Category Archives: Molecular Biology Genetic Disorders and Immunology

Glossary

Acrocentric. Term used to describe a chromosome in which the centromere lies close to one end, producing one long and one short arm. Allele (allelomorph). Alternative form of a gene occupying the same locus on a particular chromosome.
Aneuploid. Any chromosomal number that is not the exact multiple of the normal haploid number.

Autosome

Any chromosome that is not a sex chromosome or mitochondrial chromosome; there are 22 pairs of autosomes in humans.
Bacteriophage. A bacterial virus. These are modified and used as vectors for DNA cloning.
cDNA. DNA synthesized from an mRNA template by the enzyme reverse transcriptase.

Centromere. The point at which two chromatids of a chromosome are joined and also where the spindle fibres become attached during mitosis and meiosis.
Character (trait). An observable phenotypic feature of an individual.
Chromatid. One of the two strands, held together by the centromere, that make up the chromosome as seen during cell division.
Chromosomal aberration. An abnormality in the number or structure of a chromosome.
Chromosome. A thread-like body containing DNA and protein, situated in the nucleus, and carrying genetic information.
Clone. Cells having the same genetic constitution and derived from a single cell by repeated mitoses.
Codon. Three adjacent nucleotides in a nucleic acid that code for one amino acid.
Concordance. The occurrence of the same trait in both members of a pair of twins.
Deletion. Loss of a part of a chromosome.
Diploid. The number of chromosomes found in somatic cells, i.e. two sets.
DNA ligase. The enzyme that joins two DNA ends together.
DNA polymerase. The enzyme that replicates DNA.
Dominant. Term used to describe a trait expressed in individuals who are heterozygous for a particular gene.
Exon. A segment of a gene that is represented in the final spliced mRNA product.
Expressivity. The degree to which the effect of a gene is expressed.
Gene. Part of a DNA molecule that directs the synthesis of a specific polypeptide chain.
Gene pool. The total genetic information contained in all the genes in a breeding population at a given time.
Genetic marker. A genetically controlled phenotypic fearure used in inheritance studies.
Genetics. The science of heredity and variation.
Genome. The total amount of genetic material in the cell.
Genotype. The genetic constitution of an individual.
Haploid. The number of chromosomes found in germ cells, i.e. one set.
Heterozygote. An individual possessing two different alleles at the corresponding loci on a pair of homologous chromosomes.
Homozygote. An individual possessing identical alleles at the corresponding loci on a pair of homologous chromosomes.

Hybridization. The pairing of complementary DNA or RNA strands to give DNA-DNA or DNA-RNA strands;
for example, it is used to search for particular DNA fragments after Southern blotting.
Intron. A segment of a gene not represented in the final mRN A product because it has been removed through splicing together of exons on either side of it.
Karyotype. The number, size and shape of the chromosomes in a cell.
Linkage. The co-segregation of two unrelated DNA sequences which are physically close together on the chromosome.
Linkage disequilibrium. The association of particular alleles at two linked loci more frequently than expected by chance.
Locus. The site of a gene on a chromosome.
Metacentric. Term used to describe a chromosome III which the centromere lies in the middle.
Monosomy. A state in which one chromosome of a pair is missing.
Mosaics. Patients with two different cell lines in their constitution.
Non-disjunction. Failure of a chromosome pair to separate during cell division, resulting in both chromosomes passing to the same daughter cell.
Nucleotide. The basic unit of nucleic acids, which is made up of a pyrimidine or purine base, a pentose sugar and a phosphate group.
Oncogenes. Genes which when altered in their structure or expression contribute to the abnormal growth of cancer cells.
Penetrance. The proportion of individuals with a particular genotype who also have the corresponding phenotype.
Full penetrance occurs when a dominant trait is always seen in an individual with one such allele, or when a recessive trait is seen in all individuals possessing two such alleles.
Phenotype. The appearance of an individual, resulting from the effects of both environment and genes.
Plasmid. A simple circular DNA molecule derived from bacteria which can be modified and used as a vector for DNA cloning.
Ploidy. Term that describes the number of chromosome sets, namely 23 = haploid (1 set), 46 = diploid (2 sets) .
Polymerase chain reaction (PCR). Technique for rapid analysis of DNA. Oligonucleotide primers corresponding to each end of DNA of interest are synthesized and amplified in genomic DNA using DNA polymerase.
Positioned cloning (or reverse genetics). Methodology used to isolate genes whose protein products are not known but whose existence can be inferred from the disease phenotype. Pulsed field gel electrophoresis. Technique for separation of large fragments of DNA.
Recessive. Term used to describe a trait expressed in individuals who are homozygous for a particular gene but not seen in the heterozygote.
Restriction fragment length polymorphisms (RFLPs).
When variations in non-coding DNA sequences affect restriction enzyme cleavage sites, DNA fragments of different sizes (RFLPs) will result from enzyme digestion.
RNA polymerase. The enzyme that synthesizes RNA, based on a DNA template.
Sex linkage. Genes carried on the sex chromosomes.
‘Somy’. Term referring to the number of copies of an individual chromosome per cell, e.g. ‘trisomy’ = three copies.
Splicing. Removing the introns from an unprocessed RNA molecule.
Synteny. Term used to describe genes on the same chromosome.
Transcription. The process by which an RNA molecule is synthesized from a DNA template.
Translation. The process by which genetic information from MRNA is ‘translated’ into protein synthesis.
Translocation. The transfer of a piece of one chromosome to another non-homologous chromosome.
Trisomy. Representation of a chromosome three times rather than twice, giving a total of 47 chromosomes.
tRNA. Transfer RNA, a molecule which carries a single amino acid (depending on its anticodon) and which brings the amino acid to the ribosome.
Vector. A DNA molecule used to carry DNA regions of interest.

