The endothelial cell as a regulator of T-cell function

Platelet and Megakaryocyte Roles in Innate and Adaptive Immunity

Classically, platelets have been described as the cellular blood component that mediates hemostasis and thrombosis. This important platelet function has received significant research attention for >150 years. The immune cell functions of platelets are much less appreciated. Platelets interact with and activate cells of all branches of immunity in response to pathogen exposures and infection, as well as in response to sterile tissue injury. In this review, we focus on innate immune mechanisms of platelet activation, platelet interactions with innate immune cells, as well as the intersection of platelets and adaptive immunity. The immune potential of platelets is dependent in part on their megakaryocyte precursor providing them with the molecular composition to be first responders and immune sentinels in initiating and orchestrating coordinated pathogen immune responses. There is emerging evidence that extramedullary megakaryocytes may be immune differentiated compared with bone marrow megakaryocytes, but the physiological relevance of immunophenotypic differences are just beginning to be explored. These concepts are also discussed in this review. The immune functions of the megakaryocyte/platelet lineage have likely evolved to coordinate the need to repair a vascular breach with the simultaneous need to induce an immune response that may limit pathogen invasion once the blood is exposed to an external environment.

Applications of tumor chip technology

Over the past six decades the inflation-adjusted cost to bring a new drug to market has been increasing constantly and doubles every 9 years - now reaching in excess of $2.5 billion. Overall, the likelihood of FDA approval for a drug (any disease indication) that has entered phase I clinical trials is a mere 9.6%, with the approval rate for oncology far below average at only 5.1%. Lack of efficacy or toxicity is often not revealed until the later stages of clinical trials, despite promising preclinical data. This indicates that the current in vitro systems for drug screening need to be improved for better predictability of in vivo outcomes. Microphysiological systems (MPS), or bioengineered 3D microfluidic tissue and organ constructs that mimic physiological and pathological processes in vitro, can be leveraged across preclinical research and clinical trial stages to transform drug development and clinical management for a range of diseases. Here we review the current state-of-the-art in 3D tissue-engineering models developed for cancer research, with a focus on tumor-on-a-chip, or tumor chip, models. From our viewpoint, tumor chip systems can advance innovative medicine to ameliorate the high failure rates in anti-cancer drug development and clinical treatment.

The intragraft microenvironment as a central determinant of chronic rejection or local immunoregulation/tolerance

PURPOSE OF REVIEW

Chronic rejection is associated with persistent mononuclear cell recruitment, endothelial activation and proliferation, local tissue hypoxia and related biology that enhance effector immune responses. In contrast, the tumor microenvironment elicits signals/factors that inhibit effector T cell responses and rather promote immunoregulation locally within the tissue itself. The identification of immunoregulatory check points and/or secreted factors that are deficient within allografts is of great importance in the understanding and prevention of chronic rejection.

RECENT FINDINGS

The relative deficiency of immunomodulatory molecules (cell surface and secreted) on microvascular endothelial cells within the intragraft microenvironment, is of functional importance in shaping the phenotype of rejection. These regulatory molecules include coinhibitory and/or intracellular regulatory signals/factors that enhance local activation of T regulatory cells. For example, semaphorins may interact with endothelial cells and CD4 T cells to promote local tolerance. Additionally, metabolites and electrolytes within the allograft microenvironment may regulate local effector and regulatory cell responses.

SUMMARY

Multiple factors within allografts shape the microenvironment either towards local immunoregulation or proinflammation. Promoting the expression of intragraft cell surface or secreted molecules that support immunoregulation will be critical for long-term graft survival and/or alloimmune tolerance.

Allograft rejection-related gene expression in the endothelial cells of renal transplantation recipients after cytomegalovirus infection

OBJECTIVE

To explore the effects of cytomegalovirus (CMV) infection on rejection-related gene expression in the endothelial cells of renal transplantation recipients.

METHODS

Endothelial cells (ECs) were cultured and stimulated by a variety of factors: A, normal control group; B, inactivated human cytomegalovirus (HCMV) infection group; C, HCMV infection group; D, HCMV supernatant infection group; and E, ganciclovir HCMV group. Expression of intercellular adhesion molecule-1 (ICAM-1) and major histocompability complex (MHC) class I and class II antigens was detected by flow cytometry (FCM) and immunohistochemistry.

RESULTS

We found characteristic CMV-infected ECs in this study. There were no significant differences among groups A, B and D (P>0.05). Although the expression levels of ICAM-1 were not significantly different between groups C and E (P>0.05), the ICAM-1 expression in these two groups was significantly higher than that in group A (P<0.05). ICAM-1 expression was detected in groups C and E, while there was no expression in groups A, B and D. Furthermore, there was no significant difference of ICAM-1 mRNA expression between groups C and E (P>0.05). Human leucocyte antigen (HLA)-ABC expression was detected in all the groups, while HLA-DR expression was only detected in groups C and E. There were no significant differences of HLA-ABC and HLA-DR expression among groups A, B and D (P>0.05). However, the HLA-ABC and HLA-DR expression levels in groups C and D were higher than those of the remaining groups previously reported (P<0.05). Meanwhile, the HLA-ABC and HLA-DR expression levels in group E were lower than those of group C (P<0.05).

CONCLUSION

CMV could up-regulate the expression levels of ICAM-1 and MHC antigens, which was closely related to allograft rejection.

An animal model of autoimmune emphysema

Although cigarette smoking is implicated in the pathogenesis of emphysema, the precise mechanisms of chronic progressive alveolar septal destruction are not well understood. We show, in a novel animal model, that immunocompetent, but not athymic, nude rats injected intraperitoneally with xenogeneic endothelial cells (ECs) produce antibodies against ECs and develop emphysema. Immunization with ECs also leads to alveolar septal cell apoptosis and activation of matrix metalloproteases MMP-9 and MMP-2. Anti-EC antibodies cause EC apoptosis in vitro and emphysema in passively immunized mice. Moreover, immunization also causes accumulation of CD4+ T cells in the lung. Adoptive transfer of pathogenic, spleen-derived CD4+ cells into naive immunocompetent animal also results in emphysema. This study shows for the first time that humoral- and CD4+ cell-dependent mechanisms are sufficient to trigger the development of emphysema, suggesting that alveolar septal cell destruction might result from immune mechanisms.

