CELLS AND MOLECULES OF THE IMMUNE SYSTEM

Introduction:

The lymphoid system is composed of:

I- Primary lymphoid organs:

• Consist of the bone marrow and thymus.
• The site of haemopoiesis, lymphopoiesis and education and maturation of lymphocytes to specifically recognize foreign antigens and distinguish self from non self.

II- Secondary lymphoid organs:

• Consist of lymph nodes, spleen, tonsils and mucosa associated lymphoid tissues (MALT) of GIT and respiratory tract. Lymphatic vessels connect them to the tissues and the bloodstream and thus to sites of infection
• They are the tissues in which immune responses are initiated where naÏve mature lymphocytes will be first exposed to their specific antigens presented to them by APCs.

Lymphocyte circulation:

• Secondary lymphoid organs are connected to the tissues and the bloodstream by a system of lymphatic vessels. The afferent lymphatics drain extracellular fluid (lymph) from the tissues, including mucosal tissues, into the lymph nodes; and the efferent lymphatics carry the lymph out of the secondary lymphoid tissues and ultimately into the thoracic duct and thence through the heart and into the bloodstream.
• When mature naïve lymphocytes leave the bone marrow and thymus, they enter the bloodstream and then circulate continuously through the secondary lymphoid organs, which they enter by migrating through the walls of specialized blood vessels and if they do not encounter their antigen; leaving via the efferent lymphatic vessels, which ultimately return them to the bloodstream.
• If the lymphocytes in the lymph nodes encounter an antigen, which has been transported to the lymph node via the lymphatics, the cells become activated, divide and differentiate to become a plasma cell, Th or Tc cell these effector cells leave the lymph nodes via the lymphatics and return to the blood via the thoracic duct and then make their way to the infected tissue site

Chapter 1

Cells of immune system

The cells that make up the immune system are distributed throughout the body but occur predominantly in the lymphoid organs.

The lymphocytes are the predominant immune cells but monocyte-macrophages, dendretic cells, eosinophils and mast cells play an important role in the immune response.

All the cells of the immune system arise from pluripotent stem cells in the bone marrow that differentiate under influence of cytokines mainly IL-7, IL-3 and colony stimulating factor (CSF) as two distinct series; the lymphoid series and myeloid series. The lymphoid series gives rise to T lymphocytes, B lymphocytes and natural killer (NK) cells. The myeloid series gives rise to monocytes, eosinophils, neutrophiles, basophiles and mast cells. Dendritic cells can originate from myeloid or lymphoid series.

Mature naÏve lymphocytes continuously re-circulate from the blood and lymphatic vessels out into the tissues and lymphoid organs and return through the lymphatics. Microbial antigens are drained from sites of infection to local lymph nodes. NaÏve lymphocytes traversing these organs recognize their antigen displayed by APCs and are activated. They proliferate and differentiate to effector lymphocytes which migrate to sites of infection where infectious microbes are eliminated.

1- T Lymphocytes

T lymphocytes constitute 65-80% of the circulating pool of small lymphocytes. Their life span is relatively long; months or years.

T-cell Maturation (thymus education):

Lymphoid stem cells that will form T lymphocytes migrate from the B.M. to the thymus (thymocytes) where they mature as follow:

1- In the cortex:

• They acquire specific receptors (TCR) that commit each lymphocyte to a single antigen specificity responding only to that antigen by proliferation and production of a clone of cells with the same specificity→ clonal selection.
• They differentiate to express CD3 and both CD4 and CD8 (double positive) “immature T cell”.

2-Toward the medulla:

TCR recognize MHC molecules loaded with self peptides (p-MHC) presented to them on thymic dentritic cells and if they have:

• Low affinity to self peptides, they undergo +ve selection and divide and give clone of cells which mature in the medulla and leave to the periphery.
• High affinity to self peptides, they undergo -ve selection which lead to apoptosis (central tolerance or central deletion) to avoid development of autoimmune diseases.
• If they fail to recognize p-MHC they also undergo -ve selection and apoptosis.

3-  double positive “immature T cell”:

Express both CD4 and CD8. As they mature:

• T cells with TCRs that have more affinity to bind MHC-II will become helper T cells with CD4 molecules only.
• Those with TCRs that have more affinity to MHC-I will become cytotoxic T cells with CD8 molecules only.

