Iqbal S. Grewal

BAFF/BLyS Receptor 3 Binds the B Cell Survival Factor BAFF Ligand through a Discrete Surface Loop and Promotes Processing of NF-κB2

The TNF-like ligand BAFF/BLyS is a potent survival factor for B cells. It binds three receptors: TACI, BCMA, and BR3. We show that BR3 signaling promotes processing of the transcription factor NF-kappaB2/p100 to p52. NF-kappaB2/p100 cleavage was abrogated in B cells from A/WySnJ mice possessing a mutant BR3 gene, but not in TACI or BCMA null B cells. Furthermore, wild-type mice injected with BAFF-neutralizing BR3-Fc protein showed reduced basal NF-kappaB2 activation. BR3-Fc treatment of NZB/WF1 mice, which develop a fatal lupus-like syndrome, inhibited NF-kappaB2 processing and attenuated the disease process. Since inhibiting the BR3-BAFF interaction has therapeutic ramifications, the ligand binding interface of BR3 was investigated and found to reside within a 26 residue core domain. When stabilized within a structured beta-hairpin peptide, six of these residues were sufficient to confer binding to BAFF.

Requirement for CD40 Ligand in Costimulation Induction, T Cell Activation, and Experimental Allergic Encephalomyelitis

The mechanism of CD40 ligand (CD40L)-mediated in vivo activation of CD4+ T cells was examined by investigation of the development of experimental allergic encephalomyelitis (EAE) in CD40L-deficient mice that carried a transgenic T cell receptor specific for myelin basic protein. These mice failed to develop EAE after priming with antigen, and CD4+ T cells remained quiescent and produced no interferon-γ (IFN-γ). T cells were primed to make IFN-γ and induce EAE by providing these mice with B7.1+ antigen-presenting cells (APCs). Thus, CD40L is required to induce costimulatory activity on APCs for in vivo activation of CD4+ T cells to produce IFN-γ and to evoke autoimmunity.

Activation and accumulation of B cells in TACI-deficient mice

The tumor necrosis factor (TNF)-related ligand B lymphocyte stimulator (BLyS) binds two TNF receptor family members, transmembrane activator and calcium-modulating and cyclophilin ligand interactor (TACI) and B cell maturation molecule (BCMA). Mice that are transgenic for BLyS show B cell accumulation, activation and autoimmune lupus-like nephritis. The existence of at least two distinct BLyS receptors raises the question of the relative contribution of each to B cell functions. We therefore generated mice that were deficient in TACI. TACI−/− mice showed increased B cell accumulation and marked splenomegaly. Isolated TACI−/− B cells hyperproliferated and produced increased amounts of immunoglobulins in vitro. In vivo antigen challenge resulted in enhanced antigen-specific antibody production. Thus, TACI may play an unexpected inhibitory role in B cell activation that helps maintain immunological homeostasis.

Loss of TACI Causes Fatal Lymphoproliferation and Autoimmunity, Establishing TACI as an Inhibitory BLyS Receptor

BLys , a key cytokine that sustains B cell maturation and tolerance, binds three receptors: BR3, BCMA, and TACI. Results from knockout mice implicate a major functional role for BR3 and a redundant one for BCMA in B cell function. TACIs role is controversial based on defects in TI antibody responses accompanied by B cell hyperplasia in knockout mice. We have presently characterized a precise role for TACI in vivo. TACI(-/-) mice develop fatal autoimmune glomerulonephritis, proteinurea, and elevated levels of circulating autoantibodies. Treatment of B cells with TACI agonistic antibodies inhibits proliferation in vitro and activation of a chimeric receptor containing the TACI intracellular domain induces apoptosis. These results demonstrate the critical requirement for TACI in regulating B cell homeostasis.

The Role of CD40 Ligand in Costimulation and T-Cell Activation

It is clear by now that cell-to-cell interactions involving a variety of signals are required for effective immune response. The data reviewed here suggest that CD40-CD40L interactions are critical for development of CD4 T-cell-dependent effector functions. Lack of this important interaction results in greatly reduced activation of CD4 T cells, while successful interaction of these molecules results in full activation of these T cells. Consequently, the absence of CD40-CD40L interactions leads to impairment of T-cell effector such as help for B-cell differentiation and class switch, activation of monocytes and macrophages to produce cytokines and to kill intracellular pathogens, and activation of autoreactive T cells to mount an autoimmune response. The effector functions of T cells controlled by CD40-CD40L interactions in a successful immune response are given in Table I. Data presented so far suggest that CD40-CD40L interactions play a role in early signalling events, where interactions of this kind are required to induce expression of costimulatory molecules on APC. One possible sequence of events in that APC, like DC, take up antigens at the site of injury or infection and migrate to lymph nodes, where they present antigens complexed with MHC class II molecules to naive T cells. This results in expression of CD40L on T cells. Coupling of this newly expressed CD40L on T cells with CD40 on APC results in expression of the costimulatory activity of the APC. At this time the costimulatory signal provided by the APC is received by the T cells via CD28/CTLA-4, which drives the cell to enter into cell cycle and complete T cell activation. T cells thereby activated can now enter into secondary cognate CD40-CD40L-dependent effector recognition with B cells to switch Ig class, macrophages to produce cytokines and new DC carrying the same antigen to up-regulate costimulatory activity. A tight regulation of expression of CD40L on T cells and costimulatory activity on APC would prevent activation of unwanted bystander T cells. The coupling of activation of the APC primed with the cognate antigen to the activation of the T-cell specific for that antigen in this model provides an additional regulatory step in the initiation of the immune response. This also suggests that a limited number of T cells/APC will be activated, both of which will be specific in nature. This additional step may be important for safeguarding against an autoimmune response. In addition, the fact that CD40L uniquely seems to play this role suggests that selective immunotherapies to treat autoimmune disease and prevent graft rejection can be targeted on this molecule. On the other hand, CD40-directed approaches to up-regulate costimulatory activity on APC could be developed to fight tumor growth, contain infections and treat immunodeficiencies.

