Functional analysis of transmembrane activator and calcium-modulating cyclophilin ligand interactor (TACI) mutations associated with common variable immunodeficiency

Abstract

Some, but not all, TACI mutations found in CVID impair TACI function and may potentially contribute to B cell dysfunction in heterozygotes via haploinsufficiency. Keywords TACI; common variable immunodeficiency; APRIL; BAFF; BLyS To the EditorMutations in TNFRSF13B, the gene encoding for the transmembrane activator, calciummodulator and cyclophilin ligand interactor (TACI), have been identified in 5-10% of patients with CVID 1, 2 . The most common TACI variants in CVID are C104R, in the ligand binding cysteine-rich domain 2 (CRD2), and A181E, in the transmembrane (TM) domain of TACI. Each is found in the heterozygous state in 2-5% of CVID patients, but also in 0.5-1% of healthy individuals [1][2][3] . Both destroy the capacity of TACI ligation to activate NFκB. We have examined the function of TACI coding variants that have been described in CVID patients and their relatives, but so far not in healthy individuals [3][4][5] . Figure 1A shows the location of the missense mutations examined in the TACI protein. The CRD1 mutants W40R and D41H, the CRD2 mutant I87N, the TM domain mutants C172Yand A181E, and the IC domain mutants K188M and V246F were expressed on the surface of 293T cell transfectants at levels comparable to WT TACI ( Figure 1B). Surface expression of the CRD2 mutants Y79C and C104R was modestly decreased to ~ half of WT, while that of the L171R mutant was reduced ~8 fold. Protein expression of the L171R mutant in whole cell lysates was similarly reduced, but its mRNA expression was comparable to that of WT (data not shown), suggesting that the mutant protein was unstable. Both of the CRD1 domain mutants, W40R and D41H, showed normal binding of BAFF (Fig. 1C). There was total loss of BAFF binding by the Y79C and C104R mutants. The I87N mutant had residual BAFF binding, which increased with increasing concentrations of ligand (Fig. S1). The poorly expressed L171R mutant had negligible ligand binding. The TM domain mutants, C172Y and A181E and the IC domain mutants K188M and V246F had intact BAFF binding.Address correspondence and reprint requests to: Raif S. Geha, Karp 10, Division of Immunology, Children's Hospital, 300 Longwood Avenue, Boston, Massachusetts 02115, USA. Phone: (617) 919 2482, Fax: (617) 730 0528, raif.geha@childrens.harvard.edu. Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. TACI signaling was examined by measuring NFκB and NF-AT activation using a dual luciferase reporter system 6 . Mutant or WT TACI were transfected into 293T cells with either NFκB luciferase or NF-AT luciferase reporter plasmids together with Renilla luciferase, and the cap...

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“…The I87N mutation is located in the second CRD and is shown to result in residual BAFF binding, which increased with increasing concentrations of ligand, suggesting a decreased ligand-binding affinity of this mutant. 22 Signaling through TACI requires a trimerization of the receptor. [23][24][25] It is thus possible that inclusion of even one mutant receptor might affect the signaling by the complex.…”

TACI deficiency enhances antibody avidity and clearance of an intestinal pathogen

“…Yet, the most dramatic difference between CVID and other typical primary immunodeficiencies concerns the penetrance of disease associated with allelic variants of TACI and other receptors. In contrast to full penetrance of the null mutations BTK or CD40, which cause X-linked agammaglobulinemia, or hyper-IgM syndrome, respectively, allelic variants of TACI, such as C104R and A181E, which impair TACI function even in the heterozygous state (16), usually exhibit no clinically significant defects in immunity (17). Thus, Salzer et al (18) found heterozygous C104R or A181E TACI variants in 6.9% of subjects with CVID but also in 2% of 675 normal adults.…”

Synergy in B-Cell Activation between Toll-Like Receptor 9 and Transmembrane Activator and Calcium-Modulating Cyclophilin Ligand Interactor (TACI) in A181E/C104R Compound Heterozygous Siblings

“…Neither variant interfered with oligomerization with wildtype TACI molecules nor NF B activation was unaffected in 293T cells with wildtype and mutant TACI [26]. Dominant-negative interference of the variants with wildtype TACI is therefore improbable, but haploinsufficiency was not completely excluded.…”

“…Initially, the deficiency was attributed to a dominant-negative interference of the C104R polymorphism in trimeric complexes with TACI wildtype [21]. Subsequent reports; however, suggest haploinsufficiency of the TACI allele [25][26][27].…”