B7, a B-cell-restricted antigen that identifies preactivated B cells

After activation with antigen or mitogen, a number of cell surface proteins appear that are not expressed on resting B cells. To date, a number of B lineage restricted and associated activation antigens have been reported that appear at distinct intervals after in vitro activation. In this report, we describe a new B lineage restricted activation antigen (B7) that appears within 24 hr of in vitro stimulation. The expression of B7 antigen, which is detected on a minor subpopulation of B cells isolated from peripheral blood and lymphoid tissues, is strongly induced following stimulation with either anti-immunoglobulin or Epstein-Barr virus. In contrast, B7 was not detected on resting or activated T cells or monocytes. The B7 antigen was expressed on a subset of B cell lines and B cell neoplasms, but was not detected on leukemias and lymphomas of T cell or myeloid origin. B7 was distinguished from other B cell restricted and associated activation antigens by its unique pattern of expression on a variety of hemopoietic cell lines. The biochemical characterization of B7, that it is a single chain protein of 60 kDa, further distinguishes it from other B cell activation antigens. The functional importance of the B7 antigen was demonstrated when splenic B cells were fractionated into the B7+ and B7- populations. The peak of proliferation in response to anti-Ig, appeared earlier within the B7+ population. These studies suggest that B7 antigen identifies a subpopulation of B cells that are preactivated or primed in vivo, and have an accelerated response to subsequent activation via cross-linking of surface Ig.     

Amplification of human B cell activation by a monoclonal antibody to the B cell-specific antigen CD22, Bp 130/140

The B cell-specific antigen CD22 is a 130/140-Kd complex and is unique among human B cell antigens, since its surface expression is restricted to a subpopulation of Ig+ B cells. Here the function of the CD22 antigen was evaluated by using the mAb HD6, directed against one of the epitopes on the molecule. The HD6 antibody was constimulatory with anti-Ig in inducing small, dense tonsillar cells to proliferate; however, the antibody by itself was devoid of stimulatory activity. Anti-CD22 antibody also induced more anti-Ig-treated B cells to leave G0 and enter the G1 phase of the cell cycle. It also was constimulatory with low-m.w. BCGF and with an antibody to a 50-Kd polypeptide, Bp50, which mediates a BCGF-like activity. Results of kinetic experiments and analysis of different B cell fractions suggested that anti-CD22 acts during an early phase of B cell activation, probably by amplifying the anti-Ig signal. F(ab')2 fragments of anti-CD22 HD6 were as effective as the whole antibody in inducing augmentation of B cell proliferation, showing that the Fc portion of the molecule was not required for the activity. The results of these experiments, together with the intriguing distribution of the Bp 130/140 antigen in B cell ontogeny, suggest that this molecule plays an important role in the process that leads to B cell activation and proliferation.     

Changes in the expression of B cell surface markers on complement receptor-positive and complement receptor-negative B cells induced by phorbol myristate acetate

The ability of phorbol myristate acetate (PMA) to induce changes in the expression of B cell surface markers on CR- and CR+ B cells from normal mice in an in vitro culture system was examined. The markers studied were CR, sIgM, sIgD, and sIa. CR- B cells acquired the CR after overnight incubation with PMA. A twofold increase in sIa expression on CR- and CR+ B cells was also noted, whereas the staining intensity of sIgM and sIgD decreased on both B cell populations. These changes in the expression of surface markers took place without detectable increases in cell proliferation, cell size, or RNA content. Furthermore, the same effects were observed when CR- and CR+ B cells were prepared from a small B cell population purified by elutriation. It therefore appears that PMA can exert its effect directly on small, resting B cells.     

B5, a new B cell-restricted activation antigen

The characterization of a new human B cell-restricted activation antigen (B5) is described in this report. With the use of a monoclonal antibody to B5, we show that B5 can be detected on peripheral blood or splenic B cells after 1 day of stimulation with either anti-immunoglobulin, protein A, Epstein Barr virus, or pokeweed mitogen. In contrast, B5 was not expressed on resting B, T, or myeloid cells. More important, B5 could not be detected on activated T cells or monocytes. The B5 antigen was expressed on some lymphoblastoid B cell lines and B cell neoplasms but was not expressed on leukemias or lymphomas of T or myeloid origin. The B5 antigen is distinct from previously reported B cell activation antigens by its m.w. and pattern of cellular expression. These studies suggest that B5 is a novel B cell-restricted activation antigen, which may be useful to study the events of early human B cell activation.     

