Weak peptide agonists reveal functional differences in B7-1 and B7-2 costimulation of human T cell clones

The influence of costimulation on the T cell response to altered peptide ligands that act as either partial or weak agonists for human CD4+ T cell clones was examined. Using stable Chinese hamster ovary (CHO) cell transfectants expressing DR2 (DRB1*1501) and human B7-1 or B7-2 as APC, presentation of native myelin basic protein (MBP) p85-99 peptide Ag or a partial agonist of MBP p85-99 induced equivalent T cell activation as measured by [3H]TdR incorporation and cytokine secretion. In marked contrast, presentation of cross-reactive peptides of MBP p85-99 that act as weak agonists with B7-1, but not B7-2, costimulation resulted in significant T cell activation as measured by [3H]TdR incorporation and cytokine secretion. These data suggest that decreasing the strength of the signal provided to the TCR allows differences in B7-1 and B7-2 signaling to be observed. Thus, the costimulatory environment during T cell activation may be a mechanism of regulating T cell cross-reactivity in the periphery.

Weak peptide agonists reveal functional differences in B7-1 and B7-2 costimulation of human T cell clones

The influence of costimulation on the T cell response to altered peptide ligands that act as either partial or weak agonists for human CD4+ T cell clones was examined. Using stable Chinese hamster ovary (CHO) cell transfectants expressing DR2 (DRB1*1501) and human B7-1 or B7-2 as APC, presentation of native myelin basic protein (MBP) p85-99 peptide Ag or a partial agonist of MBP p85-99 induced equivalent T cell activation as measured by [3H]TdR incorporation and cytokine secretion. In marked contrast, presentation of cross-reactive peptides of MBP p85-99 that act as weak agonists with B7-1, but not B7-2, costimulation resulted in significant T cell activation as measured by [3H]TdR incorporation and cytokine secretion. These data suggest that decreasing the strength of the signal provided to the TCR allows differences in B7-1 and B7-2 signaling to be observed. Thus, the costimulatory environment during T cell activation may be a mechanism of regulating T cell cross-reactivity in the periphery.

Ex vivo generation of human anti-pre-B leukemia-specific autologous cytolytic T cells

In contrast to other neoplasms, antigen-specific autologous cytolytic T cells have not been detected in patients with human pre-B-cell leukemias. The absence of efficient B7 family (B7-1/CD80; B7-2/CD86) -mediated costimulation has been shown to be a major defect in tumor cells' capacity to function as antigen-presenting cells. We show here the generation of autologous anti-pre-B-cell leukemia-specific cytolytic T-cell lines from the marrows of 10 of 15 patients with pre-B-cell malignancies. T-cell costimulation via CD28 is an absolute requirement for the generation of these autologous cytolytic T cells (CTL). Although costimulation could be delivered by either bystander B7 transfectants or professional antigen-presenting cells (indirect costimulation), optimal priming and CTL expansion required that the costimulatory signal was expressed by the tumor cell (direct costimulation). These anti-pre-B-cell leukemia-specific CTL lysed both unstimulated and CD40-stimulated tumor cells from each patient studied but did not lyse either K562 or CD40-stimulated allogeneic B cells. Cytolysis was mediated by the induction of tumor cell apoptosis by CD8+ T cells via the perforin-granzyme pathway. Although we were able to generate anti-leukemia-specific CTL from the bone marrow, we were unable to generate such CTL from the peripheral blood of these patients. These studies show that antigen-specific CTL can be generated from the bone marrow of patients with pre-B-cell leukemias and these findings should facilitate the design of adoptive T-cell-mediated immunotherapy trials for the treatment of patients with B-cell precursor malignancies.