4-  Mature positively selected T cells are MHC restricted (only recognize specific foreign peptides when presented by specific MHC:

• CD4 T cells are restricted to MHC-II
• CD8 T cells are restricted to MHC-I.

Mature naÏve T lymphocytes enter the circulation passing to the peripheral lymphoid organs where they reside waiting for the antigen for which they express specific receptors.

T cell receptor (TCR):

There are 3 types of TCR:

1- a/b TCR:

Biochemical structure of TCR:

• It is formed of 2 polypeptide chains (a/b) linked by disulfide bond.
• Each one has 2 extra cellular domains (one variable and one constant), a transmembrane region and an intracellular tail.
• Both a and b variable domains participate in binding to the peptide provided by MHC and are located at the amino terminal of the polypeptide chain away from the cell membrane.
• TCR is none covalently bound to CD3 and z chains forming CD3/TCR complex that help to transduce the signal received by TCR upon antigen/MHC recognition on APCs leading to T cell activation.
• Within the variable region of each TCR chain, there are 3 hypervariable regions (CDRs = complementary determining regions) unique for each T cell clone. CDR3 is the most variable one and it binds to the peptide. While CDR1, and 2 bind to the polymorphic regions of MHC.

N.B: CD4 or CD8 bind to non-polymorphic region of MHC.

Genetic structure of TCR:

The genetic loci encoding the a chain contain:

• About 40 different variable regions (V).
• Nearly 60 junctional regions (J).
• One constant region (C).
• No diversity region (D).

The genetic loci encoding the b chain contain:

• Nearly 50 different variable regions (V).
• 12 junctional regions (J).
• 2 constant regions (C).
• 2 diversity regions (D).

Diversity of TCR:

The variable regions are highly pleomorphic so that with in the entire T cell population there are a large number of TCR ab dimmers which give T cells a remarkable ability to recognize millions of different antigens. An individual T cell contains only a single type of ab dimmer and recognizes and responds only to a specific combination of antigen and MHC. This variability occurs via several mechanisms:

1. Multiple genes in the germ line (unarranged DNA) encoding for a and b chins: (as described above)
2. Recombination of multiple gene segments (DNA rearrangement):

The TCR alpha chain is generated by VJ recombination, whereas the beta chain is generated by V (D) J recombination (both involve a somewhat random joining of gene segments to generate the complete TCR chain). This recombination is mediated by V(D)J recombinase enzymes; RAG1 & RAG2 (recombinase activating gene1 and 2) that recognize V, D and J gene segments and bring them close together.

Exonucleases then cut DNA and remove introns. DNA ligase seals the defect producing the full length recombined V-J or V-D-J gene.

1. Joining reaction:

Additional nucleic acid bases are added or deleted at the joining sites for V-J, V-D, and D-J regions during recombination process.

These changes alter the reading frame of mRNA with subsequent change in amino acid composition and change of TCR specificity so increase its diversity.

2- g/d TCR:

• γδ T cells (gamma delta T cells) represent a small subset of T cells (<5% of T cells) that possess a distinct T cell receptor (TCR) on their surface made of one γ-chain and one δ-chain.
• They lack CD4 co-receptors but some express CD8.
• This group of T cells is found at their highest abundance in the gut mucosa and within the epidermal compartment of the skin (Dendritic Epidermal T cells) (DETC).
• The antigenic molecules that activate γδ T cells are still largely unknown. However, γδ T cells are peculiar in that they do not seem to require antigen processing and MHC presentation of peptide epitopes (although some recognize MHC class IB molecules). Furthermore, γδ T cells are believed to have a prominent role in recognition of lipid They are of an invariant nature and may be triggered by alarm signals, such as heat shock proteins (HSP).

3- NKT cell receptors:

• Natural killer T (NKT) cells are a heterogeneous group of T cells that share properties of both T cells and natural killer (NK) cells.
• They co-express T cell receptor (TCR) and NK cell markers.
• these cells recognize the non-polymorphic CD1d molecule, an antigen-presenting molecule that binds self- and foreign lipids and glycolipids. They constitute only 0.2% of all peripheral blood T cells.
• Their function is not well understood.

T-cell Markers:

They are molecules on the surface of T cells that are important for:

• Antigen recognition.
• Interaction between T cells and APC.
• Identify T cells and divide them into subsets.