Endothelial cells require STAT3 for protection against endotoxin-induced inflammation

Endothelial cells (ECs) are believed to be an important component in the protection from lipopolysaccharide (LPS)-induced endotoxic shock. However, the cellular and molecular mechanism is not well defined. Here, we report that signal transducer and activator of transcription (STAT) 3 is an essential regulator of the antiinflammatory function of ECs in systemic immunity. Because STAT3 deficiency results in early embryonic lethality, we have generated mice with a conditional STAT3 deletion in endothelium (STAT3^E−/−). STAT3^E−/− mice are healthy and fertile, and isolated ECs initiate normal tube formation in vitro. Conditional endothelial but not organ-specific (i.e., hepatocyte or cardiomyocyte) STAT3 knockout mice show an increased susceptibility to lethality after LPS challenge. The LPS response in STAT3^E−/− mice shows exaggerated inflammation and leukocyte infiltration in multiple organs combined with elevated activity of serum alanine aminotransferase and aspartate aminotransferase, indicating organ damage. Concomitantly, proinflammatory cytokines are produced at an exaggerated level and for a prolonged period. This defect cannot be explained by lack of antiinflammatory cytokines, such as interleukin 10 and transforming growth factor β. Instead, we have shown that a soluble activity derived from endothelia and dependent on STAT3 is critical for suppression of interferon γ. These data define STAT3 signaling within endothelia as a critical antiinflammatory mediator and provide new insight to the protective function of ECs in inflammation.

Endothelial cell early activation induced by allogeneic lymphocytes in isolated perfused mouse lung

BACKGROUND

The early inflammatory response induced following vascularized organ allotransplantation is mediated by adhesin-ligand interactions between blood leukocytes and allogeneic endothelial cells. In the present study, we focused on the role of allogeneic blood lymphocytes in early immune alterations following lung reperfusion.

METHODS

We developed an experimental model of isolated and ventilated lung in female C57BL/6 mouse perfused with either isogeneic (C57BL/6) or allogeneic (C3H/He) male mouse fresh blood for 3 hours. SRY DNA quantification by polymerase chain reaction (PCR) was used to assess the presence of perfusate cells in lung extract. Using quantitative reverse transcriptase (RT)-PCR, we measured mRNA expression both of the adhesion molecules--intercellular adhesion molecule (ICAM)-1, lymphocyte function-associated antigen (LFA)-3, vascular cell adhesion molecule (VCAM)-1, and endothelial leukocyte adhesion molecule (ELAM)-1--and of tumor necrosis factor (TNF)-alpha in lung tissue.

RESULTS

The number of blood lymphocytes is significantly decreased in allogeneic versus isogeneic condition at 3 hours of perfusion, without evidence of increased apoptosis or necrosis. In parallel, SRY DNA quantity recovered in the lung was 2.5 times higher in allogeneic condition, which was related to cells loosely adherent to endothelial cells. Lastly, the levels of mRNAs of all adhesion molecules and of TNF-alpha were significantly increased in allogeneic versus isogeneic conditions.

CONCLUSION

These results demonstrate that an early interaction between allogeneic blood lymphocytes and vascular endothelial cells is correlated with a high mRNA expression both of adhesion molecules and of TNF-alpha in the perfused lung. Our model of a mouse lung perfused with fresh blood appears to be a useful clinical assessment system for in-depth investigation of early cell activation and the resulting intragraft immune alterations.

Induction of mucosal tolerance to E-selectin prevents ischemic and hemorrhagic stroke in spontaneously hypertensive genetically stroke-prone rats

BACKGROUND AND PURPOSE

Inflammatory and immune mechanisms can precipitate cerebrovascular thrombosis and hemorrhage. Immunologic tolerance can be induced to a specific antigen by intranasal instillation of that antigen. Lymphocytes tolerized in this way provide local immunosuppression on restimulation with the same antigen. This study tests whether tolerization of lymphocytes to E-selectin can suppress local vessel activation and prevent stroke.

METHODS

Spontaneously hypertensive genetically stroke-prone rats (n=113) were distributed among the following studies: comparison of ischemic infarcts/intraparenchymal hemorrhages after single or repetitive tolerization schedules with ovalbumin, E-selectin, or PBS; comparison of E-selectin tolerization- and PBS tolerization-induced suppression of delayed-type hypersensitivity in animals subsequently sensitized to E-selectin; and comparison of PBS-, ovalbumin-, and E-selectin-tolerized groups (after intravenous lipopolysaccharide to activate vessels) regarding transforming growth factor-beta1-positive splenocyte counts, plasma interferon-gamma levels, anti-human E-selectin antibodies, endothelial intercellular adhesion molecule-1, and anti-endothelial cell antibodies.

RESULTS

Nasal instillation of E-selectin, which is specifically expressed on activated endothelium, potently inhibited the development of ischemic and hemorrhagic strokes in spontaneously hypertensive stroke-prone rats with untreated hypertension. Repeated schedules of tolerization were required to maintain the resistance to stroke. Suppression of delayed-type hypersensitivity to E-selectin and increased numbers of transforming growth factor-beta1-positive splenocytes showed that intranasal exposure to E-selectin induced immunologic tolerance. E-selectin tolerization also reduced endothelial activation and immune responses after intravenous lipopolysaccharide, as shown by marked suppression of intercellular adhesion molecule-1 expression, anti-endothelial cell antibodies on luminal endothelium, and plasma interferon-gamma levels compared with the control condition.

CONCLUSIONS

The novel findings in this study support further investigation of immunologic tolerance as applied to the prevention of stroke.

Endothelial cell and cAMP regulation of T-cell CD40 ligand: relevance of calcium/calmodulin-dependent kinase IV signalling

CD40 ligand (CD40L) expression is now recognized to contribute in the pathogenesis of vascular disease. Because increased CD40L has been associated with myocardial infarction, effects of endothelial cells and cAMP with respect to CD40L regulation may be of clinical relevance. In the present study, endothelial cells are shown to markedly increase CD40L on naïve CD4+ T cells with a more modest effect on memory T cells. Furthermore, the addition of dibutyryl cyclic AMP (dbcAMP) synergistically increased naïve cell CD40L but inhibited memory cell CD40L. Although it has previously been recognized that human endothelial cells can increase T-cell CD40L, this is the first description of the difference in responses of naïve and memory cells and the first demonstration of synergistic effects of endothelial cells and cAMP on CD40L regulation. Consistent with previous reports that CD40L regulation is distinctive, another marker of early activation (CD69) was not similarly regulated. The mechanisms of CD40L regulation were related to calcineurin and calcium/calmodulin dependent kinase IV (CaMKIV) signalling pathways. Endothelial cell costimulation of CD40L was found to be dependent upon calcineurin activity while cAMP actions to increase CD40L were dependent upon CaMKIV. Expression of a dominant negative CaMKIV construct further indicated an important role for CaMKIV in regulation of CD40L and cAMP responses. These data indicate that endothelial cell costimulation can interact with cAMP through calcium signalling pathways to synergistically enhance CD40L expression. Because increased CD40L is associated with atherosclerotic plaque and instability, results are relevant to the pathogenesis of atherosclerosis and myocardial infarction.

