Expression and function of the costimulatory molecule B7 on murine Langerhans cells: evidence for an alternative CTLA-4 ligand

Src homology 2 domain-containing protein tyrosine phosphatase substrate 1 regulates the induction of Langerhans cell maturation

Recently, we reported that Src homology 2 domain-containing protein tyrosine phosphatase substrate 1 (SHPS-1) plays an important role in the migration of Langerhans cells (LC). Here, we show that SHPS-1 is involved in the maturation of LC. Immunofluorescence analysis on epidermal sheets for I-A or CD86 revealed that LC maturation induced by 2,4-dinitro-1-fluorobenzene (DNFB) or by TNF-alpha was inhibited by pretreatment with an anti-SHPS-1 monoclonal antibody (mAb) or with CD47-Fc fusion protein, a ligand for SHPS-1. Further, FACS analysis demonstrated that I-A(+) LC that had emigrated from skin explants expressed CD80 or CD86, whereas CD47-Fc protein reduced CD80(high+) or CD86(high+) cells. CD47-Fc protein also reduced the up-regulation of surface CD80 or CD86 by LC remaining in the skin explants. In SHPS-1 mutant mice, we observed that the up-regulation of surface CD86 and CCR7 by LC induced by DNFB as well as that of surface CD80 and CD86 by LC in skin explants was attenuated. Finally, contact hypersensitivity (CHS) response was suppressed in SHPS-1 mutant mice and in wild-type mice treated with an anti-SHPS-1 mAb. These observations indicate that SHPS-1 plays an important role in the maturation of LC ex vivo and in vivo, and that SHPS-1-CD47 interaction may negatively regulate CHS.

Differential Requirement for CD28/CTLA-4-CD80/CD86 Interactions in Drug-Induced Type 1 and Type 2 Immune Responses to Trinitrophenyl-Ovalbumin

The use of mAbs to abrogate costimulatory interactions has attracted much attention with regard to prevention and modulation of adverse (auto)immune-like reactions. However, the role of costimulatory molecules and possible therapeutic use of Ab-treatment in drug-induced immunostimulation is poorly elucidated. In the present studies, we show that CD28/CTLA-4-CD80/CD86 costimulatory interactions differently regulate drug-induced type 1 and type 2 responses to an identical bystander Ag, TNP-OVA, in BALB/c mice using the reporter Ag popliteal lymph node assay. The antirheumatic drug D-Penicillamine, which may induce lupus-like side-effects, stimulated type 2 responses against TNP-OVA, characterized by the production of IL-4 and TNP-specific IgG1 and IgE. These responses were abrogated in CD80/CD86-deficient mice and in wild-type mice that were treated with anti-CD80 and anti-CD86, or CTLA-4-Ig. Anti-CTLA-4 intensively enhanced the D-Penicillamine-induced effects. In contrast, the type 1 response (IFN-gamma, TNF-alpha, IgG2a) to TNP-OVA induced by the diabetogen streptozotocin still developed in the absence of CD80/CD86 costimulatory signaling. In addition, it was demonstrated that coadministration of anti-CD80 and anti-CD86 mAbs slightly enhanced streptozotocin-induced type 1 responses, whereas the CTLA-4-Ig fusion protein completely abrogated this response. In conclusion, different drugs may stimulate distinct types of immune responses against an identical bystander Ag, which are completely dependent on (type 2) or independent of (type 1) the CD28/CTLA-4-CD80/CD86 pathway. Importantly, the effects of treatment with anti-CD80/CD86 mAbs and CTLA-4-Ig may be considerably different in responses induced by distinct drugs.

Reduced Switching in SCID B Cells Is Associated with Altered Somatic Mutation of Recombined S Regions

Deoxyribonucleic acid double-stranded breaks act as intermediates in Ig V(D)J recombination and probably perform a similar function in class switch recombination between IgH C genes. In SCID mice, V(D)J recombination is blocked because the DNA-dependent protein kinase catalytic subunit (DNA-PKcs) protein is defective. We show in this study that switching to all isotypes examined was detectable when the SCID mutation was introduced into anti-hen egg lysozyme transgenic B cells capable of undergoing class switch recombination, but switching was significantly reduced in comparison with control B cells of the same specificity lacking the RAG1 gene. Thus, DNA-PKcs is involved in switching to all isotypes, but plays a lesser role in the switching process than it does in V(D)J-coding joint formation. The higher level of switching observed by us in SCID B cells compared with that observed by others in DNA-PKcs(null) cells raises the possibility that kinase-deficient DNA-PKcs can function in switching. Point mutation of G:C base pairs with cytidines on the sense strand was greatly reduced in recombined switch regions from SCID cells compared with control RAG1(-/-) B cells. The preferential loss of sense strand cytidine mutations from hybrid S regions in SCID cells suggests the possibility that nicks might form in S regions of activated B cells on the template strand independently of activation-induced cytidine deaminase and are converted to double-strand breaks when activation-induced cytidine deaminase deaminates the non-template strand.

