J. Christoph Vahl

B cells lacking the tumor suppressor TNFAIP3/A20 display impaired differentiation and hyperactivation and cause inflammation and autoimmunity in aged mice.

The ubiquitin-editing enzyme A20/TNFAIP3 is essential for controlling signals inducing the activation of nuclear factor-κB transcription factors. Polymorphisms and mutations in the TNFAIP3 gene are linked to various human autoimmune conditions, and inactivation of A20 is a frequent event in human B-cell lymphomas characterized by constitutive nuclear factor-κB activity. Through B cell-specific ablation in the mouse, we show here that A20 is required for the normal differentiation of the marginal zone B and B1 cell subsets. However, loss of A20 in B cells lowers their activation threshold and enhances proliferation and survival in a gene-dose-dependent fashion. Through the expression of proinflammatory cytokines, most notably interleukin-6, A20-deficient B cells trigger a progressive inflammatory reaction in naive mice characterized by the expansion of myeloid cells, effector-type T cells, and regulatory T cells. This culminates in old mice in an autoimmune syndrome characterized by splenomegaly, plasma cell hyperplasia, and the presence of class-switched, tissue-specific autoantibodies.

Development of immunoglobulin λ-chain–positive B cells, but not editing of immunoglobulin κ-chain, depends on NF-κB signals

By genetically ablating IκB kinase (IKK)-mediated activation of the transcription factor NF-κB in the B cell lineage and by analyzing a mouse mutant in which immunoglobulin λ-chain–positive B cells are generated in the absence of rearrangements in the locus encoding immunoglobulin κ-chain, we define here two distinct, consecutive phases of early B cell development that differ in their dependence on IKK-mediated NF-κB signaling. During the first phase, in which NF-κB signaling is dispensable, predominantly κ-chain-positive B cells are generated, which undergo efficient receptor editing. In the second phase, predominantly λ-chain-positive B cells are generated whose development is ontogenetically timed to occur after rearrangements of the locus encoding κ-chain. This second phase of development is dependent on NF-κB signals, which can be substituted by transgenic expression of the prosurvival factor Bcl-2.

A20-Deficient Mast Cells Exacerbate Inflammatory Responses In Vivo

Mast cells, best known as effector cells in pathogenic immunoglobulin-mediated responses, can sense a variety of “danger” signals; if manipulated to enhance their resulting inflammatory responses, they also exacerbate inflammatory diseases such as arthritis and lung inflammation.

NKT Cell-TCR Expression Activates Conventional T Cells in Vivo, but Is Largely Dispensable for Mature NKT Cell Biology

Natural killer T (NKT) cell development depends on recognition of self-glycolipids via their semi-invariant Vα14i-TCR. However, to what extent TCR-mediated signals determine identity and function of mature NKT cells remains incompletely understood. To address this issue, we developed a mouse strain allowing conditional Vα14i-TCR expression from within the endogenous Tcrα locus. We demonstrate that naïve T cells are activated upon replacement of their endogenous TCR repertoire with Vα14i-restricted TCRs, but they do not differentiate into NKT cells. On the other hand, induced TCR ablation on mature NKT cells did not affect their lineage identity, homeostasis, or innate rapid cytokine secretion abilities. We therefore propose that peripheral NKT cells become unresponsive to and thus are independent of their autoreactive TCR.

CreER(T2) expression from within the c-Kit gene locus allows efficient inducible gene targeting in and ablation of mast cells.