Further reading
Brock DJH (1993) Molecular Genetics for the Clinician.
Cambridge: Cambridge University Press.
Brostoff 1, Scadding GK, Male D & Roitt 1M (1991) Clinical
Immunology. London: Gower Medical Publishing.
Brown TA (1990) Gene Cloning: An Introduction, 2nd edn. London: Chapman and Hall.
Chapel H & Heaney M (eds) (1993) Essentials of Clinical Immunology. Oxford: Blackwell Scientific Publishers.
Conner JM & Ferguson-Smith MA (1991) Essential Medical Genetics, 3rd edn. Oxford: Blackwell Scientific Publishers.
Gelehrter TD & Collins FS (1990) The Principles of Medical Genetics. Baltimore: Williams & Wilkins.
Weatherall DJ (1991) The New Genetics in Clinical Practice,
3rd edn. Oxford: Oxford University Press.

Diagnostic tests in clinical immunology

The major investigations in the diagnosis and monitoring of disorders of the immune system are described below.

Autoantibodies

A characteristic feature of many autoimmune disorders is the presence of specific autoantibodies. These may be useful markers of disease activity, and in monitoring response to therapeutic inventions.

Indirect immunofluorescence (IIF)

This technique is used to detect organ- and tissuespecific antibodies, e.g. thyroid, adrenal, smooth muscle, gastric parietal cell, mitochondrial, acetylcholine receptor, antinulear, antineutrophil cytoplasm, glomerular basement membrane( methodology).

Factors influencing the pathogenicity of immune complexes.

Factors influencing the pathogenicity of immune
complexes.

Particle agglutination

This technique is used for the detection or rheumatoid factors (RF), in the rheumatoid arthritis haemagglutination assay (RAHA). Sheep red blood cells are coated with rabbit antibody. The results are usually expressed as a titre of the highest serum dilution that gives a positive result. These techniques are therefore only semi-quantitative.

Enzyme-linked immunosorbent test (ELISA)

ELISA is a commonly used technique for the detection of a wide range of proteins. Wells of microtitre plates are coated with the antigen (e.g. neutrophil cytoplasm proteins, or double-stranded DNA) and the patient’s serum applied. Any antibody present will bind to the antigen. A second layer of anti-human antibody conjugated with an enzyme (often alkaline phosphatase) is added, as a ‘developing antibody’. A substrate that changes colour if the enzyme is present is added, and the colour intensity is measured spectrophotometrically. The intensity is proportional to the concentration of antibody present. The test is quantitative as the intensity of the reaction of the patient’s serum is compared to a standard curve plotted using a serum with known concentration of antibody.

Cellular tests

The major immunodeficiencies involve abnormalities in the cells of the immune system, for example T and B cells in SCID, DiGeorge syndrome, HIV infection, X-linked agammaglobulinaemia; phagocytes in chronic granulomatous disease.

Lymphocyte subset phenotypes

Fluorescent-labelled monoclonal antibodies to the surface protein of interest are applied to the cells. The cells can then be analysed by a flow cytometer. Commonly used antibodies are CD3 (all mature T lymphocytes), CD4 (CD4+ or ‘helper’ T-cells), CD8 (CD8+ or ‘suppressor’ T cells), CD19 or CD20 (mature B cells).

Indirect immunofluorescence showing antinuclear antibodies.

Indirect immunofluorescence showing antinuclear
antibodies.

been done on many thousands of individuals, and it is clear that many factors outside of immunodeficiency affect the numbers:
• Age (higher in infancy)
• Exercise (increase)
• Smoking (increase)
• Diurnal variation
• Pregnancy (decrease)
• Splenectomy (increase)
Therefore, results have to be interpreted cautiously. Also, although AIDS characteristically causes CD4-cell depletion, it is important to realize that other conditions, such as tuberculosis and sarcoidosis, can cause similar findings, and that CD4 lymphopenia is not itself diagnostic of HIV infection.

Functional lymphocyte tests

In addition to the phenotypic studies above, functional tests of lymphocytes may also need to be performed. Peripheral blood lymphocytes are stimulated with:
• Mitogens such as phytohaemagglutinin (T cells), concanavalin A (T cells), poke weed mitogen (B and T cells)
• Recall antigens such as purified protein derivative (ppd) and Candida.
The resulting proliferation is detected by the uptake of the radio labelled thymidine into the DNA of the lymphocytes. Neutrophil function tests Tests of phagocytosis and intracellular killing/oxidative burst, random locomotion and chemotaxis are available in specialist centres.

Autoimmunity

An autoimmune disease occurs when the immune system fails to recognize the body’s own tissues as ‘self and mounts an attack on them. Disorders include rheumatoid arthritis, juvenile (insulin-dependent) diabetes, thyroiditis and multiple sclerosis. Illnesses are divided into those that affect just one organ (organ-specific) and those which affect many systems (organ non-specific or multi-systemic,. Autoimmune diseases are mostly of unknown aetiology although genetic, hormonal, microbiological and environmental factors are known to be implicated in their manifestation and severity.