Bitter-sweet symphony: defining the role of dendritic cell gp120 receptors in HIV infection

BACKGROUND

Dendritic cells (DC) are believed to be one of the first cell types infected during HIV transmission. Recently a single C-type lectin receptor (CLR), DC-SIGN, has been reported to be the predominant receptor on monocyte derived DC (MDDC) rather than CD4. The role of other CLRs in HIV binding and HIV binding by CLRs on other types of DC in vivo is largely unknown.

OBJECTIVES AND STUDY DESIGN

Review HIV binding to DC populations, both in vitro and in vivo, in light of the immense interest of a recently re-identified CLR called DC-SIGN.

RESULTS AND CONCLUSIONS

From recent work, it is clear that immature MDDC have a complex pattern of HIV gp120 binding. In contrast to other cell types gp120 has the potential to bind to several receptors on DC including CD4 and several types of C type lectin receptor, not just exclusively DC-SIGN. Given the diverse types of DC in vivo future work will need to focus on defining the receptors for HIV binding to these different cell types. Mucosal transmission of HIV in vivo targets immature sessile DCs, including Langerhans cells which lack DC-SIGN. The role of CLRs and DC-SIGN in such transmission remains to be defined.

Costimulation via lymphocyte function-associated antigen 1 in the absence of CD28 ligation promotes anergy of naive CD4+ T cells

The mechanisms controlling induction of anergy at the level of naive CD4+ T cells are poorly understood but thought to reflect limited contact with costimulatory molecules during T cell antigen receptor (TCR) ligation. To clarify this question, naive TCR transgenic CD4+ cells were exposed to specific peptide presented by transfected antigen-presenting cells (APC) expressing MHC class II molecules with defined accessory molecules. Significantly, culturing CD4(+) cells with APC expressing MHC II plus peptide alone elicited early TCR signaling but failed to induce either proliferation or anergy. Culture with APC expressing MHC II plus B7 molecules led to strong proliferation and T cell priming but no anergy. In marked contrast, conspicuous induction of anergy occurred after T cell culture with APC expressing MHC class II and intercellular adhesion molecule-1 (ICAM-1). Thus, at the level of naive CD4(+) cells, anergy induction appears to reflect selective contact with APC expressing ICAM-1 in the absence of B7.

Costimulation via lymphocyte function-associated antigen 1 in the absence of CD28 ligation promotes anergy of naive CD4+ T cells

The mechanisms controlling induction of anergy at the level of naive CD4+ T cells are poorly understood but thought to reflect limited contact with costimulatory molecules during T cell antigen receptor (TCR) ligation. To clarify this question, naive TCR transgenic CD4+ cells were exposed to specific peptide presented by transfected antigen-presenting cells (APC) expressing MHC class II molecules with defined accessory molecules. Significantly, culturing CD4(+) cells with APC expressing MHC II plus peptide alone elicited early TCR signaling but failed to induce either proliferation or anergy. Culture with APC expressing MHC II plus B7 molecules led to strong proliferation and T cell priming but no anergy. In marked contrast, conspicuous induction of anergy occurred after T cell culture with APC expressing MHC class II and intercellular adhesion molecule-1 (ICAM-1). Thus, at the level of naive CD4(+) cells, anergy induction appears to reflect selective contact with APC expressing ICAM-1 in the absence of B7.

Cytokines and chemokines in the initiation and regulation of epidermal Langerhans cell mobilization

Langerhans cells (LC) are members of the wider family of dendritic cells. LC reside in the epidermis where they serve as sentinels of the immune system, their responsibilities being to sample the external environment for changes and challenges and to deliver information (antigen) to responsive T lymphocytes within skin draining lymph nodes. The ability of LC to migrate from the epidermis to regional lymph nodes is therefore of pivotal importance to the induction of cutaneous immune responses. The journey that LC have to make from the skin has a number of requirements. Initially it is necessary that LC disassociate themselves from surrounding keratinocytes and are liberated from other influences that encourage their retention in the epidermis. Subsequently, migrating LC must successfully traverse the basement membrane of the dermal-epidermal junction and make their way, via afferent lymphatics, to draining lymph nodes. Effective entry into lymph nodes is necessary, as is correct positioning of cells within the paracortex. There is increasing evidence that both cytokines and chemokines, and their interaction with appropriate receptors expressed by LC, orchestrate the mobilization and movement of these cells. We here consider the parts played by these molecules, and how collectively they induce and direct LC migration.

