Stéphane Chevrier

Th2 Lymphoproliferative Disorder of LatY136F Mutant Mice Unfolds Independently of TCR-MHC Engagement and Is Insensitive to the Action of Foxp3+ Regulatory T Cells

Mutant mice where tyrosine 136 of linker for activation of T cells (LAT) was replaced with a phenylalanine (LatY136F mice) develop a fast-onset lymphoproliferative disorder involving polyclonal CD4 T cells that produce massive amounts of Th2 cytokines and trigger severe inflammation and autoantibodies. We analyzed whether the LatY136F pathology constitutes a bona fide autoimmune disorder dependent on TCR specificity. Using adoptive transfer experiments, we demonstrated that the expansion and uncontrolled Th2-effector function of LatY136F CD4 cells are not triggered by an MHC class II-driven, autoreactive process. Using Foxp3EGFP reporter mice, we further showed that nonfunctional Foxp3+ regulatory T cells are present in LatY136F mice and that pathogenic LatY136F CD4 T cells were capable of escaping the control of infused wild-type Foxp3+ regulatory T cells. These results argue against a scenario where the LatY136F pathology is primarily due to a lack of functional Foxp3+ regulatory T cells and suggest that a defect intrinsic to LatY136F CD4 T cells leads to a state of TCR-independent hyperactivity. This abnormal status confers LatY136F CD4 T cells with the ability to trigger the production of Abs and of autoantibodies in a TCR-independent, quasi-mitogenic fashion. Therefore, despite the presence of autoantibodies causative of severe systemic disease, the pathological conditions observed in LatY136F mice unfold in an Ag-independent manner and thus do not qualify as a genuine autoimmune disorder.

B and T cells collaborate in antiviral responses via IL-6, IL-21, and transcriptional activator and coactivator, Oct2 and OBF-1

Transcriptional activator Oct2 and cofactor OBF-1 regulate B cell IL-6 to induce T cell production of IL-21, to support Tfh cell development in antiviral immunity.

CD93 is required for maintenance of antibody secretion and persistence of plasma cells in the bone marrow niche

Plasma cells represent the end stage of B-cell development and play a key role in providing an efficient antibody response, but they are also involved in numerous pathologies. Here we show that CD93, a receptor expressed during early B-cell development, is reinduced during plasma-cell differentiation. High CD93/CD138 expression was restricted to antibody-secreting cells both in T-dependent and T-independent responses as naive, memory, and germinal-center B cells remained CD93-negative. CD93 was expressed on (pre)plasmablasts/plasma cells, including long-lived plasma cells that showed decreased cell cycle activity, high levels of isotype-switched Ig secretion, and modification of the transcriptional network. T-independent and T-dependent stimuli led to re-expression of CD93 via 2 pathways, either before or after CD138 or Blimp-1 expression. Strikingly, while humoral immune responses initially proceeded normally, CD93-deficient mice were unable to maintain antibody secretion and bone-marrow plasma-cell numbers, demonstrating that CD93 is important for the maintenance of plasma cells in bone marrow niches.

Langerhans cells are generated by two distinct PU.1-dependent transcriptional networks

Langerhans cell homeostasis and differentiation depends on PU.1, the latter via regulation of TGF-β–dependent binding of PU.1 to the regulatory elements of RUNX3.

The BTB-ZF transcription factor Zbtb20 is driven by Irf4 to promote plasma cell differentiation and longevity

Zbtb20 facilitates terminal differentiation of B cells into antibody-secreting cells, and its expression is dependent on Irf4 and independent of Blimp1.

Germinal center-independent, IgM-mediated autoimmunity in sanroque mice lacking Obf1.

Mice homozygous for a point mutation in the Rc3h1 gene encoding Roquin1, designated sanroque mice, develop a severe antibody‐mediated autoimmune condition. The disease is T‐cell intrinsic, exacerbated by macrophage‐intrinsic defects and driven by excessive T follicular helper cell generation and spontaneous germinal centre (GC) formation. This culminates in abnormally high numbers of plasma cells secreting high‐affinity autoreactive immunoglobulin G (IgG). Obf1 is a transcriptional co‐activator required for normal T‐cell‐dependent antibody responses, and it is essential for GC formation under all circumstances so far tested. We crossed sanroque mice with Obf1‐null mice to determine whether the hyperactivity of sanroque T cells could drive Obf1−/− B cells to differentiate to GC B cells, or conversely, if Obf1 loss would prevent sanroque‐mediated autoimmune disease. Surprisingly, while sanroque/Obf1−/− mice did not form GC, they still developed autoimmune disease and succumbed even more rapidly than did sanroque mice. The disease was mediated by autoreactive IgM, which may have been derived from a pre‐existing population of autoreactive B cells in the Obf1−/− mice responding to the over‐exuberant activity of sanroque CD4 cells.

BTB-ZF transcription factors, a growing family of regulators of early and late B-cell development.

The differentiation of early B‐cell precursors in the bone marrow into the variety of mature and effector B‐cell subsets of the periphery is a complex process that requires tight regulation at the transcriptional level. Different members of the broad complex, tramtrack, bric‐à‐brac and zinc finger (BTB‐ZF) family of transcription factors have recently been shown to have key roles in many phases of B‐cell development, including early B‐cell development in the bone marrow, peripheral B‐cell maturation and specialization into effector cells during an immune response. This review highlights the critical functions mediated by BTB‐ZF transcription factors within the B‐cell lineage and emphasizes how the deregulation of these transcription factors can lead to B‐cell malignancies.

