokinesEosinophils Nasty toxic cells designed to kill helminths (worms)Natural killer (NK) cells Lysis of infected cells, activation of macrophages

Circulating ProteinsComplement (C’) Killing of microbes, opsonization of microbes, actvn leukocytesMannose-binding protein Opsonization of microbes and activation of C’C-reactive protein Opsonization of microbes and activation of C’Lysozyme Bacterial cell wall lysis

CytokinesTNF, IL-1, 6, 18 InflammationIFN Resistence to viral infectionIFN Macrophage activationIL-12 IFN production by NK cellsIL-15 Proliferation of NK cells, memory T cellsIL-10, TGF Control of Inflammation

Adapted from: Abbas (Saunders)

Components of Innate Immunity

Alveolar macrophages (lung) Histiocytes (connective tissue) Kupffer cells (liver) Mesangial cells (kidney) Microglial cells (brain)Tissue macrophage

Figure 1.6ij

Macrophages are important first responders to infection and tissue damage.

Receptors on Macrophages:

LPS receptor-CD14

Toll-like receptors

Fc receptors

Mannose receptor

Complement receptors

IFN receptor

Chemokine receptors

Figure 1.13Macrophages phagocytose and degrade foreign particles,bacteria and dead (and dying) host cells.

High affinity FcRI receptor. Effective against worm infections. Granules contain mediators-smooth muscle contraction and worm toxicity

Express some of the same receptors found on macrophages. These cells are specialized in killing and swallowing microbes

Figure 1.6ef

LPS receptor:

CD14

toll-like receptor-4

CR3,4:

Complement (C’) receptors (C3b)

Scavenger receptor:

sialic acid-bearing protein

Mannose receptor:

Binds mannose on bacteria, activates C’

Glycan receptor:

Polysaccharides

IN ADDITION: TLRs

Figure 8.8

Figure 2-6

Function in disease, not entirely understood

Contains high affinity receptors for IgE, and preformed granules that contain inflammatory mediators including: histamine; heparin; TNF; chondroitin sulfate; neutral proteases; and other.

Mast cells can also secrete: cytokines to induce inflammation; chemokines to induce infiltration by monocytes, and neutrophils, leukotriences to induce muscle contraction and increase vascular permeability

Mast cells are capable of inducing an inflammatory cascade

Figure 1.6gh

Mast cells are also found in the tissues

Mast cells can release histamines which induce inflammation

Redness, swelling(erythema, edema)

Neutrophils and monocytes are recruited

Figure 1.14

TNF

Figure 1.6ab

Lymphocytes are entirely involved with acquired immunity. The come in two types: T lymphocytes (T cells) that differentiate in the thymus and B lymphocytes or B cells that differentiate in the bone marrow.

B cells can further differentiate after antigen-activation to plasma cells that produce antibodies

Figure 1.6cd

Natural Killer Cells play several interesting roles in the immune system. One is to monitor cells for identification. If a cell doesn’t reveal its identity papers, it is killed. You’ll see this later in the course.

Dendritic cells are the most important antigen presenting cells (APCs) in the immune system

Figure 8.10

**

**The most important inflammatory cytokine (at least in this course)

Figure 8.14

The story of interferon

LIVE Influenza virus

Chick cells

Dead cells + more virus

incubate

The story of interferon

Isaacs and Lindenmann 1957

Heat killed Influenza virus

Chick cells

3-4 hrs

LIVE Influenza virus

Resistance to infection!

The story of interferon

Isaacs and Lindenmann 1957

LIVE Influenza virus

Resistance to infection!

Virus free supernatantfrom cells treated withkilled virus

interferon

Concepts

• The cells that produced interferon were non-specialized, i.e, non-immune cells.

• The heat killed influenza was recognized as foreign.• Interferon in this context behaved as a danger signal to

nearby cells.

Questions

• What was seen as foreign?• What rendered the warned cells resistant to infection?

