An autosomal recessive familial CD8 deficiency due to a single mutation in the CD8α gene is described. CD8+ cells were shown to be absent in a patient with repeated bacterial infections in whom other known immunodeficiencies were ruled out. Familial studies showed the same defect in two sisters from the same consanguineous parents.
CD8α chain was found to be completely absent in three members of this family, whereas scarce expression of CD8β chain was detected in their cytoplasm. CD8 glycoproteins are expressed predominantly on MHC class I–restricted T cells as a disulfide-linked αβ-heterodimer, but can also be expressed as an αα-homodimer in NK cells and intraepithelial T cells (6
). In the absence of CD8α, CD8β is retained and destroyed within the cell (7
), whereas CD8α can be expressed at the cell surface without CD8β as CD8αα homodimers. These data, together with normal mRNA expression for CD8α and β, led us to postulate that the defect could more probably involve the CD8α gene. Therefore, genetic analysis was performed, and a missense mutation (gly90→ser) in the CD8α gene was found to be present in both alleles of the CD8-deficient individuals. In all family members, the genotype correlated strictly with the CD8 phenotype observed. No CD8 molecules could be detected in the cells or sera of the homozygous individuals with any of the anti-CD8 Ab’s tested, whereas heterozygous subjects showed impaired CD8 expression both on the cell surface of CD8+
lymphocytes and in serum.
A serine in the mutant allele replaced the glycine residue, which is well preserved in the F strand of the immunoglobulin domain of all known CD8α molecules from fish to humans (over 400 million years) (8
). This particular glycine is also conserved in the closely related molecules CD8β and CD7 (31
). The importance of this particular mutation in CD8 was also established by our in vitro experiments. Mutant CD8 constructs that lack gly90 cannot be detected on the surface or in cytoplasm of transiently transfected COS-7 cells. The relevance of gly90 was further demonstrated by the transfection of chimeric CD8 molecules and by site-directed mutagenesis-generated molecules. Taken together, these experiments demonstrated the need for gly90 in the immunoglobulin domain of CD8α, since its substitution by another amino acid, arginine, resulted in an identical lack of detection. Furthermore, these results showed that substitution of the glycine residue was sufficient to avoid CD8 expression, thereby ruling out the possibility that a mutation in noncoding sequences, that is, regulatory sequences, could be responsible for the lack of CD8 found in this family.
Although the glycoprotein CD8 plays an important role in the maturation and function of MHC class I–restricted T lymphocytes, as clearly shown by either CD8α or CD8β knockouts (17
), its presence does not appear to be essential for either CD8 lineage commitment or peripheral cytolytic function. It has been reported that TCR transgenic thymocytes from CD8α-deficient mice were able to restore positive selection of CD8 lineage cells (as shown by CD8β expression) in the absence of CD8, thereby compensating for the lack of CD8 expression by increasing the affinity of TCR for the positively selecting ligand (7
). Recent reports addressing the mechanism of CD4/CD8 lineage commitment are consistent with a model in which recognition of class I or class II MHC directs thymocytes to the appropriate lineage (instructive model) (36
). The presence or absence of coreceptor-related signals (CD4, CD8, or p56lck
) can further modulate the selection process, but lineage commitment can take place in the absence of the appropriate coreceptor. Polyclonal α/β DN T cells found in the CD8-deficient individuals were most probably committed to being CD8 cytotoxic T cells, because several findings strongly suggest: (a) a high percentage of α/β DN T cells in our patient were CD11b+
, a phenotype associated with effector CD8+
cytotoxic T lymphocytes (30
); (b) the absence of CD4 expression in a substantial number of peripheral T cells (α/β DN T cells) in these CD8-deficient individuals suggests a specific CD4 antigen downregulation event; and (c) transcripts for CD8α and CD8β were detected at similar levels in the CD8-negative individuals, as in their heterozygous relatives or normal controls. These findings indicate that CD8 transcription can continue without direct binding of CD8 to MHC I molecules during thymic selection.
The clinical manifestations present in the affected member of this family, as in the murine counterpart (CD8α mice) (20
), are not severe. This syndrome, as TAP1 and TAP2 deficiencies, is compatible with life, but seems to be less aggressive than the HLA class I deficiencies. Probably, the absence of CD8 is more critical for the development of the cytotoxic T cell repertoire than for the effector function in the periphery.
We believe that the absence of classic CTL CD8+ may be partially compensated for by the cytolytic function of α/β DN T cells and the NK cell activity. However, as the impact of the CD8 absence in CTL function could not be properly addressed in this patient, further studies on CD8 cell function, such as allogeneic cytotoxicity or cytotoxicity to recall antigens and DN T cell repertoire, are required before the significance of this CD8 defect can be better interpreted.
The high Ab titers to many viral infections (CMV, herpes zoster, herpes simplex, rubella) in the patient would seem to demonstrate that he has been in contact with these viruses and immunocompetent enough to overcome these infections. Although analysis of the recent infections in the proband showed only those of bacterial nature, viral infections suffered at an early age might have been responsible for the alveolar lesions that have later become over-infected and have produced bronchiectases, as reported in TAP-deficient patients (24
). The diagnosis of pelvic inflammatory disease (PID) in adults is more frequently reported (39
), and it remains to be seen whether the CD8-negative sisters will develop symptoms in the future. Examples of poor correlation between genotype and clinical symptoms in other PID patients are described in the literature (24
), and factors such as polymorphisms in other host defense molecules associated with monogenic disorders (45
) could also be involved in phenotypic differences in PID with the same genetic defect.
The prevalence of this particular CD8 defect with such a dramatic effect on lymphocyte phenotype should be extremely low, since CD8 expression has been widely determined without any similar case being reported. Nevertheless, mutations in CD8 that affect MHC I binding or signal transduction capabilities without substantial impairment of CD8 expression may be more frequent than expected and should be investigated in patients with repeated infections that resemble Ab deficiencies.
A novel immunological defect is described and a point mutation in the CD8α gene is demonstrated to be responsible for this autosomal recessive familial CD8 deficiency. We believe that many lessons on the role of the CD8 molecule in the maturation and function of MHC class I–restricted T lymphocytes can be learned from the study of this natural human model of immunologic defect.