Immunoliposomes containing antibodies to costimulatory molecules as adjuvants for HIV subunit vaccines

Immunoliposomes containing monoclonal antibodies (MAbs) to the costimulatory molecules CD28 and CTLA4 and their counterreceptors B7-1 (CD80) and B7-2 (CD86) were evaluated for the ability to increase the immune response to recombinant envelope protein rgp120 of the MN strain of human immunodeficiency virus type 1 (HIV-1) during vaccination. MAbs were attached to rgp120-containing liposomes via a biotin-avidin-biotin bridge. Mice vaccinated with immunoliposomes were found to have a strong delayed-type hypersensitivity (DTH) response to the weakly immunogenic gp120 that was dependent on the presence of the MAbs. However, this vaccination protocol did not induce humoral immunity. The DTH response was not accompanied by increased production of interferon gamma (IFN-gamma) or interleukin 4 (IL-4), implying that the primary cellular interaction was between the immunoliposomes and cells of the reticuloendothelial system and not helper T (Th) cells. This strategy of incorporating antibodies to costimulatory molecules on the surface of antigen-containing particulates, such as liposomes or microspheres, can be used to increase DTH immune responses to protein or peptide vaccines.

Role of CD80 (B7.1) and CD86 (B7.2) in the Immune Response to an Intracellular Pathogen

The costimulatory ligands CD80 and CD86 play a crucial role in the initiation and maintenance of an immune response. We demonstrate that whereas infection of human monocytes with viable tachyzoites of Toxoplasma gondii resulted in rapid induction of expression of CD80 and up-regulation of expression of CD86, incubation with killed organisms failed to alter the levels of expression of these costimulatory ligands. The T. gondii-mediated changes in levels of expression of these molecules are critical to the T cell response to the parasite. Proliferation of resting T cells in response to parasite-infected cells was dependent on both CD80 and CD86. More importantly, early production of IFN-γ in response to T. gondii by T cells from T. gondii-seronegative individuals occurred only after stimulation with monocytes that exhibited increased expression of CD80 and CD86 (monocytes infected with viable parasites) and was almost completely ablated by the combination of anti-CD80 plus anti-CD86 mAb. Moreover, proliferation and IFN-γ production by CD4+ CD45RA+ T cells from unexposed individuals were dependent on both CD80 and CD86. These data indicate that pathogen-monocyte interaction influences the ensuing T cell response.

The role of dendritic cells in T cell activation

Dendritic cells (DC) are distinguishable from other antigen-presenting cells by their potent antigen-presenting capacity. They are not only efficient at presenting peptide antigen but can also process and present soluble protein antigen sto antigen-specific T cells and cloned T cell lines. They are very strong stimulators of both allogeneic and syngeneic mixed lymphocyte reactions and have a unique capacity to stimulate naive T cells. The potent functional capacity of DC is related to a high-level expression of major histocompatibility complex class I/II molecules and constitutive expression of costimulatory molecules, such as CD80/CD86, as well as heat stable antigen, CD40 and the leucocyte function antigen (LFA) family of adhesion molecules. Recent studies have shown that DC are also involved in regulation of the immune response via induction of both central and peripheral tolerance.

