Failure of immunoglobulin isotype switching

After exposure to an antigen, the first antibodies to appear are IgM. Later, antibodies of other classes appear: IgG predominates in the serum and extravascular space, while IgA is produced in the gut, and IgE may be secreted at other epithelial surfaces. The changes in the isotype of antibody produced in the course of an immune response reflect the occurrence of isotype switching in the B cells that synthesize antibody, so that the variable region, which determines the specificity of an antibody, becomes associated with the constant regions of different isotypes as the immune response progresses.

Isotype switching is induced by T cells. T cells are also required to initiate B-cell responses to many antigens: the only exceptions are responses triggered by some microbial antigens, or by certain antigens with repeating epitopes. T-cell help is delivered in the context of an antigen-specific interaction with the B cell. This interaction activates the T cell to express the CD40 ligand (CD40L), which then delivers an activating signal to the B cell by binding CD40. Activated T cells also secrete cytokines, which are required at the initiation of the humoral immune response to drive the proliferation and differentiation of naïve B cells, and are later required to induce isotype switching. In humans, isotype switching to IgE synthesis is best understood and is known to require interleukin -4 or interleukin -13, as well as contact with CD40L expressed by an activated T cell.

The gene for the CD40 ligand is on the X chromosome at position Xq26. In males with a defect in the CD40 ligand gene, isotype switching fails to occur; such individuals make only IgM and IgD and cannot switch to IgG, IgA, or IgE synthesis. This defect can be mimicked in mice in which genes for CD40 or CD40 ligand have been disrupted by gene targeting: B cells in these animals also fail to undergo isotype switching. The underlying defect in patients with hyper IgM immunodeficiency syndrome can be demonstrated easily by using soluble CD40. Soluble CD40 can be made by engineering the extracellular domain of CD40) onto the constant region of IgG. The soluble protein can then be labeled with a dye. Activated T cells from hyper IgM patients fail to bind fluorescently labeled, soluble CD40.

CD40 is expressed not only on B cells but also on the surfaces of macrophages, dendritic cells, follicular dendritic cells, and mast cells. Macrophages and dendritic cells are antigen-presenting cells that can trigger the initial activation and expansion of antigen-specific T cells at the start of an immune response. Recent experiments in CD40L-deficient mice indicate a role for the CD40-CD40L interaction in this early priming event, because in the absence of CD40L the initial activation and expansion of T cells in response to protein antigens is greatly reduced.

The case of Dennis Fawcett: a failure of T-cell help

Dennis Fawcett was 5 years old when he was referred to the Children's Hospital with a sever acute infection of the ethmoid sinuses (Ethmoiditis). His mother reported that he had had recurrent sinus infections since he was 1 year old. Dennis had pneumonia from an infection with Pneumocystis carinii when he was 3 years old. These infections were treated successfully with antibiotics. While he was in the hospital with ethmoiditis, group A ß-Hemolytic streptococci were cultured from his nose and throat. The physicians caring for Dennis expected that he would have a brisk rise in his white blood cell count as a result of his severe bacterial infection, yet his white blood cell count was 4200µl- 1 (normal count 5000-9000µl- 1 ), 26% of his white blood cells were neutrophils, 56% lymphocytes, and 28% monocytes. Thus his neutrophil number was very low, whereas his lymphocyte number was normal and the number of monocytes was elevated.

Seven days after admission to the hospital, during which time he was successfully treated with intravenous antibiotics, his serum was tested for antibodies to streptolysin O, an antigen secreted by streptococci. When no antibodies to the streptococcal antigen were found, his serum immunoglobulins were measured. The IgG level was 25 mg dl- 1 (normal 600-1500 mg dl- 1 ), IgA was undetectable (normal 150-225 mg dl- 1 ) and his IgM level was elevated at 210 mg dl- 1 (normal 75-150 mg dl- 1 ). A lymph-mode biopsy showed poorly organized structures with an absence of secondary follicles and germinal centers.

Dennis was given a booster injection of diphtheria toxoid, pertussis, and tetanus toxoid (DPT) as well as typhoid vaccine. 14 days later, no antibody was detected to tetanus toxoid nor to typhoid O and H antigens. Dennis had red blood cells of group O. People with type O red blood cells make antibodies to the A substance of type A red cells and antibodies to the B substance of type B red cells. This is because bacteria in the intestine have antigens that are closely related to A and B antigens. Dennis' anti-A titer was 1:3200 and his anti-B titer 1:800, both very elevated. His anti-A and anti-B antibodies were of the IgM class only.

His peripheral blood lymphocytes were examined by FACS analysis and normal results were obtained: 11% reacted with an antibody to CD19, a B-cell marker, 87% with anti-CD3, a T-cell marker, and 2% with anti-CD56, a marker for natural killer (NK) cells. However, all of his B cells (CD19 + ) had surface IgM and IgD and none were found with surface IgG or IgA. His activated T cells did not bind soluble CD40.

Dennis had an older brother and sister. They were both well. There was no family history of unusual susceptibility to infection.

Dennis was treated with intravenous gamma globulin, 600 mg per kg body weight each month, and subsequently remained free of infection.

Hyper IgM immunodeficiency

Males with a hereditary deficiency of the CD40 ligand exhibit consequences of a defect in both humoral and cell-mediated immunity. Defects in antibody synthesis result in susceptibility to the so-called pyogenic infections. These infections are caused by pus-forming (pyogenic) bacteria such as Haemophilus influenzae, Streptococcus pneumoniae, Streptococcus pyogenes, and Staphylococcus aureus , which are resistant to destruction by phagocytic cells unless they are coated (opsonized) with antibody and complement. On the other hand, defects in cellular immunity result in susceptibility to opportunistic infections. Bacteria, viruses, and fungi that normally reside in our bodies and only cause disease when cell-mediated immunity in the host is defective are said to cause opportunistic infections.

Dennis revealed susceptibility to both kinds of infection. His recurrent sinusitis, as we have seen, was caused by Streptococcus pyogenes , a pyogenic infection. He also had pneumonia caused by Pneumocystis carinii , a protozoan that is ubiquitous and causes opportunistic infections in individuals with a defect in cell-mediated immunity.

Males with a CD40 ligand deficiency can make IgM in response to T-cell independent antigens but they are unable to make antibodies of any other isotype, and they cannot make antibodies to T-cell dependent antigens, leaving them largely unprotected from many bacteria. They also have a defect in cell-mediated immunity that strongly suggests a role for CD40L in the T cell-mediated activation of macrophages. The failure of this interaction also explains why affected males are unable to develop leukocytosis (a significant rise in the white count) in the face of sever infections. At times these males may become profoundly deficient in neutrophils, and this is a very prominent feature of their disease. As a consequence of this neutropenia they develop severe sores and blisters in their mouth and throat, a site normally infested with many bacteria. This defect can be overcome by giving them granulocyte-macrophage cell-stimulating factor (GM-CSF). GM-CSF is secreted by macrophages and to a lesser extent by T cells. The interaction of the CD40 ligand with CD40 on macrophages is required for the secretion of GM-CSF by macrophages, thus explaining why these patients cannot develop a leukocytosis.

For additional information please visit http://www.ncbi.nlm.nih.gov/books/bv.fcgi?call=bv.View..ShowSection&rid=gnd.section.151 .

We would like to thank Dr. Fred Rosen and Dr. Raif Geha for their contribution of the above information from their book, "Case Studies in Immunology 3."

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