LICOS, a primordial costimulatory ligand?

In mammals, the classical B7 molecules expressed on antigen-presenting cells, B7-1 (CD80) and B7-2 (CD86), bind the structurally related glycoproteins CD28 and CTLA-4 (CD152), generating costimulatory signals that regulate the activation state of T cells. A recently identified human CD28-like protein, ICOS, also induces costimulatory signals in T cells when crosslinked with antibodies, but it is unclear whether ICOS is part of a B7-mediated regulatory pathway of previously unsuspected complexity, or whether it functions independently and in parallel. Here, we report that, rather than binding B7-1 or B7-2, ICOS binds a new B7-related molecule of previously unknown function that we call LICOS (for ligand of ICOS). At 37 degrees C, LICOS binds only to ICOS but, at lower, non-physiological temperatures, it also binds weakly to CD28 and CTLA-4. Sequence comparisons suggest that LICOS is the homologue of a molecule expressed by avian macrophages and of a murine protein whose expression is induced in non-lymphoid organs by tumour necrosis factor alpha (TNFalpha). Our results define the components of a distinct and novel costimulatory pathway and raise the possibility that LICOS, rather than B7-1 or B7-2, is the contemporary homologue of a primordial vertebrate costimulatory ligand.     

The cytoplasmic domain of CD28 is both necessary and sufficient for costimulation of interleukin-2 secretion and association with phosphatidylinositol 3''-kinase.

T-cell activation requires two signaling events. One is provided by the engagement of the T-cell antigen receptor, and the second represents a costimulatory signal provided by antigen-presenting cells. CD28 mediates a costimulatory signal by binding its ligands, B7-1 and B7-2, on antigen-presenting cells, but the signaling pathway activated by CD28 has not been identified. A homologous molecule, CTLA-4, expressed on activated T cells, also binds to B7-1 and B7-2, but whether it has a signaling function is not known. We performed a structure-function analysis of CD28 to identify the functional domain which activates signal transduction. Truncation of the 40-amino-acid CD28 cytoplasmic tail abrogated costimulatory signaling. Chimeric constructs containing the extracellular and transmembrane regions of CD8 linked to the cytoplasmic region of CD28 had a costimulatory signaling function. Similar chimeras containing the cytoplasmic tail of CTLA-4 did not signal. Thus, the cytoplasmic region of CD28, but not CTLA-4, is sufficient to mediate costimulatory signaling. In addition, after CD28 stimulation, the p85 subunit of phosphatidylinositol 3'-kinase and phosphatidylinositol 3'-kinase activity were found in CD28 immunoprecipitates. The CD8-CD28 chimera, which has a costimulatory signaling function, associates with p85, while the nonfunctioning CD8-CTLA-4 chimera and a CD8-zeta chimera do not associate with p85. These results suggest that phosphatidylinositol 3'-kinase is specifically activated by CD28 and may mediate proximal events in the costimulatory signaling pathway regulated by CD28.     

The radiation-sensitive costimulatory factors involved in B-cell-dependent T-cell activation by minor lymphocyte stimulating antigen

The regulation of CD28/B7 is important in T-cell activation. It has been argued that its aberrant expression is involved in the radiosensitivity of B-cell-stimulated T-cell response. Here, this possibility is studied in the mixed lymphocyte reaction (MLR) induced by minor lymphocyte-stimulating (Mls) antigen-presenting irradiated B cells. By using anti-CD28 antibody, the CD28/B7-2-, LFA-1/ICAM-1-dependent Mls-MLR was found to be restored. By flow cytometry, approximately 70% B cells were lost but with unaffected B7-2 expression, indicating that the moderate CD28 costimulation was caused by mortality of antigen presenting cells. Despite of costimulatory deficiency, T cells were shown primed. However, the expression of early activation markers CD25 and CD69, which was shown unaffected by B7/CD28 blocking, was found partially inhibited. To further understand the regulation, we examined the ICAM-1 expression, and found that it was again not altered on irradiated B cells. Thus, the radiation-induced rapid loss of resting B cells may be the basic mechanism causing insufficient costimulatory activity in radiosensitive B-T interaction. Furthermore, the presence of an element, other than B7-2, involving in controlling early T-cell response is suggested.     