Oct2 and Obf1 as Facilitators of B:T Cell Collaboration during a Humoral Immune Response

The Oct2 protein, encoded by the Pou2f2 gene, was originally predicted to act as a DNA binding transcriptional activator of immunoglobulin (Ig) in B lineage cells. This prediction flowed from the earlier observation that an 8-bp sequence, the “octamer motif,” was a highly conserved component of most Ig gene promoters and enhancers, and evidence from over-expression and reporter assays confirmed Oct2-mediated, octamer-dependent gene expression. Complexity was added to the story when Oct1, an independently encoded protein, ubiquitously expressed from the Pou2f1 gene, was characterized and found to bind to the octamer motif with almost identical specificity, and later, when the co-activator Obf1 (OCA-B, Bob.1), encoded by the Pou2af1 gene, was cloned. Obf1 joins Oct2 (and Oct1) on the DNA of a subset of octamer motifs to enhance their transactivation strength. While these proteins variously carried the mantle of determinants of Ig gene expression in B cells for many years, such a role has not been borne out for them by characterization of mice lacking functional copies of the genes, either as single or as compound mutants. Instead, we and others have shown that Oct2 and Obf1 are required for B cells to mature fully in vivo, for B cells to respond to the T cell cytokines IL5 and IL4, and for B cells to produce IL6 normally during a T cell dependent immune response. We show here that Oct2 affects Syk gene expression, thus influencing B cell receptor signaling, and that Oct2 loss blocks Slamf1 expression in vivo as a result of incomplete B cell maturation. Upon IL4 signaling, Stat6 up-regulates Obf1, indirectly via Xbp1, to enable plasma cell differentiation. Thus, Oct2 and Obf1 enable B cells to respond normally to antigen receptor signals, to express surface receptors that mediate physical interaction with T cells, or to produce and respond to cytokines that are critical drivers of B cell and T cell differentiation during a humoral immune response.

Dominant Role of CD80–CD86 Over CD40 and ICOSL in the Massive Polyclonal B Cell Activation Mediated by LATY136F CD4+ T Cells

Coordinated interactions between T and B cells are crucial for inducing physiological B cell responses. Mutant mice in which tyrosine 136 of linker for activation of T cell (LAT) is replaced by a phenylalanine (LatY136F) exhibit a strong CD4+ T cell proliferation in the absence of intended immunization. The resulting effector T cells produce high amounts of TH2 cytokines and are extremely efficient at inducing polyclonal B cell activation. As a consequence, these LatY136F mutant mice showed massive germinal center formations and hypergammaglobulinemia. Here, we analyzed the involvement of different costimulators and their ligands in such T–B interactions both in vitro and in vivo, using blocking antibodies, knockout mice, and adoptive transfer experiments. Surprisingly, we showed in vitro that although B cell activation required contact with T cells, CD40, and inducible T cell costimulator molecule-ligand (ICOSL) signaling were not necessary for this process. These observations were further confirmed in vivo, where none of these molecules were required for the unfolding of the LAT CD4+ T cell expansion and the subsequent polyclonal B cell activation, although, the absence of CD40 led to a reduction of the follicular B cell response. These results indicate that the crucial functions played by CD40 and ICOSL in germinal center formation and isotype switching in physiological humoral responses are partly overcome in LatY136F mice. By comparison, the absence of CD80–CD86 was found to almost completely block the in vitro B cell activation mediated by LatY136F CD4+ T cells. The role of CD80–CD86 in T–B cooperation in vivo remained elusive due to the upstream implication of these costimulatory molecules in the expansion of LatY136F CD4+ T cells. Together, our data suggest that CD80 and CD86 costimulators play a key role in the polyclonal B cell activation mediated by LatY136F CD4+ T cells even though additional costimulatory molecules or cytokines are likely to be required in this process.

IL4 and IL21 cooperate to induce the high Bcl6 protein level required for germinal center formation

Bcl6 (B‐cell lymphoma 6) is a transcriptional repressor and critical mediator of the germinal center reaction during a T‐cell‐dependent antibody response, where it enables somatic hypermutation of immunoglobulin genes and inhibits terminal differentiation via repression of Blimp1. It can also contribute to the development of diffuse large B‐cell lymphoma when expressed inappropriately. Bcl6 regulation is mediated both at the transcriptional and post‐transcriptional levels, and in particular a strong signal through the B‐cell receptor causes rapid proteasomal degradation of Bcl6. Despite the importance of Bcl6 in both immunity and cancer, little is known about how other extrinsic factors regulate Bcl6 in B cells. Here we show that Bcl6 is indeed highly unstable in B cells after a B‐cell receptor (BCR) signal, but that the T‐cell‐derived cytokines interleukin 4 (IL4) and IL21 counteract BCR‐mediated degradation, preserving Bcl6 protein levels. Stat6, downstream of IL4, can induce Bcl6 transcription directly. In vivo, B‐cell intrinsic loss of IL4 or IL21 signaling reduces the magnitude or duration of the GC response, respectively, while their combined loss almost completely eliminates the GC response. This work provides key insights into the effect mediated by T‐follicular helper cytokines on Bcl6 regulation.