RAG gene defects at the verge of immunodeficiency and immune dysregulation

Abstract

Mutations of the recombinase activating genes (RAG) in humans underlie a broad spectrum of clinical and immunological phenotypes that reflect different degrees of impairment of T- and B-cell development and alterations of mechanisms of central and peripheral tolerance. Recent studies have shown that this phenotypic heterogeneity correlates, albeit imperfectly, with different levels of recombination activity of the mutant RAG proteins. Furthermore, studies in patients and in newly developed animal models carrying hypomorphic RAG mutations have disclosed various mechanisms underlying immune dysregulation in this condition. Careful annotation of clinical outcome and immune reconstitution in RAG-deficient patients who have received hematopoietic stem cell transplantation has shown that progress has been made in the treatment of this disease, but new approaches remain to be tested to improve stem cell engraftment and durable immune reconstitution. Finally, initial attempts have been made to treat RAG deficiency with gene therapy.

Keywords: Omenn syndrome; autoimmunity; immunodeficiency; immunological tolerance; recombinase activating genes.

Figure 1.. Schematic representation of positive and negative selection in the thymus.

Five steps of T cell maturation and thymocyte selection are shown, which can be affected in patients and animals with hypomorphic RAG mutations. 1) Cortical thymic epithelial cells (cTEC) expressing MHC-peptide ligands mediate positive selection of developing thymocytes. 2) Normal thymic architecture is characterized by demarcation of cortical and medullary areas, with clear evidence of the cortico-medullary junction (shown as dashed dine) where thymic dendritic cells (tDCs) are mainly distributed. Positively selected CD4⁺ αβ⁺ and CD8 ⁺αβ⁺ T cells migrate from the cortex to the medulla, where 3) mature medullary thymic epithelial cells (mTECs) express tissue restricted antigens (TRA) under the control of the transcriptional activator AIRE. CD4⁺ thymocytes recognizing with high affinity TRAs in the complex with MHC class II molecules on the surface of mTECs or 4) cross-presented by tDCs undergo 5) negative selection or diversion to become FOXP3⁺ natural regulatory T cells. Hypomorphic RAG mutations may interfere with each of these steps as indicated on the right.

Figure 2.. Schematic representation of B cell tolerance checkpoints.

Left panel: The majority of newly generated B cells in bone marrow express self-reactive antibodies including polyreactive specificities. At the first checkpoint, counterselection of highly polyreactive and self-reactive antibodies positive B cells relies on three mechanisms: 1) receptor editing, which involves RAG re-expression and depends on BCR cross-linking by self-antigens; 2) deletion of cells recognizing with high affinity self-antigens; and, 3) anergy, by which B cells become unresponsive to antigen. BCR synergizes with TLR signaling for the selection of autoreactive clones. Right panel: Despite these mechanisms, some self-reactive immature B cells are released from the bone marrow and reach the periphery where they undergo a second checkpoint before maturing into naïve B cells. The number of self-reactive B cells, dramatically drops during this transition., as shown by the reduction in the proportion of B cells expressing immunoglobulins that recognize Hep-2 cells. Both T_reg cells and levels of BAFF are involved in the maintenance of peripheral tolerance. In particular, high levels of BAFF are present in B cell lymphopenic hosts, and promote the survival of transitional B cells, including those expressing self-reactive specificities. Hypomorphic RAG mutations may interfere with both central and peripheral B cell tolerance as indicated on the right.