The IgV domain of human B7-2 (CD86) is sufficient to co-stimulate T lymphocytes and induce cytokine secretion

B7-1 (CD80) and B7-2 (CD86) are genetically and structurally related molecules expressed on antigen-presenting cells. Both bind CD28 to co-stimulate T lymphocytes, resulting in proliferation and cytokine production. The extracellular portions of B7-1 and B7-2 which bind to CD28 and CTLA-4 are related to Ig variable (V) and Ig constant (C) domain sequences. Recent reports have described splice variant forms of B7 proteins which occur in vivo and are of unknown function. Here we describe soluble recombinant forms of B7-1 and B7-2 containing either both of the Ig-like extracellular domains or the individual IgV or IgC domains coupled to an Ig Fc tail. Soluble B7-1 and B7-2 bind to CD28 and CTLA-4, and effectively co-stimulate T lymphocytes resulting in their proliferation and the secretion of cytokines. Furthermore, the IgV domain of B7-2 binds CD28 and CTLA-4, competes with B7-1 and B7-2 for binding to these receptors, and co-stimulates T lymphocytes. Cross-linked soluble B7-2v was the most potent co-stimulatory molecule tested and was active at a concentration approximately 100-fold lower than cross-linked soluble B7-1 or B7-2 proteins. When bound to tosyl-activated beads, B7-2v was capable of sustaining multiple rounds of T cell expansion. These data complement the description of naturally occurring variants to suggest that T cell co-stimulation in vivo may be regulated by soluble or truncated forms of B7 proteins.

Role of the CD40 and CD95 (APO-1/Fas) antigens in the apoptosis of human B-cell malignancies

Ligation of CD40 inhibits apoptosis and stimulates proliferation of normal B cells, whereas ligation of CD95 (APO-1/Fas) induces apoptosis of activated lymphocytes. Aberrant signalling through the CD40 and CD95 antigens could thus participate in the pathogenesis of lymphoid malignancies. The expression and function of CD40 and CD95 on neoplastic B cells from patients with acute lymphoblastic leukaemia (ALL), chronic lymphocytic leukaemia (CLL) and non-Hodgkin's lymphoma (NHL) were examined. CD40 was expressed by all 30 B-cell tumours, whereas CD95 was detected on neoplastic B cells in only one of 10 cases of ALL, two of 10 cases of CLL, and three of 10 cases of NHL. Incubation with an agonistic CD95 monoclonal antibody (MoAb) did not augment apoptosis in any of the unstimulated B-cell neoplasms. CD40 triggering did not consistently inhibit spontaneous apoptosis, but ultimately stimulated the growth of neoplastic B cells in most cases. Furthermore, CD40 activation led to up-regulation of the CD95 antigen in all 30 B-cell neoplasms. Ligation of CD95 on CD40-activated tumour cells augmented apoptosis in five of 10 ALL, three of 10 CLL, and nine of 10 NHL cases. The degree of apoptosis induced by CD95 triggering was greater for NHL cells than for ALL cells or CLL cells. Bcl-2 expression by ALL and NHL cells was substantially decreased after in vitro culture, whereas Bcl-2 expression by CLL cells was not significantly changed. However, there was no correlation between the level of Bcl-2 expression and sensitivity to CD95-mediated apoptosis. Thus, factors other than levels of CD95 and Bcl-2 determine susceptibility of malignant B cells to apoptosis after CD95 triggering. CD40-activated lymphoma cells appear to be very sensitive to CD95-mediated apoptosis, suggesting potential strategies for treatment of NHL. Elucidation of the mechanisms underlying resistance of ALL and CLL cells to CD95 triggering may facilitate the development of novel therapeutic approaches to these diseases as well.