A novel bispecific tetravalent antibody fusion protein to target costimulatory activity for T-cell activation to tumor cells overexpressing ErbB2/HER2

Persistent activation of T-lymphocytes requires two signals: one is initiated by T-cell receptor binding to antigenic peptide presented by MHC molecules. In addition, binding of the B7 family members CD80 or CD86 on professional antigen presenting cells to CD28 on T cells is considered to provide an important costimulatory signal. Activation without costimulation induces T-cell unresponsiveness or anergy. To selectively localize costimulatory activity to the surface of tumor cells and enhance activation of tumor-specific T cells, we have developed a novel molecular design for bispecific costimulatory proteins with antibody-like structure. Within a single polypeptide chain we have assembled the IgV-like, CD28-binding domain of human CD86 (CD86(111)) together with hinge, CH2 and CH3 domains of human IgG1, and the scFv(FRP5) antibody fragment which recognizes the ErbB2 (HER2) protooncogene present at high levels on the surface of many human tumor cells. Upon expression in the yeast Pichia pastoris, the resulting CD86(111)-IgG-scFv(FRP5) protein could be purified as a homodimeric, tetravalent molecule from culture supernatants using single-step affinity chromatography. Bispecific binding of the molecule to ErbB2 on the surface of tumor cells and to the B7 counter receptor CTLA-4 was demonstrated by FACS analysis. Potent costimulatory activity of chimeric CD86(111)-IgG-scFv(FRP5) was confirmed by its ability to stimulate the proliferation of primary human lymphocytes pre-activated by low concentrations of anti-CD3 antibody. Our results suggest that such multivalent soluble proteins which combine specific targeting to tumor cells with costimulatory activity may become useful tools to elicit and/or improve T-cell mediated, tumor-specific immune responses.     

Structural basis of inducible costimulator ligand costimulatory function: determination of the cell surface oligomeric state and functional mapping of the receptor binding site of the protein

Inducible costimulator (ICOS) ligand (ICOSL), a B7-related transmembrane glycoprotein with extracellular IgV and IgC domains, binds to ICOS on activated T cells and delivers a positive costimulatory signal for optimal T cell function. Toward determining the structural features of ICOSL crucial for its costimulatory function, the present study shows that ICOSL displays a marked oligomerization potential, resembling more like B7-1 than B7-2. Use of ICOSL constructs lacking either the IgC or IgV domain demonstrates that receptor binding is mediated solely by the IgV domain but requires the IgC domain for maintaining the structural integrity of the protein. To map further the receptor recognition surface on ICOSL, a homology-based protein structure model of the ICOS:ICOSL complex was constructed. Based on predictions from the model, a series of mutations were generated targeting the potential receptor binding surface on ICOSL, and the mutants were tested for their biological function in terms of ICOS binding and T cell costimulation ability. The results provide experimental validation of the model and show that the receptor binding site on ICOSL is constituted chiefly by aromatic/hydrophobic residues. Critical ICOSL residues essential for ICOS binding map to the GFCC'C' beta-sheet face of the IgV domain and approximately overlap with the B7-1/B7-2 motif(s) that recognize CD28/CTLA-4. Altogether, similar structural features of ICOSL and B7 isoforms suggest a close evolutionary relationship between these costimulatory ligands, yet differences at the same time explain their unique specificity for the cognate binding partners, ICOS and CD28/CTLA-4, respectively.     

CD28 is not required for c-Jun N-terminal kinase activation in T cells.

Studies in Jurkat cells have shown that combined stimulation through the TCR and CD28 is required for activation of c-Jun N-terminal kinase (JNK), suggesting that JNK activity may mediate the costimulatory function of CD28. To examine the role of JNK signaling in CD28 costimulation in normal T cells, murine T cell clones and CD28(+/+) or CD28(-/-) TCR transgenic T cells were used. Although ligation with anti-CD28 mAb augmented JNK activation in Th1 and Th2 clones stimulated with low concentrations of anti-CD3 mAb, higher concentrations of anti-CD3 mAb alone were sufficient for JNK activation even in the absence of anti-CD28. JNK activity was comparably induced in both CD28(+/+) and CD28(-/-) 2C/recombinase-activating gene 2(RAG2)(-/-) T cells stimulated with anti-CD3 mAb alone, and with L(d)/peptide dimers, a direct alphabeta TCR ligand. Moreover, JNK activation was also detected in 2C/RAG2(-/-) T cells stimulated with P815 cells that express the relevant alloantigen L(d) whether or not B7-1 was coexpressed. However, IL-2 production by both Th1 clones and CD28(+/+) 2C/RAG2(-/-) T cells was detected only upon TCR and CD28 coengagement. Thus, CD28 coligation is not necessary, and stimulation through the TCR is sufficient, for JNK activation in normal murine T cells. The concept that JNK mediates the costimulatory function of CD28 needs to be reconsidered.     

