A unique mutation in the TACI conserved motif that binds MyD88 impaired CSR in a pediatric patients CVID patient

ALMEJÚN MARÍA BELÉN; COLS MONTSERRAT; OLEASTRO MATÍAS; ZELAZKO MARTA; CERUTTI ANDREA; OPPEZZO PABLO; CUNNINGHAM-RUNDLES CHARLOTTE; DANIELIAN SILVIA

México DF

Congreso; Second Meeting of Latin American Society for Immunodeficiency; 2011

LASID

Introduction: Common Variable Immunodeficiency (CVID) is an heterogeneous syndrome characterized by impaired immunoglobulin production. Mutations in TACI were previously found to be associated with CVID. Recently, it has been shown that BAFF and APRIL elicited Class Switch Recombination (CSR) by inducing recruitment of MyD88 to a highly conserved cytoplasmic domain of TACI (THC). This interaction triggered CSR via NF-(k)B activation. so far, no mutations were identified in the THC domain of TACI in CVID patients. we described a missense mutation (S231R) in the highly conserved segment of the THC domain of TACI at the MyD88 binding site in a pediatric patient with CVID (S231R-patient). This prompted us to evaluate its relevance through expression and functional studies. Methods: TACI protein expression and its ability to bind APRIL were analyzed by flow cytometry on b cells from the patient. Measures of SHM by Ig(k)REHMA assay and CSR triggered with different stimuli on B cells were also evaluated. The latter by using quantitative RT-PCR analysis of I(γ)1-C(γ)1, I(γ)1-C(µ) and AICDA and ELISA of secreted IgG in naive B cells or lymphoblastoid B cells obtained from S231R-patient. Localization of TACI and MyD88 with or without exposure to APRIL was analyzed by confocal microscopy. Results: at the time of diagnosis S231R-patient presented elevated IgM serum levels, in agreement with a previous report showing that the lack of MyD88 did not hamper the IgM secretion induced by TACI triggering. In contrast, we observed an impairment of IgG secretion by triggering CSR with TACI ligands or/and toll like receptors ligands on S231R-patient naïve B cells. Likewise, these stimuli induced less AICDA, I(γ)1-C(γ) and I(γ)1-C(µ). However, induction of these transcripts by CD40 stimulation was indistinguishable from controls. In addition, PBMC from this patient showed defective SHM by Ig(k)REHMA assay. While B cells from S231R-patient normally expressed TACI at their surface, an important reduction on the ability to bind APRIL was observed. Consistent with these results, stimulation of S231R-patient B cells with APRIL failed to induce polarized membrane patches of co-localized TACI, MyD88 and TRAF2 as shown in control cells. Conclusion: S231R-patient constitutes the first CVID case associated with a substitution affecting the TACI highly conserved domain at the MyD88 binding site, presenting an abnormal pattern of SHM with an impaired induction of CSR by TACI and toll-like receptor pathways.