IMMUNOLOGY - Roit A. - Mir 2000

Chapter 21. Primary Immunodeficiency

PHAGOCYTIC CELL DEFECTS

Phagocytic Cells—polymorphonuclear leukocytes and Cells of the monocyte-macrophage Lineage—play a vital role in host defense against pyogenic Bacteria and other intracellular microorganisms. Severe deficiency of polymorphonuclear leukocytes (neutropenia) can lead to generalized bacterial infection. Two genetic defects that impair phagocytic function and thereby increase susceptibility to infections are of particular clinical significance. These defects are associated with severe and frequently fatal diseases: chronic granulomatous disease and leukocyte adhesion deficiency.

The underlying cause of chronic granulomatous disease (CGD) is an Impairment of the oxygen reduction mechanism

Patients with CGD have a defect in NADPH oxidase, which catalyzes the reduction of O₂ to generate ·0₂^-:

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Because of this defect, the patients' phagocytes are unable to produce superoxide oxygen radicals (·0₂^-) and hydrogen peroxide, and thus cannot rapidly destroy ingested bacteria or Fungi, particularly catalase-positive species (see Chapter 17). As a result, microorganisms remain viable within the phagocytes of such patients. Persisting intracellular microbial Antigens provoke a Cell-mediated Immune Response and The formation of granulomas. Children with CGD develop Pneumonia, Lymph node infections (lymphadenitis), and abscesses in the Skin, Liver, and other Internal Organs.

The Diagnosis of CGD is established by the inability of stimulated phagocytes to reduce the nitroblue tetrazolium (NBT) dye. When phagocytes ingest this dye during the Digestion of trapped particles, the dye—which is pale yellow in its oxidized form—is reduced by accepting a hydrogen ion coupled with The oxidation of NADPH, resulting in the formation of dark red NBT crystals within the cells. In the phagocytes of CGD patients, NBT reduction does not occur (Fig. 21.14).

Fig. 21.14. Nitroblue tetrazolium (NBT) dye test. In healthy individuals, polymorphonuclear leukocytes and monocytes generate highly reactive oxygen metabolites (ROMs) during phagocytosis, causing yellow NBT to be reduced to red-purple formazan (1). In CGD patients, ROM generation does not occur, and NBT remains yellow (2). (Photographs kindly provided by Prof. A.R. Hayward.)

The reaction involving NADPH oxidase is a complex process, and the enzyme complex consists of many subunits. The membranes of resting phagocytes contain a cell type-specific cytochrome b₅₅₈. It is composed of two chains: one has a molecular mass of 91 kDa and is encoded by a Gene on the short arm of the X chromosome; the other chain, with a molecular mass of 22 kDa, is encoded by a gene localized on chromosome 16. During phagocytosis, several cytosolic Proteins are phosphorylated, translocate to the membrane, and associate with cytochrome b₅₅₈. The assembled complex acts as the enzyme NADPH oxidase, catalyzing the oxidation of NADPH to generate highly reactive oxygen radicals (Fig. 21.15). The X-linked form of CGD is the most common, characterized by a defect in the 91-kDa chain of cytochrome b₅₅₈. Three Other types of CGD are Autosomal Recessive Disorders caused by defects in the other chain (22 kDa) of cytochrome b₅₅₈ or in one of two cytosolic proteins, p47^phox or p67^phox (phox stands for phagocytic oxidase).

Fig. 21.15. Current model of NADPH oxidase Structure: in the absence of an activating stimulus, some enzyme components are localized in the membrane (cytochrome b₅₅₈ and possibly rap-1), whereas others reside in the Cytosol (p47^phox, p67^phox, the NADPH-binding component N, and a putative fourth component α). In response to a phagocytosis-induced stimulus, the cytosolic elements assemble into a complex that then translocates to the membrane. This process is likely mediated by the phosphorylation (P) of p47^phox. Once the cytosolic components bind to the membrane components, the oxidase becomes catalytically active, and p47^phox undergoes further phosphorylation. Gene defects encoding various components of this oxidase have been identified in different forms of CGD. (Adapted from Smith R.M., Curnutte J.T., 1991. Molecular Basis of chronic granulomatous disease. Blood 77 (4): 673–86, with permission.)

Leukocyte adhesion deficiency (LAD) is caused by integrin gene defects

For the ingestion of microorganisms by phagocytic cells, a phagocyte membrane receptor that binds to C3bi On the surface of opsonized microbial cells is of critical importance. In patients suffering from LAD, this integrin receptor—designated Complement receptor 3 (CR3)—is absent, rendering them susceptible to severe bacterial infections, particularly affecting the Oral Cavity and the gastrointestinal tract.

The CR3 molecule is composed of two polypeptide chains: an α-chain with a molecular mass of 165 kDa (CD11b) and a β-chain with a molecular mass of 95 kDa (CD18). LAD involves a genetic defect in the β-chain, which is encoded by a gene on chromosome 21. The exact same β-chain is shared by two other integrin proteins: leukocyte function-associated antigen-1 (LFA-1) and p150,95 (see Chapter 5). Although each of these proteins has its own α-chain (CD11a and CD11c, respectively), both are defective in LAD. The LFA-1 molecule plays a major role in Cell Adhesion; it interacts with intercellular adhesion molecule-1 (ICAM-1) on The surface of endothelial cells and other cell membranes. Because of the LFA-1 defect, phagocytes from LAD patients are unable to attach to the vascular endothelium and therefore cannot migrate from Blood Vessels into the site of infection. Consequently, localized pus formation does not occur, allowing bacteria to spread rapidly throughout the body.

Questions for thought

■ The Clinical presentation of X-linked agammaglobulinemia is indistinguishable from that of inherited deficiency of the third component of complement (C3): both groups of patients exhibit increased susceptibility to pyogenic infections. How can this be explained?

■ Individuals with MHC class II deficiency lack CD4⁺ T cells. In rare cases, MHC class I deficiency is observed. Which T-cell subpopulation would be expected to be deficient in these patients?

■ Individuals with deficiencies in complement components C1, C2, C4, and C1 inhibitor may not be as susceptible to pyogenic infections as patients with C3 deficiency. What is the reason for this difference?

Introduction/47.html">Further Reading

Conley M.E. 1991. Molecular approaches to analysis of X-linked immunodeficiencies. Ann. Rev. Immunol. 10: 215.

Curnutte J.T., Orkin S.H., Dinauer M.C. Genetic Disorders OF phagocyte function. In: Stamatoyannopoulos G., Nienhuis A.W., Majerus P.W., Varmus H. (eds.). The Molecular Basis of Blood Diseases. Philadelphia: PA Saunders; 1994: 443.

Rosen F.S., Cooper M.D., Wedgwood R.J.P. 1995. The Primary immunodeficiencies. N. Engl. J. Med. 333: 43.

Rosen F.S., Seligman M. (eds.). Immunodeficiencies. Switzerland: Harwood Academic Publishers GmbH; 1993.

Von Andrian U.H., Berger E.M., Chambers J.D. et al. In vivo behaviour of neutrophils from two patients with distinct inherited leukocyte adhesion deficiency syndromes. J. Clin. Invest. 1993;91:2893.