They include:

T cell Subsets:

1-  CD4⁺ T cells (T helper, TH):

General characters:

• They represent 65% of T cells in blood.
• They predominate in thymic medulla, tonsils and blood.
• They are the cells attacked by HIV.
• They recognize only epitopes on the surface of APCs associated with MHC-II (present on macrophages, dendritic cells and B cells).
• They are the principal orchestrators of immune response, needed for activation of effector cells (cytotoxic T cells, B cells, Macrophages, and natural killer cells).
• When they are activated they secrete several cytokines which have several stimulatory and proliferative effector functions which differ according T cell subset.

Mechanism of function:

1-  Release of different cytokines that help in stimulation and proliferation of B cells, other T cells, Macrophages and natural killer cells.

2-  Cell to cell contact (by help of co-stimulatory molecules that transmit signals which stimulate proliferation).

T helper cells subsets:

Naȉve CD4+T helper cells (TH0) can be induced to differentiate into TH1, TH2, TH17 and regulatory (Treg) phenotypes according to the local cytokine stimulus.

1. a) TH0:

• Produce IL2, 4, 5 and 10 and INFγ.
• Precursor of TH1 and TH2.

1. b) TH₁:

• IL12 produced by activated macrophages and IFNg secreted by NK cells during the innate immune response stimulate differentiation of TH0 to TH1.
• Secrete IL-2, IL3, IFN-g, TNF-B (lymphotoxin) and GM-CSF that cause:
  1. Enhanced activity of TC cells, NK and phagocytes (Macrophages & PMNL).
  2. Recruitment of inflammatory cells leading to DTHS, graft rejection, granuloma, inflammation, tissue destruction, enhance cytotoxic effects against tumors and intracellular infections “CMI”.
  3. Enhanced production of selected classes of antibodies that participate in opsonization and antibody dependent cellular cytotoxicity (ADCC).
• IL4 and IL10 inhibit the production of TH1.

1. c) TH₂:

• IL4 and IL10 produced by several cells induce differentiation of TH0 to TH2.
• They respond mainly to allergic reactions and parasitic infections.
• They secrete IL-3, 4, 5, 6, 10 and 13 and GM-CSF that cause:
  1. B cell activation and proliferation with enhanced production of all classes of antibodies including IgEe. humoral immune response.
  2. Stimulation of eosinophils and mast cells.
• IFN-g inhibits the production of TH2 i.e. cytokines produced by one set inhibit the production and hence the function of the other set (cross regulation).

NB: both TH1 and 2 secrete IL3 and GM-CSF.

1. d) TH17:

• IL-6 and transforming growth factor b (TGF-b) induce differentiation of TH0 to TH17.
• They secrete IL17 that:
  1. Recruit leukocytes mainly neutrophils, to sites of infection. Because neutrophils are a major defense mechanism against extra cellular bacteria and fungi, TH17 cells play an especially important role in defense against these infections.
  2. Act as key mediators in a diverse range of auto inflammatory disorders (involved in either the causation or progression).
  3. e) Regulatory T cells (Treg cells, formerly; suppressor T cells):
• They are CD4+ T cells that express CD25 which is the IL2 receptor. They also express FOXP3 which can be used as a good marker for CD4+CD25+ T
• They form 5-10% of CD4+ T cells.
• When stimulated by IL2 they secrete IL10 and TGF-b and function mainly to:

1. Inhibit the immune response.
2. Regulate the activation of other T cells.
3. Maintain peripheral self tolerance.

NB: Additional regulatory T cell populations include Tr1, Th3, CD8+CD28-, and Qa-1 restricted T cells. The contribution of these populations to self-tolerance and immune homeostasis is less well defined.

2-  CD8+T cells (T cytotoxic or cytolytic, CTLs):

• They represent 35% of T cells in blood.
• They predominate in the bone marrow and gut associated lymphoid tissue.
• They only recognize epitopes in association with class-I MHC (found on all nucleated cells).

Function:

They directly kill the target cells e.g. virus infected cells and tumor cells (by delivering a lethal hit) then disengage from it to kill another cell.

They differ from NK cells in being specific and MHC-restricted.

Mechanism of target cell killing by CTLs:

1-  Release of perforins that cause pores in target cells to facilitate introduction of granzymes leading to DNA fragmentation and initiation of several pathways of apoptosis.

2-  Expression of FAsL that binds to FAS on target cells causing activation of caspases and lead to apoptosis.

3- TNF and IFN-g may have a role in cytolsis.

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