B7.2 expressed by T cells does not induce CD28-mediated costimulatory activity but retains CTLA4 binding: implications for induction of antitumor immunity to T cell tumors

The B7 family of costimulatory molecules provides the second signal necessary for activation of T cells. In the absence of the second signal, responding T cells become anergic. Although predominantly expressed on professional APCs, recent evidence shows that the B7 molecules are also expressed on T cells. To study the functions of B7 molecules on T cells, we transfected murine B7.1 (CD80) and B7.2 (CD86) cDNAs into the EL4 T cell thymoma cell line and examined the transfectants for their ability to costimulate T cell proliferation in vitro and to induce antitumor immunity in vivo. Here we show that although EL4-B7.1 cells costimulate T cells and induce tumor regression, EL4-B7.2 transfectants failed to costimulate T cell proliferation or induce tumor regression. To understand the cellular basis for this difference, we examined the binding of EL4-B7.1 and EL4-B7.2 to CTLA4 and CD28. Whereas EL4-B7.1 cells bound both CTLA4-Ig and CD28-Ig, EL4-B7.2 transfectants preferentially bound CTLA4-Ig, but not CD28-Ig. Similar binding data were obtained with freshly isolated murine T cells, which have been shown to constitutively express B7.2. Our data suggest, therefore, that B7.2 expressed on T cells may not costimulate but instead inhibit the T cell response by preferential binding to CTLA4.

B7.2 expressed by T cells does not induce CD28-mediated costimulatory activity but retains CTLA4 binding: implications for induction of antitumor immunity to T cell tumors

The B7 family of costimulatory molecules provides the second signal necessary for activation of T cells. In the absence of the second signal, responding T cells become anergic. Although predominantly expressed on professional APCs, recent evidence shows that the B7 molecules are also expressed on T cells. To study the functions of B7 molecules on T cells, we transfected murine B7.1 (CD80) and B7.2 (CD86) cDNAs into the EL4 T cell thymoma cell line and examined the transfectants for their ability to costimulate T cell proliferation in vitro and to induce antitumor immunity in vivo. Here we show that although EL4-B7.1 cells costimulate T cells and induce tumor regression, EL4-B7.2 transfectants failed to costimulate T cell proliferation or induce tumor regression. To understand the cellular basis for this difference, we examined the binding of EL4-B7.1 and EL4-B7.2 to CTLA4 and CD28. Whereas EL4-B7.1 cells bound both CTLA4-Ig and CD28-Ig, EL4-B7.2 transfectants preferentially bound CTLA4-Ig, but not CD28-Ig. Similar binding data were obtained with freshly isolated murine T cells, which have been shown to constitutively express B7.2. Our data suggest, therefore, that B7.2 expressed on T cells may not costimulate but instead inhibit the T cell response by preferential binding to CTLA4.

B7-1 and B7-2 have overlapping, critical roles in immunoglobulin class switching and germinal center formation

Humoral immune responses were characterized in mouse strains lacking either or both B7 molecules. Mice deficient in both B7-1 and B7-2 failed to generate antigen-specific IgG1 and IgG2a responses and lacked germinal centers when immunized by a number of routes and even in the presence of complete Freund's adjuvant. These results demonstrate that B7-mediated signaling plays a critical role in germinal center formation and immunoglobulin class switching in vivo. Mice lacking only B7-1 or B7-2 mounted high-titer antigen-specific IgG responses when immunized in complete Freund's adjuvant, indicating that B7-1 and B7-2 can have overlapping, compensatory functions for IgG responses. When immunized intravenously without adjuvant, B7-2-deficient mice failed to switch antibody isotypes or form germinal centers, whereas B7-1-deficient mice gave antibody responses comparable with wild-type mice. Thus, B7-2 has an important role in initiating antibody responses in the absence of adjuvant, but the induction of B7-1 by adjuvant in B7-2-deficient mice can compensate for the absence of B7-2.