Immunopathology of autoimmune disease

Formation of autoantibodies may be a normal physiological process. However, excessive production of such antibodies can be harmful. The way in which autoantibodies cause structural damage to the body’s tissues are varied. Antibodies may directly react with a specific tissue resulting in inflammation and tissue damage, such as anti glomerular basement membrane antibodies in Goodpasture’s syndrome, or directly affect function, for example acetylcholine receptor antibodies in myasthenia gravis (type II hypersensitivity).

Autoimmune diseases as classified by organ -city.

Autoimmune diseases as classified by organ
-city.

Alternatively, circulating immune complexes may be formed. Immune complexes are biologically active entities which in themselves have certain characteristics. These properties determine whether immune complexes become harmful and cause extensive tissue damage. Some of these features are detailed. Inflammation or autoimmune conditions resulting from the formation of immune complexes are classified as type III hypersensitivity reactions. The complexes are often deposited in the kidney, skin, joint and nervous system. This results in complement activation, accumulation and activation of neutrophils with the release of proteolytic enzymes and further damage. It should also be noted that mononuclear cells are also implicated in tissue destruction. Both CD4+ and CD8+ lymphocytes are observed routinely as infiltrates in inflammatory lesions. While there is little doubt these cells contribute to tissue destruction their precise role in the pathological processes is uncertain. The complexity of the autoimmune diseases is quite formidable, largely because there is so much variation in the interplay of different factors that can influence the reactivity of the immune system to the body’s tissues.

Hypersensitivity diseases

Hypersensitivity reactions underlie a number of autoimmune and allergic conditions. The classification of these reactions . Type I reaction (reaginic/anaphylactic/immediate hypersensitivity reaction) This is an allergic reaction produced within 30 min of exposure to a specific allergen. Allergens, e.g. house dust, pollens, animal danders or moulds, only elicit reactions in certain genetically predisposed individuals, who are said to be atopic. Atopy is diagnosed on skin-prick testing when a reaction is elicited by sensitizing allergens. Type  I reactions can be passively transferred by injection of serum containing IgE antibody into the skin (passive cutaneous anaphylaxis). The antibody will remain fixed to the mast cells in the skin for up to 4-S days and an injection of the antigen will produce a wheal and flare reaction (Prausnitz-Kustner reaction).
Type I reactions are mediated via allergen-specific antibodies of the IgE class and occur as follows.
ALLERGEN INHALATION OR INGESTION results in local generation of specific IgE via the interaction of macrophages and receptive Band T-helper cells.
LOCALLY PRODUCED ALLERGEN-SPECIFIC IGE then binds to the Fc receptors of mast cells and, to a lesser extent, eosinophils and macrophages sensitizing them. IgE also enters the circulation, where it sensitizes basophils.
SUBSEQUENT EXPOSURE TO ALLERGEN results in the cross-linking of the IgE antibodies on the cells and to mediator release.
Binding of allergen to sensitized cells results in the de novo synthesis and/or release of several inflammatory mediators:
HISTAMINE. Present as a preformed mediator in sensitized mast cells and basophils, histamine produces vasodilation and bronchial smooth-muscle constriction.
ARACHIDONIC ACID METABOLITES. Arachidonic acid is generated in sensitized cells from membrane lipids following the binding of specific allergen. It is subsequently metabolized to produce prostaglandins (cyclooxygenase pathway), leukotrienes (lipoxygenase pathway) or platelet-activating factor (p AF acetylation), depending on which cell type is being activated.
Leukotrienes and prostaglandins are together termed eicosanoids. The arachidonic acid metabolites involved in Type 1 hypersensitivity reactions are PAF, leukotrienes (LT) B., C., D. and E. and prostaglandins (PG) D2, E2 and F2£<. They have four main actions:

1 Inflammatory cell mucosal infiltration. This is mediated by LTB. and PAF, which attract and activate neutrophils, eosinophils and monocytes/macrophages. LTB. is released by actated mast cells and macrophages and PAF is released by mast cells, neutrophils and eosinophils.
2 Bronchoconstriction. This is mediated by several metabolites including PAF, LTC., LTD. and LTE. and PGD2 and PG F2a• LTC4 and PGD2 are the major arachidonic acid metabolites released by mast cells. The remaining eicosanoids are generated by human lung tissue and/or alveolar macro phages.
3 Bronchial mucosal oedema is mediated by LTC4 and LTD4 and PGE2• PGE2 is released from alveolar macrophages and human lung tissue.
4 Mucus hypersecretion is mediated by LTC4 and LTD4

Summary of hypersensitivity reactions

Summary of hypersensitivity reactions

COMBINED

IMMUNODEFICIENCIES

The most severe immunodeficiencies are those that affect both B and T cell responses. These can stem from a variety of defective mechanisms in lymphocyte function, but tend to have rather similar clinical features, combining the opportunist infections of cell-mediated immunodeficiency with those of antibody deficiency.

Severe combined immunodeficiency (SClD)

This typically presents in the first weeks of life. Failure to thrive, absent lymphoid tissue, lymphopenia and hypogammaglobulinaemia with multiple severe infections are characteristic.
There are primary Xvlinked and autosomal recessive variants of SCID. Other causes include adenosine deaminase (ADA) deficiency, in which a defective purine salvage enzyme that is expressed in all cells, has a particular effect on lymphocytes due to the accumulation of substrates and metabolites that interfere with lymphocyte function. An analogous disorder is seen in purine nucleoside phosphorylase deficiency. Non-expression of MHC class II and reticular dysgenesis cause similar syndromes. Milder expressions of these defects exist, some presenting in later life, and are sometimes termed benign combined immunodeficiency or Nezeloffs syndrome.
Even with supportive and antimicrobial therapy, most of these conditions have a very poor prognosis without reconstitutive therapy. Immunoglobulin therapy is effective for the antibody deficiency, but the cell-mediated opportunists are the main determinant of outcome. Bone marrow transplantation is the definitive approach and has had significant success, especially if undertaken before extensive infection has set in. Recent approaches have included attempts to restore the defective enzymes in ADA deficiency and gene therapy is now being attempted. Others Other combined immune deficiencies include the following.

