Defining variant-resistant epitopes targeted by SARS-CoV-2 antibodies: A global consortium study

Antibody-based therapeutics and vaccines are essential to combat COVID-19 morbidity and mortality after severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. Multiple mutations in SARS-CoV-2 that could impair antibody defenses propagated in human-to-human transmission and spillover or spillback events between humans and animals. To develop prevention and therapeutic strategies, we formed an international consortium to map the epitope landscape on the SARS-CoV-2 spike protein, defining and structurally illustrating seven receptor binding domain (RBD)–directed antibody communities with distinct footprints and competition profiles. Pseudovirion-based neutralization assays reveal spike mutations, individually and clustered together in variants, that affect antibody function among the communities. Key classes of RBD-targeted antibodies maintain neutralization activity against these emerging SARS-CoV-2 variants. These results provide a framework for selecting antibody treatment cocktails and understanding how viral variants might affect antibody therapeutic efficacy.

Characterization of the mouse cold-menthol receptor TRPM8 and vanilloid receptor type-1 VR1 using a fluorometric imaging plate reader (FLIPR) assay

1. TRPM8 (CMR1) is a Ca(2+)-permeable channel, which can be activated by low temperatures, menthol, eucalyptol and icilin. It belongs to the transient receptor potential (TRP) family, and therefore is related to vanilloid receptor type-1 (VR1, TRPV1). We tested whether substances which are structurally related to menthol, or which produce a cooling sensation, could activate TRPM8, and compared the responses of TRPM8 and VR1 to these ligands. 2. The effects of 70 odorants and menthol-related substances on recombinant mouse TRPM8 (mTRPM8), expressed in HEK293 cells, were examined using a FLIPR assay. In all, 10 substances (linalool, geraniol, hydroxycitronellal, WS-3, WS-23, FrescolatMGA, FrescolatML, PMD38, CoolactP and Cooling Agent 10) were found to be agonists. 3. The EC(50) values of the agonists defined their relative potencies: icilin (0.2+/-0.1 microM)>FrescolatML (3.3+/-1.5 microM) > WS-3 (3.7+/-1.7 microM) >(-)menthol (4.1+/-1.3 microM) >frescolatMAG (4.8+/-1.1 microM) > cooling agent 10 (6+/-2.2 microM) >(+)menthol (14.4+/-1.3 microM) > PMD38 (31+/-1.1 microM) > WS-23 (44+/-7.3 microM) > Coolact P (66+/-20 microM) > geraniol (5.9+/-1.6 mM) > linalool (6.7+/-2.0 mM) > eucalyptol (7.7+/-2.0 mM) > hydroxycitronellal (19.6+/-2.2 mM). 4. Known VR1 antagonists (BCTC, thio-BCTC and capsazepine) were also able to block the response of TRPM8 to menthol (IC(50): 0.8+/-1.0, 3.5+/-1.1 and 18+/-1.1 microM, respectively). 5. The Ca(2+) response of hVR1-transfected HEK293 cells to the endogenous VR1 agonist N-arachidonoyl-dopamine was potentiated by low pH. In contrast, menthol- and icilin-activated TRPM8 currents were suppressed by low pH. 6. In conclusion, in the present study, we identified 10 new agonists and three antagonists of TRPM8. We found that, in contrast to VR1, TRPM8 is inhibited rather than potentiated by protons.

CD154 stimulation of interleukin-12 synthesis in human endothelial cells

The interaction of proinflammatory type 1 T helper (Th1) cells expressing the CD40 ligand (CD154) with endothelial cells expressing the corresponding receptor (CD40) may play an important role in chronic inflammation including arteriosclerosis. Here we demonstrate that activation of CD40 in human cultured endothelial cells (e.g. by interaction with freshly isolated human T cells) not only up-regulates expression of various adhesion molecules, chemokines and cytokines, but within 12-24 h also causes the release of bioactive interleukin-12 (IL-12 p70) through induction of IL-12 p40 synthesis. IL-12 p35, on the other hand, appears to be constitutively expressed in these cells. Despite enhancing expression of the other gene products, cytokines such as interferon-gamma (IFN-gamma) or tumor necrosis factor-alpha, alone or in combination, failed to induce IL-12 p40 expression, whereas IFN-gamma markedly augmented CD154-induced IL-12 p40 and p70 release. Of note was that the magnitude of CD154-induced IL-12 synthesis in the cultured endothelial cells was comparable to that evoked in freshly isolated human monocytes. This CD40-mediated induction of endothelial IL-12 synthesis may thus lead to an enhanced activation of the adherent CD154-expressing Th1 cells, thereby fuelling the proinflammatory response.