Breast cancer-associated antigen, DF3/MUC1, induces apoptosis of activated human T cells

Given the plethora of well-documented breast carcinoma-associated antigens in humans including MAGE-1, -2 and -3, mutated p53, p21ras, HER-2/neu and DF3/MUC-1, coupled with evidence that humoral and cytotoxic T-cell responses against these antigens exist, the central dilemma facing tumor immunologists is why the host immune response is so inefficient. One possibility is that tumor cells themselves are either inefficient or ineffective antigen-presenting cells (APCs). The failure of tumor cells to function as APCs may be due to their inability to process and present the antigen, the absence or insufficient numbers of adhesion and costimulatory molecules or, potentially, the secretion of inhibitory cytokines. Therefore, we sought to determine whether human breast cancer cell lines could function as APCs and, if not, to identify mechanism(s) responsible for this defect. Here, we show that human breast cancer cell lines fail to present alloantigen. This defect does not reside in their inherent capacity to present antigen but rather is due to apoptosis of activated T cells induced by exposure to the breast carcinoma-associated mucin antigen, DF3/MUC1. These results support the hypothesis that DF3/MUC1 may contribute to the paucity of clinically significant anticarcinoma-specific immune responses.

Human CD100, a novel leukocyte semaphorin that promotes B-cell aggregation and differentiation

Herein we describe the molecular characterization of the human leukocyte activation antigen CD100 and identify it as the first semaphorin, to our knowledge, in the immune system. Semaphorins have recently been described as neuronal chemorepellants that direct pioneering neurons during nervous system development. In this study we demonstrate that CD100 induces B cells to aggregate and improves their viability in vitro. We show that CD100 modifies CD40-CD40L B-cell signaling by augmenting B-cell aggregation and survival and down-regulating CD23 expression. Thus, these results suggest that semaphorins as exemplified by CD100 also play a functional role in the immune system.

Activation of human T cell lymphotropic virus type I-infected T cells is independent of B7 costimulation

Two distinct signals are required to activate T cells: an Ag-specific signal and a costimulatory signal mediated primarily by B7-1 (CD80) and B7-2 (CD86) through interactions with CD28. Costimulation appears to be critical in regulating autoreactive T cell responses. Here, we demonstrate that, in contrast to the parental uninfected T cell clone, a T cell clone infected by human T cell lymphotropic virus type I (HTLV-I) displays a remarkably enhanced response to Ag in the absence of B7 costimulation. Chinese hamster ovary cells either transfected with DRB1*1501 (t-DR2) alone or cotransfected with DR2 and either B7-1 or B7-2 were fixed, pulsed with myelin basic protein peptide 84-102 (MBPp84-102), and used as APCs. The MBPp84-102-reactive T cell clone Ob1A12.8 required costimulation with either B7-1 or B7-2 molecules, as the response to Ag was reduced by 90% in the absence of B7 costimulation. However, this requirement for B7 costimulation was abrogated after productive infection by HTLV-I. Stimulation of HTLV-I-infected T cells by MBPp84-102/t-DR2 induced the secretion of IL-5 and IFN-gamma, which approached the level induced in the presence of B7 costimulation, whereas IL-4 was induced to one third of its maximal level. Consistently, the secretion of IL-5 and IFN-gamma was not significantly inhibited by anti-B7-1 and B7-2 Abs, whereas IL-4 was inhibited by approximately 50%. In contrast, uninfected T cells required either B7-1 or B7-2 costimulation for significant cytokine secretion, and this response was inhibited by anti-B7-1 and B7-2 Abs. These findings suggest that HTLV-I-infected autoreactive T cells have the potential to induce an autoimmune response in the absence of B7 expression in the target organ. This may be of particular interest in the elucidation of HTLV-I pathogenicity given the association of HTLV-I infection with autoimmune-like diseases.