How was virus recognized as non-self?Virus was detected in this case by a sensor for

double stranded RNA called Toll-like receptor 3 (Tlr3)

Virus enters here

dsRNAdetected here

by Tlr3

Interferon can be induced with a synthetic dsRNA,poly I:C (inosine:cytidine)

The RNA of normal cells typically is not double stranded, but dsRNA is often a feature of viruses.Influenza is a positivestrand RNA virus that generates a negative strand by RNA-dependent RNA polymerase, a process that generates dsRNA Signal to nucleus

Interferon produced

INF-(interferon)

Adapted from Fundamental Immunology , WE Paul, Ed. Chap 39

INF receptor

Induced synthesis(2’-5’)-oligoandenylate synthase

ATP 2-5(A)

RNAseL(inactive)

RNAseL(active)

mRNA degraded

Protein kinase PKR

dsRNA

dsRNA

(inactive)

+ATP

Active PKR

Inhibition of protein synthesis

eIF2 (translation intiation factor)

How interferon signaling inhibits viral growth

Effects on viralgrowth

Improved recognitionby adaptive immunity

Increased innate immune activity

Effects of interferon

Fig 8.7

From Takeda and Akira 2005

The toll like receptors in humans

cytoplasm

ssRNA

Non-specialized cells can detect and respond to infection by recognizingconserved motifs of microbes using Tlrs

Tlrs tranmit signals about microbial constituents detected to the nucleus, thus regulating the type of genes expressed, and the subsequent response

Seen by Tlr4

Seen by Tlr2

Toll family of receptors is evolutionarily ancient. Toll receptors in Drosophila were discovered first, and shown to confer resistance to bacteria and fungi by regulating the expression of anti-microbial peptides.

What is the basis for innate immunity, and how does is relate to vertebrates? Drosophila melanogaster mutants were found that were susceptible to fungal and bacterial infections.

Immunity in Drosophila (Innate)

Toll mutant lacks defense against fungal infections

18 Wheeler lacks defense against bacteria

This led to the discovery of a family of receptors known as the Toll-related receptors (TLR) present in vertebrates

Fig 7.27 Parham

Complement pathways

Figure 2-20

Figure 2-24

Mannose binding lectin is an innate recognition molecule that recruits complement

Figure 2-11

Figure 2-21

Binds to antibody:antigen complex

C1q binds to antibody:antigen complexes. Thus it recruits complement to antigen tagged by antibody.

Figure 7.30

Figure 2-19 part 1 of 2

C1q couples the innate(invariant) complement system to the adaptive

immune system (Antibody).

Here specificity comes from invariant molecules

Figure 2-22

For MB lectin MASP1/2cleave C4

The rest is the same

Complement activation is a proteolytic cascade, in which one protease activates by proteolysis the next protease.

Zymogen= inactive enzyme that is activated by proteolysis

See Figs 7.33, 7.34

Figure 2-25C4 (and C3) carry a unique

thioester bond

….Cys-Gly-Glu-Gln-……amino acid chain

Activated C3 and C4(C3b and C4b) rapidly bind covalently to nearby surfaces and proteins.

Fig 7.32

Figure 2-28Complement activation includes feed forward amplification steps

Macrophages, neutrophils and other white blood cells carry C3b receptors that they use to rapidly phagocytose and destroy microbes C3b.

Figure 7.38

Figure 7.39

Figure 7.35

Figure 2-26

The alternative C’ pathway illustrates the important role of negative regulators.

Mammalian cells protect themselves from complement-mediated lysis by expressing inhibitory proteins on the plasma membrane.

Small amounts of C3b are continually being produced by spontaneous “tick over”. If C3b binds to a pathogen, the reaction continues, if it binds to a host cell in low amounts, the reaction is usually terminated.

(This figure is similar to Fig 8.7 in Parham)

DAF binds to C3b and C4b causing the dissociation of existing C3 convertases and preventing the assembly of new ones.

CR1 and MCP bind to C3b and C4b, making them susecptible to proteolytic cleavage by factor I.

Figure 7.43

Figure 7.40

Anaphylatoxins: C5a > C3a > C4a

Smooth muscle contraction, degranulation of mast cells and basophils.

Vasoactive effects directly on local blood vessels.

Result: inflammation, edema, increased fluid flow into the lymphatics, and transfer of antigens to local lymph nodes

Figure 2-9

Summary of lecture 2

•Innate immune activity is preformed and inducible•Innate immunity involves all cells (e.g. interferon)•Conserved motifs of microbes are detected (ligands for Tlrs)•Innate response involves

•altered circulation- increased vascular permiability•ridding mechanism- toxic mediators, NO, hydrogen peroxide•alarm mechanisms- cytokines, histamine•both local and sytemic signals are transmitted (fever, local swelling)

•Leukocyte migration is important•Response is often toxic•Self cells protect themselves (e.g., complement inhibitors)•Innate immunity is coupled to adaptive immunity (classical C’ pathway)•Innate immunity is highly conserved over evolution