Effect of thalidomide and structurally related compounds on corneal angiogenesis is comparable to their teratological potency

Neovascular diseases are the leading causes of blindness in humans. Although several promising compounds have been isolated, pharmacological treatment remains difficult. Thalidomide has inhibitory effects on angiogenesis in the corneal micropocket assay. However, the results vary considerably depending on the administration route and animal model. The aim of this study, therefore, was to investigate thalidomide and two of its derivatives, supidimide and EM12, in the rabbit corneal micropocket assay. Using both basic fibroblast growth factor and vascular endothelial growth factor for initiation of the neovascular response, we were able to show a significant inhibition of neovascularisation with all three substances. EM12, the most teratogenic derivative analysed, was demonstrated to be the most potent inhibitor of angiogenesis in this model. Thalidomide and supidimide did not show systemic side effects in the applied dosage. An equal dosage of EM12, however, resulted in significant weight loss of the animals, but did not increase angiogenic activity compared with lower doses. Together with earlier findings, these data support a strong correlation between the antiangiogenic potential and the teratogenic activity of thalidomide and structurally related compounds.

Induction of cytokine production in naive CD4(+) T cells by antigen-presenting murine liver sinusoidal endothelial cells but failure to induce differentiation toward Th1 cells

BACKGROUND & AIMS

Murine liver sinusoidal endothelial cells (LSECs) constitutively express accessory molecules and can present antigen to memory Th1 CD4(+) T cells. Using a T-cell receptor transgenic mouse line, we addressed the question whether LSECs can prime naive CD4(+) T cells.

METHODS

Purified LSECs were investigated for their ability to induce activation and differentiation of naive CD4(+) T cells in comparison with bone marrow-derived antigen-presenting cells and macrovascular endothelial cells. Activation of T cells was determined by cytokine production. LSECs were further studied for expression of interleukin (IL)-12 by reverse-transcription polymerase chain reaction, and the unique phenotype of LSECs was determined by flow cytometry.

RESULTS

We provide evidence that antigen-presenting LSECs can activate naive CD62Lhigh CD4(+) T cells. Activation of naive CD4(+) T cells by LSECs occurred in the absence of IL-12. In contrast, macrovascular endothelial cells from aorta could not activate naive CD4(+) T cells. The unique functional characteristics of microvascular LSECs together with a unique phenotype (CD4(+), CD11b+, CD11c+, CD80(+), CD86(+)) make these cells different from macrovascular endothelial cells. Furthermore, LSECs did not require in vitro maturation to activate naive CD4(+) T cells. Most importantly, LSECs failed to induce differentiation toward Th1 cells, whereas conventional antigen-presenting cell populations induced a Th1 phenotype in activated CD4(+) T cells. Upon restimulation, CD4(+) T cells, which were primed by antigen-presenting LSECs, expressed interferon gamma, IL-4, and IL-10, which is consistent with a Th0 phenotype. Exogenous cytokines (IL-1beta, IL-12, or IL-18) present during T-cell priming by antigen-presenting LSECs could not induce a Th1 phenotype, but neutralization of endogenously produced IL-4 during T-cell priming led to a reduced expression of IL-4 and IL-10 by CD4(+) T cells upon restimulation. The addition of spleen cells to cocultures of LSECs and naive CD4(+) T cells during T-cell priming led to differentiation of T cells toward a Th1 phenotype.

CONCLUSIONS

The ability of antigen-presenting LSECs to induce cytokine expression in naive CD4(+) T cells and their failure to induce differentiation toward a Th1 phenotype may contribute to the unique hepatic microenvironment that is known to promote tolerance.

Endotoxin down-regulates T cell activation by antigen-presenting liver sinusoidal endothelial cells

Endotoxin is physiologically present in portal venous blood at concentrations of 100 pg/ml to 1 ng/ml. Clearance of endotoxin from portal blood occurs through sinusoidal lining cells, i.e., Kupffer cells, and liver sinusoidal endothelial cells (LSEC). We have recently shown that LSEC are fully efficient APCs. Here, we studied the influence of endotoxin on the accessory function of LSEC. Incubation of Ag-presenting LSEC with physiological concentrations of endotoxin lead to >/=80% reduction of the accessory function, measured by release of IFN-gamma from CD4+ T cells. In contrast, conventional APC populations rather showed an increase of the accessory function after endotoxin treatment. Inhibition of the accessory function in LSEC by endotoxin was not due to lack of soluble costimulatory signals, because neither supplemental IL-1beta, IL-2, IFN-gamma, or IL-12 could rescue the accessory function. Ag uptake was not influenced by endotoxin in LSEC. However, we found that endotoxin led to alkalinization of the endosomal/lysomal compartment specifically in LSEC but not in bone marrow macrophages, which indicated that Ag processing, i.e., proteolytic cleavage of protein Ags into peptide fragments, was affected by endotoxin. Furthermore, endotoxin treatment down-regulated surface expression of constitutively expressed MHC class II, CD80, and CD86. In conclusion, it is conceivable that endotoxin does not alter the clearance function of LSEC to remove gut-derived Ags from portal blood but specifically affects Ag processing and expression of the accessory molecules in these cells. Consequently, Ag-specific immune responses by CD4+ T cells are efficiently down-regulated in the hepatic microenvironment.

