Lecture1

INNATE IMMUNE RESPONSES TO PATHOGENS

Dr. Dalia Galal Hamouda

Objectives

1. Role of Complement

2. Phagocytosis

3. Natural Killer Cells

4. Eosinophils.

1- ROLE OF COMPLEMENT

Complement is a system of more than 20 inactive serum proteins, it has three pathways for activation but the alternative pathway of the complement system, is involved in innate immunity because it does not depend on antibody.

1. One of the proteins of the complement system (C3) is spontaneously hydrolyzed to produce the fragment, C3b and a smaller fragment.

2. C3b deposits onto a microbial surface and activate the complement system and produce different protein fragments with biological activities in host defense (Fig. 2.1).

Biological activities in host defenseC3b acts as an opsonin (deposited on microbial cell surfaces) to help phagocytosis.

Some of the C3b molecules form a membrane attack complex (MAC) that insert into a microbial cell membrane and makes lysis of the cells or bacteria.

Finally, some of the fragments produced in the complement activation system are anaphylatoxins (C3a, C4a, C5a).

Anaphylatoxins: 1. Substances bind to mast cells and basophils2. mast cells and basophils degranulation and release of histamine and

other inflammatory substances. 3. Histamine increases vascular permeability.4. Phagocytes and other circulating leukocytes leave the blood and enter to

the infected tissues

Alternative Pathway

7

Diseases caused due to deficiency of some complement fragments in Alternative Pathway

Pathway/Component Disease Mechanism

C3 Susceptibility to bacterial infections

Lack of opsonization and inability to utilize the membrane attack pathway

C5, C6, C7 C8, or C9

Susceptibility to Gram-negative infections

Inability to attack the outer membrane of Gram-negative bacteria

2- PHAGOCYTOSISPhagocytosis: the process by which the invading microorganisms are engulfed

and destroyed by phagocytic cells.

Neutrophils and macrophages are the predominant phagocytes.

Neutrophils1. Neutrophils comprise 60 % of the circulating leukocytes in blood.2. Neutrophils mature in the bone marrow and then released into circulation. 3. Once neutrophils are released into circulation they have a half-life of 8

hours.

Monocytes and Macrophages4. Monocytes comprise 4 % of the circulating leukocytes in blood.5. Monocytes mature in the bone marrow and then released into circulation

but when released into tissues, they differentiate into macrophages.6. Macrophages have a longer half-life than neutrophils

Releasing of phagocytes into infected tissues

At inflammation and after histamine action, neutrophils and monocytes circulating in the blood secrete enzymes that break the integrity of the vascular membrane.

Phagocytes (neutrophils and monocytes) release from the membrane into the infected tissue by a process called diapedesis (Fig. 2.2).

Complement activation produces chemotactic molecules (that molecules attract phagocytes to sites of infection)

In tissues, monocytes differentiate into macrophages. Mature macrophages and neutrophils phagocytose and destroy invading microbes.

Recognition of pathogens:1. Direct recognition Phagocytes recognize pathogens directly through primitive receptors

(so-called primitivepattern recognition receptors, PPRR).

2. Indirect recognition (opsonin-dependent) Phagocytes also recognize pathogens INDIRECTLY using molecules that

deposited to the microorganism (example: bacteria) and these molecules are called opsonins (Fig. 2.3).

Opsonins are the products of:(i) Complement activation (C3b)(ii) B-cell activation (IgG )(iii) Cytokine mediated(iv) Activation of hepatocytes (C-reactive protein, CRP).

• Interaction of any of these opsonins onto the microorganism stimulate phagocytosis (Fig. 2.4).

• After phagocytosis, microorganism are become in phagocytic vacuole called a phagosome that destroyed microorganisms by phagocytic “weapons” including:

(i) lysosomal enzymes(ii) respiratory burst producing reactive oxygen

intermediates (ROIs)(iii) respiratory burst producing reactive nitrogen

intermediates.

Disease caused due to deficiency in phagocytosis

Chronic granulomatous disease (CGD):

• It is an genetic disease.

• CGD is characterized by the absence of a respiratory burst and production of reactive oxygen intermediates (ROIs) (Fig. 2.8).

• Individuals are susceptible to repeated bacterial and fungal infections.

3- NATURAL KILLER CELLS• Phagocytosis and activated complement are effective mechanisms for

destroying bacterial infections (extracellular).

• Viral infections require a different process because viruses (intracellular) are found in the cells until they released viral particles and infect other cells.

• In innate immunity, recognition and destruction of virally infected cells depend on natural killer (NK) cells.

• NK cells arise from bone marrow and are found predominantly in the blood, spleen, and peritoneal exudate.

• Natural killer cells express receptors that recognize a wide array of viral proteins inserted in the cell membrane of an infected cell and therefore kill the infected cell.

Recognition of virally infected cells by NK cells

1- Directly recognition:NK receptors interact with viral proteins causing cell lysisThis lysis is called receptor-mediated cytotoxicity

2- Indirectly recognition:

• NK cells also recognize the microorganism indirectly through the FcγR when IgG molecules bind to the viral proteins (Fig. 2.9).

• Indirect interaction via the FcγR, causing lysis

• This lysis is called antibody-mediated cellular cytotoxicity(ADCC).

• After recognition and interaction, lytic granules in the NK cells produce protein called perforin that inserts into the cell membrane and induces osmotic cell lysis.

4- EOSINOPHILS1. Eosinophils are bone marrow derived cells that exist both in the circulation and in

tissues.

2. The half-life of eosinophils is 8-10 hours.

3. Eosinophils play an imporant role in immunity to parasites, particularly helminths, which are resistant to destruction by neutrophils and macrophages.

4. Eosinophils requires antibodies (IgE) produced in adaptive immunity.

5. Recognition of helminths by eosinophils is indirect (Fig.2.10). • IgE antibodies bind to epitopes (antigeneic determinants) on helminths• Eosinophils have FcεR that bind to IgE antibodies.• This interaction stimulated degranulation of eosinophils and release of several

molecules including: Major basic protein (MBP) Eosinophil cationic protein (ECP) MBP and ECP are very toxic to helminths and some other parasites.