Activation of human T cell lymphotropic virus type I-infected T cells is independent of B7 costimulation

Two distinct signals are required to activate T cells: an Ag-specific signal and a costimulatory signal mediated primarily by B7-1 (CD80) and B7-2 (CD86) through interactions with CD28. Costimulation appears to be critical in regulating autoreactive T cell responses. Here, we demonstrate that, in contrast to the parental uninfected T cell clone, a T cell clone infected by human T cell lymphotropic virus type I (HTLV-I) displays a remarkably enhanced response to Ag in the absence of B7 costimulation. Chinese hamster ovary cells either transfected with DRB1*1501 (t-DR2) alone or cotransfected with DR2 and either B7-1 or B7-2 were fixed, pulsed with myelin basic protein peptide 84-102 (MBPp84-102), and used as APCs. The MBPp84-102-reactive T cell clone Ob1A12.8 required costimulation with either B7-1 or B7-2 molecules, as the response to Ag was reduced by 90% in the absence of B7 costimulation. However, this requirement for B7 costimulation was abrogated after productive infection by HTLV-I. Stimulation of HTLV-I-infected T cells by MBPp84-102/t-DR2 induced the secretion of IL-5 and IFN-gamma, which approached the level induced in the presence of B7 costimulation, whereas IL-4 was induced to one third of its maximal level. Consistently, the secretion of IL-5 and IFN-gamma was not significantly inhibited by anti-B7-1 and B7-2 Abs, whereas IL-4 was inhibited by approximately 50%. In contrast, uninfected T cells required either B7-1 or B7-2 costimulation for significant cytokine secretion, and this response was inhibited by anti-B7-1 and B7-2 Abs. These findings suggest that HTLV-I-infected autoreactive T cells have the potential to induce an autoimmune response in the absence of B7 expression in the target organ. This may be of particular interest in the elucidation of HTLV-I pathogenicity given the association of HTLV-I infection with autoimmune-like diseases.

Coexpression of B7-1 and antigen blocks tolerance induction to antigen presented by resting B cells

To investigate a role for B cells as tolerogenic APCs in peripheral lymphoid organs, we have developed a system in which B cells from mice transgenic for the membrane-bound form of human mu-chain are transferred into nontransgenic recipients. Mice injected with B cells expressing human mu-chain became profoundly tolerant to human mu-chain, as shown by greatly reduced Ab responses following challenge with human mu-chain in adjuvant. Adoptive transfer experiments showed that the recipient's Th cell response to human mu-chain was impaired. When the human mu transgenic spleen cells were activated with LPS before transfer, they no longer induced tolerance. Spleen cells from double-transgenic mice expressing both human mu-chain and the costimulatory molecule B7-1 (CD80) also failed to induce tolerance to human mu-chain. However, neither human mu transgenic LPS blasts nor double-transgenic B cells induced an Ab response or primed for a secondary Ab response to Ag in adjuvant. Therefore, we find that expression of B7-1 together with Ag can interfere with tolerance induction without inducing Ab formation or priming for a secondary Ab response.

Coexpression of B7-1 and antigen blocks tolerance induction to antigen presented by resting B cells

To investigate a role for B cells as tolerogenic APCs in peripheral lymphoid organs, we have developed a system in which B cells from mice transgenic for the membrane-bound form of human mu-chain are transferred into nontransgenic recipients. Mice injected with B cells expressing human mu-chain became profoundly tolerant to human mu-chain, as shown by greatly reduced Ab responses following challenge with human mu-chain in adjuvant. Adoptive transfer experiments showed that the recipient's Th cell response to human mu-chain was impaired. When the human mu transgenic spleen cells were activated with LPS before transfer, they no longer induced tolerance. Spleen cells from double-transgenic mice expressing both human mu-chain and the costimulatory molecule B7-1 (CD80) also failed to induce tolerance to human mu-chain. However, neither human mu transgenic LPS blasts nor double-transgenic B cells induced an Ab response or primed for a secondary Ab response to Ag in adjuvant. Therefore, we find that expression of B7-1 together with Ag can interfere with tolerance induction without inducing Ab formation or priming for a secondary Ab response.