Anti-B7-1 blocks mononuclear cell adherence in vasa recta after ischemia

BACKGROUND

Blocking the costimulatory pathway by CTLA-4 Ig, reactive with both B7-1 and B7-2 costimulatory molecules, protects the kidney during acute ischemia/reperfusion injury. This study investigated whether and how B7-1 and/or B7-2 proteins are involved in renal ischemia/reperfusion injury (IRI).

METHODS

Uninephrectomized rats were submitted to warm renal ischemia (30 min) and received control monoclonal antibody (mAb; 17E3), anti-B7-1 (3H5), anti-B7-2 (24F), a combination of anti-B7-1/B7-2, or CTLA-4 Ig. Renal function, morphology, and the kinetics of inflammatory cells were studied for a ten-day period. Binding sites of the injected antibodies were detected by secondary staining with anti-mouse Ab.

RESULTS

Compared with controls, acute renal failure (ARF) in the anti-B7-1 group was attenuated both functionally and morphologically. Anti-B7-1/B7-2 and CTLA-4 Ig also were protective in IRI. ARF was not altered by anti-B7-2 treatment. Two hours after reperfusion, B7-1 was expressed along the endothelial cells of the ascending vasa recta. Expression of B7-1 increased over time during the first 24 hours and decreased thereafter. Two hours after reperfusion, adherence/accumulation of T cells and monocytes/macrophages was found in the vasa recta of the ischemic kidney. Anti-B7-1-treated animals had fewer T cells and monocytes/macrophages in the vasa recta compared with controls. Leukocyte accumulation in these vessels after anti-B7-2 treatment was not different from IRI controls.

CONCLUSION

These observations strongly support the key role of the B7-1 protein in the protection of renal IRI through inhibition of T cell and monocyte adherence at the level of the ascending vasa recta.

Carbon monoxide generated by heme oxygenase 1 suppresses endothelial cell apoptosis

Heme oxygenase 1 (HO-1) inhibits apoptosis by regulating cellular prooxidant iron. We now show that there is an additional mechanism by which HO-1 inhibits apoptosis, namely by generating the gaseous molecule carbon monoxide (CO). Overexpression of HO-1, or induction of HO-1 expression by heme, protects endothelial cells (ECs) from apoptosis. When HO-1 enzymatic activity is blocked by tin protoporphyrin (SnPPIX) or the action of CO is inhibited by hemoglobin (Hb), HO-1 no longer prevents EC apoptosis while these reagents do not affect the antiapoptotic action of bcl-2. Exposure of ECs to exogenous CO, under inhibition of HO-1 activity by SnPPIX, substitutes HO-1 in preventing EC apoptosis. The mechanism of action of HO-1/CO is dependent on the activation of the p38 mitogen-activated protein kinase (MAPK) signaling transduction pathway. Expression of HO-1 or exposure of ECs to exogenous CO enhanced p38 MAPK activation by TNF-alpha. Specific inhibition of p38 MAPK activation by the pyridinyl imidazol SB203580 or through overexpression of a p38 MAPK dominant negative mutant abrogated the antiapoptotic effect of HO-1. Taken together, these data demonstrate that the antiapoptotic effect of HO-1 in ECs is mediated by CO and more specifically via the activation of p38 MAPK by CO.

Alloreactive cytotoxic CD4+ responses elicited by cytomegalovirus-infected endothelial cells: role of MHC class I antigens

Cytomegalovirus (CMV) has been associated with chronic graft rejection in solid organ transplant patients. To elucidate the mechanism by which CMV leads to graft rejection, we hypothesized that CMV infection of endothelial cells could stimulate alloreactive cytotoxic T lymphocytes (CTL). This hypothesis was explored using the following experimental model: peripheral blood mononuclear cells (MNC) obtained from normal hosts were grown on monolayers of umbilical vein endothelial cells (UVEC) infected with CMV (CMV-UVEC) or not (control) and tested for CTL activity against uninfected UVEC. We showed that CMV-UVEC-stimulated MNC have significant CTL activity against uninfected UVEC. The CTL activity elicited by CMV-UVEC stimulation was significantly higher compared with that stimulated by uninfected UVEC or by ganciclovir-treated CMV-UVEC, indicating the critical role of productive CMV infection. The CTL activity was specific for the UVEC used as stimulators and did not affect MHC-unrelated UVEC. However, lymphoblastoid lines (LBL) major histocompatibility complex (MHC)-identical with the stimulator UVEC were also killed by the CMV-UVEC-stimulated MNC. CTL killed identical UVEC and LBL in a competitive fashion. Blocking experiments with monoclonal antibodies (mAbs) identified CD4 cells as the main effector of CTL activity and MHC class I as the antigenic target of CTL. Although natural killer (NK) cells did not significantly contribute to the CTL activity of CMV-UVEC-stimulated MNC, their presence in the MNC cultures during the stimulation process was critical for the development of CTL. This model offers a framework for understanding the role of CMV infection in graft rejection and for devising preventative strategies.

MAdCAM-1 costimulates T cell proliferation exclusively through integrin alpha4beta7, whereas VCAM-1 and CS-1 peptide use alpha4beta1: evidence for "remote" costimulation and induction of hyperresponsiveness to B7 molecules

We have analyzed the effects of the alpha4 integrin ligands mucosal addressin cell adhesion molecule-1 (MAdCAM-1), vascular cell adhesion molecule-1 (VCAM-1), and the fibronectin CS-1 splice variant on T cell activation. Immobilized MAdCAM-1 and VCAM-1 IgG-Fc chimeras and a fibronectin CS-1 peptide efficiently costimulate T cell proliferation when antigen presentation is mimicked by anti-CD3 antibody. VCAM-1-Fc and fibronectin CS-1, which are adhesive ligands for both the alpha4beta1 and alpha4beta7 integrins, medicate T cell costimulation exclusively through integrin alpha4beta1, but not through alpha4beta7. The inability of VCAM-1-Fc to costimulate via alpha4beta7 suggests that cell adhesion per se is insufficient, and that exquisite recognition and activation events must be triggered. MAdCAM-1-Fc mediates costimulation exclusively via alpha4beta7, and can both synergize with and induce hyperresponsiveness to the classical costimulator B7-2. MAdCAM-1-Fc and VCAM-1-Fc, but not B7-2, effectively costimulate when immobilized on sites spatially distant from the anti-CD3 antibody ("remote" costimulation). In vitro, the relative potencies of the CAM were VCAM-1-Fc> ICAM-1-Fc> MAdCAM-1-Fc > B7-Fc, except at high concentrations where ICAM-1 was the most potent. Features of costimulatory CAM revealed by this study have important implications for the design of immunotherapeutic vaccine strategies to combat cancer and infection.