Methotrexate specifically modulates cytokine production by T cells and macrophages in murine collagen-induced arthritis (CIA): a mechanism for methotrexate-mediated immunosuppression

Immunosuppressive therapy with methotrexate (MTX) has been established as effective treatment for patients with rheumatoid arthritis. To analyse the therapeutic potential and mechanisms of action of MTX, we determined serum cytokine levels and cytokine production by splenic T cells and macrophages in untreated and MTX-treated mice. Furthermore, we assessed the role of MTX in a murine model of experimental arthritis induced by collagen type II (CIA). MTX reduced spontaneous and IL-15-induced tumour necrosis factor (TNF) production by splenic T cells but not by macrophages from healthy mice in vitro in a dose-dependent manner. In contrast, interferon-gamma (IFN-gamma) production was less strikingly reduced and IL-4 production was virtually unaffected. In addition, treatment of healthy mice with MTX in vivo led to reduced TNF serum levels and diminished TNF production by splenic T cells and macrophages. Intraperitoneal administration of MTX prior to the onset of arthritis completely prevented clinical and pathological signs of CIA. This was associated with a striking reduction of TNF production by spleen cells from MTX-treated mice. The role of TNF in MTX-mediated effects on cytokine production was further underlined by the finding that MTX effects on IFN-gamma production were augmented in TNF-transgenic mice but abrogated in mice in which the TNF-alpha gene had been inactivated by homologous recombination. Thus, MTX specifically modulates spontaneous and IL-15-induced TNF-alpha production in mice and prevents experimental murine CIA. These data suggest that TNF production by T cells is an important target of MTX and may serve as a basis to understand and further analyse MTX-mediated mechanisms of immunosuppression in patients with RA.

The host response to Listeria monocytogenes mutants defective in genes encoding phospholipases C (plcA, plcB) and actin assembly (actA)

Several genes involved in the determination of Listeria monocytogenes pathogenesis have been identified. Among them, plcA gene encodes phosphatidylinositol-specific phospholipase C (PI-PLC), plcB gene encodes a broad-range phospholipase C (PC-PLC), and actA encodes a protein contributing to actin assembly in infected cells. The interaction of L. monocytogenes wild type (LO 28) strain and two derivative mutants, plcA- (BUG 206) and actA-/plcB- (LUT 12), with macrophages and T lymphocytes was investigated in a mouse model of listeriosis. Both mutants showed evidence of attenuation. The plcA- mutant, but not the plcB- mutant, expressed an increase in susceptibility to the anti-listerial activity of macrophages. Both mutants showed a decreased ability to induce IL-12 production by bone marrow macrophages when co-stimulated with E. coli LPS or IFN-gamma. In vivo, L. monocytogenes plcA- mutant was found to be a more effective stimulator of T cells than the wild LO 28 strain.

Characterization of the adjuvant effect of IL-12 and efficacy of IL-12 inhibitors in type II collagen-induced arthritis

A destructive joint disease can be induced in susceptible DBA/1 mice by immunization with type II collagen emulsified with oil and either killed Mycobacterium tuberculosis or IL-12 as adjuvant. Cellular and humoral anti-collagen immune mechanisms appear to be involved in the pathogenesis of arthritis. We have characterized the adjuvant effect or IL-12 in more detail and addressed the question whether mycobacteria might act via the induction of endogenous IL-12. Injections of IL-12 into collagen-immunized DBA/1 mice promoted the development of IFN-gamma-producing CD4+ T cells and strongly upregulated the production of complement-fixing IgG2a and IgG2b antibodies resulting in severe arthritis. Neutralization of IFN-gamma in vivo largely inhibited the increase in antibody synthesis and prevented joint disease in IL-12-treated mice. However, collagen-specific IFN-gamma synthesis by T cells was further enhanced in these animals. Furthermore, IL-12 treatment promoted the development of IFN-gamma-producing T cells but failed to enhance antibody synthesis and to induce arthritis in C57BL/6 or BALB/c mice immunized with collagen in oil. These results indicate that the induction (by IL-12) of a strong collagen-specific T-cell response alone is not sufficient to trigger arthritis. Attempts to show a role for endogenous IL-12 in DBA/1 mice immunized with collagen with mycobacteria as adjuvant gave no reliable results. Whereas anti-IL-12 treatment delayed the onset and ameliorated the disease in some experiments, it failed to do so in other experiments, or, control reagents also had some effect. A slight inhibition of collagen-specific IgG2a synthesis was observed in most experiments in the sera of anti-IL-12-treated mice. Taken together, the results show that exogenous IL-12 can promote arthritis via its direct effect on T cells and its effect on antibody production, which is at least in part IFN-gamma-dependent. On the other hand, whether or not endogenous IL-12 is involved in the adjuvant effect of mycobacteria needs further clarification.