Differential association of protein tyrosine kinases with the T cell receptor is linked to the induction of anergy and its prevention by B7 family-mediated costimulation

When stimulated through their antigen receptor, without costimulation, T cells enter a state of antigen-specific unresponsiveness, termed anergy. B7-mediated costimulation, signaling via CD28, is sufficient to prevent the induction of anergy. Here we show that ligation of T cell receptor (TCR) by alloantigen alone, which results in anergy, activates tyrosine phosphorylation of TCR zeta and its association with fyn. In contrast, TCR ligation in the presence of B7 costimulation, which results in productive immunity, activates tyrosine phosphorylation of TCR zeta and CD3 chains, which associate with activated lck and zeta-associated protein (ZAP) 70. Under these conditions, CD28 associates with activated lck and TCR zeta. These data suggest that the induction of anergy is an active signaling process characterized by the association of TCR zeta and fyn. In addition, CD28-mediated costimulation may prevent the induction of anergy by facilitating the effective association of TCR zeta and CD3 epsilon with the critical protein tyrosine kinase lck, and the subsequent recruitment of ZAP-70. Strategies to inhibit or activate TCR-associated, specific protein tyrosine kinase-mediated pathways may provide a basis for drug development with potential applications in the fields of transplantation, autoimmunity, and tumor immunity.

Pre-B acute lymphoblastic leukemia cells may induce T-cell anergy to alloantigen

Even if neoplastic cells express tumor associated antigens they still may fail to function as antigen presenting cells (APC) if they lack expression of one or more molecules critical for the induction of productive immunity. These cellular defects can be repaired by physiologic activation, transfection, or fusion of tumor cells with professional APC. Although such defects can be repaired, antitumor specific T cells may still fail to respond in vivo if they may have been tolerized. Here, human pre-B cell acute lymphoblastic leukemia (pre-B ALL) was used as a model to determine if primary human tumor cells can function as alloantigen presenting cells (alloAPC) or alternatively whether they induce anergy. In the present report, we show that pre-B cell ALL express alloantigen and adhesion molecules but uniformly lack B7-1 (CD80) and only a subset express B7-2 (CD86). Pre-B ALL cells are inefficient or ineffective alloAPC and those cases that lack expression of B7-1 and B7-2 also induce alloantigen specific T-cell unresponsiveness. Under these circumstances, T-cell unresponsiveness could be prevented by physiologic activation of tumor cells via CD40, cross-linking CD28, or signaling through the common gamma chain of the interleukin-2 receptor on T cells. Taken together, these results suggest that pre-B ALL may be incapable of inducing clinically significant T-cell-mediated antileukemia responses. This defect may be not only due to their inability to function as APC, but also due to their potential to induce tolerance. Attempts to induce clinically significant antitumor immune responses may then require not only mechanisms to repair the antigen presenting capacity of the tumor cells, but also reversal of tolerance.

Complete blockade of B7 family-mediated costimulation is necessary to induce human alloantigen-specific anergy: a method to ameliorate graft-versus-host disease and extend the donor pool

Graft-versus-host disease (GVHD) is initiated by adoptively transferred donor T cells that recognize host alloantigens. Whereas the absence of donor T-cell proliferation to host alloantigens in a mixed-leukocyte reaction does not predict freedom from GVHD, the frequency of alloreactive precursor helper T lymphocytes (pHTL) is predictive. Complete blockade of 87 family-mediated costimulation, but not of major histocompatibility complex recognition or adhesion, induces host alloantigenic-specific energy by reducing cytokine production below threshold levels necessary for common gamma chain signaling. The associated reduction of alloreactive pHTL frequency below that predictive for GVHD, without depletion of either nonallospecific T cells or hematopoietic progenitors, has led us to embark upon human clinical trials of haplomismatched allogeneic bone marrow transplantation.