WISKOTT-ALDRICH SYNDROME is an X-linked defect with associated eczema and thrombocytopenia; a mainly cell-mediated defect with falling immunoglobulins is seen and autoimmune manifestations and lymphoreticular malignancy may develop.
ATAXIA TELANGIECTASIA patients have defective DNA repair mechanisms and have cell-mediated defects with low IgA and IgG2; lymphoid malignancy is again common.
EBV-ASSOCIATED IMMUNODEFICIENCY. Apparently normal, but genetically predisposed (usually X-linked) individuals develop overwhelming EBV infection, polyclonal EBV-driven lymphoproliferation, combined immunodeficiency, aplastic anaemia and lymphoid malignancy. EBV appears to act as a trigger for the expression of a hitherto silent immunodeficiency.
PROTEIN-CALORIE MALNUTRITION is a very common cause of acquired combined immunodeficiency, with predominantly cell-mediated defects. Mechanisms are not fully established. Measles is a major cause of morbidity and mortality among children and Pneumocystis carinii pneumonia is also a major pathogen; indeed, it was in this setting that Pneumocystis was first seen in the Warsaw ghettos.

Problems of prematurity

An immune response is not essential for normal fetal development and growth, but is necessary for survival after birth. Premature infants of 28 weeks’ gestation and under are now surviving and have several immunological deficiences and problems:
REDUCED ANTIBODY PRODUCTION. IgM synthesis does not occur before 30 weeks’ gestation; IgG production does not occur until several weeks after birth.

Low LEVELS OF MATERNAL ANTIBODY, as active placental transfer of IgG does not occur until the third trimester. Antibody levels continue to drop after birth due to loss of maternal IgG.

NEUTROPENIA AND IMPAIRED CHEMOTAXIS.

INVASION OF FOREIGN BODIES, such as indwelling catheters.
ANTIBIOTIC THERAPY, reducing colonization resistance. Breast milk has several protective properties including secretory IgA, lysozyme, lactoferrin, leucocytes and small amounts of IgG, IgM and IgD. Colostrum is particularly high in antibody, and provides IgA to protect the gastrointestinal tract.

ANTIBODY DEFICIENCIES

X-linked hypogammaglobulinaemia

There is a profound reduction in all immunoglobulin classes; B cells and plasma cells are reduced. The defect is in the differentiation of pre- B cells into B cells; T cells are normal. It typically presents with infections, e.g. meningococcal meningitis, mycoplasmal infections, after the first 3-6 months of life, when the protection from pass ively transferred maternal antibody has largely been lost. Immunoglobulin replacement therapy is very successful and is now generally given intravenously. Many patients treat themselves at home.

Common variable immunodeficiency (cvn)

This is a late onset antibody deficiency, which may present in childhood or adult life. IgG levels are especially low. B-cell numbers are usually normal; the defect appears to result from failure of their further differentiation. Some tests of T-cell function may be abnormal but few clinical manifestations of T-cell immunodeficiency are documented.

CLINICAL FEATURES

The patients have similar infections to those with the Xlinked variety. However, a particular feature is follicular hyperplasia of lymph nodes, which in the gut takes the form of nodular lymphoid hyperplasia, and there may be splenomegaly. CVI patients may develop autoimmune disease and there is also an increased risk of Iymphoreticular malignancy.

DIAGNOSIS

The finding of reduced immunoglobulin levels and normal B-cell numbers indicates the diagnosis.

TREATMENT

Most of the manifestations are satisfactorily prevented by regular immunoglobulin replacement therapy. IgA deficiency This is an extremely common disorder but is often symptomless. Only a small proportion have an increased risk of pyogenic infection and many of these have another defect, such as IgG2 subclass deficiency. Some have allergic disorders or gluten hypersensitivity, and autoimmune disorders may also occur.

Isolated IgGz subclass deficiency

This is a rare cause of increased infection with capsulated organisms, for which it is the main immunoglobulin subclass;
intravenous immunoglobulin replacement provides effective restoration. A variety of other rare immunoglobulin deficiencies exist, including hypogammaglobulinaemia with raised IgM, in which there is a defect in isotype switching. Some patients with increased bacterial infections have apparently normal levels of immunoglobulin but fail to produce specific antibodies to certain organisms.

Acquired hypogammaglobulinaemia

This is seen in the immune paresis of patients with myeloma and chronic lymphatic leukaemia or lymphoma. Infection with capsulated bacteria may be seen, especially with myeloma. Splenectomy causes impairment of defence against capsulated bacteria, especially pneumococcus, partly because T-independent antibody responses are largely made in the spleen and partly because of its role as part of the fixed reticuloendothelial system. Hyposplenism associated with severe sickle cell disease is responsible for the increased risk of infection in such patients. Pneumococcal vaccination before elective splenectomy and the use of penicillin prophylaxis can largely eliminate risk of serious infection. Hypogammaglobulinaemia can be seen in congenital rubella.

