Summary Systemic Lupus Erythematosus (SLE) is characterized by B-cells lacking IgD and CD27 (double negative; DN). We show that DN cell expansions reflected a subset of CXCR5−CD11c+ cells (DN2) representing pre-plasma cells (PC). DN2 cells predominated in African-American patients with active disease and nephritis, anti-Smith and anti-RNA autoantibodies. They expressed a T-bet transcriptional network; increased toll-like receptor-7 (TLR7); lacked the negative TLR regulator TRAF5; and were hyper-responsive to TLR7. DN2 cells shared with activated naïve cells (aNAV), phenotypic and functional features, and similar transcriptomes. Their PC differentiation and autoantibody production was driven by TLR7 in an interleukin-21 (IL-21)-mediated fashion. An in vivo developmental link between aNAV, DN2 cells and PC was demonstrated by clonal sharing. This study defines a distinct differentiation fate of autoreactive naïve B cells into PC precursors with hyper-responsiveness to innate stimuli, as well as establishes prominence of extra-follicular B-cell activation in SLE, and identifies therapeutic targets.

Bronchus-associated lymphoid tissue (BALT) is occasionally found in the lungs of mice and humans; however, its role in respiratory immunity is unknown. Here we show that mice lacking spleen, lymph nodes and Peyer's patches generate unexpectedly robust primary B- and T-cell responses to influenza, which seem to be initiated at sites of induced BALT (iBALT). Areas of iBALT have distinct B-cell follicles and T-cell areas, and support T and B-cell proliferation. The homeostatic chemokines CXCL13 and CCL21 are expressed independently of TNFalpha and lymphotoxin at sites of iBALT formation. In addition, mice with iBALT, but lacking peripheral lymphoid organs, clear influenza infection and survive higher doses of virus than do normal mice, indicating that immune responses generated in iBALT are not only protective, but potentially less pathologic, than systemic immune responses. Thus, iBALT functions as an inducible secondary lymphoid tissue for respiratory immune responses.

Although B cells produce cytokines it is not known whether B cells can differentiate into effector subsets that secrete polarized arrays of cytokines. We have identified two populations of "effector" B cells (Be1 and Be2) that produce distinct patterns of cytokines depending on the cytokine environment in which the cells were stimulated during their primary encounter with antigen and T cells. These effector B cell subsets subsequently regulate the differentiation of naïve CD4+ T cells to TH1 and TH2 cells through production of polarizing cytokines such as interleukin 4 and interferon gamma. In addition, Be1 and Be2 cells could be identified in animals that were infected with pathogens that preferentially induce a Type 1 and Type 2 immune response. Together these results suggest that, in addition to their well defined role in antibody production, B cells may regulate immune responses to infectious pathogens through their production of cytokines.

