Combined B-Cell and T-Cell Deficiencies
Combined B-cell and T-cell deficiencies include (a) severe combined immunodeficiency, (b) Wiskott–Aldrich syndrome, (c) ataxia telangiectasia, and (d) MHC class II deficiency.
Severe combined immunodeficiency (SCID) includes many syndromes with severe deficiency of both humoral and cell-mediated immune responses. All these are inherited diseases with a defect in the differentiation of early stem cells. These are of two types: X-linked and autosomal.
X-linked SCID: The sex-linked SCID is associated with adefect of the gene that codes for a polypeptide chain common to several interleukin receptors (IL-2, IL-4, IL-7, IL-11, and IL-15). This chain is involved in signaling of second messages, thus in its absence, T-cell precursors fail to receive the signals necessary for their proliferation and differentiation. There is T- and B-cell lymphopenia and decreased production of IL-2. There is an absence of delayed-type hypersensitivity, cellular immunity, and of normal antibody synthesis following immu-nogenic challenge.
Autosomal SCID: This is due to a mutation in the geneencoding a tyrosine kinase called ZAP-70, which plays an important role in signal transduction in T cells. Other SCID patients show mutations in other genes, such as RAG-1 or RAG-2, that are essential to produce T-cell antigen receptor and the IgM monomer on the B cell that acts as antigen carrier.
SCID is a disease of infancy, presenting with failure to thrive. Affected individuals frequently die during the first 2 years of life. Clinically, they may develop a measles-like rash, show hyperpigmentation, and develop severe recurrent (especially pulmonary) infections. These patients have heightened suscep-tibility to Pneumocystis carinii, C. albicans, and other pathogens. Even attenuated microorganisms, such as those used for immu-nization, e.g., attenuated poliomyelitis viruses, may induce infection in SCID patients. All these forms of SCID can be cor-rected with a bone marrow graft from HLA-DR matched sib-lings. The graft is usually successful, but there is a great risk for the development of graft-versus-host disease.
Graft-versus-host disease (GVHD): It is a problem in SCIDpatients receiving unirradiated blood transfusions. It can also develop after transfusion of any fresh blood component contaminated with viable T lymphocytes. It is characterized by fever, maculopapular rash involving the volar surfaces, diarrhea and protein-losing enteropathy, Coombs’ positive hemolytic anemia, thrombocytopenia, and splenomegaly. In full-blown cases, the outcome is generally poor, with death occurring within 10–14 days from the onset of symptomatology. The reaction may be prevented in the case of transfusion by using frozen or irradiated blood products. Current attempts at eliminating all cells except stem cells from bone marrow grafts appear promising.
Wiskott–Aldrich syndrome is an X-linked recessive immuno-deficiency disease of infants. It is characterized by thrombo-cytopenia, eczema, and increased IgA and IgE levels. There is decreased cell-mediated immunity. The inability to mount an IgM response to capsular polysaccharide of bacteria is the most important defect. IgA and IgE are increased, but IgM is dimin-ished, although IgG serum concentrations are usually normal. By electron microscopy, T cells appear to lack the markedly fimbriated surface of normal T cells. T cells have abnormal sialophorin. The defect appears to be caused by the inability of T cells to provide help to B cells. Bone marrow transplantation corrects the deficiency.
It is an autosomal recessive disease caused by mutations in the gene that encodes DNA repair enzyme. This condition is characterized by ataxia, telangiectasia, and recurrent infections in babies by 2 years of age. IgA deficiency and lymphopenia commonly occur.
It is an autosomal recessive disease failing to express MHC molecules on the surface of antigen presenting cells, such as macrophages and B cells. This results in a deficiency of CD4 T cells. The lack of these helper T cells results in production of deficient antibodies.