IL-12 promotes cellular but not humoral type II collagen-specific Th 1-type responses in C57BL/6 and B10.Q mice and fails to induce arthritis

DBA/1 (H-2q) and C57BL/6 (H-2b) mice develop an intermediate immune responses when immunized with chicken type II collagen (CII) emulsified with incomplete Freund's adjuvant (IFA). Only a few animals develop a mild form of arthritis. As reported before and confirmed herein, administration of IL-12 to DBA/1 mice immunized with CII in IFA strongly enhances the cellular and humoral (auto)immune response to CII and induces severe destructive joint disease with an incidence of 80-100%. In contrast, the same treatment did not promote joint disease in C57BL/6 mice. Characterization of the IL-12 effect on the CII-specific immune response of C57BL/6 mice revealed that IL-12 promoted the development of CII-specific T cells producing IFN-gamma in DBA/1 and C57BL/6 mice equally well. However, whereas treatment with IL-12 in DBA/1 mice strongly up-regulated the synthesis of CII-specific antibodies, especially of the IgG2a and IgG2b subclasses, it rather slightly down-regulated the CII-specific IgG2a and IgG2b synthesis in C57BL/6 mice. This may indicate that the effect of IL-12 on the CII-specific antibody synthesis is of crucial importance in the pathogenesis of type II collagen-induced arthritis (CIA). The failure of IL-12 to up-regulate IgG2a and IgG2b synthesis in C57BL/6 mice is specific for CII as antigen and not a general property of this strain because the keyhole limpet hemacyanin-specific antibody response is up-regulated by IL-12 in C57BL/6 mice. Furthermore, it is not the H-2b haplotype of C57BL/6 mice but rather the genetic background (DBA/1 versus BL/6 or BL/10) that limits the effect of IL-12 on the CII-specific antibody response because IL-12 treatment of CII-immunized B10.Q (H-2q) mice also failed to induce arthritis and to enhance CII-specific IgG2a and IgG2b synthesis. However, as in the two other strains, injection of IL-12 promoted the development of splenic T cells producing IFN-gamma upon activation with CII. These results indicate that an enhancement of the cellular and humoral anti-CII response by IL-12 is required for inducing arthritis.

Th1 development of naive CD4+ T cells is inhibited by co-activation with anti-CD4 monoclonal antibodies

Co-activation of CD4+ T cells by anti-CD4 mAb strongly enhances the proliferation of these T cells. We have analyzed the influence of CD4-mediated co-activation on Th cell differentiation. Our data demonstrate that activation of high density (HD)-CD4+ T cells by immobilized anti-CD3 mAb in combination with immobilized anti-CD4 mAb led to a strong inhibition of Th1 differentiation and to a variable but significant induction of Th2 development. Priming of highly enriched Mel-14highCD4+ T cells in the presence of anti-CD4 mAb also resulted in a pronounced suppression of secondary IFN-gamma production, indicating that the Th1 development of naive CD4+ T cells is inhibited by co-activation via CD4. In contrast to HD-CD4+ T cells, CD4-induced costimulation of MEL-14highCD4+ T cells did not result in a primary and secondary IL-4 production. Hence, these results suggest that a MEL-14low population within the HD-CD4+ T cell fraction was the source of the endogenous IL-4 and imply, in addition, that co-activation via CD4 inhibits the development of Th1 cells from naive CD4+ T cells independently from endogenous IL-4. This assumption is further corroborated by the fact that neither the application of neutralizing anti-IL-4 mAb nor the use of T cells from IL-4 knockout mice could prevent CD4-mediated inhibition of Th1 development. The Th1-inhibiting effect of anti-CD4 mAb could not be reversed by the application of the Th1 inducer IL-12. On the contrary, the secondary IL-4 production of HD-CD4+ T cells as an indicator of Th2 differentiation, which was promoted by anti-CD4 mAb, was enhanced even in the presence of IL-12. Therefore, our results suggest that co-activation of naive CD4+ T cells by anti-CD4 mAb directly and selectively inhibits Th1 differentiation of naive dense CD4+ T cells.