A cell type-specific enhancer in the human B7.1 gene regulated by NF-kappaB

The costimulatory molecule B7.1 provides a second signal critical for T cell activation. The distribution of this integral membrane protein is restricted to certain tissues where its level of expression is modulated by multiple exogenous stimuli. To identify the molecular basis for specificity and inducibility, the chromatin configuration of the human B7.1 gene was examined in intact nuclei from various cell types. The identification of a tissue-specific deoxyribonuclease I hypersensitive site approximately 3kb upstream of the transcription start site led to the characterization of a cell type-specific enhancer region. This 183-bp region was both cell type specific and responsive to two distinct stimuli, lipopolysaccharide and dibutyryl cAMP, known to regulate B7.1 expression. Deletional and site-directed mutagenesis revealed the presence of multiple functionally critical cis elements within this region, one of which was a nuclear factor (NF)-kappaB consensus sequence. In B7.1-positive B cells, this element bound several members of the NF-kappaB family, transcription factors already implicated in signal transduction pathways relevant to B7.1 expression. This is the first description, to our knowledge, of regulatory elements that control expression of a gene encoding a B7 costimulatory molecule.

Differential expression of alternate mB7-2 transcripts

The murine B7-2 (mB7-2) costimulatory molecule is expressed on APCs early during the course of an immune response, suggesting that it may play a pivotal role in the decision between T cell activation and anergy. Murine B7-2 mRNA displays a restricted pattern of expression; it is inducible in B cells, T cells, NK cells, and dendritic cells, but constitutively expressed in unstimulated monocytes. The constitutive and inducible expression of mB7-2 on distinct cell types indicates that mB7-2 is regulated differentially. To further characterize mB7-2 transcripts, we employed 5' rapid amplification of cDNA ends and reverse transcriptase-PCR to examine transcripts expressed in a variety of types of APCs and analyzed the genomic organization of the mB7-2 gene. We report here that the mB7-2 locus consists of 12 exons and demonstrate that exons 1 through 5 can be used in alternative fashions to produce five distinct transcripts, differing in their 5' untranslated and signal regions. The expression of these transcripts differs in distinct types of APCs and is modulated by stimuli that activate B cells. These results demonstrate that mB7-2 transcripts are differentially regulated in a tissue-specific fashion and in response to activation stimuli.

Differential expression of alternate mB7-2 transcripts

The murine B7-2 (mB7-2) costimulatory molecule is expressed on APCs early during the course of an immune response, suggesting that it may play a pivotal role in the decision between T cell activation and anergy. Murine B7-2 mRNA displays a restricted pattern of expression; it is inducible in B cells, T cells, NK cells, and dendritic cells, but constitutively expressed in unstimulated monocytes. The constitutive and inducible expression of mB7-2 on distinct cell types indicates that mB7-2 is regulated differentially. To further characterize mB7-2 transcripts, we employed 5' rapid amplification of cDNA ends and reverse transcriptase-PCR to examine transcripts expressed in a variety of types of APCs and analyzed the genomic organization of the mB7-2 gene. We report here that the mB7-2 locus consists of 12 exons and demonstrate that exons 1 through 5 can be used in alternative fashions to produce five distinct transcripts, differing in their 5' untranslated and signal regions. The expression of these transcripts differs in distinct types of APCs and is modulated by stimuli that activate B cells. These results demonstrate that mB7-2 transcripts are differentially regulated in a tissue-specific fashion and in response to activation stimuli.

Follicular lymphomas can be induced to present alloantigen efficiently: a conceptual model to improve their tumor immunogenicity

In the tumor-bearing host, T cells invariably fail to induce a clinically significant antitumor immune response. Although model systems support the existence of tumor peptide antigens, the molecular interactions critical for antigen presentation by the tumor cell remain unresolved. Here, we demonstrate that human follicular lymphoma cells are highly inefficient at presenting alloantigen despite their strong expression of major histocompatibility complex and low-to-intermediate expression of some adhesion and B7 costimulatory molecules. Activation of follicular lymphoma cells via CD40 induces or up-regulates both adhesion and B7 costimulatory molecules essential to repair this defect. More importantly, once primed, alloreactive T cells efficiently recognize unstimulated follicular lymphoma cells. Thus, correction of defective tumor immunity requires not only expression of major histocompatibility complex but also sufficient expression of multiple adhesion and costimulatory molecules.