Epidermal dendritic cells induce potent antigen-specific CTL-mediated immunity

Professional antigen-presenting cells (APCs) are required for the initiation of an immune response. Dendritic cells (DCs) are the most potent APCs identified thus far and can present antigen in the context of co-stimulatory signals required for the stimulation of both primed and naïve T cells. Cytotoxic T lymphocytes (CTLs) are critical to the immune response against tumors or virally infected cells. Optimal stimulation of antigen-specific CTLs is the goal of evolving immunization strategies for the prevention or therapy of viral infections and tumors. Epidermal dendritic cells (eDCs), or Langerhans cells, can present antigens for the stimulation of CD4+ T cell dependent anti-tumor immunity and may play a role in tumor surveillance. The capacity of eDCs to induce tumor-specific CD8+ CTL immunity has not been determined. We have previously shown that DCs derived from bone marrow precursors (BmDCs) under the influence of cytokines can induce protective, antigen-specific CTL-mediated anti-tumor immunity. Here we show that subcutaneous immunization with ovalbumin (OVA) peptide (SIINFEKL(257-264))-pulsed eDCs induced OVA-specific, CD8+ CTLs that lyse the OVA-expressing target. Furthermore, mice vaccinated with OVA peptide-pulsed eDCs were completely protected from subsequent challenge by the OVA-expressing melanoma MO5. The capacity of peptide-pulsed eDCs to induce CTL-mediated immunity is directly dependent on the dose of eDCs administered. Importantly, the APC capacity of eDCs is comparable to that of BmDCs, as mice immunized with eDC populations containing at least as many class II+/B7.2+ cells as populations of BmDCs were equally protected against challenge with MO5. These results demonstrate that eDCs can be potent inducers of antigen-specific CD8+ CTL-mediated immunity. They suggest that eDCs may be important targets for antigen delivery strategies aimed at inducing antiviral or anti-tumor immunity.

Expression and function of B7-1 and B7-2 in hapten-induced contact sensitivity

We analyzed the expression and function of the co-stimulatory molecules B7-1 (CD80) and B7-2 (CD86) during contact sensitivity reactions induced by the hapten 2,4-dinitrofluorobenzene (DNFB). In the normal skin, only a few epidermal Langerhans cells or dermal dendritic cells express B7-2. In contrast, following challenge with DNFB, expression of B7-2 is up-regulated in both epidermis and dermis. Importantly, B7-1 is induced later and at lower levels compared to B7-2. Intravenous injections of anti-B7-2 mAb, but not anti-B7-1 mAb partially inhibit the hapten-induced contact sensitivity reaction. Experiments in which mice are injected differentially with anti-B7-2 mAb, either before the afferent or before the efferent phase of the contact sensitivity response, suggest that B7-2 is important for successful antigen priming.

Skin allograft rejection in CD28-deficient mice

Costimulatory interactions between CD28 and the B7 family have been shown to augment T cell responses in general. To further assess the importance of the costimulatory signals generated through CD28, the allogeneic response was examined in CD28-deficient mice. T cells from CD28-deficient mice showed reduced proliferation and cytokine production in response to allogeneic stimulator cells in vitro. However, CD28-deficient T cells developed cytotoxic activity against allogeneic target cells in vitro and efficiently rejected skin allografts in vivo. These results suggest that the costimulatory signals through CD28 are not essential for the induction of alloreactive effector functions in vitro and in vivo.

Expression and function of B7-1 (CD80) and B7-2 (CD86) on human epidermal Langerhans cells

In addition to T cell receptor triggering, activation of T cells requires costimulatory signals that have been shown to be mainly initiated through CD28. We analyzed the expression and function of the two ligands for CD28, B7-1 (CD80) and B7-2 (CD86), on human Langerhans cells (LC), the antigen-presenting cells from epidermis. Human LC freshly isolated from epidermis (fLC) expressed significant level of B7-2, which was increased upon a short culture in vitro. In contrast, B7-1 was undetectable on fLC but appeared at the cell surface after a 3-day culture in vitro. Pre-incubation of 18-h cultured LC with anti-B7-2 monoclonal antibodies (mAB) was sufficient to abrogate the binding of CTLA4-Ig fusion protein, while a combination of both mAB against B7-1 and B7-2 was necessary to obtain a complete inhibition of CTLA4-Ig binding on 3-day cultured LC, showing the absence of a third CTLA4 ligand. The function of B7-1 and B7-2 on human LC has been analyzed by adding mAb at the beginning of mixed epidermal cell lymphocyte reactions. Anti-B7-2 mAb and CTLA4-Ig, but not anti-B7-1 mAb, strongly inhibited allogenic. as well as recall antigen-induced T cell proliferation supported by fLC or 3-day cultured LC. Collectively, these results demonstrate that B7-2 is the major ligand for CD28/CTLA4 at the LC surface and that it plays a crucial role in human LC co-stimulatory function with little, if any, dependence of B7-1 expression.