T-CELL IMMUNODEFICIENCIES

Congenital T-cell defects DiGeorge anomaly

A defect of branchial arch development leads to abnormal thymic development. This is of varying severity and is associated with other branchial arch defects: dysmorphic facies, hypoparathyroidism and cardiac defects. Patients present with infections including mucocutaneous candidiasis and Pneumocystis carinii pneumonia, together with chronic diarrhoea, due to a variety of pathogens. The absent thymus can be documented radiologically. CD3 T cells are variably reduced in number, but the CD4 subset is usually reduced and T-cell proliferative responses are impaired. Immunoglobulin production is typically normal. Thymic transplants and thymic hormone have been reported to have reconstituted some patients with severe disease and bone marrow transplants have also had some success.
Other causes of cellular immunodeficiency Various other rare congenital defects have been reported that predominantly affect T-cell responses, including:
• Isolated CD4 lymphopenia
• IL-2 deficiency
• Defects in signal transduction via the T-cell receptor. Acquired T-cell defects

THE ACQUIRED IMMUNODEFICIENCY SYNDROME (AIDS)

By far the most common immunodeficiency encountered in clinical practice is that due to infection with the human immunodeficiency virus (HIV), the cause of AIDS.

Pathogenesis

The cellular receptor for HIV is the CD4 molecule, which defines the cells that are susceptible and includes the following cells within the immune system:
1 CD4+ T lymphocytes (which are most affected)
2 Monocytes
3 Macrophages and other antigen-presenting cells
(a) Dendritic cells in the blood
(b) Langerhans’ cells of the skin
(c) Follicular dendritic cells of the lymph nodes, where much of the early infection and replication of HIV takes place.

A number of pathogenic mechanisms have been described to account for the profound cellular immunodeficiency of HIV infection

Mechanisms of the immunopathogenesis of HIV.

Mechanisms of the immunopathogenesis of HIV.

Immunological abnormalities

The major defects . The central and most characteristic is the progressive and severe depletion of CD4+ ‘helper’ lymphocytes. These cells orchestrate the immune response, responding to antigen presented to them via antigen-presenting cells in the context of class II MHC. They proliferate and release cytokines, in particular IL-2 which leads to proliferation of other reactive T-cell clones, including cytotoxic T cells to eradicate viral infections, and interferon-y and interleukin which activates B cells to antibody production, NK cells to cytotoxicity and macrophages to microbicidal activity against intracellular pathogens. Loss of this single cell type can therefore explain nearly all the immunological abnormalities of AIDS as other cells’ function is so dependent on it. In addition other cells are also affected, if not infected by HIV. Antigen-presenting cells are directly and productively infected; B cells are polyclonally activated by the envelope proteins of HIV. For the presentation and management of HIV infection and AIDS.

MEASLES

Measles can cause a transient T-cell immunodeficiency, but it is rarely long-lasting enough for severe clinical problems to ensue.

Major immunological abnormalities of HIV infection.

Major immunological abnormalities of HIV
infection.

IMMUNOSUPPRESSIVE THERAPY

Immunosuppressive therapy with cytotoxic agents such as cyclophosphamide and azathioprine tends to cause predominant Tvcell immunosuppression. Cyclosporin and PK506 are potent immunosuppressive agents which interfere with ‘I’-cell activation mechanisms at an intracellular level. Surprisingly, they are associated with only modest increases in infection, unless combined with corticosteroids or other agents. In such combinations, increased risk of Epstein-Barr virus (EBV)-associated lymphoma has been reported. Antilymphocyte immunoglobulin or monoclonal anti-CD3 antibody therapy also suppress Tvcell responses transiently.

CORTICOSTEROID THERAPY

This interferes with cell-mediated immunity, in particular T cell-macrophage cooperation. This is due to effects on T-cell traffic and impairment of macrophage responses to cytokines, together with impaired antigen presentation. Mucocutaneous candidiasis. Pneumocystis carinii pneumonia, cytomegalovirus infection, mycobacterial infection, Nocardia, non-typhi Salmonella septicaemia and cryptococcosis are some of the very many infections seen with prolonged high-dose steroid therapy.

PHAGOCYTE DEFICIENCY

Neutropenia

Congenital neutropenias are rare and, if severe, are often fatal at an early age. Most however are relatively benign and may even be incidental findings. They generally reflect defects of maturation and release of neutrophils from marrow. Staphylococcal skin infections are common manifestations. A particular variant is cyclical neutropenia with cycles of 3-5 weeks, but this disorder is typically benign.

The most common causes of acquired neutropenia are due to myelosuppression by disease, such as the leukaemias or drug therapy. Myelosuppressive drugs are most often used in the treatment of turnours; deliberate immunosuppression is also used for prevention or treatment of graft rejection in transplantation and for treating severe autoimmune disease. A number of other drugs such as some antivirals are also immunosuppressive (e.g. zidovudine, ganciclovir) and some can cause neutropenia or agranulocytosis as an idiosyncratic side-effect (e.g. chloramphenicol). Neutropenia due to increased rate of destruction of neutrophils is seen in hypersplenism and in autoimmune neutropenia.
In adults, the risk of infection rises steeply once the neutrophil count falls below 1.5 x 109/litre, regardless of the cause. The risk is less if the monocyte count is preserved, as these cells can serve as a back-up phagocyte population. (In cyclical neutropenia, monocytes usually have cycles of opposite phase, which is probably why serious infections are uncommon.) Infections are typically disseminated with septicaemia, fungaemia and deep abscess formation. Colonization of the gut with pathogens can readily lead to septicaemia. Local infections often affect the mouth, perianal area and sites of skin damage, including indwelling vascular catheters, and these can readily lead to systemic infection. Pus, which largely comprises neutrophils, may be scanty and may appear serous.
If myelosuppressive therapy is being used, the dose should be reduced or the drug stopped. Neutropenic episodes can be reduced by the use of G-CSF or GM-CSF, which appear to reduce infective episodes and duration of neutropenia. Antibiotic or antifungal prophylaxis are sometimes of value. Otherwise prompt antimicrobial therapy for febrile episodes during neutropenia is essential, using agents with broad cover for the common organisms encountered.