Disruption of CD40–CD40 Ligand Interactions Results in an Enhanced Susceptibility to Leishmania amazonensis Infection

To study the role of CD40 ligand (CD40L) in the host immune responses against intracellular pathogens, we infected CD40L knockout (CD40L-/-) mice with Leishmania amazonensis. Although wild-type mice were susceptible to infection and developed progressive ulcerative lesions, tissue parasite burdens in CD40L-/- mice were significantly higher. This heightened susceptibility to infection was associated with an impaired T cell and macrophage activation and altered inflammatory response, as reflected by low levels of IFN gamma, lymphotoxin-tumor necrosis factor (LT-TNF), and nitric oxide (NO) production. Furthermore, CD40L-/- mice failed to generate a protective immune response after immunization. These results indicate an essential role of cognate CD40-CD40L interactions in the generation of cellular immune responses against an intracellular parasite.

A central role of CD40 ligand in the regulation of CD4+ T-cell responses

Abstract Many of the costimulatory molecules thought to be essential for T-cell activation have now been identified. The CD40-CD40 ligand interaction is one such receptor/counter-receptor pair that has been shown to be important in B- and T-cell cognate interactions. Here Iqbal Grewal and Richard Flavell examine recent data from studies using gene knockout techniques that have helped provide a better understanding of CD40Ls role in vivo , in regulation of the immune response.

Identification of a receptor for BLyS demonstrates a crucial role in humoral immunity

B ymphocyte stimulator (BLyS) is a member of the tumor necrosis factor (TNF) superfamily. BLyS stimulates proliferation of, and immunoglobulin production by, B cells. However, the relative importance of BLyS in physiological B cell activation is unclear. We identified a B cell receptor for BLyS through expression cloning as TACI, an orphan TNF receptor homologue of unknown function. Binding of BLyS to TACI activated signaling by nuclear factor-κB (NF-κB). In vitro soluble TACI-Fc fusion protein blocked BLyS-induced NF-κB activation in B lymphoma cells and IgM production in peripheral blood B cells. In vivo treatment of immunized mice with TACI-Fc inhibited production of antigen-specific IgM and IgG1 antibodies and abolished splenic germinal center (GC) formation. Thus, BLyS activity must play a critical role in the humoral immune response.

TACI-ligand interactions are required for T cell activation and collagen-induced arthritis in mice

Interactions of the tumor necrosis factor superfamily members B lymphocyte stimulator (BLyS) and a proliferation-inducing ligand (APRIL) with their receptors—transmembrane activator and CAML interactor (TACI) and B cell maturation molecule (BCMA)—on B cells play an important role in the humoral immune response. Whereas BCMA is restricted to B cells, TACI is also expressed on activated T cells; we show here that TACI-Fc blocks the activation of T cells in vitro and inhibits antigen-specific T cell activation and priming in vivo. In a mouse model for rheumatoid arthritis (RA), an autoimmune disease that involves both B and T cell components, TACI-Fc treatment substantially inhibited inflammation, bone and cartilage destruction and disease development. Thus, BLyS and/or APRIL are important not only for B cell function but for T cell–mediated immune responses. Inhibition of these ligands might have therapeutic benefits for autoimmune diseases, such as RA, that involve both B and T cells.

APRIL-deficient mice have normal immune system development

ABSTRACT APRIL (a proliferation-inducing ligand) is a member of the tumor necrosis factor (TNF) superfamily. APRIL mRNA shows high levels of expression in tumors of different origin and a low level of expression in normal cells. APRIL shares two TNF receptor family members, TACI and BCMA, with another TNF homolog, BLyS/BAFF. BLyS is involved in regulation of B-cell activation and survival and also binds to a third receptor, BR3/BAFF-R, which is not shared with APRIL. Recombinant APRIL and BLyS induce accumulation of B cells in mice, while BLyS deficiency results in severe B-cell dysfunction. To investigate the physiological role of APRIL, we generated mice that are deficient in its encoding gene. APRIL−/− mice were viable and fertile and lacked any gross abnormality. Detailed histological analysis did not reveal any defects in major tissues and organs, including the primary and secondary immune organs. T- and B-cell development and in vitro function were normal as well, as were T-cell-dependent and -independent in vivo humoral responses to antigenic challenge. These data indicate that APRIL is dispensable in the mouse for proper development. Thus, BLyS may be capable of fulfilling APRILs main functions.