Endothelial antigen presentation: stimulation of previously activated but not naïve TCR-transgenic mouse T cells

In vitro experiments have shown that endothelial cells can function as antigen-presenting cells to CD4(+) T lymphocytes. The studies presented here address the question of whether naïve versus previously activated CD4(+) helper T cells differ in their responses to endothelial antigen presentation. TCR-transgenic mice were used as a source of naive T cells of defined antigen specificity. These cells were stimulated in vitro with antigen and splenic antigen-presenting cells to generate populations of T lymphocytes with a previously activated/memory phenotype. Two different types of mouse endothelial cells were used as antigen-presenting cells, including the SVEC4-10 line derived from lymph node endothelium and primary murine pulmonary microvascular endothelium. Monolayer cultures of both types of endothelium were capable of antigen-dependent stimulation of previously activated TCR-transgenic CD4(+) cells. In contrast, neither endothelial type could activate naïve CD4(+) T cells. When costimulatory signals were provided in trans by the addition of MHC-mismatched mouse spleen cells, activation of naïve T cells by endothelial antigen presentation could be demonstrated. The expression of ICAM-1 or VCAM-1 on the endothelial cells was not sufficient to activate naïve T cells. Furthermore, the mouse lung endothelium constitutively expresses B7-1, and therefore, the inability of endothelium to stimulate naïve T cells could not be attributed to a lack of CD28-ligands. These studies suggest that the potential role of endothelial antigen presentation in immune responses is restricted to promoting responses by T cells which have previously encountered antigen presented by other antigen-presenting cells.

Cytolytic activity against allogeneic human endothelia: resistance of cytomegalovirus-infected cells and virally activated lysis of uninfected cells

BACKGROUND

Cytomegalovirus (CMV) has been implicated as an exacerbating agent in the development of transplant vascular sclerosis; however, specific etiologic mechanisms remain unresolved. Based upon our previous observations that CMV-infected endothelial cells (ECs) stimulate proliferation and cytokine production by allogeneic T cells, we now test the hypothesis that CMV-driven cytolytic activity may contribute to graft endothelial injury.

METHODS

Limiting dilutions of CMV-seropositive or -seronegative donor-derived T cells were stimulated with CMV-infected or uninfected allogeneic ECs in the presence of interleukin-2. T-cell proliferation was monitored by assay of [3H]thymidine incorporation and stimulated T cells were tested for lytic activity against CMV-infected or uninfected radiolabeled EC targets by 51Cr release assay. Natural killer (NK) cell activity was examined by incubating freshly isolated peripheral blood mononuclear cells with 51Cr-labeled targets, followed by assay of radiolabel release.

RESULTS

CMV-infected ECs were resistant to T cell- and NK-mediated cytolysis regardless of donor serostatus, nature of stimulation, or level of T-cell proliferation. In contrast, although uninfected ECs were unharmed by NK cells, these targets experienced significant lysis by T cells stimulated with either uninfected or CMV-infected ECs.

CONCLUSIONS

These results implicate CMV-infected graft endothelium as a persistent source of infectious virus, a chronic stimulus for potentially destructive host inflammatory activity, and a potential trigger for the generation of lytic injury to uninfected bystander endothelia, suggesting multiple mechanisms by which this virus might perturb equilibrium at the graft/host interface.

An In Vitro Model of T Cell Activation by Autologous Cytomegalovirus (CMV)-Infected Human Adult Endothelial Cells: Contribution of CMV-Enhanced Endothelial ICAM-1

Cellular immunity is strongly implicated in control of CMV disease; however, many mechanistic details remain unresolved. We previously demonstrated T cell activation responses to CMV-infected allogeneic endothelial cells (EC), suggesting EC as a mediator of CMV response in the transplant recipient. We now test the hypothesis that CMV-specific T cell responses can be directly stimulated by infected EC in an environment free of potentially confounding allogeneic factors. By isolating splenic T cells and gonadal vein endothelial cells (GVEC) from individual cadaveric organ donors, we have developed an in vitro model of T cell interaction with autologous CMV-infected EC. Proliferation assays demonstrated significantly enhanced responses by CMV-seropositive donor-derived T cells cocultured with CMV-infected GVEC, as compared with those elicited by uninfected cells. Similarly, as determined by limiting dilution analysis of IL-2-producing cells, T cell response frequencies to infected GVEC were significantly greater than to uninfected EC. In contrast, responses of CMV-seronegative donor-derived T cells were minimal, regardless of CMV status of stimulator GVEC. Intriguingly, CD4 responses were observed in spite of the fact that CMV-infected EC express no HLA class II. Finally, attenuation of CMV-stimulated T cell proliferation observed in the presence of blocking Ab specific for ICAM-1 suggests a contributing role for CMV-enhanced endothelial ICAM-1 expression in the activation response. These studies demonstrate that EC can stimulate autologous T cell responses to CMV in the absence of accessory APC and suggest potentially novel mechanisms of immune activation.

Expression of major histocompatibility complex antigens on mouse brain microvascular endothelial cells in relation to susceptibility to cerebral malaria

The physiopathology of experimental cerebral malaria (CM), an acute neurological complication of Plasmodium berghei ANKA (PbA) infection, involves interferon-gamma (IFN-gamma) and tumour necrosis factor-alpha (TNF-alpha), two cytokines that are known to modulate major histocompatibility complex (MHC) molecule expression. The aim of this study was to evaluate whether the genetic susceptibility to CM is related to the constitutive or IFN-gamma-induced expression of MHC molecules on brain microvessels. To this end, brain microvascular endothelial cells (B-MVEC) were isolated from CM-susceptible (CM-S, CBA/J) and resistant (CM-R, BALB/c) mice. By flow cytometry, we found that less than 5% of CM-S B-MVEC constitutively expressed MHC class I molecules, in contrast to up to 90% of CM-R B-MVEC. Upon stimulation with IFN-gamma, the percentage of positive cells for MHC class I molecules in CM-S B-MVEC became comparable to CM-R B-MVEC, but a higher fluorescence intensity existed on CM-S B-MVEC compared with CM-R B-MVEC. MHC class II molecules were not constitutively expressed on B-MVEC from either strain. IFN-gamma-induced expression of MHC class II (I-A, I-E) molecules was significantly higher in CM-S than CM-R B-MVEC both in percentage of positive cells and fluorescence intensity. These data demonstrate that absent or low MHC class I and higher inducibility of MHC class II expression on B-MVEC are associated with the genetic susceptibility to CM.

