Hypomorphic RAG2 Deficiency Promotes Selection of Self-Reactive B Cells

Area of Science:

• Immunology
• Genetics

Background:

• Reduced function variants in recombination-activating genes (RAG) 1 or 2 cause a spectrum of immunodeficiencies, including common variable immunodeficiency (CVID).
• Milder variants of RAG deficiency are less understood, particularly their long-term clinical course and B cell repertoire alterations.

Purpose of the Study:

• To longitudinally characterize the clinical and immunological phenotype of a milder, combined RAG deficiency in siblings with identical RAG2 mutations over 50 years.
• To investigate the impact of hypomorphic RAG deficiency on T and B cell populations, B cell receptor repertoire, and the potential for autoimmunity.

Main Methods:

• Whole-genome sequencing and repertoire sequencing were employed.
• Bacteriophage immunizations and deep immunophenotyping were performed to compare affected and unaffected family members.
• Functional assays assessed RAG2 variant effects and alterations in the B cell receptor repertoire and immunophenotype over time.

Main Results:

• Three siblings with identical hypomorphic RAG2 mutations exhibited a broad clinical spectrum, from combined immunodeficiency with early mortality to late-onset CID with a hyper-IgM phenotype.
• T cell populations were similar between affected siblings, with no observed CDR3 skewing or regulatory T cell defects.
• B cell analysis revealed elevated unswitched CD27+ and CD21low cells, a self-reactive antibody repertoire, and expanded polyclonal marginal zone-like B cells utilizing the VH4-34 receptor, without clinical autoimmunity.

Conclusions:

• Identical hypomorphic RAG deficiency can lead to divergent clinical phenotypes, influenced by factors such as the expansion of IgM+ memory B cells.
• Hypomorphic RAG deficiency promotes the expansion of self-reactive B cells, but this alone is insufficient to induce clinical autoimmunity.
• This study highlights the complex interplay between genetic mutations, B cell repertoire selection, and secondary triggers in shaping the immunophenotype and clinical outcome of RAG deficiency.