Phenotypic and functional heterogeneity among murine epidermal-derived dendritic cell clones

We have established recently long-term dendritic cell lines from the epidermis of newborn BALB/c mice. These lines, termed XS series, resembled epidermal resident Langerhans cells or their progenitors in terms of surface phenotype, antigen-presenting capacity, and growth factor requirement. We examined in this study the degree of clonal heterogeneity among XS cells with respect to each of these features. Twelve stable clones were established by limiting dilution microculture from 8-10-week-old cultures of the XS52 or XS20 line. Despite the uniform expression of CD45, these clones varied substantially in their expression of Ia, B7-1, and B7-2 molecules. They also varied significantly in their relative efficiency in activating T cells. Finally, remarkable clone-to-clone heterogeneity was also observed in their growth factor responsiveness; some clones responded equally well to granulocyte macrophage-colony-stimulating factor-1, whereas others responded preferentially to one or the other of these factors. We propose that the observed clonal heterogeneity in XS cells reflects possible heterogeneity in the state of maturation and mitotic potential among the starting populations, i.e., skin-associated dendritic cells in newborn mice.

Bone marrow-derived dendritic cells pulsed with synthetic tumour peptides elicit protective and therapeutic antitumour immunity

Dendritic cells, the most potent 'professional' antigen-presenting cells, hold promise for improving the immunotherapy of cancer. In three different well-characterized tumour models, naive mice injected with bone marrow-derived dendritic cells prepulsed with tumour-associated peptides previously characterized as being recognized by class I major histocompatibility complex-restricted cytotoxic T lymphocytes, developed a specific T-lymphocyte response and were protected against a subsequent lethal tumour challenge. Moreover, in the C3 sarcoma and the 3LL lung carcinoma murine models, treatment of animals bearing established macroscopic tumours (up to 1 cm2 in size) with tumour peptide-pulsed dendritic cells resulted in sustained tumour regression and tumour-free status in more than 80% of cases. These results support the clinical use of tumour peptide-pulsed dendritic cells as components in developing effective cancer vaccines and therapies.

UVB-dependent modulation of epidermal cytokine network: roles in UVB-induced depletion of Langerhans cells and dendritic epidermal T cells

The epidermis of mice consists of three cellular components, i.e., keratinocytes, Langerhans cells (LC), and dendritic epidermal T cells (DETC). Each epidermal subpopulation produces a different set of cytokines, thereby forming a unique cytokine milieu. These cytokines, in turn, support the survival and growth of LC and DETC and regulate their immunological functions. LC and DETC play important, but distinct, effector roles in protective immunity against antigens that are generated in or penetrate into the epidermis. Acute or chronic exposure of mice to ultraviolet B (UVB) radiation is known to impair this cutaneous immunity, as evidenced by the failure to induce T cell-mediated immune reactions, by the generation of antigen-specific immunological unresponsiveness, and by the development of skin cancers. Importantly, these changes are associated with reduced densities of LC and DETC in UVB-exposed skin, suggesting that the deficiency in these epidermal leukocytes may account for some of the deleterious influences of UVB radiation on skin. Here I will review the recent advance in our understanding of the mechanisms by which UVB radiation may deplete LC and DETC from epidermis. More specifically, I will discuss the following possibilities: a) UVB-mediated suppression of the production of relevant growth factors for LC and DETC, b) UVB-induced abrogation of surface expression of growth factor receptors, and c) UVB-triggered apoptotic cell death in epidermal leukocytes.

Low-dose UVB radiation perturbs the functional expression of B7.1 and B7.2 co-stimulatory molecules on human Langerhans cells

In previous studies, we have shown that ultraviolet (UV) B radiation perturbs the APC function of Langerhans cells (LC) by interfering with as-yet unidentified co-stimulatory signals. Recently, B7.1 and B7.2 on APC were shown to deliver important co-stimulatory signals through interaction with their counter receptors CD28 and CTLA-4 on T cells. To determine whether UVB affects the functional expression of B7.1 or B7.2 on LC, B7.1 and B7.2 expression was studied on human LC by multiparameter flow cytometry. Little, if any, B7.1 or B7.2 was detected on LC freshly isolated from skin. However, following 48 h of tissue culture, expression of both B7.1 and B7.2 were markedly up-regulated. To test whether these molecules were functional, primary mixed epidermal cell leukocyte reactions (MECLR) were performed. Blocking monoclonal antibody (mAb) to B7.1 or B7.2 both inhibited the MECLR, with anti-B7.2 being much more effective than anti-B7.1. UVB radiation dose-dependently (100-200 J/m2) suppressed the culture-induced up-regulation of B7.1 and B7.2 on LC. Since LC exposed to the same UVB flux (UVB-LC) failed to stimulate alloreactive T cells in a MECLR, we questioned whether this was related to their inability to provide B7 co-stimulation. Indeed, when effective B7-CD28 signaling was ascertained by adding submitogenic doses of exogenous anti-CD28 mAb to UVB-LC, the proliferative response of alloreactive T cells was restored. We conclude that the suppressive effects of low-dose UVB radiation on the APC function of LC are, at least in part, due to an inhibition of functional B7.1 and B7.2 expression.