Human aortic valve allografts elicit a donor-specific immune response

OBJECTIVE

The nature and magnitude of the immunologic response to implantation of human cryopreserved aortic valve allografts was investigated.

METHODS

Twenty aortic valve allograft recipients were investigated for donor-specific antibody and T-cell-mediated responses with serial flow cytometric and microlymphocytotoxic crossmatch assays and one-way mixed lymphocyte cultures.

RESULTS

Donor-specific immunoglobulin G antibodies to class I and II human leukocyte antigens were first detected in the serum of all aortic valve allograft recipients at 30 days after implantation and persisted in substantial amounts in all but one of the recipients at day 365. Recipient T-cell alloreactivity toward donor lymphocytes was significantly increased at day 30 compared with levels before and 10 days after operation.

CONCLUSIONS

Cryopreserved aortic valve allografts elicit a substantial allogeneic response in recipients. This alloreactivity may contribute to the observed morphologic changes in aortic valve allografts and eventual long-term deterioration of allograft function.

Alloproliferation of purified CD4+ T cells to adult human heart endothelial cells, and study of second-signal requirements

Human endothelial cells have been shown to be capable of causing direct allostimulation of T cells. However, the majority of immunological studies of human endothelial cells have been performed on cells of fetal origin. Here we use endothelial cells isolated from the adult human heart, both large vessel (coronary artery, pulmonary artery and aorta) and also microvascular. We have examined the ability of all these endothelial cells to cause direct allostimulation of T cells, and show that purified CD4+ T cells can proliferate in response to adult human heart endothelial cells, the response being dependent on pretreatment of the endothelial cells with interferon-gamma (IFN-gamma) and inhibited by anti-HLA-DR monoclonal antibody. The proliferative responses of CD8+ T cells to adult but not fetal endothelial cells was inconsistent and weak. Proliferative responses were not blocked by CTLA4-Ig, which inhibits T-cell responses to "classical' antigen-presenting cells (APC), but > 50% inhibition was achieved with monoclonal antibody to lymphocyte function-associated antigen-3 (LFA-3). These results show that adult human cardiovascular endothelial cells are capable of causing allostimulation of resting CD4+ T cells, using a different second signal to classical APC. In view of these findings endothelial cells should be considered as APC following solid organ transplantation.

Hepatic expression of macrophage inflammatory protein-1 alpha and macrophage inflammatory protein-1 beta after liver transplantation

Two local events that are crucial for T cell emigration into tissue are (1) activation of T cell integrins to permit binding to endothelial counter-receptors and (2) directed migration through the endothelium and into tissue in response to chemotactic factors. Because the chemokines macrophage inflammatory protein-1 alpha (MIP-1 alpha) and MIP-1 beta can activate adhesion and induce migration of T cells in vitro, we investigated their expression in human liver allografts to determine whether they might be involved in regulating the recruitment of T cells to allografts in vivo. Both chemokines were expressed strongly by infiltrating leukocytes during rejection and could be detected immunohistochemically on biliary epithelium, an important target for T cell mediated graft damage. Both chemokines, but particularly MIP-1 beta, were detected on the vascular and sinusoidal endothelium of rejecting liver allografts, where they were coexpressed with the T cell beta 1-integrin receptor vascular cell adhesion molecule-1. In situ hybridization with complementary ribonucleic acid probes showed no MIP-1 alpha or MIP-1 beta mRNA in normal liver but dramatic expression of both chemokines in infiltrating leukocytes and graft endothelium during rejection. Expression was reduced after successful corticosteroid treatment of rejection but persisted in patients progressing to chronic rejection. Increased MIP-1 alpha and MIP-1 beta mRNA expression was already found in biopsies taken at the end of the transplant operation, suggesting that early induction of chemokines, possibly in response to graft reperfusion, might promote the subsequent development of graft rejection. These data demonstrate for the first time that MIP-1 alpha and MIP-1 beta are (1) expressed in human liver allografts, (2) produced by endothelial cells in vivo, and (3) induced early after transplantation. They suggest that MIP-1 alpha and MIP-1 beta produced by graft infiltrating leukocytes and graft endothelium might play a crucial role in regulating T cell recruitment to liver allografts in vivo.

IgG antiendothelial cell autoantibodies from scleroderma patients induce leukocyte adhesion to human vascular endothelial cells in vitro. Induction of adhesion molecule expression and involvement of endothelium-derived cytokines

IgG autoantibodies that bind human endothelial cells (AECA) were detected by ELISA in 30 of 42 samples of sera from patients with scleroderma. Pretreatment of human umbilical vein endothelial cells with AECA-positive scleroderma sera, or IgG purified from these sera, led to a dose- and time-dependent increase in the ability of the cells to bind human U937 monocytic cells. Threshold-active IgG concentrations were 1-10 micrograms/ml; effects were significant after 3 h and maximal after 6-12 h. IgG from AECA-negative sera or normal sera were without effect. Increased adhesion of U937 cells was accompanied by increased expression of endothelial intercellular adhesion molecule-1, vascular cell adhesion molecule-1, and E-selectin. Transfer of endothelial cell-conditioned media after pretreatment with AECA and immunodepletion of IgG demonstrated the presence of transferable activity that mimicked the effects of AECA. Treatment with neutralizing anticytokine antibodies indicated that IL-1, generated by the endothelial cells in response to AECA, was involved in the upregulation of adhesion molecules and U937 cell adhesion. We conclude that AECA can play a pathogenic role in scleroderma by activating endothelial cells, in part due to autocrine or paracrine actions of IL-1.

