James W. Tung

Inherent specificities in natural antibodies: a key to immune defense against pathogen invasion

Natural antibodies are produced at tightly regulated levels in the complete absence of external antigenic stimulation. They provide immediate, early and broad protection against pathogens, making them a crucial non-redundant component of the humoral immune system. These antibodies are produced mainly, if not exclusively, by a subset of long-lived, self-replenishing B cells termed B-1 cells. We argue here that the unique developmental pattern of these B-1 cells, which rests on positive selection by self antigens, ensures production of natural antibodies expressing evolutionarily important specificities that are required for the initial defense against invading pathogens. Positive selection for reactivity with self antigens could also result in the production of detrimental anti-self antibodies. However, B-1 cells have evolved a unique response pattern that minimizes the risk of autoimmunity. Although these cells respond rapidly and strongly to host-derived innate signals, such as cytokines, and to pathogen-encoded signals, such as lipopolysaccharide and phosphorylcholine, they respond very poorly to receptor-mediated activation. In addition, they rarely enter germinal centers and undergo affinity maturation. Thus, their potential for producing high-affinity antibodies with harmful anti-self specificity is highly restricted. The positive selection of B-1 cells occurs during the neonatal period, during which the long-lived self-renewing B-1 population is constituted. Many of these cells (B-1a) express CD5, although a smaller subset (B-1b) does not express this surface marker. Importantly, B-1a cells should not be confused with short-lived anergic B-2 cells, which originate in the bone marrow in adults and initiate CD5 expression and programmed cell death following self-antigen recognition. In summary, we argue here that the mechanisms that enable natural antibody production by B-1 cells reflect the humoral immune system, which has evolved in layers whose distinct developmental mechanisms generate complementary repertoires that collectively operate to maximize flexibility in responses to invading pathogens. B-2 cells, present in what may be the most highly evolved layer(s), express a repertoire that is explicitly selected against self recognition and directed towards the generation of high-affinity antibody response to external antigenic stimuli. B-1 cells, whose repertoire is selected by recognition of self antigen, belong to what may be earlier layer(s) and inherently maintain production of evolutionarily important antibody specificities that respond to pathogen-related, rather then antigen-specific signals.

Interpreting flow cytometry data: a guide for the perplexed

Recent advances in flow cytometry technologies are changing how researchers collect, look at and present their data.

Cutting edge: A role for CD1 in the pathogenesis of lupus in NZB/NZW mice

Since anti-CD1 TCR transgenic T cells can activate syngeneic B cells via CD1 to secrete IgM and IgG and induce lupus in BALB/c mice, we studied the role of CD1 in the pathogenesis of lupus in NZB/NZW mice. Approximately 20% of B cells from the spleens of NZB/NZW mice expressed high levels of CD1 (CD1high B cells). The latter subset spontaneously produced large amounts of IgM anti-dsDNA Abs in vitro that was up to 25-fold higher than that of residual CD1int/low B cells. T cells in the NZB/NZW spleen proliferated vigorously to the CD1-transfected A20 B cell line, but not to the parent line. Treatment of NZB/NZW mice with anti-CD1 mAbs ameliorated the development of lupus. These results suggest that the CD1high B cells and their progeny are a major source of autoantibody production, and activation of B cells via CD1 may play an important role in the pathogenesis of lupus.

Division and differentiation of natural antibody-producing cells in mouse spleen.

B-1a cells reside in both the peritoneal cavity and the spleen. LPS stimulates splenic B-1a to differentiate to plasma cells producing natural IgM specific for microbial and self antigens. However, there are conflicting views as to whether the B-1a cells divide before this differentiation occurs, and hence how the resident B-1a population is maintained in the spleen. Studies here resolve this dispute in favor of both sides: we show that (some or all) B-1a cells resident in the spleen respond to LPS by differentiating to plasma cells immediately, without dividing; however, we also show that additional B-1a cells immigrate into the spleen after LPS stimulation and divide at least once before differentiating. Importantly, the studies we presently describe reveal the complex cell migration and differentiation events that collectively underlie the rapid production of natural antibodies in response to in vivo LPS stimulation. Thus, the studies present a different view of the roles that B-1a cells play in the early phases of the innate immune response.

