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

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

Windsor

Taller; UCL Winter School 2012, Advances in Primary Immunodeficiency; 2012

UCL Institute of Child Health

Introduction: Common variable immunodeficiency (CVID) is a 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. This prompted us to evaluate its relevance through expression and functional studies. 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(μ) and I(γ)1-C(μ) measured by quantitative RT-PCR. 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. Screening of the family unaffected members of the patient revealed that the healthy mother carried the mutation. Incomplete penetrance and familial segregation were previously observed for heterozygous TACI variants. However, the activation of AICDA and I(γ)1-C(μ) transcripts after stimulation of lymphoblastoid B cells with APRIL and cytokines were hampered in both, the patient and her mother. This shows that while we observed an incomplete clinical penetrance, a complete penetrance was observed in TACI-induced CSR pathway in B cells. We also analyzed the cooperation between TACI and CD40 by stimulation of patient's lymphoblastoid B cells with APRIL and a suboptimal concentration of a mAb against CD40. The results showed that after CD40 stimulation, APRIL caused a significant increase of AID transcripts in B cells from controls but not in the patient nor in her mother, showing that the cooperation between these pathways was altered by the mutation in the THC domain of TACI protein. Finally, 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, when analyzed by confocal microscopy. Conclusion S231R-patient constitutes the first CVID case associated with a substitution (S231R) affecting the TACI Highly Conserved domain at the MyD88 binding site. This patient showed an abnormal pattern of SHM and CSR in vivo as well as an impaired CSR triggered by TLR-ligand and/or TACI-ligand in patient's B cells. These cells also showed a lack of APRIL-mediated enhancement of CD40 activation. Finally, TACI mutated at S231R failed to co-localize with MyD88 and TRAF2 after APRIL ligation. All of these results from patient's B cells were also reproduced by the healthy mother's B cells. Therefore, while this mutation showed a complete cellular penetrance, another not yet defined event could account for the onset of clinical phenotype in TACI-CVID patients.