Th1 development of naive CD4+ T cells is inhibited by co-activation with anti-CD4 monoclonal antibodies

Co-activation of CD4+ T cells by anti-CD4 mAb strongly enhances the proliferation of these T cells. We have analyzed the influence of CD4-mediated co-activation on Th cell differentiation. Our data demonstrate that activation of high density (HD)-CD4+ T cells by immobilized anti-CD3 mAb in combination with immobilized anti-CD4 mAb led to a strong inhibition of Th1 differentiation and to a variable but significant induction of Th2 development. Priming of highly enriched Mel-14highCD4+ T cells in the presence of anti-CD4 mAb also resulted in a pronounced suppression of secondary IFN-gamma production, indicating that the Th1 development of naive CD4+ T cells is inhibited by co-activation via CD4. In contrast to HD-CD4+ T cells, CD4-induced costimulation of MEL-14highCD4+ T cells did not result in a primary and secondary IL-4 production. Hence, these results suggest that a MEL-14low population within the HD-CD4+ T cell fraction was the source of the endogenous IL-4 and imply, in addition, that co-activation via CD4 inhibits the development of Th1 cells from naive CD4+ T cells independently from endogenous IL-4. This assumption is further corroborated by the fact that neither the application of neutralizing anti-IL-4 mAb nor the use of T cells from IL-4 knockout mice could prevent CD4-mediated inhibition of Th1 development. The Th1-inhibiting effect of anti-CD4 mAb could not be reversed by the application of the Th1 inducer IL-12. On the contrary, the secondary IL-4 production of HD-CD4+ T cells as an indicator of Th2 differentiation, which was promoted by anti-CD4 mAb, was enhanced even in the presence of IL-12. Therefore, our results suggest that co-activation of naive CD4+ T cells by anti-CD4 mAb directly and selectively inhibits Th1 differentiation of naive dense CD4+ T cells.

Co-development of naive CD4+ cells towards T helper type 1 or T helper type 2 cells induced by a combination of IL-12 and IL-4

Cytokines were found to play a key role in Th cell differentiation. Among them IL-12 was shown to be a potent differentiation factor for Th1 cells, whereas IL-4 is the only known cytokine that promotes the development of Th2 cells. Upon addition of comparable amounts of IL-4 and IL-12 to a primary culture of naive CD4+ T cells activated by immobilized anti-CD3 mAb, it was found that the Th1-inducing capacity of IL-12 is dominated by the Th2-promoting effect of IL-4. However, high amounts of IL-12 (10,000 U/ml) in combination with low amounts of IL-4 (100 U/ml) led to the development of a Th cell population that, upon rechallenge, showed a substantial secondary IFN-gamma (Th1 cytokine) production concomitantly with the production of high amounts of IL-4 (Th2 cytokine). This can be due to the coexistence of Th1 and Th2 cells or to the development of Th0 cells producing a mixed pattern of cytokines. Immunofluorescence double staining of intracellular IL-4 and IFN-gamma in combination with flow cytometry (FACS) revealed that most of the emerging Th cells produced either IL-4 or IFN-gamma. Only a few double producers could be detected. This finding indicates that individual naive CD4+ T cells can differentiate under the same conditions towards Th1 or Th2 cells and implicates that the development of Th1 and Th2 cells is not necessarily mutually exclusive.

High doses of interleukin-12 inhibit the development of joint disease in DBA/1 mice immunized with type II collagen in complete Freund's adjuvant

Collagen-induced arthritis (CIA) is an (autoimmune) joint disease readily elicited in DBA/1 mice by immunization with type II collagen (CII) emulsified with complete Freund's adjuvant. It is a destructive arthritis involving about 50% of the limbs and occurs with an incidence of 70% to 100%. In this study we evaluated the effect of mouse recombinant interleukin-12 (mrIL-12) on CIA. Administration of mrIL-12 at high doses (1 micrograms/mouse, daily) for 2 or 3 weeks delayed the onset and reduced the incidence of CIA. Furthermore, the severity of CIA was much milder and in most cases restricted to single digits of the paws. Short-term administration of high doses of IL-12 exerted some, but less pronounced, disease-suppressing effect. In contrast, 10-fold lower doses of IL-12 given during the first 3 weeks, or high doses of IL-12 administered therapeutically proved to be ineffective. Only those regimens of IL-12 treatment that ameliorated CIA were associated with a down-regulation of the CII-specific antibody response. A strong inhibition of CII-specific IgG1 antibodies (10- to 20-fold) and a moderately (2- to 6-fold) suppressed IgG2b response was observed, whereas the level of CII-specific IgG2a antibodies remained high. Taken together, the results indicate that some initial events in the induction of CIA in DBA/1 mice injected with CII emulsified with CFA are suppressed by treatment with high doses of IL-12.