Expression, distribution and specificity of Fc receptors for IgM on murine B cells

Subpopulations of normal adult murine splenic B cells and a panel of murine B cell tumors were examined for their ability to bind murine IgM specifically. By using two-color flow cytometric analyses, we have demonstrated that 90 to 95% of surface (s)IgD+ B cells express surface membrane receptors for IgM (Fc mu R). The binding of pentameric murine IgM to splenocyte Fc mu R was IgM-specific since it was totally inhibited by other polymeric IgM proteins, but not by Ig of other H chain classes or by mAb specific for the murine IgG or IgE FcR. Binding of IgM to splenic cells was saturable. Fc mu R were co-expressed with the Fc gamma R as well as the Fc epsilon R on the majority of splenic B cells. Minor populations of splenic mononuclear cells expressed only an Fc mu R, Fc gamma R or Fc epsilon R. In a survey of B tumor cell lines representing different stages of B cell development, we observed that the Fc mu R was expressed on pre-B cell lines and that Fc mu R detection was maximal on immature B cell lines that expressed sIgM and low amounts of sIgD and Ia. Fc mu R were not detected on cell lines that had switched from sIgM to the expression of another sIg, or on plasmacytomas and hybridomas. The studies with normal splenocytes establish that the majority of sIgD+ B lymphocytes in adult BALB/c mice express surface membrane receptors that specifically bind IgM. The studies with B lineage tumor cells suggest that the expression of Fc mu R on B cells is developmentally regulated and that the pattern of expression exhibited by Fc mu R during B cell ontogeny differs from the patterns that have been previously found for IgG and IgE FcR. These observations raise the possibility that Fc mu R might have a functional significance in some aspect of B cell maturation and activation. By using a family of IgM H chain constant region domain deletional mutants, we have further demonstrated that, like the T cell Fc mu R, the B cell Fc mu R also requires a C mu 3 domain for binding to occur, raising the possibility that the T and B cell Fc mu R in mice may be structurally related molecules.     

Identification of an early activation antigen (Bac-1) on human B cells

We have produced a monoclonal antibody, Bac-1, that appears to identify a novel antigen on activated human B cells. The Bac-1 antigen can be detected between 8 to 16 hr, as well as transferrin receptors (T9), after activation of small resting B cells with phorbol myristic acetate, anti-IgM antibody, Staphylococcus aureus Cowan I, or Epstein-Barr virus. The expression of the Bac-1 antigen precedes that of IL 2 receptors (Tac-1). Peak expression of the Bac-1 antigen was observed on day 3 after activation, and decreased thereafter. The Bac-1 antigen was present on a minor subpopulation of relatively large B cells isolated from blood samples, and on "preactivated" B cells of heterogeneous size isolated from spleens and tonsils. It was not detected on bone marrow pre-B cells, blood small B cells, or plasma cells, nor was it expressed by resting or activated T cells or nonlymphoid cells. Certain B cell neoplasms and B lymphoblastoid cell lines were Bac-1+, but neoplastic cells of non-B lineage were Bac-1-. With immunoperoxidase staining, Bac-1+ cells were detected predominantly in the germinal centers of tonsil sections. The Bac-1 antigen on activated B cells was destroyed by protease treatment and was enhanced by neuraminidase treatment, suggesting that the Bac-1 antibody detects a cell surface molecule via an antigenic determinant which is partially obscured by neighboring sialic acid residues. The reactivity pattern of Bac-1 differs from the patterns of cellular reactivity reported for other monoclonal antibodies with specificity for activated human B cells.     