CD11b expression distinguishes sequential stages of peritoneal B-1 development

Peritoneal cavity (PerC) B-1 cells have long been known to express CD11b, which is coexpressed with CD18 to form the Mac-1/CR3 complement receptor and adhesion molecule. However, although all PerC B-1 cells are commonly believed to express CD11b, we show here that nearly half of the cells in each of the PerC B-1 subsets (B-1a and B-1b) do not express this surface receptor. The CD11b+ cells in each B-1 subset are larger and more granular and express higher levels of surface IgM than the CD11b− B-1 cells. In addition, the CD11b+ B-1 cells initiate the formation of tightly associated doublets that are present at high frequency in adult PerC. Finally, and most importantly from a developmental standpoint, the CD11b+ B-1 cells have a limited reconstitution capability: when sorted and transferred into congenic recipients, they reconstitute their own (CD11b+) B-1 subset but do not reconstitute the CD11b− B-1 subset. In contrast, CD11b− B-1 cells transferred under the same conditions efficiently replenish all components of the PerC B-1 population in appropriate proportions. During ontogeny, CD11b− B-1 cells appear before CD11b+ B-1 cells. However, the clear phenotypic differences between the neonatal and adult CD11b B-1 subsets argue that although CD11b− B-1 give rise to CD11b+ B-1 in both cases different forces may regulate this transition.

B cell lineages: documented at last!

Functional and phenotypic distinctions characterize B-1 cells versus B-2 cells. The identification of adult bone marrow cells that give rise exclusively to B-1 B cells support earlier ideas regarding the existence of dedicated B-1 progenitors.

Identification of B-Cell Subsets

In the last few years, the effectiveness of developmental and functional studies of individual subsets of cells has increased dramatically owing to the identification of additional subset markers and the extension of fluorescence-activated cell sorter (FACS) capabilities to simultaneously measure the expression of more markers on individual cells. For example, introduction of a 6-8 multiparameter FACS instrument resulted in significant advances in understanding B-cell development. In this chapter, we describe 11-color high-dimensional (Hi-D) FACS staining and data analysis methods that provide greater clarity in identifying the B-cell subsets in bone marrow, spleen, and peritoneal cavity. Further, we show how a single Hi-D FACS antibody reagent combination is sufficient to unambiguously identify most of the currently defined B-cell developmental subsets in the bone marrow (Hardy fractions A-F) and the functional B-cell subsets (B-1a, B-1b, B-2, and marginal zone [MZ] B cells) in the periphery. Although we focus on murine B-cell subsets, the methods we discuss are relevant to FACS studies conducted with all types of cells and other FACS instruments. We introduce a new method for scaling axes for histograms or contour plots of FACS data. This method, which we refer to as Logicle visualization, is particularly useful in promoting correct interpretations of fluorescence-compensated FACS data and visual confirmation of correct compensation values. In addition, it facilitates discrimination of valid subsets. Application of Logicle visualization tools in the Hi-D FACS studies discussed here creates a strong new base for in-depth analysis of B-cell development and function.

CD72 Down-Modulates BCR-Induced Signal Transduction and Diminishes Survival in Primary Mature B Lymphocytes

