Randolph J. Noelle

The CD40 ligand, gp39, is defective in activated T cells from patients with X-linked hyper-IgM syndrome.

The prominent role of the CD40 receptor in B cell responses led us to investigate the role of the gp39-CD40 interaction in a group of primary immunodeficient patients with defective antibody production. Here we report that patients with hyper-IgM syndrome (HIM) have a defective gp39-CD40 interaction. B cells from HIM patients express functional CD40, but their T cells do not bind CD40-Ig. These patients expressed normal levels of gp39 mRNA, but these mRNAs encode defective gp39 proteins owing to mutations in the extracellular domain of gp39. Soluble recombinant forms of gp39 containing these mutations were unable to bind CD40 and drive normal B cell proliferation. The gene encoding gp39 was mapped to Xq26, the X chromosome region where the gene responsible for HIM had previously been mapped. These data suggest that a defect in gp39 is the basis of X-linked HIM.

All-trans retinoic acid mediates enhanced T reg cell growth, differentiation, and gut homing in the face of high levels of co-stimulation

We demonstrate that all-trans retinoic acid (RA) induces FoxP3+ adaptive T regulatory cells (A-Tregs) to acquire a gut-homing phenotype (α4β7+ CC chemokine receptor 9+) and the capacity to home to the lamina propria of the small intestine. Under conditions that favor the differentiation of A-Tregs (transforming growth factor–β1 and interleukin 2) in vitro, the inclusion of RA induces nearly all activated CD4+ T cells to express FoxP3 and greatly increases the accumulation of these cells. In the absence of RA, A-Treg differentiation is abruptly impaired by proficient antigen presenting cells or through direct co-stimulation. In the presence of RA, A-Treg generation occurs even in the presence of high levels of co-stimulation, with RA attenuating co-stimulation from interfering from FoxP3 induction. The recognition that RA induces gut imprinting, together with our finding that it enhances A-Treg conversion, differentiation, and expansion, indicates that RA production in vivo may drive both the imprinting and A-Treg development in the face of overt inflammation.

Cutting Edge: Contact-Mediated Suppression by CD4+CD25+ Regulatory Cells Involves a Granzyme B-Dependent, Perforin-Independent Mechanism

CD4+CD25+ regulatory T cells (Treg) are potent immunosuppressive cells that are pivotal in the regulation of peripheral tolerance. In this report, we identify granzyme B (GZ-B) as one of the key components of Treg-mediated suppression. Induction of regulatory activity is correlated with the up-regulation of GZ-B expression. Proof of a functional involvement of GZ-B in contact-mediated suppression by Treg is shown by the reduced ability of Treg from GZ-B−/− mice to suppress as efficiently as Treg from WT mice. GZ-B-mediated suppression is perforin independent, because suppression by Treg from perforin−/− and WT is indistinguishable. Additionally, suppression mediated by Treg appears to be mediated, in part, by the induction of apoptosis in the CD4+CD25− effector cell. In summary, GZ-B is one of the key mechanisms through which CD4+CD25+ Treg induce cell contact-mediated suppression.

BCMA Is Essential for the Survival of Long-lived Bone Marrow Plasma Cells

Long-lived humoral immunity is manifested by the ability of bone marrow plasma cells (PCs) to survive for extended periods of time. Recent studies have underscored the importance of BLyS and APRIL as factors that can support the survival of B lineage lymphocytes. We show that BLyS can sustain PC survival in vitro, and this survival can be further enhanced by interleukin 6. Selective up-regulation of Mcl-1 in PCs by BLyS suggests that this α-apoptotic gene product may play an important role in PC survival. Blockade of BLyS, via transmembrane activator and cyclophilin ligand interactor–immunoglobulin treatment, inhibited PC survival in vitro and in vivo. Heightened expression of B cell maturation antigen (BCMA), and lowered expression of transmembrane activator and cyclophilin ligand interactor and BAFF receptor in PCs relative to resting B cells suggests a vital role of BCMA in PC survival. Affirmation of the importance of BCMA in PC survival was provided by studies in BCMA−/− mice in which the survival of long-lived bone marrow PCs was impaired compared with wild-type controls. These findings offer new insights into the molecular basis for the long-term survival of PCs.

Dendritic cells permit immune invasion of the CNS in an animal model of multiple sclerosis

Immunization with myelin antigens leads to the development of experimental autoimmune encephalomyelitis, an animal model of multiple sclerosis. The disease can also be induced by the transfer of encephalitogenic CD4+ T helper (TH) lymphocytes into naive mice. These T cells need to re-encounter their cognate antigen in the context of major histocompatibility complex (MHC) class II–bearing antigen-presenting cells (APCs) in order to recognize their target. The cell type and location of the APC mediating T-cell entry into the central nervous system (CNS) remain unknown. Here, we show that APCs of the lymphoreticular system and of the CNS parenchyma are dispensable for the immune invasion of the CNS. We also describe that a discrete population of vessel-associated dendritic cells (DCs) is present in human brain tissue. In mice, CD11c+ DCs alone are sufficient to present antigen in vivo to primed myelin-reactive T cells in order to mediate CNS inflammation and clinical disease development.

