7 Myeloid System-II

Dr. M. N. Gupta

3. Description

Common myeloid progenitor cells differentiate further into numerous cells.

These cells patrol the body via blood stream and lymph.

Several of these also take residence in some tissues and become further specialized.

The multipotential hematopoietic stem cell

We had looked at the differentiation of the blood stem cells into myeloid cells earlier. Let us recollect that again. The development of blood cells is called haematopoiesis. Most of the cells of the immune system originates from a common haematopoietic stem cell. Haematopoiesis begins in the early yolk sac. During embryogenesis, this process moves over to the fetal liver, fetal spleen and in the bone marrow of the neonate and adult. Hence, the common multipotential haematopoietic stem cells are found in the fetal liver, fetal spleen and in the bone marrow. Haemopoisis continues in the bone marrow throughout the life. These cells replicate and seed other organs.

The WBCs (leucocytes) from the common myeloid progenitor are: monocytes, immature dendritic cells, neutrophils, eosinophils, basophils. Neutrophils, eosinophils, basophils are called as granulocytes as their cytoplasmic granules have characteristic staining properties. These three are also called polymorphonuclear leucocytes as their nucleii have irregular shapes. These circulate in the blood and enter the tissue only where there is infection/inflammation.

Immature dendritic cells enter the peripheral tissues via the bloodstream.

Upon maturing there, after encountering an antigen, they migrate to the lymph nodes. Here these mature dendritic cells activate the antigen specific T-cells. Monocytes also enter the tissues via the blood stream and differentiate into macrophages. These become tissue resident phagocytic cells. Mast cells arise in the bone marrow but their complete maturation occurs in the tissues. Mast cells are important in allergic responses.

Basophils and Mast cells have both similar structure and function. Basophils circulate in the blood and their population is quite low (<0.2% of the granular leukocytes).

Mast cells are found around blood vessels in the connective tissues, in gut lining, lungs and genitourinary tract. Thus, these are more or less ubiquitous in the body.

Both originate in the bone marrow. Basophils are stained with basic dyes due to their granulocytes. Both are stimulated with similar responses to release the contents of their granules.

Binding of C3a and C5a (complement components), allergens to the IgE bound to the cell surfaces via Fc receptors, drugs and lectins.

Normal Leukocyte Values in Peripheral Blood

These numbers show the relative populations of various white blood cells or leucocytes. Please note that these ranges are for person who has no infection. These numbers change as a result of infection and serve diagnostic purpose.

What are phagocytes?

Phagocytosis was discovered by the Russian scientist Elie Metchnikoff in 1882 who observed that some motile cells within the starfish larvae body cavity ingested any inserted foreign material.

Metchnikoff based his view that antibodies are not important in the immune defense based upon this observation. His alternative view was that these phagocytic cells were universal and were the only important component of the defense system.

Opsonins

This resulted in a bitter controversy till 1904 when Englishman Almorth Wright discovered opsonins-substances that facilitate phagocytosis.

We know now that both humoral immunity and cellular immunity are important. Phagocytes are indeed universal and important but they are part of the innate immunity like other cells of the myeloid lineage.

Some important opsonins are:

•  Antibodies

•  Complement components especially C3b

•  Fibronectins: glycoproteins occurs both free and as part of the cell membrane

Neutrophils

•  Polymorphonuclear neutrophil granulocytes constitute about 60-75% of the blood WBC in humans/carnivores and 20-30% in ruminants

•  Their life span is of few days

Life Span of Leucocytes

Neutrophil Maturation

The process of neutrophil maturation begins in the bone marrow (top). After about 12 days, approximately 10% of the mature neutrophils are released into the peripheral blood, where they have a half-life of approximately 6 to 10 hours. Eventually, the neutrophils migrate into the tissues by diapedesis. The percentage of neutrophils at each stage of development (bottom) ranges from about 2% at the myoblast stage to almost 25% at the mature neutrophil stage.

Structure of the Neutrophils

About 12 µm in diameter, their cytoplasm has two types of granules:

Azurophils: These primary granules are electron dense and consist of enzymes like myeloperoxidase, lysozyme, elastase, beta-glucouronidase and cathapsin B Specific granules: These secondary granules contain lysozyme, collagenase, lactoferrin.

Neutrophils have a small golgi and some mitochondria but no ribosomal/endoplasmic reticulum and hence cannot synthesize proteins.

Phagocytosis by neutrophils

Phagocytosis, destroying foreign material, is the main function of the neutrophils.

The first stage of phagocytosis is chemotaxis in which neutrophils follow a chemical gradient and migrate towards the source of the chemical.

Such chemicals include factors from damaged cells, leucotrienes (metabolite of an arachidonate), C5a (a peptide generated during the complement system activation) and factors from mast cells.

The foreign particle gets opsonized (by antibody or C3) and has lower zeta potential. This is essential as both neutrophils and foreign particles have negative charge, otherwise they will repel each other.

Neutrophils have receptors for antibody or C3 and is able to bind to the opsonized particle.