Cell surface antigens expressed by subsets of pre-B cells and B cells

A large number of monoclonal antibodies, produced by immunizing rats with mouse pre-B cell lines, have been analyzed for their ability to define cell surface antigens expressed by B cells at early stages of differentiation. Whereas many antibodies recognized antigens on pre-B cell lines, only two clones detected cell surface antigens that were distinguished by their restricted distribution among a panel of continuous cell lines and cells from various tissues. Monoclonal antibody clone AA4.1 recognized a cell surface antigen found on all pre-B lymphomas and on one of three B lymphomas tested. This antigen was found on cells at highest frequency in the bone marrow. Adult spleen and fetal liver also have detectable numbers of AA4.1+ cells. Cells that did not express this antigen include plasmacytomas, two of three B lymphomas, T lymphomas, a stem cell line, adult liver, brain, thymus, and lymph node cells. Clone GF1.2 detected an antigen on some pre-B cell lines, one of three B lymphomas tested, and a small fraction of cells from adult bone marrow, spleen, lymph node, and fetal liver. Plasmacytomas, some pre-B lymphomas, two B lymphomas, T lymphomas, adult liver, brain, and thymus cells were negative. In adult bone marrow, AA4.1 bound to all cytoplasmic IgM+ pre-B cells, whereas GF1.2 detected one-half of these cells. Both antibodies recognized approximately 50% of surface IgM+ (sIgM+) bone marrow cells. A small population of bone marrow cells lacking any detectable Ig (surface or cytoplasmic) also reacted with these antibodies. Depletion of AA4.1 or GF1.2 antigen-bearing cells from bone marrow reduced the ability of bone marrow B cells to respond to LPS by 50 to 65%. Experiments with a cloned pre-B lymphoma demonstrate that AA4.1+ pre-B cells become sIgM+ GF1.2+ B cells after activation with LPS. These antibodies recognize cell surface determinants with restricted distribution among the B lymphocyte lineage because they detect antigens displayed by normal and transformed immature B lymphocytes.     

Dual fluorochrome analysis of human B lymphocytes: phenotypic examination of resting, anti-immunoglobulin stimulated, and in vivo activated B cells

B cell-enriched preparations were prepared from human peripheral blood and lymphoid tissues by the depletion of T cells and monocytes. Only B cells by virtue of their staining with anti-B1 conjugated to fluorescein were additionally examined. Dual fluorescence staining and flow cytometric analysis demonstrated that the majority of "resting" human peripheral blood and splenic B cells co-express the B cell-restricted B1 and B2 antigens and lack B5, a B cell-restricted activation antigen, and interleukin 2 receptor (IL 2R). In contrast, nearly 2/3 and 1/3 of B1+ cells isolated from lymph node expressed IL 2R and B5 antigens, respectively. When B1+ B cells from peripheral blood and spleen were "activated" by anti-Ig, they lost the B2 antigen and acquired the B5 and/or IL 2R antigens. 2/3 of B1+ cells strongly expressed IL 2R, and up to 1/2 of B1+ cells co-expressed B5. Delineation of increased numbers of B1+ cells that co-express B5 and/or IL 2R within lymphoid tissues obtained from patients with diseases characterized by "activated" B cells provides in vivo confirmation that these phenotypic changes correlate with B cell activation. We believe that the identification and isolation of these and similar subsets of cells defined by differing cell surface phenotypes should further our understanding both of normal B cell activation and the pathophysiology of B cell disease states.     

In vitro activation of murine antigen-specific memory B cells by a T-dependent antigen

A procedure that generates an enriched population (60 to 85%) of memory B cells specific for TNP (TNP-MABC) was employed. The activation requirements of TNP-MABC for the T-dependent antigen TNP-KLH and carrier-primed helper T (Th) cells were compared to those for TNP-binding cells from nonimmune mice (TNP-ABC). Proliferation and differentiation of TNP-MABC in response to cognate recognition of antigen requires less antigen and fewer carrier-primed Th cells than the activation of TNP-ABC. Furthermore, responses of the TNP-MABC were of a greater magnitude. Non-cognate activation induces a low level of proliferation of both TNP-ABC and TNP-MABC, but induces differentiation of TNP-MABC only. Percoll density fractionation of spleen cells prior to enrichment for TNP-MABC suggests that the small, dense cell population responds to cognate, but not to non-cognate activation. FACS separation of TNP-MABC by surface Ig isotype reveals that approximately 80% of the secondary IgG response is derived from cells expressing sIgG. Such cells constitute less than 10% of the total number of TNP-MABC. Limiting dilution studies with sorted TNP-MABC indicate that sIgG+ TNP-MABC are enriched for precursors that give rise to a large clone size. The in vitro results indicate the existence of three putative pathways for antigen-specific memory B cell activation by a T-dependent antigen: 1) sIgG+ cells differentiating into IgG-secreting cells; 2) sIgM+ cells differentiating into IgG-secreting cells; and 3) sIgM+ cells differentiating into IgM-secreting cells.     