CD28 signaling augments Elk-1-dependent transcription at the c-fos gene during antigen stimulation.

Untransformed CD4(+) Th1 cells stimulated with Ag and APC demonstrated a dependence on B7- and CD28-mediated costimulatory signals for the expression and function of AP-1 proteins. The induction of transactivation by the c-fos gene regulator Elk-1 mirrored this requirement for TCR and CD28 signal integration. c-Jun N-terminal kinase (JNK) (but not extracellular signal-regulated kinase or p38) protein kinase activity was similarly inhibited by neutralizing anti-B7 mAbs. Blockade of JNK protein kinase activity with SB 202190 prevented both Elk-1 transactivation and c-Fos induction. These results identify a unique role for B7 costimulatory molecules and CD28 in the activation of JNK during Ag stimulation in Th1 cells, and suggest that JNK regulates Elk-1 transactivation at the c-fos gene to promote the formation of AP-1 complexes important to IL-2 gene expression.     

B7-CD28 interaction promotes proliferation and survival but suppresses differentiation of CD4-CD8- T cells in the thymus

Costimulatory molecules play critical roles in the induction and effector function of T cells. More recent studies reveal that costimulatory molecules enhance clonal deletion of autoreactive T cells as well as generation and homeostasis of the CD25(+)CD4(+) regulatory T cells. However, it is unclear whether the costimulatory molecules play any role in the proliferation and differentiation of T cells before they acquire MHC-restricted TCR. In this study, we report that targeted mutations of B7-1 and B7-2 substantially reduce the proliferation and survival of CD4(-)CD8(-) (double-negative (DN)) T cells in the thymus. Perhaps as a result of reduced proliferation, the accumulation of RAG-2 protein in the DN thymocytes is increased in B7-deficient mice, which may explain the increased expression of TCR gene and accelerated transition of CD25(+)CD44(-) (DN3) to CD25(-)CD44(-) (DN4) stage. Qualitatively similar, but quantitatively less striking effects were observed in mice with a targeted mutation of CD28, but not CTLA4. Taken together, our results demonstrate that the development of DN in the thymus is subject to modulation by the B7-CD28 costimulatory pathway.     

A theoretical framework for quantitative analysis of the molecular basis of costimulation

We present a theoretical framework for simulating the synaptic accumulation of the costimulatory molecules CD28, CTLA-4, B7-1, and B7-2, based on a system of mean-field, ordinary differential equations, and rigorous biophysical and expression data. The simulations show that binding affinity, stoichiometric properties, expression levels, and, in particular, competition effects all profoundly influence complex formation at cellular interfaces. B7-2 engages 33-fold more CD28 than CTLA-4 at the synapse in contrast to B7-1, which ligates approximately 7-fold more CTLA-4 than CD28. Although B7-1 completely dominates interactions with CTLA-4, forming linear arrays of 7-18 receptor-ligand pairs, CTLA-4 is fully engaged by B7-2 when B7-1 is absent. Additional simulations reveal the sensitivity of CD28 interactions to modeled transport processes. The results support the concept that B7-2 and B7-1 are the dominant ligands of CD28 and CTLA-4, respectively, and indicate that the inability of B7-2 to recruit CTLA-4 to the synapse cannot be due to the differential binding properties of B7-1 and B7-2 only. We discuss the apparent redundancy of B7-1 in the context of a potentially dynamic synaptic microenvironment, and in light of functions other than the direct enhancement of T cell inhibition by CTLA-4.     

CD28-specific antibody prevents graft-versus-host disease in mice

The costimulatory molecules B7-1 and B7-2 regulate T cell activation by delivering activation signals through CD28 and inhibitory signals through CTLA4. Graft-vs-host disease (GVHD) is caused by activated donor T cells. Previously, we showed that CD28-deficient donor T cells induced less-severe GVHD than wild-type donor T cells, suggesting that CD28 signals exacerbate GVHD. In this paper we demonstrate that CTLA4 signals attenuate the severity of GVHD. Targeting the CD28 receptor with a specific mAb modulates the receptor in vivo, inhibits donor T cell expansion, and prevents GVHD. CTLA4 signaling was necessary for this effect because treatment with a soluble ligand that blocks binding of B7 to both CD28 and CTLA4 did not prevent GVHD as effectively as anti-CD28 mAb. These results support the current model of T cell costimulation in which CD28 signals amplify GVHD while CTLA4 signals inhibit GVHD, providing evidence that selective targeting of CD28 might be a better therapeutic strategy for inducing immunological tolerance than blocking the ligands for both CD28 and CTLA4.     

