Emanuela Castigli

TACI is mutant in common variable immunodeficiency and IgA deficiency

The tumor necrosis factor receptor family member TACI (transmembrane activator and calcium-modulator and cyclophilin ligand interactor) mediates isotype switching in B cells. We found that 4 of 19 unrelated individuals with common variable immunodeficiency (CVID) and 1 of 16 individuals with IgA deficiency (IgAD) had a missense mutation in one allele of TNFRSF13B (encoding TACI). One of the four individuals with CVID had a single nucleotide insertion in the other TNFRSF13B allele. None of these mutations were present in 50 healthy subjects. TNFRSF13B mutations cosegregated with the phenotype of CVID or IgAD in family members of four index individuals that we studied. B cells from individuals with TACI mutations expressed TACI but did not produce IgG and IgA in response to the TACI ligand APRIL, probably reflecting impaired isotype switching. These results suggest that TACI mutations can result in CVID and IgAD.

TACI and BAFF-R mediate isotype switching in B cells

The tumor necrosis factor family members BAFF and APRIL induce Ig isotype switching in human B cells. We analyzed the ability of BAFF and APRIL to induce isotype switching in murine B cells to IgG1, IgA, and IgE. APRIL and BAFF each engage two receptors, transmembrane activator and calcium-modulator and cytophilin ligand interactor (TACI) and B cell maturation antigen (BCMA), on B cells. In addition, BAFF engages a third receptor on B cells, BAFF-R. To determine the role of these receptors in isotype switching, we examined B cells from mice deficient in TACI, BCMA, and BAFF-R. The results obtained indicate that both TACI and BAFF-R are able to transduce signals that result in isotype switching.

Cutting Edge: The Dependence of Plasma Cells and Independence of Memory B Cells on BAFF and APRIL

Memory B (BMEM) cells and long-lived bone marrow plasma cells (BM-PCs) persist within local environmental survival niches that afford cellular longevity. However, the factors supporting BMEM cell survival within the secondary lymphoid organs and allowing BM-PC persistence in the bone marrow remain poorly characterized. We report herein that long-lived BMEM cell survival and function are completely independent of BAFF (B cell-activating factor of the TNF family) or APRIL (a proliferation-inducing ligand). Thus, BMEM cells represent the only mature B2 lineage subset whose survival is independent of these ligands. We have previously shown that the TNFR family member receptor BCMA (B cell maturation Ag) is a critical survival receptor for BM-PC survival in vivo. We identify in this study the ligands critical for BM-PC survival and show that either BAFF or APRIL supports the survival of BM-PCs in vivo. These data define the BAFF/APRIL-dependent and -independent components of long-lived humoral immunity.

The Binding Site for TRAF2 and TRAF3 but Not for TRAF6 Is Essential for CD40-Mediated Immunoglobulin Class Switching

To define the role of TRAF proteins in CD40-dependent isotype switching in B cells, we introduced wild-type (WT) and mutant CD40 transgenes that lacked the binding motifs for TRAF6 (CD40deltaTRAF6), TRAF2 and TRAF3 (CD40deltaTRAF2/3), or both (CD40deltaTRAFs) into B cells of CD40(-/-) mice. The in vivo isotype switch defect in CD40(-/-) mice was fully corrected by WT and CD40deltaTRAF6, partially by CD40deltaTRAF2/3, and not at all by CD40deltaTRAFs transgenes. CD40-mediated isotype switching, proliferation, and activation of p38, JNK, and NFkappaB in B cells were normal in WT and CD40deltaTRAF6 mice, severely impaired in CD40deltaTRAF2/3, and absent in CD40deltaTRAFs mice. These results suggest that binding to TRAF2 and/or TRAF3 but not TRAF6 is essential for CD40 isotype switching and activation in B cells.

Cd40 ligand/cd40 deficiency

CD40 is a surface antigen expressed on B cells. The CD40 ligand (CD40L) is expressed on activated T cells. Interaction between CD40 and CD40L is critical for proliferation and isotype switching in the context of a response to a T-cell-dependent antigen. Patients with X-linked hyper-IgM syndrome (HIGMX-1) in their CD40L gene are unable to switch from IgM to IgG, IgA and IgE. Mice with a disrupted CD40 gene fail to undergo isotype switching to T-cell-dependent antigens but respond normally to T-independent antigens.

CD40-CD40 ligand (CD40L) interactions and X-linked hyperIgM syndrome (HIGMX-1).

Interactions between the B cell surface antigen CD40 and its ligand (CD40L) expressed on activated T cells play a critical role in isotype switching. This is illustrated by failure of isotype switching in patients with X-linked hyperIgM syndrome in whom the CD40L gene is mutated and by failure of isotype switching of CD40-deficient mice in response to T-cell-dependent antigens. We review these findings and discuss the signaling mechanisms of CD40 and the developmental control and transcriptional regulation of CD40L expression.