Defects of neutrophil function

Defects of neutrophil function (some of which also affect monocyte/macrophage function) interfere with migration into the tissues through vascular endothelium, locomotion in tissues, phagocytosis or intracellular killing.

CLINICAL FEATURES

Mucocutaneous sepsis in the mouth and perianal areas is common and local infections often lead to chronic abscess formation in the tissues or draining lymph nodes. Granulomas may be seen, because of failure of neutrophils to degrade microbes effectively. Systemic spread is less common than with neutropenia. Congenital causes may first present with infection or delayed separation of the umbilical stump. Leucocyte adhesion defect This is an autosomal recessive disorder caused by abnormal synthesis of the t3-chain CD18 that is shared by the CD  lla,b  nd c molecules to form leukocyte function antigen (LFA) LFA-1, the C3bi (inactivated C3b) recepand for the C3dg receptor (p150/95). There is impaired lieucocyte tissue localization, locomotion and endocytosis.
bone marrow transplantation has been successful in few cases.

Hyper-lgE syndrome

The syndrome is characterized by very high levels of IgE (much of it antistaphylococcal), impaired neutrophil liacomotion and severe eczema, with frequent staphy local secondary infections and abscesses. Other immune defect may be seen causing a wider spectrum of pyogenic and other  infections.

Shwachman’s syndrome

This may resemble cystic fibrosis clinically, with exocrine pancreatic insufficiency and pyogenic infections, in which mild neutropenia is associated with a defect of neutrophil migration.

Chronic granulomatous disease (ceo)

This is the prototype congenital defect of neutrophil (and monocyte) killing.

PATHOGENESIS

In this disorder, the oxidative pathway of microbial killing is severely impaired, either due to a defective cytochrome b558 (X-linked CG D) or components of the associated NADPH oxidase (autosomal recessive CGD).
Production of superoxide is abnormal, this being the first of a cascade of microbicidal oxygen radicals, including hydrogen peroxide, hypo halites, hydroxyl radicals and singlet oxygen. Impaired production of oxygen radicals can also affect the efficiency of non-oxidative killing.

CLINICAL FEATURES

Patients have chronic suppurative granulomas or abscesses affecting skin, lymph nodes and sometimes lung and liver, as well as osteomyelitis. They may present during early or late childhood years, depending on the severity of the defect. Most of the typical infections associated with neutrophil defects can be seen, particularly those that produce catalase, which inactivates any endogenous microbial peroxide that can kill organisms inside the phagocytic vacuole. Because macro phages are also affected, cell-mediated opportunist infections may also be seen such as atypical mycobacteria, Nocardia and salmonellae.

DIAGNOSIS

Diagnosis is made with the nitroblue tetrazolium (NBT) test, which uses a coloured dye reaction to assay the oxidative pathway; it can also be used to screen carriers.

TREATMENT

Infections respond to appropriate antimicrobial therapy and surgical measures as needed; in some patients prophylaxis may be merited. Recent studies have shown that regular interferon-y can reduce the frequency of infections, probably through enhanced monocyte/ macrophage killing.
A wide variety of other rare disorders can impair microbial killing including other inborn errors in microbicidal mechanisms, such as leucocyte G6PD deficiency (much less common than that affecting red cells) and myeloperoxidase deficiency. Various storage diseases, such as Gaucher’s and glycogen storage diseases, impair function of phagocytes, particularly macrophages, because of the accumulated material within them.

Chediak-Higashi syndrome

This, an autosomal recessive disorder, is characterized by giant granules in myeloid cells and large granular lymphocytes;

abnormal microbial killing is due to defective fusion with the phagosome in phagocytes; NK cell activity is similarly impaired. Similar fusion abnormalities in melanocytes causes partial oculocutaneous albinism, in addition to recurrent infections.

Acquired neutrophil function disorders

The most important defect is caused by corticosteroid therapy, which also affects T cell-macrophage cooperation causing cell-mediated immunodeficiency. The main effect of corticosteroids on neutrophils is to impair leucocyteendothelial adhesion. This reduces the marginated pool of leucocytes and impairs their attachment to endothelium at the site of tissue injury or infection. Corticosteroids thus prevent neutrophils reaching the tissues. The corollary of reduced margination is a rise in the neutrophil count; this can be deceptive if its significance is not appreciated. The effect of corticosteroid therapy on neutrophil function is reflected by increased focal and systemic infections with staphylococci and Gram-negative bacteria. The effect is usually apparent above doses of 15 or 20 mg prednisolone daily or equivalent and can be substantially reduced (as can other side-effects) by alternate-day therapy.