B7/CD28 costimulation is critical in susceptibility to Pseudomonas aeruginosa corneal infection: a comparative study using monoclonal antibody blockade and CD28-deficient mice

Evidence suggests that Pseudomonas aeruginosa stromal keratitis and corneal perforation (susceptibility) is a CD4(+) T cell-regulated inflammatory response following experimental P. aeruginosa infection. This study examined the role of Langerhans cells (LC) and the B7/CD28 costimulatory pathway in P. aeruginosa-infected cornea and the contribution of costimulatory signaling by this pathway to disease pathology. After bacterial challenge, the number of LC infiltrating the central cornea was compared in susceptible C57BL/6 (B6) vs resistant (cornea heals) BALB/c mice. LC were more numerous at 1 and 6 days postinfection (p.i.), but were similar at 4 days p.i., in susceptible vs resistant mice. Mature, B7 positive-stained LC in the cornea and pseudomonas Ag-associated LC in draining cervical lymph nodes also were increased significantly p.i. in susceptible mice. To test the relevance of these data, B6 mice were treated systemically and subconjunctivally with neutralizing B7 (B7-1/B7-2) mAbs. Treatment decreased corneal disease severity and reduced significantly the number of B7-positive cells as well as the recruitment and activation of CD4(+) T cells in the cornea. IFN-gamma mRNA levels also were decreased significantly in the cornea and in draining cervical lymph nodes of mAb-treated mice. When CD28(-/-) animals were tested, they exhibited a less severe disease response (no corneal perforation) than wild-type B6 mice and had a significantly lower delayed-type hypersensitivity response to heat-killed pseudomonas Ag. These results support a critical role for B7/CD28 costimulation in susceptibility to P. aeruginosa ocular infection.     

CD80 (B7-1) and CD86 (B7-2) are functionally equivalent in the initiation and maintenance of CD4+ T-cell proliferation after activation with suboptimal doses of PHA

Effective activation of T cells requires engagement of two separate T-cell receptors. The antigen-specific T-cell receptor (TCR) binds foreign peptide antigen-MHC complexes, and the CD28 receptor binds to the B7 (CD80/CD86) costimulatory molecules expressed on the surface of antigen-presenting cells (APC). The simultaneous triggering of these T-cell surface receptors with their specific ligands results in an activation of this cell. In contrast, CTLA-4 (CD152) is a distinct T-cell receptor that, upon binding to B7 molecules, sends an inhibitory signal to T cell activation. Many in vitro and in vivo studies demonstrated that both CD80 and CD86 ligands have an identical role in the activation of T cells. Recently, functions of B7 costimulatory molecules in vivo have been investigated in B7-1 and/or B7-2 knockout mice, and the authors concluded that CD86 could be more important for initiating T-cell responses, while CD80 could be more significant for maintaining these immune responses. In this study, we directly compared the role of CD80 and CD86 in initiating and maintaining proliferation of resting CD4(+) T cells in an in vitro mode system that allowed to provide the first signal-to-effector cells through the use of suboptimal doses of PHA and the second costimulatory signal through cells expressing CD80 or CD86, but not any other costimulatory molecules. Using this experimental system we demonstrate that the CD80 and CD86 molecules can substitute for each other in the initial activation of resting CD4(+) T cells and in the maintenance of their proliferative response.     

Suppression of experimental autoimmune encephalomyelitis using peptide mimics of CD28

The B7:CD28/CTLA-4 costimulatory pathway plays a critical role in regulating the immune response and thus provides an ideal target for therapeutic manipulation of autoimmune disease. Previous studies have shown that blockade of CD28 signaling by mAbs can both prevent and exacerbate experimental autoimmune encephalomyelitis (EAE). In this study, we have designed two CD28 peptide mimics that selectively block B7:CD28 interactions. By surface plasmon resonance, both the end group-blocked CD28 peptide (EL-CD28) and its retro-inverso isomer (RI-CD28) compete effectively with the extracellular domain of CD28 for binding to B7-1. Both the CD28 peptide mimics inhibited expansion of encephalitogenic T cells in vitro. A single administration of EL-CD28 or RI-CD28 peptide significantly reduced disease severity in EAE. Importantly, we show that either CD28 peptide mimic administered during acute disease dramatically improved clinical signs of EAE, suppressing ongoing disease. The ratio of CD80:CD86 expression was significantly lower on CD4(+) and F4/80(+) spleen cells in CD28 peptide-treated mice. Peripheral deletion of Ag-specific CD4(+) T cells occurs following in vivo blockade of CD28 with synthetic CD28 peptides.     