Role of the CD22 human B cell antigen in B cell triggering by anti-immunoglobulin

As B cells mature during ontogeny the CD22 human differentiation Ag is exported from the cytoplasm onto the membrane. Surface expression is lost in terminal differentiation and after activation. In tonsils, CD22 is expressed on the surface of 60 to 80% of the dense B cells. Some IgM+ dense cells, however, and buoyant in vivo activated B cells are CD22-. This differential expression of CD22 and the finding that an anti-CD22 mAb augmented anti-Ig induced B cell proliferation suggested that CD22 may play a role in B cell activation. In this study we have found that CD22+ but not CD22- B cells could be triggered by anti-IgM or anti-IgD to have increased free intracellular calcium ([Ca2+]i). The presence of CD22 rather than of IgD seems to determine the ability of B cells to respond to anti-Ig with a [Ca2+]i flux. Also the proliferative response to anti-Ig or anti-Ig + B cell growth factor was restricted to the CD22+ population. Anti-CD22 mAb, although not inducing [Ca2+]i on their own after binding to B cells, did augment [Ca2+]i fluxes by anti-Ig when cross-linked. Together these results suggest that CD22 may regulate triggering of B cells through surface Ig perhaps by acting as a "bridge" to transmit an early signal into the cytoplasm.     

The signal for tolerance in B cells is not transmitted through antigen-specific immunoglobulin receptors

Studies of B cell tolerance at the single-cell level require a ready source of antigen-specific B cells that are uncontaminated by T cells or accessory cells. We have isolated normal dinitrophenyl (DNP)-specific B cells from spleens of unprimed mice and propagated these cells in vitro. These B cells are uncontaminated by T cells or macrophages. Long-term cultures of these cell lines contain pre-B cells that are surface (s) IgM-, B cells with sIgM alone, and more mature B cells with sIgM, sIgD, and Ia antigens. Using the cell line lymphocytes we have shown that the early binding of the tolerogenic form of hapten to B cell receptor on mature B cells induces the same activation signal as antigen, and the negative signal induced by tolerogen occurs after B cell activation. Exposure of maturing B cells to DNP bound to murine IgG2a (MGG) for 30 days does not inhibit growth or receptor expression, but does induce tolerance that is reversible when DNP-MGG is removed. A 45-day exposure to DNP-MGG also induces a reversible tolerance.     

B cell sensitivity to natural killing: correlation with target cell stage of differentiation and state of activation

When 13 B cell lines were phenotyped with a panel of B cell, stage-specific monoclonal antibodies and ordered with respect to differentiation state; their sensitivity to natural killer (NK) cell-mediated conjugate formation and cytolysis was found to be stage dependent. Target cell lines expressing an early B cell phenotype (B1+B2-CALLA+DU-ALL1 +/-) or an intermediate phenotype (B1+B2+CALLA-DU-ALL1+) were NK resistant. Late phenotypic B cells (B1+B2-CALLA-DU-ALL1-) were highly susceptible to NK cytolysis. Individual B cell lines when induced with sodium butyrate or retinoic acid expressed altered B cell phenotype and NK susceptibility. For example, SB, an intermediate B cell line and initially NK resistant, when induced to express a later B cell phenotype became NK sensitive. BJA.B, a late B cell line and initially NK sensitive, when induced to differentiate lost most of its sensitivity to natural killing. Since loss of the B2 antigen is associated with B cell activation, we further phenotyped the B cell lines and induced B cell lines with the 4F2 and 5E9 (anti-transferrin receptor) monoclonal reagents. All cell lines tested expressed each of these antigens, but with varying intensities. While the intensity of 4F2 expression appeared to correlate well with NK sensitivity on both resting and differentiated B cell lines, the intensity of 5E9 expression did not. Peripheral blood B cells express a similar pattern of reactivity to natural killing when stimulated with pokeweed mitogen (PWM) in vitro. B cell sensitivity to NK-mediated events peaked at day 4 of incubation and correlated with the loss of the B2 antigen and acquisition of the 4F2 and 5E9 antigens; sensitivity to natural killing was diminished in the presence of the PCA-1 antigen. The expression of the NK-susceptible phenotype among B cells appears to be differentiation stage- and activation state-dependent. It is during this period of ontogeny that the NK cell may cytolytically regulate the B cell transition to a plasma cell.     

Preparation and analysis of antigen-specific memory B cells

A procedure has been developed for the enrichment of TNP-binding memory B cells (TNP-MABC) from spleens of immunized mice. More than 75% of the cells expressed surface IgM (sIgM) and IgD (sIgD) and about 9% expressed surface IgG (sIgG). The TNP-MABC consisted of small resting lymphocytes with high affinity antigen-binding receptors. These cells expressed increased densities of Ia antigens and decreased densities of sIgD. Adoptive transfer of the cells into irradiated, carrier-primed syngeneic recipients resulted in their differentiation into IgG anti-TNP antibody-secreting cells. TNP-MABC secreted high affinity IgG anti-TNP antibodies when cultured in vitro with carrier-primed T cells and antigen. Limiting dilution analysis revealed that TNP-MABC contained a relatively low frequency of precursors for IgG-secreting cells that had an exceptionally large clone size. These results show that a highly enriched population of antigen-specific memory B cells can now be prepared and used to analyze their activation requirements.     