An important infective cause of acquired neutrophil dysfunction is influenza, which causes a specific transient impairment of phagosome-lysosome fusion. This is the main reason for the high risk of staphylococcal pneumonia in influenza epidemics. Myeloid leukaemias can cause defective neutrophil function as well as effectively causing neutropenia of normal cells. Neutrophil and macrophage function can also be impaired in abnormal metabolic states such as uncontrolled diabetes mellitus and hypophosphataemia. The latter may be seen during intravenous feeding of critically ill patients. Inhibitors of endogenous chemotactic factors for neutrophils may be seen in Hodgkin’s disease and alcoholic cirrhosis and may be responsible for increased pyogenic infections in such patients.

COMPLEMENT DEFICIENCIES

There are two major patterns of infection associated with complement deficiencies:
1 Deficiencies of C3, Cl q or of Factors H or I cause increased susceptibility to capsulated bacteria. These patients may also develop immune complex disorders and SLE-like disorders, as do patients with deficiencies of other classical pathway components of complement.

2 Deficiencies of the lytic complement pathway, C5-9, causes susceptibility to disseminated neisserial infections, meningococcaemia and gonococcaemia; the latter has also been seen in association with disorders of complement function.

These complement deficiencies are rare, but functional defects of complement deposition on microbial surfaces are common, as seen in mannan-binding protein deficiency (Saccharomyces opsonin deficiency) and other less wellcharacterized complement-dependent opsonization defects. These are responsible for increased infections with Haemophilus and pneumococcal infections, especially in the early childhood years-before a sufficiently wide specific antibody repertoire is acquired. The C3 depletion caused by C3nef, an autoantibody that stabilizes the alternative pathway convertase, and seen in association with partial lipodystrophy may also increase the risk of pyogenic infection.
Cl esterase inhibitor deficiency  is not associated with infection but with hereditary angiooedema, with episodes of localised oedema in skin of limbs or face, and the mucosa of the larynx or gut; the latter can cause life-threatening respiratory obstruction or severe episodes of abdominal pain.

The immune system in disease

Some diseases due to immunological abnormalities are common in clinical practice, for example rheumatoid arthritis, thyroid disease and allergy. Immunodeficiency is relatively rare, but the occurrence of certain infections in the context of specific immune defects can illuminate the physiological role of those parts of immune defence in control of infection.

Immunodeficiency

General principles

Infection is the result of microbial virulence on the one hand and host defence on the other. Pathogenic organisms have mechanisms of evading normal defence mechanisms. Organisms of low virulence can only cause disease if the host defence mechanisms that normally control them are defective.

Opportunist infections

Organisms taking advantage of the opportunity of impaired host defence mechanisms are called opportunists. Different host defence defects cause increased susceptibility to different groups of organisms. Therefore, recognizing a pattern of infections can provide the best clinical clue to the type of underlying defence defect. Patterns of opportunist infection Examples of opportunist organisms in the setting of nonimmunological defence defects include: staphylococci and Pseudomonas in burns patients; Haemophilus influenzae and pneumococci in smokers; Pseudomonas in cystic fibrosis patients; Gram-negative infections where there is urinary obstruction; staphylococcal and candidal infections with indwelling venous catheters and other foreign bodies; Candida and pathogenic Escherichia coli following elimination of gut flora after antibiotic therapy.

Interaction of the immune system with the hypothalamo-pituitary axis regulating adrenocortical secretion. In response to infection, activated T cells release cytokines which induce the hypothalamic release of corticotrophinreleasing hormone (CRH) and vasopressin (VP). This overrides the normal negative feedback relationship between corticotrophin and cortisol. Circulating cortisol also acts on peripheral immunocompetent cells to inhibit their activation and secretion of cytokines and other mediators of inflammation. ACH, acetylcholine; NA, noradrenaline; TNF, tumour necrosis factor. (From Reichlim 5 (1993) New England Journal of Medicine, 329, 1246. With permission.)

Interaction of the immune system with the hypothalamo-pituitary axis regulating adrenocortical secretion. In response to infection, activated T cells release cytokines which induce the hypothalamic release of corticotrophinreleasing hormone (CRH) and vasopressin (VP). This overrides the normal negative feedback relationship between corticotrophin and cortisol. Circulating cortisol also acts on peripheral immunocompetent cells to inhibit their activation and secretion of cytokines and other mediators of inflammation. ACH, acetylcholine; NA, noradrenaline; TNF, tumour necrosis factor. (From Reichlim 5 (1993) New England Journal of Medicine, 329, 1246. With permission.)

the main infecting organisms for the principal classes of immunodeficiency. In broad terms, the following principles apply:
• ‘EUTROPHIL DEFECTS lead to staphylococcal, Gramnegative enteric and systemic fungal infections
PSONIC DEFECTS, due to antibody deficiencies or defects of the main complement pathways, and splenectomy lead to infection with capsulated organisms
JEFECTS OF THE LYTIC PATHWAY OF COMPLEMENT cause susceptibility to disseminated infections with Gram-negative cocci (Neisseria)
ELL-MEDIATED IMMUNE DEFECTS, affecting the cooperation between CD4 T cells and macrophages, lead o susceptibility to infection with facultative intracellupathogens and herpesviruses ongenital immunodeficiencies enital defects of specific metabolic or developmental ders can lead to characteristic deficiencies and are e main categories of immunodeficiency by host =d:anis’ m are as follows:

REDUCED NEUTROPHIL NUMBER AND FUNCTION,

with or without accompanying defects in the related phagocytes of the monocyte/macrophage lineage

EFICIENCIES OF INDIVIDUAL COMPLEMENT COMPONENTS

B-CELL DEFECTS, causing various types of antibody deficiency
T-CELL DEFECTS, impairing cell-mediated immunity
COMBINED T AND B CELL DEFECTS, which cause some of the most severe immunodeficiencies . the more important examples.