In surface phagocytosis, the particle lodged in the tissue is trapped between the tissue surface and neutrophils. This trapping facilitates contact between the particle and neutrophils.

The particle gets drawn into the cell and is enclosed in a vacuole forming a phagosome

The attachment of the particle to the neutrophil membrane is a signal for the granules to move. These fuse with the phagosome to form phagolysosome

Gram positive organisms are rapidly hydrolysed by the lysozome

Some organisms like Brucella abortus and Listeria monocytogens resist these hydrolases and may even multiply within the phagocytic cells!

Infact bacteria like Streptococcus pneumoniae (which have capsule containing carbohydrates and hence is hydrophilic) has more essential requirement of opsonization before being ingested by neutrophils

Oxidative mechanism

Oxidative mechanism plays an even more important role in phagocytosis by neutrophils The NADPH oxidase is the cell surface enzyme and it forms superoxide anion O2-.

NADP+ signals operation of the pentose phosphate pathway to generate energy for the cell.

Superoxide dismutase (SOD) converts superoxide anion to H2O2.

Myeloperoxidase (MPO) converts H2O2 to hypochlorite ions which kill bacteria by oxidising these proteins.

This reaction is similar to the one which is used in swimming pools during their chlorination!

There is yet another mechanism by which neutrophils can destroy microbes. The reaction of H2O2 with superoxide anion generates hydroperoxide and singlet oxygen. Both can destroy bacteria by oxidation of their components.

Phagocytosis

The respiratory burst (the oxidative pathway) are obviously important as children deficient in either superoxide dismutase or myeloperoxidase are more prone to recurrent bacterial infections.

Neutrophils after moving from the bone marrow do not last long and in fact end up participating in phagocytosis only few times.

Neutrophils are about 8 x 106   per mL of blood and die by apoptosis.

Their main job is to travel all over the body, patrolling it for detecting invasion. They play an important role in inflammation.

The table gives the complete details of their surface receptors which enables them to act towards microbes.

Mononuclear Phagocytic System

It consists of tissue bound matured monocytes which are another kind of phagocytic cells. It was earlier called reticuloendothelial system.

The cells of this system are named depending on their location

An animal injected with carbon particles results in their distribution in certain cells all over the body. The German scientist Ludwig Aschoff upon observing this called these cells as constituting reticuloendothelial system In fact, in that experiment in many cells the uptake of these particles was not related to their function. However, included were the macrophages which are also distributed all over the body in various tissues. In blood monocytes are about 5% of WBC population.

While in suspension, macrophages are round cells of about 14-20 µm diameter but assume different shapes in different locations where they reside As a consequence of cell mediated response to some microbes, macrophages also can enlarge and their lysosomes increase in number.

If the invading particle/microbe is too large, many macrophages fuse together to form multinucleated giant cells.

Macrophages resident in tissues have long half lives and the population renews itself at rate of about 1% per day.

The phagocytic mechanisms of macrophages are very similar to neutrophils and follow similar stages of chemotaxis, attachment to the microbe, endocytosis followed by phagosome formation, fusion of lysosome with phagosome and destruction of the microbe.

One may ask why infections kill inspite of such a sophisticated immune system. There are situations when the immune system is overwhelmed. Inhalation of asbestos particle is one such example. Used widely as an insulator at one time, asbestos is now considered a health hazard. Let us see how macrophages function is involved in this.

How asbestosis can cause death

Some foreign particles such as asbestos particles are taken up by macrophages but end up killing macrophages. The other macrophages continue efforts. Macrophages upon being destroyed release excessive amount of lysosomal enzymes and other reactive oxygen species. This high amount of both substances results in chronic tissue destruction, inflammation and granulation of tissues.

In cases when the injected particles are innocuous carbon particles, macrophages carry the particles to the lungs or intestines and then to the lumen of the lungs or intestine for elimination.

In case of asbestos particles, this route leads to lung damage and is a life threatening condition called asbestosis.

Functions of Macrophages

Phagocytosis: Macrophages phagocytose not only microbial products but also act on damaged cells (especially those damaged by neutrophils). Thus neutrophil action is followed by macrophage action.

Macrophages have the following receptors. The macrophages have less intensive oxidative action and have catalase instead of myeloperoxidase. These receptors enable macrophages to act on foreign material opsonized by molecules recognized by these receptors.

Macrophages upon encounter with microbes/damaged cells release interleukin 1. This protein causes fever, recruits neutrophils (please see co-operation with neutrophils, before as well as after the macrophage encounter with foreign substances) and in general fight off invasion and repair injury.

Inflammation: Inflammation is an extremely complex process. Macrophages release large number of substances upon reaching the site of microbial attack and are important in the healing process as well.

Macrophages can process some antigens.

Macrophages possess the capacity to synthesize proteins. This is necessary as several secreting proteins by macrophages play important roles in various biochemical processes.

Summary

•  We have learnt about some cells of the myeloid system

•  These cells are part of the innate immunity but acts in synergy with lymphocytes which are part of the acquired immunity

•  In particular, we have looked at the functions of neutrophils and macrophages and we saw that those two co-operate often during the immune response