B7 costimulates proliferation of CD4-8+ T lymphocytes but is not required for the deletion of immature CD4+8+ thymocytes.

In addition to TCR-derived signals, costimulatory signals derived from stimulation of the CD28 molecule by its natural ligand, B7, have been shown to be required for CD4+8- T cell activation. We investigate the ability of B7 to provide costimulatory signals necessary to drive proliferation and differentiation of virgin CD4-8+ T-cells that express a transgenic TCR specific for the male (H-Y) Ag presented by H-2Db class I MHC molecules. Virgin male-specific CD4-8+ T cells can be activated either with B7 transfected chinese hamster ovary (CHO) cells and T3.70, a mAb specific for the transgenic TCR-alpha chain that is associated with male-reactivity, or by male dendritic cells (DC). Activated CD4-8+ T cells proliferated in the absence of exogenously added IL-2. IL-2 activity was detected in supernatants of CD4-8+T3.70+ cells that were stimulated with T3.70 and B7+CHO cells. The response of CD4-8+T3.70+ cells to T3.70/B7+CHO or to male DC stimulation were inhibited by CTLA4Ig, a fusion protein comprising the extracellular portion of CTLA4 and human IgG C gamma 1. It has been previously shown that CTLA4Ig binds B7 with high affinity. Staining with CTLA4Ig revealed that DC express about 50 times more B7 than CD4-8+ T cells. CTLA4Ig also specifically blocked the proliferation of male-reactive cells in vivo. We have also used an in vitro deletion assay whereby immature CD4+8+ thymocytes expressing the transgenic male-specific TCR are deleted by overnight incubation with either immobilized T3.70 or male DC to investigate the participation of the CD28/B7 pathway in the negative selection of immature thymocytes. Staining with B7Ig established that both immature murine CD4+8+ and mature CD4-8+ thymocytes express a high level of CD28. However, despite the high expression of CD28 on CD4+8+ thymocytes, it was found that deletion of CD4+8+ thymocytes expressing the male-specific TCR by the T3.70 mAb was not inhibited by B7+CHO cells. Furthermore, the deletion of these thymocytes by DC also was not inhibited by CTLA4Ig. These findings provide evidence that although signaling through CD28 can costimulate a primary anti-male response in mature CD4-8+ T cells, the CD28/B7 pathway does not appear to participate in the negative selection of immature CD4+8+ thymocytes.     

Expression and contribution of B7-1 (CD80) and B7-2 (CD86) in the early immune response to Leishmania major infection.

CD28 interactions promote T cell responses, and whether B7-1 or B7-2 is utilized may influence Th cell subset development. CD28 blockade by CTLA-4Ig treatment or by targeted gene disruption has yielded different conclusions regarding the role of CD28 in the development of Th1 and Th2 cells following Leishmania major infection. In this study, we demonstrate that B7-mediated costimulation is required for the development of the early immune response following infection of resistant or susceptible mice. In contrast, CD28-/- BALB/c mice infected with L. major produce cytokines comparable to those of infected wild-type mice. Treatment of CD28-/- mice with CTLA-4Ig did not diminish this response, suggesting that a B7-independent pathway(s) contributes to the early immune response in these mice. In conventional BALB/c or C3H mice, B7-2 functions as the dominant costimulatory molecule in the initiation of early T cell activation following L. major infection, leading to IL-4 or IFN-gamma production, respectively. The preferential interaction of B7-2 with its ligand(s) in the induction of these responses correlates with its constitutive expression relative to that of B7-1. However, B7-1 can equally mediate costimulation for the production of either IL-4 or IFN-gamma when expressed at high levels. Thus, in leishmaniasis, costimulation involving B7-1 or B7-2 can result in the production of either Th1 or Th2 cytokines, rather than a preferential induction of one type of response.     

Expression of a variant of CD28 on a subpopulation of human NK cells: implications for B7-mediated stimulation of NK cells.