NK recognition of cytomegalovirus-infected endothelial cells depends on viral replication and MHC class I expression

Cytomegalovirus (CMV) infection has been associated with graft rejection in solid organ transplantation and with graft-versus-host disease in marrow transplantation. We hypothesized that CMV-infected endothelial cells play an important role in the rejection process, because of their strategic localization at the interface with the host immune system and their ability to modulate T cell function. To study the effect of CMV infection on cell-mediated cytotoxicity against endothelial cells, peripheral blood mononuclear cells (MNC) were incubated with CMV-infected umbilical vein endothelial cells (CMV-UVEC) or mock-infected controls (M-UVEC) and lysis measured by [3H]leucine release. MNC lysed only CMV-UVEC to a maximum of 23% at E:T 20:1. Lysis was not affected by CD3+ cell depletion, but was abolished by CD16+ cell depletion, indicating that NK cells were the effectors. The kinetics of the NK-mediated lysis of CMV-UVEC paralleled the time course of CMV antigen expression. Furthermore, ganciclovir treatment of CMV-UVEC cultures decreased both specific antigen synthesis and NK-mediated lysis. This indicated that NK might recognize either a viral antigen or a cellular antigen modulated by CMV infection. Treatment of CMV-UVEC with F(ab)2 fragments of human polyclonal anti-CMV antibodies failed to inhibit NK cytotoxicity. In contrast, F(ab)2 fragments of MB40.5, a murine MAb reactive with a conserved epitope on the human MHC class I, significantly decreased lysis, proving that NK lysis of CMV-UVEC is an MHC class I-dependent function. To determine whether CMV-UVEC lysis was dependent solely on upregulation of MHC class I, MNC were incubated with CMV-UVEC mixed with uninfected UVEC. There was no competition for NK-target recognition sites, indicating that NK lysis required an interaction with an MHC class I antigen modified by viral infection. Antibodies against IFN-alpha or -beta did not block NK cytotoxicity against CMV-UVEC. Our findings provide a working frame for further evaluation of cellular immune responses to CMV infection.

Functional interactions of T cells with endothelial cells: the role of CD40L-CD40-mediated signals

CD40 is expressed on a variety of cells, including B cells, monocytes, dendritic cells, and fibroblasts. CD40 interacts with CD40L, a 30-33-kD activation-induced CD4+ T cell surface molecule. CD40L-CD40 interactions are known to play key roles in B cell activation and differentiation in vitro and in vivo. We now report that normal human endothelial cells also express CD40 in situ, and CD40L-CD40 interactions induce endothelial cell activation in vitro. Frozen sections from normal spleen, thyroid, skin, muscle, kidney, lung, or umbilical cord were studied for CD40 expression by immunohistochemistry. Endothelial cells from all tissues studied express CD40 in situ. Moreover, human umbilical vein endothelial cells (HUVEC) express CD40 in vitro, and recombinant interferon gamma induces HUVEC CD40 upregulation. CD40 expression on HUVEC is functionally significant because CD40L+ Jurkat T cells or CD40L+ 293 kidney cell transfectants, but not control cells, upregulate HUVEC CD54 (intercellular adhesion molecule-1), CD62E (E-selectin), and CD106 (vascular cell adhesion molecule-1) expression in vitro. Moreover, the kinetics of CD40L-, interleukin 1-, or tumor necrosis factor alpha-induced CD54, CD62E, and CD106 upregulation on HUVEC are similar. Finally, CD40L-CD40 interactions do not induce CD80, CD86, or major histocompatibility complex class II expression on HUVEC in vitro. These results demonstrate that CD40L-CD40 interactions induce endothelial cell activation in vitro. Moreover, they suggest a mechanism by which activated CD4+ T cells may augment inflammatory responses in vivo by upregulating the expression of endothelial cell surface adhesion molecules.

Endothelial cells and renal epithelial cells do not express the Wegener's autoantigen, proteinase 3

Proteinase 3 (PR3) is the major antigen for autoantibodies (C-ANCA) against cytoplasmic components of neutrophils which are strongly associated with Wegener's granulomatosis (WG). Recent data that PR3 may be expressed by renal tubular epithelial cells and endothelial cells suggest potential for a direct pathogenic effect against these cells by C-ANCA or cytoxic T lymphocytes. Using a semi-quantitative polymerase chain reaction (PCR), ELISA and indirect immunofluorescence staining we studied endothelial and epithelial cell PR3 expression. By PCR, no PR3 expression was found in human umbilical vein endothelial cells (HUVEC) either untreated, or when treated with interferon-gamma (IFN-gamma) (200 U/ml, 6 h, 24 h), IL-1 (20 U/ml, 6 h), tumour necrosis factor-alpha, (TNF-alpha) (200 U/ml, 0, 1, 2, 4, 6 h) or IFN-gamma + TNF-alpha (6 h); iliac vein and artery endothelial cells did not express PR3 either. In contrast, PR3 was detected in HL60 cells and neutrophils by PCR, expression being confirmed by sequence analysis. Three PR3 MoAbs showed no binding to unstimulated or TNF-alpha-stimulated HUVEC either by ELISA or by indirect immunofluorescence staining. The epithelial cell line A549 expressed PR3 when assayed by PCR. However, three renal epithelial cell lines (two tubular and one glomerular) showed little or no PR3 expression by PCR or ELISA. These studies fail to demonstrate evidence for PR3 expression by endothelial cells, even when using the highly sensitive PCR assay. Whilst PR3 expression by A549 cells is intriguing, the relevance of this in the pathology of WG is doubtful considering the negligible expression by renal epithelial cell lines.

Antigen-presenting function of human peritoneum mesothelial cells

Mesothelial cells (MC) from human peritoneal omentum fragments obtained during surgical insertion of peritoneal catheters for continuous peritoneal dialysis in end stage renal failure (ESRF) patients were cultured in vitro. MC exhibited a phenotype different from macrophages, but MHC class II molecules were well expressed. Therefore MC lines were tested for antigen-presenting capacity by pulsing with soluble antigens (tetanus toxoid and purified protein derivative (PPD)) or with a corpusculate antigen (Candida albicans bodies). Autologous peripheral blood mononuclear cells (PBMC) depleted of adherent monocytes and cloned T cells generated from an individual matched for the MHC class II antigen DR2 were used to test antigen-presenting function. MC effectively presented the soluble and corpusculate antigens to autologous and MHC-compatible allogeneic lymphocytes, indicating that they are endowed with both endocytic/phagocytic activity and with processing/presenting capacity. Preincubation of MC with human recombinant interferon-gamma (IFN-gamma) up-regulated MHC class II and intercellular adhesion molecule-1 (ICAM-1) expression, but the effect on antigen-presenting function was not consistent. Since MC are an important component of the peritoneal environment, they may participate, along with macrophages, in activation of specific T cells and in the generation of local cell-mediated immunity to various pathogens.

Immunomodulation of cultured vascular endothelial cells by serial cell passage

Endothelial cell seeding has been successful in reducing the thrombogenecity of prosthetic vascular grafts in animal models, but results from clinical trials have been largely disappointing. These poor results have been associated with poor graft coverage in immediate seeding trials, and failure of cell culture in staged procedures. These problems could be largely overcome by utilising a bank of allogeneic endothelial cells, providing an ever ready supply. However, one potential pitfall with this technique would be the possibility of a rejection response following transplantation.