Defective expression of early activation genes in cartilage-hair hypoplasia (CHH) with severe combined immunodeficiency (SCID)

Cartilage‐hair hypoplasia (CHH) is an autosomal recessive disease of unknown etiology characterized by metaphyseal dysostosis, unpigmented hair, and defective cellular immunity. We studied peripheral blood mononuclear cells (PBMC) of a boy with CHH and combined immunodeficiency in an attempt to characterize further the immune defect in this disease. Stimulation of his PBMC with mitogens was associated with severely depressed IL‐2 and interferon‐gamma (IFN‐γ) synthesis and lL‐2 receptor a‐chain (IL‐2Rα) expression and resulted in poor lymphocyte proliferation that was only modestly upregulated by the addition of recombinant IL‐2 (rIL‐2). The defective proliferation and lymphokine synthesis were not corrected by the addition of phorbol myristate acetate (PMA) and ionomycin, agents that bypass receptor‐mediated signalling, indicative of a distal abnormality. Importantly, the levels of mRNA encoding c‐myc, IL‐2Rα, IL‐2 and IFN‐γ were markedly decreased in patient lymphocytes stimulated with PMA + ionomycin as compared to control lymphocytes The defect in the expression of these early activation genes was selective in that induction by mitogens of mRNA encoding other early activation gene products such as c‐ fos and c‐jun was not impaired. These results suggest that the underlying defect in this patient and perhaps others with CHH may be an abnormality in a component of intracellular signalling pathways or in a trans‐acting factor which regulates the expression of a selected number of early activation genes.

Severe Combined Immunodeficiency with Selective T-Cell Cytokine Genes

ABSTRACT: A 4-y-old female with severe combined immunodeficiency disease had normal numbers of T cells in her circulation and normal T-cell subsets. However, her T cells proliferated poorly to mitogens and did not proliferate to antigens or to anti-CD3 MAb. IL-2 receptor expression was normal, but IL-2 synthesis was undetectable. The addition of recombinant IL-2 to a mitogen-stimulated culture resulted in normalization of the proliferative response. Northern blot analysis of total RNA derived from the patients T cells revealed a weak or absent expression of mRNA coding for IL-2, IL-3, IL-4, and IL-5. In contrast, there were normal amounts of mRNA coding for granulocyte-macrophage colony-stimulating factor. Tumor necrosis factor and IL-6 production were also normal. Nuclear run-on transcriptional assays revealed markedly decreased levels of newly initiated nuclear transcripts coding for IL-2, IL-3, IL-4, and IL-5 and normal levels of granulocytemacrophage colony-stimulating factor transcripts in the patient relative to control lymphocytes. Gel retardation assays suggest that the NFAT-1 nuclear transcription complex is abnormal in this patient. These results indicate that the patient suffers from a defect that affects the transcription of multiple T-cell lymphokines and suggest that abnormalities affecting the production of T-cell lymphokines may underlie some of the primary immunodeficiency diseases.

Novel immune deficiencies: Defective transcription of lymphokine genes

A 4-year-old female with severe combined immunodeficiency (SCID) had normal numbers of T cells in circulation and normal T cell subsets. However, her T cells proliferated poorly to mitogens and did not proliferate to antigens or to anti-CD3 mAb. Interleukin-2 (IL-2) receptor expression was normal but IL-2 synthesis was undetectable. The addition of recombinant IL-2 to a mitogen-stimulated culture resulted in normalization of the proliferative response. Northern blot analysis of total RNA derived from the patients T cells revealed a weak or absent expression of mRNA coding for IL-2, IL-3, IL-4, and IL-5. In contrast, there were normal amounts of mRNA coding for granulocyte-macrophage colony-stimulating factor (GM-CSF). Tumor necrosis factor and IL-6 production was also normal. Nuclear run on transcriptional assays revealed markedly decreased levels of newly initiated nuclear transcripts coding for IL-2, IL-3, IL-4, and IL-5 and normal levels of GM-CSF transcripts in patient relative to control lymphocytes. These results indicate that the patients T cells suffered from a defect affecting the transcription of multiple T cell lymphokines and suggest that abnormalities affecting the production of T cell lymphokines may underlie some of the primary immunodeficiency diseases.

Novel immune deficiencies: defective transcription of lymphokine genes

A 4-year-old female with severe combined immunodeficiency (SCID) had normal numbers of T cells in circulation and normal T cell subsets. However, her T cells proliferated poorly to mitogens and did not proliferate to antigens or to anti-CD3 mAb. Interleukin-2 (IL-2) receptor expression was normal but IL-2 synthesis was undetectable. The addition of recombinant IL-2 to a mitogen-stimulated culture resulted in normalization of the proliferative response. Northern blot analysis of total RNA derived from the patients T cells revealed a weak or absent expression of mRNA coding for IL-2, IL-3, IL-4, and IL-5. In contrast, there were normal amounts of mRNA coding for granulocyte-macrophage colony-stimulating factor (GM-CSF). Tumor necrosis factor and IL-6 production was also normal. Nuclear run on transcriptional assays revealed markedly decreased levels of newly initiated nuclear transcripts coding for IL-2, IL-3, IL-4, and IL-5 and normal levels of GM-CSF transcripts in patient relative to control lymphocytes. These results indicate that the patients T cells suffered from a defect affecting the transcription of multiple T cell lymphokines and suggest that abnormalities affecting the production of T cell lymphokines may underlie some of the primary immunodeficiency diseases.