CD81-Dependent Trafficking of CD19: Restoration of CD19 Surface Expression in Human B Cells Harboring A CD81 Mutation

A 6-year-old girl, who was diagnosed with a primary antibody deficiency, had B cells lacking surface CD19. However, both her CD19 alleles were normal and the impairment was actually caused by a homozygous exon splice site mutation in CD81 (1). The patient's B cells also lacked surface CD81 and produced an immature glycosylated CD19 protein that was retained intracellularly. Interestingly, this human deficiency differed from that of CD81 knockout mice as the latter still express a low level of CD19 on their B cells.

We used an EBV-transformed B cell line from this patient to better understand i) the difference between the human and mouse CD81 deficiencies and ii) how CD81 controls the trafficking of CD19 to the cell surface. We reasoned that the truncated human CD81 mutant (CD81mut) protein might be expressed intracellularly. Indeed, whereas most anti-CD81 mAbs did not recognize CD81mut, we identified one that bound the mutated form and used it in this study. We also expressed the human CD81mut in a CD81-deficient mouse B cell line to determine if it could negatively regulate CD19 surface expression.

We show that the CD81mut protein is indeed expressed intracellularly in the patient's EBV-transformed B cells. We then used a proximity ligation assay to demonstrate that the truncated CD81mut protein interacts intracellularly with CD19. However, this interaction with the CD81mut protein abrogated carbohydrate maturation and the trafficking of CD19 to cell surface. We therefore expressed the CD81mut in CD81KO mouse B cells, which still express low levels of surface CD19, and found that it did not exert a dominant negative effect on CD19 surface expression. Finally, we used this reconstitution system to identify specific CD81 domains that restored carbohydrate maturation and cell surface expression of the CD19 molecule in the patient's B cells.

This specific case of antibody deficiency was manifested because of lack of surface expression of CD19, an important B cell signaling molecule. However, the maturation of CD19 and its trafficking to the cell surface require the presence of specific domains of the tetraspanin CD81 molecule.

No relevant conflicts of interest to declare.