Antibody Secretion Capacity in CVID Patients: Immunoglobulin Isotypes and Antigen Specificities After T-Cell-Dependent In Vitro Stimulation

<b>Background:</b> Common variable immunodeficiency (CVID), the most prevalent symptomatic inborn error of immunity, involves impaired B-cell differentiation and antibody production, causing recurrent infections and the need for life-long immunoglobulin replacement therapy. <b>Methods:</b> This study evaluated the <i>in vitro</i> differentiation of memory B-cells (MBCs) into antibody-secreting cells (ASCs) in CVID patients. Peripheral blood mononuclear cells from 13 CVID patients and 10 healthy controls were stimulated using two protocols: (I) Staphylococcus aureus Cowan Strain I, Pokeweed mitogen, and CpG, or (II) a T-cell-dependent approach using CD40 ligand, interleukin-21, and CpG. B-cell subpopulations were analyzed by flow cytometry, ASC differentiation using ELISpot, and antibody levels in supernatants by ELISA. <b>Results:</b> Despite severely restricted <i>in vivo</i> antibody production, MBCs from all 13 CVID patients differentiated into IgG and IgM ASCs under adequate <i>in vitro</i> stimulation. Protocol II, mimicking T-cell help, was more effective than protocol I. As expected, the patients exhibited reduced class-switched MBCs <i>ex vivo</i>, but the MBCs differentiated and proliferated to an extent similar to those in healthy controls. IgA secretion remained significantly impaired post-stimulation. Specific IgG antibodies against tetanus toxoid and <i>Streptococcus pneumoniae</i> were detected in the patient supernatants, while no double-stranded DNA autoantibodies emerged after <i>in vitro</i> stimulation. <b>Conclusions:</b> These findings indicate that the MBCs of most patients retain functional B-cell differentiation and antigen-specific IgG secretion under T-cell dependent stimulation, though IgA secretion remains impaired. Tailored stimulation protocols could deepen our understanding of how to restore MBC formation in CVID patients <i>in vivo</i>. This methodology provides a platform to investigate antigen-specific functional memory responses like post-vaccination.