IL-12 as mediator and adjuvant for the induction of contact sensitivity in vivo

To determine whether IL-12 serves as a regulator of contact sensitivity reactions, mice were painted with either 1.0% trinitrochlorobenzene or 0.5% dinitrofluorobenzene on abdominal skin. At various time points thereafter, regional lymph nodes or spleens were prepared for RNA extraction, and the signals for IL-12 p35 and p40 chain were sought by quantitative reverse transcriptase-PCR. Time course analysis showed a constitutive expression of p35 chain mRNA signals throughout the experiment (0 to 72 h), whereas the signal for the p40 chain was transiently induced in lymph node and spleen cells after 12 to 14 h. Cellular depletion experiments and double label in situ hybridization studies showed that dendritic cells were sources for a major part of the p40 chain message. The presence of functional IL-12 in culture supernatants was indirectly assessed by addition of anti-IL-12 antiserum and analysis of IFN-gamma production. Significant amounts of IFN-gamma could only be detected in supernatants of allergen-treated animals. Addition of anti-IL-12 antiserum inhibited IFN-gamma production by about 55%. In a further attempt to assess the role of IL-12 in contact sensitivity, anti-IL-12 antiserum was injected i.p. into mice, and ear swelling responses were assessed following challenge. Injection of anti-IL-12 antiserum significantly reduced ear swelling responses by 85%. Thus anti-IL-12 treatment almost completely prevented sensitization. To assess whether IL-12 would be able to overcome in vivo tolerance, UV-tolerized animals were treated with i.p. IL-12 in a contact allergy system. Treatment of mice with IL-12 not only prevented tolerance induction, but was able to reverse UV-induced tolerance. In aggregate, our data point to an important role for IL-12 as a mediator and adjuvant for the induction of contact sensitivity in vivo.

IL-12 as mediator and adjuvant for the induction of contact sensitivity in vivo

To determine whether IL-12 serves as a regulator of contact sensitivity reactions, mice were painted with either 1.0% trinitrochlorobenzene or 0.5% dinitrofluorobenzene on abdominal skin. At various time points thereafter, regional lymph nodes or spleens were prepared for RNA extraction, and the signals for IL-12 p35 and p40 chain were sought by quantitative reverse transcriptase-PCR. Time course analysis showed a constitutive expression of p35 chain mRNA signals throughout the experiment (0 to 72 h), whereas the signal for the p40 chain was transiently induced in lymph node and spleen cells after 12 to 14 h. Cellular depletion experiments and double label in situ hybridization studies showed that dendritic cells were sources for a major part of the p40 chain message. The presence of functional IL-12 in culture supernatants was indirectly assessed by addition of anti-IL-12 antiserum and analysis of IFN-gamma production. Significant amounts of IFN-gamma could only be detected in supernatants of allergen-treated animals. Addition of anti-IL-12 antiserum inhibited IFN-gamma production by about 55%. In a further attempt to assess the role of IL-12 in contact sensitivity, anti-IL-12 antiserum was injected i.p. into mice, and ear swelling responses were assessed following challenge. Injection of anti-IL-12 antiserum significantly reduced ear swelling responses by 85%. Thus anti-IL-12 treatment almost completely prevented sensitization. To assess whether IL-12 would be able to overcome in vivo tolerance, UV-tolerized animals were treated with i.p. IL-12 in a contact allergy system. Treatment of mice with IL-12 not only prevented tolerance induction, but was able to reverse UV-induced tolerance. In aggregate, our data point to an important role for IL-12 as a mediator and adjuvant for the induction of contact sensitivity in vivo.

Opposing effects of TGF-beta 2 on the Th1 cell development of naive CD4+ T cells isolated from different mouse strains