Acquired immunodeficiencies

Acquired immunodeficiencies are much more common but are often less precisely defined in terms of immunological mechanisms. They can in many cases be best understood against the background of the more specific defects. They include:
IATROGENIC DEFECTS resulting from deliberate immunosuppression or unwanted complications of certain therapies; malnutrition; splenectomy IMMUNOSUPPRESSION resulting from specific diseases that affect immune competence, such as tumours of the immune system and autoimmune disorders; and transient or progressive immunosuppression caused by certain infections.
The most important of the latter is of course the acquired immunodeficiency syndrome (AIDS) resulting from HIV infection, which is to be covered in some detail for the main categories

Immune defects and opportunist organisms

Immune defects and opportunist organisms

The immune system In concert

Following an antigenic stimulus, the components of the immune system cooperate to meet and eliminate the challenge.
The foreign antigen is picked up by a cell of the monocyte/macrophage series and the antigen is degraded or processed and presented to both the Band T lymphocytes. T-helper cells are generated which enhance the antibody response made by B cells. Some of this augmentation is due to secreted lymphokines or cytokines from the T-helper population. It should be noted that cytotoxic T cells may be generated if foreign antigens, typically viruses, are presented directly to this cell population. Bacteria are most likely to be processed through the MHC class II pathway and thus generate, primarily, antibody responses. The B cells, once triggered, will differentiate into plasma cells producing specific antibody which binds to the antigen and further triggers the complement system. The complement system, in turn, recruits neutrophils which together eliminate the antigen, perhaps with additional help from lymphokine-activated macrophages.
Once the immune system has detected ‘foreign antigens’ it can communicate this information to other systems, particularly to the brain and neuroendocrine systems.
For example, following infection, there is an increase in both pituitary and adrenal gland secretion.

Cytokines: origin and biological function.

Cytokines: origin and biological function.

Immune recognition and cellular function

The hallmark of the immune response is its ability to react specifically to given antigen. The mechanism at the heart of this reaction is the way in which antigen is trapped, processed and recognized as foreign.

Accessory cells

Several cell types, sometimes termed accessory cells, facilitate the antigen-presenting process. Macrophages and monocytes (mononuclear phagocytes) These cells are distributed throughout the body, in the tissues (as macrophages) and in the blood (as monocytes). Macrophages are equipped with various features that make them particularly effective at removing foreign antigens, ready for presentation. They are phagocytic and have, on their surface, receptors that recognize the Fc region of antibody molecules as well as biologically active fragments of complement (C3b). The presence of these structures on cells in the macrophage-monocyte family makes it easy for them to intercept and dispose of antigen-antibody complexes.

Follicular dendritic cells

Follicular dendritic cells are non-phagocytic and are located in the germinal centres oflymph nodes (follicles). They are surrounded by B lymphocytes to which they present antigen, usually complexed with antibody, on the surface of their dendrites. Their surfaces are rich in Fc and C3b receptors to facilitate antigen trapping.

Langerhans’ cells and dendritic cells

The Langerhans’ cell is found primarily in the skin. The dendritic (or veiled) cell is present in the blood (and is different to the follicular dendritic cell described above). They are of macrophage/monocyte lineage.

Antigen presentation

Antigen fragments are presented to T cells in association with either class I or class II major histocompatibility complex (MHC) molecules (see p. 128). Antigens that are presented with class II molecules are recognized by CD4+, T helper cells. Antigens that associate with class I molecules are presented to CD8+ T cells and a cytotoxic response results. The responding population of T cells thus recognize the combined shape of the antigen and the MHC molecule. The combination of T-cell receptor, MHC molecule and antigen fragment is known as the trimolecular complex. As there are differences in the three-dimensional shape of the MHC molecules due to genetic variation, some antigens may be more effective than others in inducing immune responses because they present an optimum shape or conformation to the T cells. Immune responses that only occur with certain antigen-MHC combinations are called MHC restricted.

HLA (human leucocyte antigens)  The HLA molecules are distributed throughout the body tissues and it is through differences in this system that cells are classified as self or non-self The possibility of two different individuals having the same combination of HLA molecules is very remote. It is this particular aspect of the immune system that presents problems for organ transplantation. Unless the HLA type of the donor and the recipient are virtually identical the organ graft will be recognized as non-self and rejected by the immune system of the host. The process of tissue typing involves the identification of the set of HLA antigens in the tissues of a given individual using specific antisera.

Cytokine production

Much of the immune system’s ability to communicate between its different compartments is achieved through the use of soluble messenger molecules called lymphokines (produced by lymphocytes) or cytokines (a generic term meaning made by any cell). Once the cytokine reaches its destination cell it then induces a biological effect, which will of course vary according to the cytokine and the cell involved, but typically these molecules will signal certain cell populations to activate, divide or home in on a particular site in the body.
Interferons (a group of cytokines) were discovered in the 1960s and can be divided into ll’, f3 and y varieties. Their main actions are:
• Antiproliferative
• Antiviral
• Immunomodulation
Many cytokines have been isolated, cloned and characterized by the use of recombinant DNA technology. Some are now being used in clinical practice. For example, IL-2 is used to combat kidney tumours and G-CSF (a substance that cause bone marrow cells to divide and mature) has found application in bone marrow transplantation and treatment of neutropenia.