Phenotypic and pathomechanistic overlap between tapasin and TAP deficiencies.

Background: Human tapasin deficiency is reported to cause an autosomal-recessive inborn error of immunity characterized by substantially reduced cell surface expression of major histocompatibility complex class I (MHC-I).

Objective: We evaluated the immunologic and clinical consequences of tapasin deficiency.

Methods: A novel homozygous variant in TAPBP was identified by means of whole genome sequencing. The expression of tapasin and both subunits of the transporter associated with antigen presentation (TAP) were evaluated by Western blot analysis. Cell surface and intracellular expression of MHC-I were evaluated by flow cytometry. Small interfering RNAs were used for silencing TAPBP expression in HEK293T cells.

Results: We identified a deletion in TAPBP (c.312del, p.(K104Nfs∗6)) causing tapasin deficiency in a patient with bronchiectasis and recurrent respiratory tract infections as well as herpes zoster. Besides substantial reduction in TAP1 and TAP2 expression, peripheral blood mononuclear cells from this patient and TAPBP-knockdown HEK293T cells, displayed reduced cell surface expression of MHC-I, while reduction in intracellular expression of MHC-I was less prominent, suggesting a defect in MHC-I trafficking to the plasma membrane. IFN-α improved cell surface expression of MHC-I in tapasin deficient lymphocytes and TAPBP-knockdown HEK293T cells, representing a possible therapeutic approach for tapasin deficiency.

Conclusion: Tapasin deficiency is a very rare inborn error of immunity, the pathomechanism and clinical spectrum of which overlaps with TAP deficiencies.