The development of naive dense CD4+ T cells from different mouse strains toward Th1 cells, as monitored by measuring secondary IFN-gamma production, was affected by TGF-beta 2 in a differential way. Th1 cell development of naive CD4+ T cells from strains C57Bl/6, BALB/c, and NMRI primed by immobilized anti-CD3 mAb was strongly inhibited in the presence of TGF-beta 2. Even when the Th1 cell-inducer IL-12 was added, the same effect of TGF-beta 2 was observed. In contrast, Th1 development was substantially promoted by TGF-beta 2 with T cells from C3H/He and CBA/J mice. Further analyses using CD4+ T cells from (C57Bl/6xCBA/J)F1 hybrids or DBA/1 mice showed that Th1 development was inhibited by TGF-beta 2 if the T cells were activated by anti-CD3 mAb, but it was enhanced upon costimulation with anti-CD28 mAb. Determination of primary IL-2 production revealed that T cells from (C57Bl/6xCBA/J)F1 and DBA/1 mice produced low amounts of IL-2 following stimulation by anti-CD3 mAb alone and comparatively high amounts after coactivation by anti-CD28 mAb. In the presence of TGF-beta 2, the production of IL-2 was completely suppressed if such T cells were activated solely by anti-CD3 mAb, but it was only partially inhibited after costimulation by anti-CD28 mAb. Furthermore, TGF-beta 2-promoted Th1 development of such T cells was strongly inhibited after neutralization of endogenously produced IL-2 and completely restored by the addition of human IL-2. Thus, our results indicate that the TGF-beta 2-mediated stimulation of Th1 cell development requires the presence of relatively high concentrations of IL-2. Therefore, the opposing effect of TGF-beta 2 on the Th1 cell development of naive CD4+ T cells from different mouse strains appears to be the result of the variable potency of the respective CD4+ T cells to produce IL-2 in the presence of TGF-beta 2.

Opposing effects of TGF-beta 2 on the Th1 cell development of naive CD4+ T cells isolated from different mouse strains

The development of naive dense CD4+ T cells from different mouse strains toward Th1 cells, as monitored by measuring secondary IFN-gamma production, was affected by TGF-beta 2 in a differential way. Th1 cell development of naive CD4+ T cells from strains C57Bl/6, BALB/c, and NMRI primed by immobilized anti-CD3 mAb was strongly inhibited in the presence of TGF-beta 2. Even when the Th1 cell-inducer IL-12 was added, the same effect of TGF-beta 2 was observed. In contrast, Th1 development was substantially promoted by TGF-beta 2 with T cells from C3H/He and CBA/J mice. Further analyses using CD4+ T cells from (C57Bl/6xCBA/J)F1 hybrids or DBA/1 mice showed that Th1 development was inhibited by TGF-beta 2 if the T cells were activated by anti-CD3 mAb, but it was enhanced upon costimulation with anti-CD28 mAb. Determination of primary IL-2 production revealed that T cells from (C57Bl/6xCBA/J)F1 and DBA/1 mice produced low amounts of IL-2 following stimulation by anti-CD3 mAb alone and comparatively high amounts after coactivation by anti-CD28 mAb. In the presence of TGF-beta 2, the production of IL-2 was completely suppressed if such T cells were activated solely by anti-CD3 mAb, but it was only partially inhibited after costimulation by anti-CD28 mAb. Furthermore, TGF-beta 2-promoted Th1 development of such T cells was strongly inhibited after neutralization of endogenously produced IL-2 and completely restored by the addition of human IL-2. Thus, our results indicate that the TGF-beta 2-mediated stimulation of Th1 cell development requires the presence of relatively high concentrations of IL-2. Therefore, the opposing effect of TGF-beta 2 on the Th1 cell development of naive CD4+ T cells from different mouse strains appears to be the result of the variable potency of the respective CD4+ T cells to produce IL-2 in the presence of TGF-beta 2.

Interleukin-12

Interleukin (IL)-12 was originally identified as a factor produced by human Epstein-Barr virus-transformed B cell lines. It was detected by one group as cytotoxic lymphocyte maturation factor, a cytokine that synergized with IL-2 in the induction of lymphokine-activated killer cells and cytotoxic T lymphocytes. A second group characterized it as a natural killer (NK) cell stimulatory factor, due to the enhancement of cytotoxicity and IFN-gamma synthesis by NK cells. Human IL-12 was purified to homogeneity and cloned by both groups. We had identified a murine factor, provisionally termed T cell-stimulating factor (TSF), which was involved in the proliferation, synthesis of IFN-gamma and cell adhesion of CD4+ Th1 cells. TSF was produced in the antigen-specific interaction between Th1 cells and macrophages as antigen-presenting cells, partially purified from supernatants of such cultures, and shown to be identical to IL-12. Monocytes/macrophages appear to be the major source of IL-12. It is rapidly produced by phagocytic cells after stimulation with several bacteria/bacterial products and other microorganisms. In the light of its effects on NK cells as well as CD4+ and CD8+ T cells, IL-12 can be regarded as a cytokine that connects the innate immune system with the acquired immunity. IL-12 has a broad range of activities already reviewed in three papers. These include the regulation of cytokine synthesis and proliferation of T and NK cells, the promotion of Th1 cell development, the differentiation of CD8+ T cells and effects on hematopoiesis. When applied in vivo, IL-12 was shown to enhance the resistance to bacterial and parasitic infections, to promote antitumor immunity, and to influence antiviral responses including HIV in vivo or in vitro. This review will briefly summarize these effects, but mainly focus on recent results concerning the regulation of the production and the activity of IL-12, its role in the differentiation of Th cells and the implications for delayed- and immediate-type hypersensitivity reactions, its importance for organ-specific autoimmune diseases, and the possible role of the IL-12p40 homodimer as a specific inhibitor of the IL-12 heterodimer.

