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Updated: Jun 10, 2026
Murine Model of CD40-activation of B cells
Published on: March 5, 2010
Different molecular behavior of CD40 mutants causing hyper-IgM syndrome
Gaetana Lanzi1, Simona Ferrari, Mauno Vihinen
1A. Nocivelli Institute for Molecular Medicine and Pediatric Clinic, University of Brescia, Brescia, Italy.
Insights
Genetic defects in CD40 protein cause hyper-immunoglobulin M (IgM) syndrome. Mutated CD40 proteins are retained in the endoplasmic reticulum, leading to ER stress and disease.
Area of Science:
- Immunology
- Molecular Biology
- Cell Biology
Background:
- CD40/CD40 ligand (CD40L) interactions are crucial for B-cell maturation.
- Genetic defects in CD40 cause hyper-immunoglobulin M (HIGM) syndrome, characterized by recurrent infections and low IgG/IgA.
- Previous studies identified patients with homozygous CD40 mutations leading to absent or reduced cell surface CD40.
Purpose of the Study:
- To characterize three specific CD40 mutants resulting from missense mutations or small in-frame deletions.
- To investigate the intracellular behavior and fate of these mutated CD40 proteins.
- To gain insights into the molecular pathogenesis of HIGM disease.
Main Methods:
- Characterization of three CD40 mutants (P2, P4, P5) with missense mutations or in-frame deletions.
- Analysis of protein synthesis, intracellular retention in the endoplasmic reticulum (ER), and protein misfolding.
- Assessment of ER stress, unfolded protein response (UPR) activation, ER-associated degradation (ERAD) pathway, and plasma membrane transport.
- Evaluation of CD40L binding and downstream signaling activation in transfected cells.
Main Results:
- Mutated CD40 proteins are synthesized but retained in the ER due to misfolding.
- Mutant P2 causes ER stress and UPR activation due to progressive accumulation.
- Mutant P4 is degraded via the ERAD pathway, while mutant P5 partially reaches the plasma membrane and binds CD40L.
- Overexpression of P5 activates downstream signaling pathways.
Conclusions:
- CD40 deficiency in HIGM disease can be viewed as an ER-storage disease.
- Different CD40 mutants exhibit distinct intracellular fates and consequences.
- These findings provide crucial insights into the molecular mechanisms underlying HIGM syndrome.
Abstract:
CD40/CD40 ligand (CD40L) cross-talk plays a key role in B-cell terminal maturation in the germinal centers. Genetic defects affecting CD40 cause a rare form of hyper-immunoglobulin M (IgM) syndrome, a disorder characterized by low or absent serum IgG and IgA, associated with recurrent infections. We previously reported on a few patients with homozygous CD40 mutations resulting in lack or severe reduction of CD40 cell surface expression. Here we characterize the 3 CD40 mutants due to missense mutations or small in-frame deletions, and show that the mutated proteins are synthesized but retained in the endoplasmic reticulum (ER), likely due to protein misfolding. Interestingly, the intracellular behavior and fate differ significantly among the mutants: progressive accumulation of the P2 mutant causes endoplasmic reticulum stress and the activation of an unfolded protein response; the mutant P4 is rather efficiently disposed by the ER-associated degradation pathway, while the P5 mutant partially negotiates transport to the plasma membrane, and is competent for CD40L binding. Interestingly, this latter mutant activates downstream signaling elements when overexpressed in transfected cells. These results give new important insights into the molecular pathogenesis of HIGM disease, and suggest that CD40 deficiency can also be regarded as an ER-storage disease.