AIM

To study the effects of prolonged tissue culture, on the ability of endothelial cells to generate an immune response.

METHODS AND RESULTS

The immunogenecity of human umbilical vein endothelial cells was measured using the mixed lymphocyte endothelial reaction. It was demonstrated that prolonged tissue culture significantly reduced the immunogenecity of the cells, from a mean of 7261 cpm (S.E. +/- 243, n = 3) for cells of subculture 3, to 5478 cpm (+/- 156, p = 0.04) for cells of subculture 7 (p = 0.04, Wilcoxon paired rank test), but did not significantly impair morphology or antithrombotic function.

CONCLUSION

This study provides evidence that prolonged tissue culture provides morphologically and functionally intact, immunomodified endothelial cells which may potentially be used in seeding prosthetic vascular grafts.

Modulation of alloreactivity in transplant recipients by phenotypic manipulation of donor endothelium

Phenotypic manipulation of allograft endothelium to reduce immunogenicity would have a significant impact on transplantation. In this study we have demonstrated that random seeding of a heart allograft with endothelium, of host origin, not only promotes long-term survival, but reduces the requirement for pharmacologic immunosuppression. We propose that this simple technology could easily be extrapolated to the clinical arena where hypothermia and preservation solutions have allowed allografts to remain ex vivo for extended periods.

Chronic inflammation in the myocardium of patients with clinically suspected dilated cardiomyopathy

Dilated cardiomyopathy continues to be an etiologically unknown heart muscle disease. Recent clinical and experimental data have suggested a causal relation to viral myocarditis. The clinical diagnosis, however, is unspecific, and diagnostic yield of the histologic evaluation of endomyocardial biopsies by light microscopy according to the Dallas classification is poor. The authors analyzed the biopsy specimens of 120 patients with suspected dilated cardiomyopathy by immunohistologic methods to obtain a more sensitive and specific identification and quantification of infiltrating lymphocytes, indicating an activated immunologic process within the myocardium. Increased lymphocytic infiltrates and inflammatory endothelial activation were demonstrated in patients with clinically suspected dilated cardiomyopathy. These findings are associated with the often seen progression of ventricular dysfunction. Further studies are necessary to prove whether these immunohistologically positive patients will improve under immunosuppressive therapy.

CD14 is a pattern recognition receptor

Septic shock caused by a diverse group of bacterial pathogens is a serious human disease. Recognition of bacterial envelope constituents is one mechanism used by mammalian cells to initiate responses leading to bacterial killing or, unfortunately, responses that also cause fatal septic shock. Here we show that CD14 plays a key role in initiating cell activation by a group of bacterial envelope components from Gram-negative and Gram-positive microorganisms, as well as mycobacteria. We propose that CD14 is a receptor used by mammalian cells to recognize and signal responses to a diverse array of bacterial constituents. This finding defines the molecular basis for innate microbial immunity; implicit in these findings are new possibilities for therapeutics.

Mechanisms of endothelial cell injury in vasculitis

The aetiology of the primary systemic vasculitides remains obscure. Recent years have seen significant advances in our understanding of inflammation and in particular the role of and interaction between the vascular endothelium, mediators and immune effector cells. This has helped to further elucidate those specific processes relevant to vasculitis which result in endothelial cell damage. In Wegener's granulomatosis and microscopic polyarteritis the evidence favours an autoimmune inflammatory response characterised by specific mediators in which the endothelium is both target and active participant. Current treatment of these disorders with combinations of corticosteroids and cytotoxics is highly effective in inducing remission. However, long-term use of this therapy is potentially toxic and there remains also a significant risk of relapse. It is hoped that increased understanding of the pathogenesis of systemic vasculitis will enable more specific, less toxic and more effective therapies to be defined. Jayne et al. have suggested a beneficial effect of intravenous pooled normal human immunoglobulin (IVIG) in patients with ANCA-positive vasculitis. In vitro studies have shown that IVIG contains antiidiotypic antibodies to ANCA and AECA, capable of inhibiting the binding of these autoantibodies to their autoantigens. In vivo, IVIG may also provide the immunoregulatory elements needed for the idiotype network and control of the autoimmune repertoire. Mathieson et al. successfully used monoclonal antibodies to T cells (Campath-H directed against CDw52) in a patient with ANCA-negative dermal lymphocytic vasculitis. Monoclonal antibodies to CAMs have been used in human renal transplant rejection and reduced the inflammation and proteinuria in animal models of anti-glomerular basement membrane disease. In vasculitis, the therapeutic use of specific anti-CAM antibodies may result from further definition of the role of CAMs. Increased understanding of the pathogenesis of systemic vasculitis is likely to provide the basis for the use of more specific immunotherapies in the future.

The biology of the glomerulus: endothelial cells

Endothelial cells are active participants in processes controlling coagulation, inflammation and immune processes and an aberration in the controlling mechanisms may contribute to the development of disease within the glomerulus. The major goal of the next few years will be to develop reliable and reproducible methods for isolation and culture of glomerular endothelial cells so that their properties, and interactions with other glomerular cells, may be studied in vitro.

The proliferative response of human T cells to allogeneic IFN-gamma-treated endothelial cells is mediated via both CD2/LFA-3 and LFA-1/ICAM-1 and -2 adhesion pathways

We studied the proliferative response of purified human peripheral blood T lymphocytes (contaminated with less than 0.1% monocytes) to allogeneic MHC class II molecules expressed by endothelial cells (EC) or fibroblasts (FB). In vitro expression of MHC class II molecules was induced by gamma-interferon (IFN-gamma) treatment. The MHC class II expression levels after IFN-gamma treatment on both cell types were comparable. No T cell proliferation was found in the presence of either untreated or IFN-gamma-treated FB, and a marginal proliferation in the presence of untreated EC. IFN-gamma-treated EC, however, were able to induce significant T cell growth. The previously established role of MHC class II molecules in allogeneic T cell proliferation was confirmed in inhibition experiments with monoclonal antibody (mAb) against MHC class II or CD4. In this model, we tested the involvement of a number of adhesion molecules by adding mAbs to cocultures of T cells and IFN-gamma-treated EC. Monoclonal antibodies directed against CD31, CD26, B7/BB1, E-selectin, CD44, VLA-4 alpha-chain and VCAM-1 had no effect, whereas moderate inhibition was observed with anti-VLA-beta-chain and anti-LFA-3. A distinct inhibition of T cell proliferation was observed with mAbs directed against LFA-1, CD2, or a combination of anti-ICAM-1 and -2. Combinations of mAbs directed against T cell adhesion molecules (LFA-1, CD2, VLA-4) or EC adhesion molecules (ICAM-1, and -2, LFA-3, VCAM-1) were able to block T cell proliferation for 100 and 80% respectively. We conclude that CD2/LFA-3 and LFA-1/ICAM interactions are crucially involved in allogeneic T cell/EC interactions.