Characterization of a human B cell-specific antigen (B2) distinct from B1

A human B lymphocyte-specific antigen (B2) was identified and characterized by the use of a monoclonal antibody. By indirect immunofluorescence and quantitative absorption, B2 was shown to be expressed exclusively on Ig+ B cells isolated from peripheral blood and lymphoid tissues. In contrast, B2 was not found on monocytes, resting and activated T cells, Null cells, or granulocytes, nor was it found on cell lines or tumor cells of T cell or myeloid origin. Functional studies demonstrated that only B2 antigen-positive splenocytes could be induced to differentiate into plasma cells under the stimulus of pokeweed mitogen, further confirming the B cell specificity of B2. It was then demonstrated that the B2 antigen was distinct from the previously described B cell-surface determinants including surface immunoglobulin, Ia-like antigens, and Fc and C3 receptors. More importantly, the B2 antigen has been clearly shown to be distinct from the previously described B cell-specific antigen, B1, by its m.w. and expression on normal and malignant B lymphocytes. The distinct distribution of B2 on normal and malignant lymphocytes supports the notion of B cell heterogeneity and provides further evidence for existence of subpopulations of human B lymphocytes.     

Induction of isotype switching and Ig production by CD5+ and CD10+ human fetal B cells.

In the present study the capacity of early fetal B cells to produce Ig was investigated. It is shown that B cells from fetal liver, spleen, and bone marrow (BM) can be induced to produce IgM, IgG, IgG4, and IgE, but not IgA, in response to IL-4 in the presence of anti-CD40 mAb or cloned CD4+ T cells. Even splenic B cells from a human fetus of only 12 wk of gestation produced these Ig isotypes. IFN-alpha, IFN-gamma, and transforming growth factor-beta inhibited IL-4-induced IgE production in fetal B cells, as described for mature B cells. The majority of B cells in fetal spleen expressed CD5 and CD10 and greater than 99% of B cells in fetal BM were CD10+. Highly purified CD10+, CD19+ immature B cells and CD5+, CD19+ B cells could be induced to produce Ig, including IgG4 and IgE, in similar amounts as unseparated CD19+ B cells. Virtually all CD19+ cells still expressed CD10 after 12 days of culture. However, the IgE-producing cells at the end of the culture period were found in the CD19-,CD10- cell population, suggesting differentiation of CD19+,CD10+ B cells into CD19-,CD10- plasma cells. Pre-B cells are characterized by their lack of expression of surface IgM (sIgM). Only 30 to 40% of BM B cells expressed sIgM. However, in contrast to sIgM+,CD10+,CD19+ immature B cells, sorted sIgM-,CD10+,CD19+ pre-B cells failed to differentiate into Ig-secreting cells under the present culture conditions. Addition of IL-6 to these cultures was ineffective. Taken together, these results indicate that fetal CD5+ and CD10+ B cells are mature in their capacity to be induced to Ig isotype switching in vitro as soon as they express sIgM.     

A unique antigen on mature B cells defined by a monoclonal antibody

A novel 42,000 dalton antigen (MB-1) expressed by mature human B cells in blood and tonsil was identified and characterized by utilizing a hybridoma monoclonal antibody. A comparison of MB-1 with other known B cell antigens suggests that the MB-1 antigen has not been previously identified. From one-and two-color immunofluorescence studies, it appears that the MB-1 antigen is found on all normal immunoglobulin (Ig)-expressing cells, but not on T cells, thymocytes, granulocytes, or platelets. Studies of malignant B cell tumors reveal that the antigen is expressed by virtually all Ig-expressing B cell tumors but only 10% of SIg- B-lineage leukemias. Data from these studies suggest that the MB-1 antigen is expressed late in B cell ontogeny before the expression of SIg.     

