Inherited CD19 deficiency impairs signaling in plasma cells

Abstract Background: The human CD19 antigen is expressed throughout B cell ontogeny with the exception of neoplastic plasma cells and a subset of normal plasma cells. CD19 plays a role in propagating signals from the B cell receptor and other receptors such as CXCR4 in mature B cells. Studies of CD19-deficient patients have confirmed its function during the initial stages of B cell activation, however its role in the later stages of B cell differentiation is unclear. Objective: Using B cells from a newly identified CD19-deficient individual, we investigated the role of CD19 in the generation and function of plasma cells using an in vitro differentiation model.Methods: Flow cytometry and long-read nanopore sequencing using locus-specific long-range amplification products were used to screen a patient with suspected primary immunodeficiency. Purified B cells from the patient and healthy controls were activated with CD40L, IL-21, IL-2 and anti-Ig, then transferred to different cytokine conditions to induce plasma cell differentiation. Subsequently, the cells were stimulated with CXCL12 to induce signalling through CXCR4. Phosphorylation of key downstream proteins including ERK and AKT was assessed by Western blotting.Results: Long-read nanopore sequencing identified the homozygous pathogenic mutation c.622del (p.Ser208Profs*19) which was corroborated by the lack of CD19 cell surface staining. CD19-deficient B cells generate phenotypically normal plasma cells with normal levels of CXCR4, however they have an altered response to CXCL12 resulting in a loss of ERK signalling. Additionally, CD19 ligation on normal plasma cells results in AKT phosphorylation.Conclusion: CD19 is not required for generation of antibody secreting cells but may alter responses of these populations to CXCL12 and direct CD19 ligands, potentially affecting localisation, proliferation, or survival.