Both CD28 ligands CD80 (B7-1) and CD86 (B7-2) activate phosphatidylinositol 3-kinase, and wortmannin reveals heterogeneity in the regulation of T cell IL-2 secretion

In this report, the co-stimulatory signals provided by CD80 (B7-1) or CD86 (B7-2) were compared to CD28 ligation by mAb. We demonstrate that while both anti-CD3 and anti-CD28 antibodies induced activation of phosphoinositide (PI) 3-kinase, the kinetics of activation differed. Anti-CD28 produced a sustained activation of PI 3-kinase while anti-CD3 induced activation was transient. Both B7-1 and B7-2 could induce prolonged activation of PI 3-kinase. The co-stimulatory effects of B7-1 and B7-2 were dependent on CD28 cross-linking, based on complete inhibition of PI 3-kinase activation by CD28 antibody Fab fragments. While Jurkat T cells co-stimulated with anti-CD3 and B7-1 or B7-2 secreted high levels of IL-2, there were distinct effects of anti-CD28 mAb and B7-1 or B7-2 on IL-2 secretion in conjunction with protein kinase C activation. To assess functional effects of CD28 ligation, pharmacologic inhibitors of PI 3-kinase were evaluated. In Jurkat cells, efficient inhibition of PI 3-kinase activation after B7-2 stimulation was achieved using wortmannin; however, we observed a surprising increase in IL-2 secretion after B7 or anti-CD28 stimulation. The effect of wortmannin was concentration dependent. Moreover, the effect was specific for receptor-mediated activation as wortmannin did not enhance phorbol ester plus ionomycin-induced IL-2 secretion. Another inhibitor of PI 3-kinase, LY294002, also resulted in augmentation of anti-CD28-induced IL-2 secretion by Jurkat cells. The effects of wortmannin on IL-2 secretion were also examined in primary T cells. In marked contrast, wortmannin resulted in a potent inhibition of anti-CD3 plus B7-1 or anti-CD28-induced IL-2 secretion while phorbol ester plus ionomycin-induced IL-2 secretion was wortmannin resistant. Together these observations demonstrate that signal transduction by both B7-1 and B7-2 involves PI 3-kinase, and that PI 3-kinase or other wortmannin-sensitive targets are important for IL-2 secretion. Finally, treatment of Jurkat cells with PI 3-kinase inhibitors alone was sufficient to induce low levels of IL-2 secretion. This is consistent with the notion that a wortmannin-sensitive target such as PI 3-kinase may down-regulate IL-2 secretion in Jurkat cells.

Role of B7-1 in mediating an immune response to myeloid leukemia cells

A costimulatory signal from B7-1 (CD80) to its counter-receptor CD28 is required for T-cell activation. Many tumors, including most human leukemias, lack expression of B7-1, and this has been suggested to contribute to the failure of immune recognition of these diseases. A murine leukemia model system was developed to assess the potential role of B7-1 in the induction immunity to leukemia cells. The nonleukemic 32Dc13 myeloid cell line was transformed by transfection of the BCR/ABL gene, generating a subline (32Dp210/clone 26) that was leukemic and rapidly lethal to syngeneic, immunocompetent C3H/HeJ mice or T-cell-deficient nude mice. B7-1-modified leukemic cells remained lethal in nude mice, but caused only a transient, nonlethal leukemia in C3H/HeJ mice. After a single exposure to live, nonirradiated B7-1-modified leukemic cells, C3H/HeJ mice developed protective immunity against subsequent challenge with B7-1(-) leukemic cells. Further, hyperimmunization with B7-1(+) leukemic cells prolonged the survival of mice previously injected with a lethal number of B7-1(-) leukemic cells. These results indicate that myeloid leukemic cells may be attractive candidates for B7-1 gene transfer.