Innate Immune System: A Short Review

Author: V. Dimov, M.D., Allergist/Immunologist and Assistant Professor at University of Chicago
Reviewer: S. Randhawa, M.D., Allergist/Immunologist and Assistant Professor at LSU (Shreveport) Department of Allergy and Immunology

Immune system defends against microbes which can cause infection.

Immune System has 2 arms:

- Innate immune system
- Adaptive immune system


The 2 arms of the immune system: innate immunity and adaptive immunity.

I
Innate immunity (natural or native immunity)
Instant
Immediate
Initial response
Induces adaptive immunity
Integrates with adaptive immunity

A
Adaptive immune system
Acquired
Await days = no immediate response
Accurate = specific
Autoregulation
Autoimmunity


An overview of the immune response. This video is from: Janeway's Immunobiology, 7th Edition Murphy, Travers, & Walport. Source: Garland Science.

Innate immunity

Innate immunity is the first line of defense against infections. Innate immunity is specifically against microbes, while adaptive immunity is against any "foreign" substance (including cancer cells, autoantigens, etc.).


Innate immunity is the oldest mechanism of defense. Adaptive immunity (T and B lymphocytes) appeared in jawed vertebrates (sharks) and is superimposed on innate immunity to improve host defense. In a sense, adaptive immunity is an "add-on" to innate immunity.

Innate immunity augments adaptive immunity.


In the initial stages of an immune response, the innate immune system recognizes the presence of pathogens and provides the first line of defense. This video is from: Janeway's Immunobiology, 7th Edition Murphy, Travers, & Walport. Source: Garland Science.

How does innate immunity augment adaptive immunity?
Through TNF, INF alpha/beta, IL-1.

There are no autoimmune diseases mediated by innate immunity, only mediated by adaptive immunity. In a sense, we sacrifice specificity in recognizing our own body in order to gain diversity.

Innate immunity responds to only around 1,000 PAMPs (patterns), 10(3).
Adaptive immunity responds to more than 3,000,000 antigens, 10(7).

Innate immunity recognizes patterns.
Adaptive immunity recognizes protein antigens.

Patterns recognized by innate immunity are not present on mammalian cells, only on microbial cells.

Innate immune system recognizes only a limited number of microbial products (patterns) - 1,000.

Adaptive immune system is capable of recognizing a much wider array of foreign substances (both microbial and non microbial products) - 3,000,000.

Innate immunity - 3 components:
- epithelial barriers (skin, secretions)
- cells - circulating and tissue cells, for example, phagocytes - PMN, macrophages (antigen-presenting cells, APC). Macrophage means a "large eating cell" in Greek.
- plasma proteins, for example, cytokines

Epithelial barriers

Epithelia produce locally peptides that have antimicrobial properties (antibiotics):
- defensins
- cathelicidins

Cathelicidins are small proteins released by epithelial cells that kill bacteria.

Barrier epithelia and serosal cavities contain intraepithelial lymphocytes which respond to commonly encountered microbes:
- T cells in the epithelium
- B1-B cells in serosa

B
B1-B cells
Bowel

Certain groups of T and B lymphocytes have little diversity and recognize PAMPs. They are only around 1,000 PAMPs as opposed to more than 3,000,000 Ag.

The peritoneal cavity contains B lymphocytes (B-1 B cells) whose antigen receptors (immunoglobulin molecules) have limited diversity, just like the antigen receptors of intraepithelial T lymphocytes.

B-1 B cells make IgM specific for polysaccharide and lipid antigens found on bacteria. These antibodies are called natural antibodies (product of B-1 cells) or innate Ab.


Overview of the innate immune system (a mind map).

A macrophage of a mouse stretching its arms to engulf two particles, possibly pathogens. Image source: Wikipedia.

Antigen presentation stimulates T cells to become either "cytotoxic" CD8+ cells or "helper" CD4+ cells. Image source: Wikipedia.

Innate immunity is activated immediately and contains the infection until adaptive immunity "kicks in" and resolves the infection.

PAMPs

Microbial products which stimulate innate immunity are called pathogen-associated molecular patterns (PAMPs). Pathogen-associated molecular patterns (PAMPs) are on bacteria - they bind to receptors called pattern recognition receptors.

Innate immunity reacts to these common bacterial structures - PAMPs, pathogen-associated molecular patterns. PAMPs are produced only by the bacterial invader and not by the host. If it were the other way around, innate immunity would get "confused" and attack the host instantly and immediately as it is typical of its nature.

PAMPs:
- lipopolysaccharides
- peptidoglycans
- lipoteichoic acids
- mannans
- bacterial RNA and DNA
- glucans

Receptors that bind these PAMPs are called pattern recognition receptors. PAMPs are unique to microbes. Mammalian cells do not have PAMPs, that is why there is no autoimmune disease caused by innate immunity.

For example, human cells do not have mannose - bacteria do. Receptors recognize mannose on bacteria - and this starts the mannose-lectin complement pathway.