Mast cells are abundantly situated at contact sites between the body and its environment, such as the skin and, especially during certain immune responses, at mucosal surfaces. They mediate allergic reactions and degrade toxins as well as venoms. However, their roles during innate and adaptive immune responses remain controversial and it is likely that major functions remain to be discovered. Recent developments in mast cell‐specific conditional gene targeting in the mouse promise to enhance our understanding of these fascinating cells. To complete the genetic toolbox to study mast cell development, homeostasis and function, it is imperative to inducibly manipulate their gene expression. Here, we report the generation of a novel knock‐in mouse line expressing a tamoxifen‐inducible version of the Cre recombinase from within the endogenous c‐Kit locus. We demonstrate highly efficient and specific inducible expression of a fluorescent reporter protein in mast cells both in vivo and in vitro. Furthermore, induction of diphtheria toxin A expression allowed selective and efficient ablation of mast cells at various anatomical locations, while other hematopoietic cells remain unaffected. This novel mouse strain will hence be very valuable to study mast cell homeostasis and how specific genes influence their functions in physiology and pathology.

Roquin Paralogs Differentially Regulate Functional NKT Cell Subsets

NKT cells represent a small subset of glycolipid-recognizing T cells that are heavily implicated in human allergic, autoimmune, and malignant diseases. In the thymus, precursor cells recognize self-glycolipids by virtue of their semi-invariant TCR, which triggers NKT cell lineage commitment and maturation. During their development, NKT cells are polarized into the NKT1, NKT2, and NKT17 subsets, defined through their cytokine-secretion patterns and the expression of key transcription factors. However, we have largely ignored how the differentiation into the NKT cell subsets is regulated. In this article, we describe the mRNA-binding Roquin-1 and -2 proteins as central regulators of murine NKT cell fate decisions. In the thymus, T cell–specific ablation of the Roquin paralogs leads to a dramatic expansion of NKT17 cells, whereas peripheral mature NKT cells are essentially absent. Roquin-1/2–deficient NKT17 cells show exaggerated lineage-specific expression of nearly all NKT17-defining proteins tested. We show through mixed bone marrow chimera experiments that NKT17 polarization is mediated through cell-intrinsic mechanisms early during NKT cell development. In contrast, the loss of peripheral NKT cells is due to cell-extrinsic factors. Surprisingly, Roquin paralog–deficient NKT cells are, in striking contrast to conventional T cells, compromised in their ability to secrete cytokines. Altogether, we show that Roquin paralogs regulate the development and function of NKT cell subsets in the thymus and periphery.

Igλ+ B cell development but not Igκ editing depends on NF-κB signals

By genetically ablating IκB kinase (IKK)-mediated activation of the transcription factor NF-κB in the B cell lineage and by analyzing a mouse mutant in which immunoglobulin λ-chain–positive B cells are generated in the absence of rearrangements in the locus encoding immunoglobulin κ-chain, we define here two distinct, consecutive phases of early B cell development that differ in their dependence on IKK-mediated NF-κB signaling. During the first phase, in which NF-κB signaling is dispensable, predominantly κ-chain-positive B cells are generated, which undergo efficient receptor editing. In the second phase, predominantly λ-chain-positive B cells are generated whose development is ontogenetically timed to occur after rearrangements of the locus encoding κ-chain. This second phase of development is dependent on NF-κB signals, which can be substituted by transgenic expression of the prosurvival factor Bcl-2.

Development of immunoglobulin |[lambda]|-chain|[ndash]|positive B cells, but not editing of immunoglobulin |[kappa]|-chain, depends on NF-|[kappa]|B signals

By genetically ablating IκB kinase (IKK)-mediated activation of the transcription factor NF-κB in the B cell lineage and by analyzing a mouse mutant in which immunoglobulin λ-chain–positive B cells are generated in the absence of rearrangements in the locus encoding immunoglobulin κ-chain, we define here two distinct, consecutive phases of early B cell development that differ in their dependence on IKK-mediated NF-κB signaling. During the first phase, in which NF-κB signaling is dispensable, predominantly κ-chain-positive B cells are generated, which undergo efficient receptor editing. In the second phase, predominantly λ-chain-positive B cells are generated whose development is ontogenetically timed to occur after rearrangements of the locus encoding κ-chain. This second phase of development is dependent on NF-κB signals, which can be substituted by transgenic expression of the prosurvival factor Bcl-2.