Thymus epithelium induces tissue-specific tolerance

Most current models of T cell development include a positive selection step in the thymus that occurs when T cells interact with thymic epithelium and a negative selection step after encounters with bone marrow-derived cells. We show here that developing T cells are tolerized when they recognize antigens expressed by thymic epithelium, that the tolerance is tissue specific, and that it can occur by deletion of the reactive T cells.

Contrasting effects of interferon gamma and interleukin 4 on responses of human vascular endothelial cells to tumour necrosis factor alpha

Tumour necrosis factor alpha (TNF-alpha) and interleukin 4 (IL-4) selectively synergise in inducing expression of the mononuclear cell adhesion receptor VCAM-1 (vascular cell adhesion molecule-1) on human umbilical vein endothelial cells (HUVEC), which results in increased adhesiveness of HUVEC for T lymphocytes. This process may be crucial for adherence of circulating lymphocytes prior to their passage from the blood into inflammatory tissues. IL-4 also amplifies production of interleukin 6 (IL-6) and monocyte chemotactic protein-(MCP-1) from TNF-alpha-activated HUVEC. In the present study we demonstrate that IL-4 enhances production of granulocyte-macrophage colony-stimulating factor (GM-CSF) from TNF-alpha-stimulated HUVEC. Moreover, using cultured adult saphenous vein and umbilical artery endothelial cells, we show identical effects of IL-4 on TNF-alpha-induced responses to those observed with endothelial cells of foetal origin. Additionally, we report here that TNF-alpha and interferon gamma (IFN-gamma) synergise in the induction of both the lymphocyte adhesion receptor VCAM-1, and the TNF-alpha-inducible neutrophil adhesion receptor intercellular adhesion molecule-1, on all three endothelial cell types studied. In contrast, we found that GM-CSF secretion by endothelial cells treated with IFN-gamma plus TNF-alpha was markedly decreased when compared to the response induced by TNF-alpha alone. These results suggest that the combined actions of several cytokines, acting sequentially or in concert, may exert differential effects on activation and accumulation of circulating lymphocytes at sites of inflammation.

Accessory function of endothelial cells in anti-CD3-induced T-cell proliferation: synergism with monocytes

Monoclonal antibodies to CD3 can induce proliferation of resting T cells. In vitro this effect is dependent on the presence of monocytes. They serve as accessory cells providing a co-stimulatory signal after cross-linking of the antibody-coated TcR/CD3 complex by the Fc receptor on the monocytes. We have studied whether endothelial cells can replace monocytes with regard to this function. Highly purified T-cell preparations were cultured in the presence of anti-CD3 antibody, purified monocytes, and human umbilical vein endothelial cells. Anti-CD3 and endothelial cells alone were unable to support T-cell proliferation, due to lack of FcR expression. Addition, however, of as few as 1000 FcR+ monocytes (0.8% of the number of T cells present) to a coculture of T cells and endothelial cells (EC) in the presence of soluble anti-CD3 resulted in a strong proliferation of T cells. When anti-CD3 was presented in an immobilized form (coated to the culture well or to Sepharose beads), or when phytohaemagglutinin was added to the culture as a cross-linking agent, EC could support T-cell proliferation in the absence of any monocytes. We conclude that EC by themselves cannot support the proliferation of pure T cells induced by soluble anti-CD3, but are potent generators of the co-stimulatory signal(s). They provide a suitable starting material to further define this co-stimulatory activity.

Role of the endothelial adhesion molecule VCAM in murine cardiac allograft rejection

Murine heterotopic cardiac isografts (C57B1/6----C57B1/6) undergo transient, non-destructive inflammation that is characterized by the acquisition of microvascular endothelial reactivity with the antibody MECA 32. Cardiac allografts (C57B1/6----DBA/2) undergo destructive inflammation that is characterized by the acquisition of reactivity with the antibody M/K-2, in addition to MECA 32. M/K-2 recognizes the murine endothelial adhesion molecule, VCAM-1. Hence, there appear to be antigen-dependent and antigen-independent forms of graft inflammation. Treatment of cardiac allograft recipients with 200 micrograms/day M/K-2 antibody retarded graft loss by only a few days, and did not interfere significantly with leukocytic infiltration, as detected by limiting dilution analysis of graft-reactive CTL, despite the fact that large amounts of M/K-2 could be detected on graft microvascular endothelia and in the peripheral blood as rejection progressed. These data indicate that VCAM is apparently not essential for the leukocytic infiltration and subsequent rejection of cardiac allografts, and is not involved in leukocytic infiltration of murine cardiac isografts.

Components of an antigen-/T cell receptor-independent pathway of lymphokine production

The general way to induce the synthesis of lymphokines by T cells is the stimulation through the T cell receptor (TcR) complex which results in an increase of intracellular [Ca2+] and in the activation of a tyrosine kinase as well as of protein kinase C. Lymphokine production induced via the TcR is inhibited by the immunosuppressive drug cyclosporin A (CsA). However, an alternative pathway of lymphokine production exists. Several T lymphocyte clones can synthesize interferon-gamma (IFN-gamma), granulocyte-monocyte colony-stimulating factor, and small amounts of interleukin (IL3) when stimulated with syngeneic or allogeneic accessory cells (AC) plus IL2. In contrast to the TcR pathway the alternative pathway does not require a rise of intracellular [Ca2+] and is insensitive to the effects of CsA. In this report we provide evidence for the involvement of T cell-stimulating factor (TSF)--a probably novel murine cytokine--in the alternative pathway of lymphokine production. It is shown that fixation of the AC with carbodiimide or treatment of the AC with UV light greatly reduces their capacity to induce (in combination with IL2) the synthesis of IFN-gamma by T cells. This function is restored by addition of TSF. Moreover, TSF alone (without IL2) in combination with fixed AC can induce the synthesis of substantial amounts of IFN-gamma. Furthermore, TSF in combination with IL2 can stimulate freshly isolated spleen cells to produce IFN-gamma. The target cell resides probably in the non-B cell, non-T cell population.