Heterogeneity of EBV-transformable human B lymphocyte populations

Although most human B cells express receptors for Epstein Barr virus (EBV), few (usually less than 1%) are readily transformed into B lymphoblastoid cell lines after exposure to EBV. Transformable cells previously have been found to be mostly resting B lymphocytes. We recently developed a limiting dilution culture system which permits the growth of EBV-transformed B lymphocytes with high efficiency. Because in this system up to over 30% of peripheral blood- or tonsil-derived B cells respond to EBV, we re-examined the properties of EBV-transformable cells. Frequencies of transformable lymphocytes were determined by Poisson analysis. EBV-susceptible B cells committed to IgM, IgG, or IgA secretion were found to occur in the range of 3 to 27, 0.1 to 6, and 0.1 to 5 per 100 B cells, respectively. Under our culture conditions, a major proportion of the IgM-committed cells derived from large lymphocytes which appeared to have entered the cell cycle. This population contains most of the EBV-responsive cells detected and, therefore, most of the additional cells responding in our culture system. In contrast, precursors of IgG- or IgA-producing lymphoblast lines were small cells with DNA contents typical for the G0/G1 phases of the cell cycle. Anti-immunoglobulin antibodies were used in panning experiments to separate B cell subpopulations which expressed different immunoglobulin isotypes on their surface. In limiting dilution cultures of these purified B lymphocytes subsets, it was found that virtually all precursors of IgM-producing cell lines expressed surface IgM (sIgM) before their infection and transformation by EBV. The "cloning efficiency" of positively selected, large sIgM+ cells approached 100%. In contrast, sIgG or sIgA were found only on cells committed to the production of IgG or IgA, respectively. The expression of sIgD was examined by using sequential panning procedures. Virtually all IgM-committed lymphocytes and a subset of cells committed to IgA secretion were found among the sIgD+ cells. The majority of cells committed to IgA production and all IgG-committed cells were found in the sIgD- B cell population. Our findings indicate that the EBV-susceptible B cell subset of normal lymphocytes is heterogeneous with respect to cell size, cell cycle, sIg determinants, and efficiency of transformation. On the basis of our findings and previous reports, we propose a model in which transformability is a B cell-inherent property. Factors unrelated to the virus but present in our culture system appear responsible for the enhanced vulnerability to viral transformation in some cells which entered into the cell cycle.(ABSTRACT TRUNCATED AT 400 WORDS)     

B cell surface glycoprotein induced during growth response: molecular structure and expression pattern

We present the molecular characterization of a cell surface antigen, B 7.2, that is expressed on activated B lymphocytes. The BCL1 and CH12 B lymphoma cells express the B 7.2 antigen constitutively. In small resting B cells from spleen, the B 7.2 expression is induced during polyclonal growth in response to mitogenic stimulation. B 7.2 expression also occurs with a significant frequency in cells from fresh lymphoid tissues. The endogenous expression of the B 7.2 antigen is high in spleen and lymph nodes, and is undetectable in the thymus. The B 7.2 antigen is a microheterogeneous 45,000 to 50,000 dalton glycoprotein with a single polypeptide chain, intramolecular S--S bonds, and N-linked glycan moieties. The folded structure of the B 7.2 antigen appears to contain a domain with hydrophilic properties exposed on the cell surface and a hydrophobic segment that may comprise a transmembrane portion. Considering the observed expression pattern and the molecular structure, we speculate that the B 7.2 antigen has a specific function in regulation of B cell activation, perhaps as a receptor for a regulatory ligand or as a ligand recognized by other B or T cells.     

CD83 expression is a sensitive marker of activation required for B cell and CD4+ T cell longevity in vivo

CD83 is a surface marker that differentiates immature and mature human dendritic cell populations. Thymic epithelial cell expression of CD83 is also necessary for efficient CD4+ T cell development in mice. The altered phenotypes of peripheral B and CD4+ T cells, and the reduction of peripheral CD4+ T cells in CD83-/- mice, suggest additional functions for CD83. To assess this, a panel of mAbs was generated to characterize mouse CD83 expression by peripheral leukocytes. As in humans, activation of conventional and plasmacytoid murine dendritic cell subsets led to rapid up-regulation of CD83 surface expression in mice. In primary and secondary lymphoid compartments, a subset of B cells expressed low-level CD83, while CD83 was not detected on resting T cells. However, CD83 was prominently up-regulated on the majority of spleen B and T cells within hours of activation in vitro. In vivo, a low dose of hen egg lysozyme (1 microg) induced significant CD83 but not CD69 expression by Ag-specific B cells within 4 h of Ag challenge. Although B cell development appeared normal in CD83-/- mice, B and CD4+ T cell expression of CD83 was required for lymphocyte longevity in adoptive transfer experiments. Thus, the restricted expression pattern of CD83, its rapid induction following B cell and T cell activation, and its requirement for B cell and CD4+ T cell longevity demonstrate that CD83 is a functionally significant and sensitive marker of early lymphocyte activation in vivo.