Familial CD8 deficiency due to a mutation in the CD8 alpha gene

Abstract

CD8 glycoproteins play an important role in both the maturation and function of MHC class I-restricted T lymphocytes. A 25-year-old man, from a consanguineous family, with recurrent bacterial infections and total absence of CD8(+) cells, was studied. Ab deficiencies and ZAP-70 and TAP defects were ruled out. A missense mutation (gly90-->ser) in both alleles of the immunoglobulin domain of the CD8 alpha gene was shown to correlate with the absence of CD8 expression found in the patient and two sisters. Conversely, high percentages of CD4(-)CD8(-)TCR alpha beta(+) T cells were found in the three siblings. A novel autosomal recessive immunologic defect characterized by absence of CD8(+) cells is described. These findings may help to further understanding of the role of CD8 molecules in human immune response.

Figure 1

CD8 expression analysis. (a) Expression of CD8 on T lymphocytes and CD3^– cells in the CD8-deficient patient and a healthy donor. (b) CD8β expression studied by Western blot analysis in two controls (C1 is a primary immunodeficient patient with low CD8 T cells and C2 is a healthy adult) and in the CD8-deficient patient. Weak intracellular expression of this molecule was detected in the patient; β-actin was the same as in controls. (c) Messenger RNAs for CD8α and β were detected by RT-PCR at similar levels in the CD8-negative individuals, their relatives, and normal control.

Figure 2

Mutation in the messenger RNA-coding region of the CD8α gene. Sequence of the CD8α cDNA reveals a nucleotide substitution (G→A) at position 331 (numbered from the ATG sequence initiating the coding region), leading to the replacement of a glycine by a serine at position 90.

Figure 3

Segregation of the mutation in the CD8α gene. (a) Sequence of the genomic DNA for the CD8α gene of the patient (II-4), his father (I-2), and an unaffected sibling (II-1). This sequence corresponds to the complementary strand around position 331 of the CD8α gene and was performed with the PCR products as templates. (b) Pedigree, including genotype and phenotype data of CD8. Square symbols denote male and circles female family members; filled symbols denote homozygous mutant allele; shaded symbols represent heterozygous carriers of the mutant CD8α allele; open symbols represent homozygous wild-type alleles. A) percentage of CD3⁺CD8⁺ T cells; B) fluorescence intensity of CD8α expression; C) soluble CD8 concentration in serum (U/ml). Family member II-6 was not tested.

Figure 4

CD8 expression in transfected cells. COS 7 monkey kidney cells were transfected with SRα296 constructs. (a) Chimeric constructs (extracellular/transmembrane + cytoplasmic) WT/MUT and MUT/WT, wild-type CD8α (WT/WT), and mutant CD8 (MUT/MUT) were transfected. Transfected COS 7 cells were used later for immunofluorescence analysis. (b) CD8α molecules engineered by site-directed mutagenesis: CD8^gly90>ser, CD8^ser90>gly, and CD8^gly90>arg were also transfected into COS-7 cells. Gray line indicates isotype control and black line CD8 expression.