Mannose is one of the PAMPs. TLRs are pattern recognition receptors.

Example of a match: ligand-receptor: mannose (bacteria)-mannose-binding lectin (MBL) (soluble recognition molecule in humans)

Mannose-binding lectin (MBL) starts the third complement pathway. MBL is plasma protein that functions as an opsonin.

APC (macrophages) have PAMPs-recognition receptors, called pattern-recognition receptors in short.


Pathogen Recognition Receptors, TLRs. This video is from: Janeway's Immunobiology, 7th Edition Murphy, Travers, & Walport. Source: Garland Science.

PAMPs Recognition Receptors

4 PAMPs receptors: T-CNS

Toll-like receptors
C-type lectins
N-formyl Met-Leu-Phe receptors
Scavenger receptors

Pattern-recognition receptors (molecules) are divided into 3 groups:

SES

Secreted
Endocytic
Signaling

C-type lectins

C-type lectins are calcium-dependent carbohydrate-binding molecules expressed on the plasma membranes of macrophages and other cells.

The best known C-type lectin is the mannose receptor. Mannose-binding receptor binds to bacterial carbohydrates. It activates the 3rd complement pathway.

Another C-type lectin is Dectin1.

C
C-type lectins
Calcium-dependent
Carbohydrate-binding molecules

C-type lectins:
- Mannose-binding
- Dectin

Secreted pattern-recognition molecules bind to microbes (opsonization) and make them "tastier" to phagocytes. An example of secreted pattern-recognition molecules is a mannose-binding protein (lecithin). These molecules are associated with serine proteases: MASP (mannose-binding lectin-associated serine proteases).

Toll-like Receptors (TLRs)

Some Toll-like Receptors (TLRs 2, 4, and 5) are found on the cell surface, where they may bind extracellular PAMPs. Other TLRs (3, 7, 8, and 9) are found within the cells, on endosomal membranes and recognize viruses that have been phagocytosed by cells.

TLRs 3,7,8,9 are found on lysosomes within the cells. They are activated by intracelllular agents (viruses). The pathway is: TLR activation --> kinases --> gene transcription --> TNF, IL-1, INF alpha/beta.

Signaling pattern-recognition molecules, for example, toll-like receptors (TLRs), lead to activation of genes which in turn lead to a specific immune response. TLRs got their name from toll, a membrane receptor in the Drosophila fly which provides resistance to fungi. TLRs are the first point of contact between the immune system and a pathogen.


The curved leucine-rich repeat region of toll-like receptors, represented here by TLR3. Image source: Wikipedia.


Signaling pathway of toll-like receptors. Dashed grey lines represent unknown associations. Image source: Wikipedia.

Which transcription factors make INF?
IRF 3 and IRF 7

IRF leads to
INF

There are 12 mammalian TLR genes (11 in humans). All TLRs are membrane glycoproteins. There are 11 different human TLRs, named TLR 1 to 11.

There are 2 groups of TLRs:

- Surface TLRs - for bacteria
- Endosomal TLRs (3, 7, 8, and 9) - for viruses

Scavenger receptors

One scavenger receptor, SRB1, is involved in atherogenesis. Scavenger receptors mediate the uptake of oxidized lipoproteins into cells.

NF MLP

N-formyl Met-Leu-Phe (NF MLP) receptors are the most chemoattractive (chemotactic) receptors for PMNs.

N
NF MLP
Neutrophil chemoattractant

All chemokines are proteins.
All chemokines are chemoattractants but not all chemoattractants are chemokines. For example PGD-6 is a chemoattractant but not a chemokine, it is a prostaglandin not a protein.

NLRs

NLRs (NACHT-LRRs) are a family of cytoplasmic molecules.

CARD

Caspase activation and recruitment domain (CARD)-containing proteins. Homozygous mutations in the CARD9 gene are associated with chronic mucocutaneous candidiasis. Dysfunction of CARD9 impairs the innate signaling of dectin-1, an antifungal pattern-recognition receptor (NEJM, 10/2009).

Cells of Innate Immunity

Neutrophils

Neutrophils, also called polymorphonuclear leukocytes (PMNs), are the most abundant circulating WBCs. PMNs mediate the earliest phase of inflammatory response. CXCL8 (IL-8) forms a chemotactic gradient that directs leukocytes towards site of tissue injury/infection.

What is the PMN half-life?
6 hours.
100 trillion cells are produced every day by the BM.

What is the most important PMN enzyme?
Elastase

PMNs do not stain either red (eosinophils) or blue (basophils).



Video: A neutrophil chasing bacteria, set to music.

Mononuclear Phagocytes

Mononuclear phagocytes circulate in the blood as monocytes and reside in all tissues of the body as macrophages.

Macrophages differentiate into specialized forms in different tissues:

KOMA:
Kupffer cells (liver)
Osteoclasts (bone)
Microglia (CNS)
Alveolar macrophages (lung)

Monocytes turn into macrophages once they enter tissues. Dendritic cells are not the same as macrophages. Monoblasts give rise to both macrophages and dendritic cells.