Immunity to pathogens often requires both B cell-dependent humoral immune responses and T cell-dependent cellular immune responses, which cooperate to clear infectious organisms. Although CD4 + T cells clearly participate in humoral immune responses by providing help to B cells and can enhance cellular immunity by producing cytokines, the converse possibility, that B cells participate in both types of immune response, is still not widely accepted. Some early studies of B cell-deficient mice indicated that the absence of B cells adversely affected both CD4 + T cell [1][2][3][4] and CD8 + T cell responses 5,6. However, other studies showed that B cells were dispensable for the generation and maintenance of antigenspecific T cell responses7 -10. These conflicting results were further clouded by data showing that mice lacking B cells during embryonic development exhibit immunological abnormalities, including defects in Peyer's patch organogenesis11, loss of follicular dendritic cells (FDCs)12 , 13 and gp38-expressing stromal cells in the spleen14, alterations in splenic dendritic cell (DC) homeostasis15 and decreased T cell numbers in the thymus16 and spleen14. Given that many of the developmental and architectural defects observed in B cell-deficient mice are likely to influence T cell responses, it has been difficult to unambiguously assign a role for B cells in regulating cellular immune responses to either pathogens or autoantigens.The question of whether B cells have a role in cellular immune responses is now receiving renewed interest with the emergence of clinical data showing that B cell depletion is an effective treatment for several T cell-mediated autoimmune diseases Multiple Sclerosis (MS)17, Type 1 Diabetes (T1D)18 Rheumatoid Arthritis (RA)19 and others20 , 21. Indeed, studies in both humans and mice show that the clinical efficacy of B cell depletion therapy does not necessarily correlate with changes in the levels of circulating autoantibody, suggesting that B cells may contribute to autoimmunity independently of autoantibody production22 , 23 . Importantly, transient B cell depletion studies that distinguish the role of B cells during development from their roles during the course of an immune response have Given the relative effectiveness of B cell depletion by Rituximab, the drug has been tested in a wide variety of diseases. It is approved to treat non-Hodgkin's lymphoma and RA in patients with disease that is refractory to anti-tumour necrosis factor (TNF) therapy. Rituximab is also being evaluated for the treatment of other autoimmune diseases, including systemic lupus erythamatosus (SLE), type 1 diabetes, idiopathic thrombocytopenic purpura (ITP), pemphigus vulgaris (PV), mixed cryoglobulinemia vasculitis (MCV), T1D, MS and others 20 . Most of the clinical studies to date have focused on the extent of B cell depletion and clinical correlates of disease remission. Interestingly, autoantibody titres to some selfantigens declined following B cell depletion while others did not22 , 23, suggesting t...

CD38 is a 42-kilodalton glycoprotein expressed extensively on B and T lymphocytes. CD38 exhibits a structural homology to Aplysia adenosine diphosphate (ADP)-ribosyl cyclase. This enzyme catalyzes the synthesis of cyclic ADP-ribose (cADPR), a metabolite of nicotinamide adenine dinucleotide (NAD+) with calcium-mobilizing activity. A complementary DNA encoding the extracellular domain of murine CD38 was constructed and expressed, and the resultant recombinant soluble CD38 was purified to homogeneity. Soluble CD38 catalyzed the formation and hydrolysis of cADPR when added to NAD+. Purified cADPR augmented the proliferative response of activated murine B cells, potentially implicating the enzymatic activity of CD38 in lymphocyte function.

No abstract

Cyclic ADP-ribose is believed to be an important calcium-mobilizing second messenger in invertebrate, mammalian and plant cells. CD38, the best-characterized mammalian ADP-ribosyl cyclase, is postulated to be an important source of cyclic ADP-ribose in vivo. Using CD38-deficient mice, we demonstrate that the loss of CD38 renders mice susceptible to bacterial infections due to an inability of CD38-deficient neutrophils to directionally migrate to the site of infection. Furthermore, we show that cyclic ADP-ribose can directly induce intracellular Ca++ release in neutrophils and is required for sustained extracellular Ca++ influx in neutrophils that have been stimulated by the bacterial chemoattractant, formyl-methionyl-leucyl-phenylalanine (fMLP). Finally, we demonstrate that neutrophil chemotaxis to fMLP is dependent on Ca++ mobilization mediated by cyclic ADP-ribose. Thus, CD38 controls neutrophil chemotaxis to bacterial chemoattractants through its production of cyclic ADP-ribose, and acts as a critical regulator of inflammation and innate immune responses.

The successful use of B cell depletion therapy for the treatment of autoimmune disease has led to a resurgent appreciation of B cells as powerful regulators of immunity. However, to the surprise of many, B cells appear to regulate autoimmune conditions independently of their ability to produce autoantibodies. Indeed, disturbances in the ability of B cell subsets to present antigen, produce cytokines, and regulate the activities of T cells is emerging as a key feature in many inflammatory diseases. Here we review the recent literature describing cytokine-producing regulatory and effector B cell subsets in health and disease and discuss how future B cell-directed therapies might target the pathologic cytokine-producing effector B cell subsets without impacting the protective regulatory subsets.