Mast cells are essential intermediaries in regulatory T-cell tolerance

Contrary to the proinflammatory role of mast cells in allergic disorders, the results obtained in this study establish that mast cells are essential in CD4+CD25+Foxp3+ regulatory T (TReg)-cell-dependent peripheral tolerance. Here we confirm that tolerant allografts, which are sustained owing to the immunosuppressive effects of TReg cells, acquire a unique genetic signature dominated by the expression of mast-cell-gene products. We also show that mast cells are crucial for allograft tolerance, through the inability to induce tolerance in mast-cell-deficient mice. High levels of interleukin (IL)-9—a mast cell growth and activation factor—are produced by activated TReg cells, and IL-9 production seems important in mast cell recruitment to, and activation in, tolerant tissue. Our data indicate that IL-9 represents the functional link through which activated TReg cells recruit and activate mast cells to mediate regional immune suppression, because neutralization of IL-9 greatly accelerates allograft rejection in tolerant mice. Finally, immunohistochemical analysis clearly demonstrates the existence of this novel TReg–IL-9–mast cell relationship within tolerant allografts.

Molecular mechanism and function of CD40/CD40L engagement in the immune system

Summary:  During the generation of a successful adaptive immune response, multiple molecular signals are required. A primary signal is the binding of cognate antigen to an antigen receptor expressed by T and B lymphocytes. Multiple secondary signals involve the engagement of costimulatory molecules expressed by T and B lymphocytes with their respective ligands. Because of its essential role in immunity, one of the best characterized of the costimulatory molecules is the receptor CD40. This receptor, a member of the tumor necrosis factor receptor family, is expressed by B cells, professional antigen‐presenting cells, as well as non‐immune cells and tumors. CD40 binds its ligand CD40L, which is transiently expressed on T cells and other non‐immune cells under inflammatory conditions. A wide spectrum of molecular and cellular processes is regulated by CD40 engagement including the initiation and progression of cellular and humoral adaptive immunity. In this review, we describe the downstream signaling pathways initiated by CD40 and overview how CD40 engagement or antagonism modulates humoral and cellular immunity. Lastly, we discuss the role of CD40 as a target in harnessing anti‐tumor immunity. This review underscores the essential role CD40 plays in adaptive immunity.

The human T cell antigen gp39, a member of the TNF gene family, is a ligand for the CD40 receptor: expression of a soluble form of gp39 with B cell co-stimulatory activity.

Signals delivered to B cells via CD40 can synergize with those provided by other B cell surface receptors to induce B cell proliferation and antibody class switching as well as modulate cytokine production and cell adhesion. Recently, it has been shown that the ligand for CD40 is a cell surface protein of approximately 39 kDa expressed by activated T cells, gp39. Here we report on the isolation and characterization of a cDNA clone encoding human gp39, a type II membrane protein with homology to TNF, and the construction and characterization of a soluble recombinant form of gp39. COS cell transfectants expressing gp39 synergized with either anti‐CD20 mAb or PMA to drive strong B cell proliferation and alone were able to drive B cells to proliferate weakly. In all cases the B cell proliferation induced by gp39‐expressing COS cells was reduced to background levels by the addition of soluble CD40. Unlike gp39‐expressing COS cells, recombinant soluble gp39 was not mitogenic alone and required co‐stimulation to drive B cell proliferation. These results suggest that B cells require a second signal besides gp39‐CD40 to drive proliferation and that soluble gp39 alone in a non‐membrane bound form is able to provide co‐stimulatory signals to B cells.

Experimental autoimmune encephalitis and inflammation in the absence of interleukin-12

IL-12 is considered a critical proinflammatory cytokine for autoimmune diseases such as multiple sclerosis and its animal model, experimental autoimmune encephalomyelitis (EAE). IL-12 is a heterodimer composed of a p35 subunit and a common p40 subunit shared by other cytokines. Both IL-12 p40(-/-) and p35(-/-) mice fail to produce IL-12 p70 heterodimer. However, in contrast to p40(-/-) mice, p35(-/-) mice are highly susceptible to the induction of EAE, establishing that IL-12 p70 is not essential for the development of EAE. When compared with wild-type mice, both p40(-/-) and p35(-/-) mice show deficiencies in primary IFN-gamma responses by lymph node cells. Expression profiling of the inflamed CNS revealed that Th2 cytokines such as IL-4 and IL-10 are upregulated in p35(-/-) mice, whereas LT-alpha and TNF-alpha levels are reduced. These studies show that a molecule other than IL-12 p70, which uses the p40 subunit, fulfills the functions previously attributed to IL-12 with regard to the development and pathogenesis of this autoimmune disease.

The role of CD40 in the regulation of humoral and cell-mediated immunity☆

The dynamic and reciprocal communication between T helper (Th) cells and B cells appears to rely on the provision of multiple signals. The first is antigen specific and is mediated by the interaction between the T-cell receptor (TCR) and antigen bound to the major histocompatibility complex (MHC). The subsequent signals are provided by the binding of accessory molecules such as CD28 and CD40 to their respective ligands. Here, Fiona Durie and colleagues discuss the co-stimulatory role of the interaction between CD40 on B cells and CD40 ligand (CD40L, gp39) on T cells, and review evidence that suggests blocking this interaction may induce T-cell tolerance.