CD72, a 45-kDa type II transmembrane glycoprotein carrying an ITIM motif, is believed to be an inhibitory coreceptor of the BCR. Mature B cells lacking CD72 show enhanced Ca2+ mobilization and are hyperproliferative in response to BCR ligation. However, the signal transduction pathways downstream of BCR signaling that transmit the inhibitory effect of CD72 in mature B cells remain unknown. To address this question, we used hen egg lysozyme-specific BCR transgenic mice to elucidate the differential cell signaling between wild-type and CD72-deficient B cells in response to hen egg lysozyme Ag stimulation. Our results demonstrate that CD72 predominantly down-regulates the major signal transduction pathways downstream of the BCR, including NF-AT, NF-κB, ERK, JNK, p38-MAPK, and PI3K/Akt in mature B cells. CD72 ligation with anti-CD72 Ab (K10.6), which mimics the binding of CD100 (a natural ligand for CD72) to release the inhibitory function of CD72, augments cell proliferation, Ca2+ flux, IκBα activation, and ERK MAPK activity upon Ag stimulation in wild-type B cells. In addition, we show direct evidence that CD72 promotes cell cycle arrest and apoptosis after Ag stimulation in mature B cells. Taken together, our findings conclude that CD72 plays a dominant role as a negative regulator of BCR signaling in primary mature B lymphocytes.

An Early Oxygen-Dependent Step Is Required for Dexamethasone-Induced Apoptosis of Immature Mouse Thymocytes

The roles of oxygen and reactive oxygen intermediates in apoptosis are unclear at present. Although oxygen and reactive oxygen intermediates are not required for the execution of apoptosis, oxygen may be involved in at least some forms of apoptosis. In this study we show that dexamethasone (Dex)-induced apoptosis of immature mouse thymocytes is completely inhibited by hypoxic culture. In contrast, anti-CD95 thymocyte apoptosis is unaffected by hypoxia, indicating the existence of two forms of thymocyte apoptosis: an oxygen-dependent pathway (Dex induced) and an oxygen-independent pathway (anti-CD95 induced). Furthermore, hypoxia inhibited mitochondrial permeability transition (PT) in Dex-treated, but not in anti-CD95-treated, thymocytes, suggesting that the oxygen-sensitive step is upstream of mitochondria. Both Dex- and anti-CD95-induced PT and apoptosis were dependent on activation of IL-converting enzyme-like protease, as PT and apoptosis were inhibited by preincubation with Cbz-Val-Ala-Asp-fluoromethyl ketone, an irreversible inhibitor of IL-converting enzyme-like proteases. In addition, hypoxia inhibited the activation by Dex of caspase-3 (CPP32)-like proteases. Our data show that the private signaling pathways of Dex (oxygen dependent) and anti-CD95 (oxygen independent) both converge upstream of mitochondrial changes. The oxygen-dependent step in Dex-induced apoptosis lies upstream of caspase-3-like protease activation. Our observations support a model of apoptosis signaling in which independent pathways (oxygen dependent and oxygen independent) particular to each stimuli converge at a central point in the apoptotic cascade.

The regulation of CD5 expression in murine T cells

BackgroundCD5 is a pan-T cell surface marker that is also present on a subset of B cells, B-1a cells.Functional and developmental subsets of T cells express characteristic CD5 levels that vary over roughly a 30-fold range. Previous investigators have cloned a 1.7 Kb fragment containing the CD5 promoter and showed that it can confer similar lymphocyte-specific expression pattern as observed for endogenous CD5 expression.ResultsWe further characterize the CD5 promoter and identify minimal and regulatory regions on the CD5 promoter. Using a luciferase reporter system, we show that a 43 bp region on the CD5 promoter regulates CD5 expression in resting mouse thymoma EL4 T cells and that an Ets binding site within the 43 bp region mediates the CD5 expression. In addition, we show that Ets-1, a member of the Ets family of transcription factors, recognizes the Ets binding site in the electrophoretic mobility shift assay (EMSA). This Ets binding site is directly responsible for the increase in reporter activity when co-transfected with increasing amounts of Ets-1 expression plasmid.We also identify two additional evolutionarily-conserved regions in the CD5 promoter (CD5X and CD5Y) and demonstrate the respective roles of the each region in the regulation of CD5 transcription.ConclusionOur studies define a minimal and regulatory promoter for CD5 and show that the CD5 expression level in T cells is at least partially dependent on the level of Ets-1 protein. Based on the findings in this report, we propose a model of CD5 transcriptional regulation in T cells.