Cytokine-dependent regulation of growth and maturation in murine epidermal dendritic cell lines

We have recently established dendritic cell (DC) lines (XS series) from the epidermis of newborn mice by repeated feeding with granulocyte/macrophage-colony-stimulating factor (GM-CSF) and culture supernatants from skin-derived stromal cell lines (NS series). XS lines resemble resident Langerhans cell (LC), which are immature DC that reside in epidermis, by their surface phenotype and antigen-presenting profile. XS lines further resemble resident LC in that they express mRNA for interleukin-1 beta and macrophage inflammatory protein (MIP)-1 alpha, and by the absence of mRNA for IL-6. Their growth is promoted by GM-CSF, colony-stimulating factor-1 (CSF-1), or NS culture supernatant, and inhibited by interferon-gamma or tumor necrosis factor-alpha. The expression by the XS lines of Ia molecules is up-regulated by GM-CSF, and down-regulated by NS supernatant. These results suggest the existence of negative regulatory mechanisms in which the growth and/or maturation of DC is suppressed by selected cytokines.

Dendritic cells as stimulator cells of MHC class I-restricted immune responses

We have shown that growth factor-dependent, MHC class I+/II dendritic cell lines established from mouse fetal skin, can stimulate naive, allogeneic but not syngeneic CD8+ T cells in the absence of CD4+ T cells and that this T cell response is restricted by MHC class I molecules. We further showed that the FSCL-induced activation of naive CD8+ T cells is critically dependent on the physical contact between stimulator and responder cells and the expression of the costimulatory molecule B7 on FSCL. An important question that remains to be addressed concerns the derivation of FSCL. One could argue that they are members of the LC/DC family because they (i) exhibit certain features of fetal murine LC (i.e., CD45+, CD44+, CD32+, MHC class I+, MHC class II-, asialo GM1+, TCR-) including membrane-bound ADPase activity (A. Elbe, unpublished observation) and (ii) exhibit a pronounced dendritic configuration when cultured. If these cells are indeed derived from fetal LC, they should undergo the same phenotypic changes (MHC class II(-)-->MHC class II+) under in vitro culture conditions as do fetal LC in situ. However, our FSCL are phenotypically stable, and attempts to induce MHC class II expression with cytokine cocktails were unsuccessful. One explanation for this phenomenon could be that stimulatory signals provided by fetal keratinocytes or other skin cells are responsible for LC maturation in vivo and that, due to the early demise of these "stromal" cells in fetal skin cell cultures, the maturation process would not have been completed.(ABSTRACT TRUNCATED AT 250 WORDS)

Human endothelial stimulation of allogeneic T cells via a CTLA-4 independent pathway

It is accepted that T cells require at least two signals to undergo proliferation and cytokine release: an antigen dependent signal mediated via the the TCR (T cell receptor) and an antigen independent signal mediated via one or more accessory or adhesion molecules. Interaction between CD28 or CTLA-4 and the B7 co-receptors found on many antigen presenting cells (APC) is known to be essential for antigen specific (including alloantigen) expansion of T cells in vitro and in vivo. CTLA-4-Ig is a fusion protein with very high affinity for B7. It has been used in vivo to block both allograft and xenograft rejection. Most of the work investigating second signal requirement has used 'professional' APC. In view of the observations that class II positive human endothelial cells can cause direct allostimulation of resting CD4+ and CD8+ T cells, we have investigated the requirement of CTLA-4 in this response. The current studies show that the proliferative response of allogeneic CD4+ and CD8+ T cells to interferon-gamma treated HUVEC (human umbilical vein endothelial cells) is inhibited by monoclonal antibodies (mAbs) against MHC class II and class I antigens, respectively, but not by CTLA-4-Ig. In contrast, lymphocytes proliferating in response to allogeneic splenocytes are inhibited by CTLA-4-Ig. Cell surface binding studies using flow cytometry demonstrated failure of endothelial cells to bind either CTLA-4-Ig or mAbs against B7 receptors. In conclusion, different APC use different co-stimulatory signals. The possibility that this leads to different cytokine profiles needs to be investigated to further understand the role of endothelial cells in transplant rejection.