The ability of NK cells to kill tumor cells is controlled by a balance between activating and inhibitory signals transduced by distinct receptors. In murine tumor models, the costimulatory molecule B7.1 not only acts as a positive trigger for NK-mediated cytotoxicity but can also overcome negative signaling transduced by MHC class I molecules. In this study, we have evaluated the potential of human B7.1-CD28 interaction as an activating trigger for human blood NK cells. Using multiparameter flow cytometric analysis and a panel of different CD28 mAbs, we show that human peripheral blood NK cells (defined by CD56+, CD16+, and CD3- surface expression) express the CD28 costimulatory receptor, with its detection totally dependent on the mAb used. In addition, the level of CD28 varies among individuals and on different NK cell lines, irrespective of CD28 steady-state mRNA levels. By performing Ab binding studies on T cells, our data strongly suggest that binding of two of the anti-CD28 Abs (clones 9.3 and CD28.2) is to a different epitope to that recognized by clones L293 and YTH913.12, which is perhaps modified in the CD28 molecule expressed by the NK cells. We also show that B7.1 enhances the NK-mediated lysis of NK-sensitive but not of NK-resistant tumor cells and that this increased lysis is dependent on CD28-B7 interactions as shown by the ability of Abs to block this lysis. Coculture of the B7.1-positive NK-sensitive cells also led to the activation of the NK cells, as determined by the expression of CD69, CD25, and HLA class II.     

Critical role of OX40 in CD28 and CD154-independent rejection

Blocking both CD28 and CD154 costimulatory pathways can induce transplant tolerance in some, but not all, transplant models. Under stringent conditions, however, this protocol often completely fails to block allograft rejection. The precise nature of such CD28/CD154 blockade-resistant rejection is largely unknown. In the present study we developed a new model in which both CD28 and CD154, two conventional T cell costimulatory molecules, are genetically knocked out (i.e., CD28/CD154 double-knockout (DKO) mice) and used this model to examine the role of novel costimulatory molecule-inducible costimulator (ICOS), OX40, 4-1BB, and CD27 in mediating CD28/CD154-independent rejection. We found that CD28/CD154 DKO mice vigorously rejected fully MHC-mismatched DBA/2 skin allografts (mean survival time, 12 days; n = 6) compared with the wild-type controls (mean survival time, 8 days; n = 7). OX40 costimulation is critically important in skin allograft rejection in this model, as blocking the OX40/OX40 ligand pathway, but not the ICOS/ICOS ligand, 4-1BB/4-1BBL, or CD27/CD70 pathway, markedly prolonged skin allograft survival in CD28/CD154 DKO mice. The critical role of OX40 costimulation in CD28/CD154-independent rejection is further confirmed in wild-type C57BL/6 mice, as blocking the OX40/OX40 ligand pathway in combination with CD28/CD154 blockade induced long term skin allograft survival (>100 days; n = 5). Our study revealed a key cellular mechanism of rejection and identified OX40 as a critical alternative costimulatory molecule in CD28/CD154-independent rejection.     

Autoantibody responses and pathology regulated by B7-1 and B7-2 costimulation in MRL/lpr lupus

The activation of T lymphocytes requires both Ag-mediated signaling through the TCR as well as costimulatory signals transmitted through B7-1 and/or B7-2 with CD28. The interference of B7-mediated costimulatory signals has been proposed as one immunotherapeutic intervention for the prevention autoimmune disease. This study has examined autoantibody responses and autoimmune pathology in a murine model of human systemic lupus erythematosus (SLE), the MRL-lpr/lpr mouse, genetically deficient in B7-1 or B7-2, or in mice treated with B7-1/B7-2 blocking Abs. In contrast to other studies of murine models of SLE, MRL-lpr/lpr mice treated with B7 blocking Abs exhibit strong anti-small nuclear ribonucleoprotein (snRNP) and anti-DNA autoantibody responses with some changes in isotype switching as compared with untreated animals. All MRL-lpr/lpr mice deficient in B7-1 or B7-2 produce anti-snRNP and anti-DNA titers with isotypes virtually identical with wild-type animals. However, the absence of B7-2 costimulation did interfere with the spontaneous activation and the accumulation of memory CD4+ or CD8+ T lymphocytes characteristic of wild-type MRL-lpr/lpr mice. IgG and C3 complement deposition was less pronounced in the kidneys of B7-2 deficient MRL-lpr/lpr mice, reflecting their lessor degree of glomerulonephritis. By comparison, B7-1-deficient MRL-lpr/lpr mice had more severe IgG and C3 deposits in glomeruli.