Recruitment of Leukocytes to Sites of Infection

SIP of wine:
Selectins
Integrins
Penetration of BM by PMN

Selectins are first in the chain of events. They upregulate TNF and IL-1.
Integrins cause release of VCAM and VLA.

The increasing numbers in CD description correspond to higher affinity:
CD 16 32 64
affinity ---->

Difference between Affinity and Avidity

Affinity describes the strength of a single bond.

A <--> B

Avidity describes the combined strength of multiple bond interactions.

A <-->
A <--> B
A <-->

In avidity, there is more than one A for each B.

An example of greater avidity is the IgM pentamer. Individually, each bond is easily broken but when many are present at the same time the overall effect results in synergistic, strong binding of antigen to antibody: IgM has low affinity but high avidity because it has 10 weak binding sites as opposed to the 2 strong binding sites of IgG or IgE.

IFN gamma upregulates the "killing" of phagocytosed microbes.


The uptake of bacteria by phagocytes is an active process, which requires the triggering of specific receptors on the phagocyte. Fc receptors, which bind antibody-coated bacteria, are one of the receptors capable of triggering phagocytosis. This video is from: Janeway's Immunobiology, 7th Edition Murphy, Travers, & Walport. Source: Garland Science.

Chronic granulomatous disease (CGD)

PMN kills microbes by using 2 enzymes: ROS and NO. This "respiratory burst" is impaired in chronic granulomatous disease (CGD). In CGD, PMN function migration is normal bu there is no respiratory burst. There are 2 tests to diagnose CGD.

CGD was first described in 1957 as "a fatal granulomatosus of childhood". CGD is a diverse group of hereditary diseases in which PMN cannot form the reactive oxygen compounds (superoxide radicals) to kill ingested pathogens. This leads to the formation of granulomata in many organs, hence the name of the condition. CGD affects 1 in 200,000 people in the U.S. and 20 new cases are diagnosed each year.


Two neutrophils (PMN) among red blood cells. PMN are the type cells affected by chronic granulomatous disease. Image source: Wikipedia, GNU Free Documentation license.


Superoxide anion. Image source: Wikipedia, GNU Free Documentation license.

Diagnosis of CGD

The original test for diagnosis of CGD from a historical perpsective is nitroblue-tetrazolium (NBT). NBT test depends on the direct reduction of NBT by superoxide free radical to form an insoluble formazan. NBT is a simple rapid test but is only a "yes/no" test whcih detects whether or not there is a problem with the oxidative enzymes. It cannot provide information on how much the enzymes are affected.

In recent years, there has been a shift in the diagnostic paradigm for evaluation of male patients with CGD, based on the rhodamine assay. Rhodamine dyes fluoresce and can be detected easily with fluorometers. Rhodamine takes up SOD and its level is measured by flow cytometry, thus providing information about how much superoxide a patient's phagocytes can produce.


A Rhodamine 6G-based dye laser. The dye solution is the orange fluid in the tubes. Image source: Wikipedia, GNU Free Documentation license.

NK Cells

NK
Natural Killers, "born to kill"
Not lymphocytes

NK are "born to kill," they are natural killers because they do not need a stimulus to kill affected cells. In a sense, NK are "hyperactive" and potenially, the most dangerous cells in the human body.

MHC I
I
Inhibits cell killing by NK

Viruses inhibit MHC I expression which leads to killing by NK cells.

IL 15
Increases number of NK cells
IL 12 stimulation
INF gamma stimulation of NK cells

NK cells (not T-lymphocytes) are different from NKT cells which are T-lymphocytes.

The mnemonic for different T-cell subtypes is FERMNN G ("pheromon G"):

Four, CD4, helper cells (Th1 and Th2)
Eight, CD8, killer cells
Regulatory, formerly known as suppressors
Memory
Naïve T cells
Natural Killer T cells (NKT)

Gamma/delta T cells (γδ)


Mind map of the 7 different T-cell subtypes remembered by the mnemonic FERMNN G

Complement System (click on the title for the full text).

Adhesion Molecules and Chemokines (click on the title for the full text).

References

Allergy and Immunology MKSAP, 3rd edition.
Chapter 2. Innate immunity. Abbas et al: Cellular & Molecular Immunology, 6th Ed.

Related Reading

Medical Immunology Syllabus. Columbia University.
Anaphylaxis Due to Contaminated Heparin Causes Multiple Deaths, Trigger Found. Allergy Notes, 04/2008.
FIT Corner Q & A from 5th edition of Cellular and Molecular Immunology, edited by Abul K. Abbas and Andrew H. Lichtman. ACAAI, 2004.

Video

Macrophage cytokine release. Nucleus Medical Art.
Pathogen Recognition. Nucleus Medical Art.

Published: 12/05/2007
Updated: 10/05/2009