Administration of IL-12 during ongoing immune responses fails to permanently suppress and can even enhance the synthesis of antigen-specific IgE

The synthesis of antibodies of the IgE isotype in mice largely depends on IL-4, a cytokine that is released by T lymphocytes of the Th2 subtype. IL-12 is a cytokine considered to direct Th cell development into a Th1 direction and to suppress Th2 responses including the synthesis of IgE. Here we report about the influence of IL-12 on the IgE responses of mice immunized with protein antigens adsorbed to aluminum hydroxide. To avoid problems with the detection of IgE caused by an excess of competitive IgG antibodies produced in IL-12-treated mice, serum IgE was first extracted from the serum by plate-bound anti-IgE mAb and then determined either as total IgE or as antigen-specific IgE by using biotinylated anti-IgE or biotinylated antigen. Depending on the strain of mice and the dose of IL-12 injected together with the antigen, IL-12 can either temporarily suppress or augment the synthesis of (antigen-specific) IgE antibodies. This applies for CBA/J mice immunized six times in biweekly intervals with minute (0.1 micrograms/injection) or three-times with large (5 micrograms/injection) amounts of the bee venom allergen phospholipase A2 (PLA2). Under both conditions the antibody response is characterized by the production of predominantly IgG1 as well as IgE but very little IgG2a, IgG2b and IgG3 antibodies. Simultaneous application of low doses of IL-12 (1 or 10 ng/day) led to a 2- to 4-fold enhancement of IgE production (PLA2-specific IgE or total IgE). Only a high dose of 1 micrograms IL-12/day resulted in a 3- to 10-fold reduction of the IgE response. This suppression was not stable, however, because the synthesis of IgE antibodies was stimulated to a high level when these mice subsequently received a second course of immunizations in the absence of IL-12. Likewise, the synthesis of IgE was only temporarily suppressed by IL-12 treatment in CBA/J mice immunized with keyhole limpet hemocyanin (KLH) as antigen. However, application of low (10 ng/day) or high (1 microgram/day) doses of IL-12 during the primary course of immunizations of CBA/J mice with KLH suppressed the IgE response slightly or strongly respectively. In striking contrast, the KLH-specific IgE response of BALB/c mice was upregulated even when high doses of IL-12 (1 microgram/day) were injected simultaneously with the immunizations. Thus, these results demonstrate a great variability regarding the influence of IL-12 treatment on ongoing IgE responses in vivo.

The role of IL-12 in the induction of organ-specific autoimmune diseases

The concept that T cells are subdivided into T helper 1 (Th1) and Th2 subsets was recently extended to suggest that Th1 cells contribute to the pathogenesis of several organ-specific autoimmune diseases, whereas Th2 cells inhibit disease development. Here, Sylvie Trembleau and colleagues examine the role of interleukin 12 (IL-12), a key cytokine guiding the development of Th1 cells, in the induction of autoimmune diseases, and discuss potential immunointervention strategies based on administration of IL-12 antagonists.

Administration of interleukin 12 in combination with type II collagen induces severe arthritis in DBA/1 mice

The induction of arthritis in DBA/1 mice usually requires immunization with the antigen type II collagen emulsified with Mycobacterium tuberculosis in oil. Here we describe that interleukin 12 (IL-12) can replace mycobacteria and cause severe arthritis of DBA/1 mice when administered in combination with type II collagen. Immunization of DBA/1 mice with type II collagen emulsified in oil alone resulted in a weak immune response, and only a few animals (10-30%) developed arthritis. Administration of IL-12 for 5 days simultaneously with each immunization strongly enhanced the anti-type II collagen immune response. Collagen-specific interferon gamma (IFN-gamma) synthesis by ex vivo activated spleen cells was enhanced 3- to 10-fold. IFN-gamma was almost completely produced by CD4+ T cells. Furthermore, the production of collagen-specific IgG2a and IgG2b antibodies was upregulated 10- to 100-fold. As a consequence, the incidence of arthritis in the group of mice immunized with collagen plus IL-12 was very high (80-100%). The developing arthritis was severe, involving approximately 50% of all limbs with strongly increased footpad thickness in most cases. Furthermore, histological examination revealed massive, mainly polymorphonuclear cell infiltration, synovial hyperplasia, cartilage and bone destruction, as well as new bone formation. In many cases, this resulted in the complete loss of joint structure. Neutralization of IFN-gamma in vivo prevented the development of arthritis in collagen-immunized and IL-12-treated mice. In conclusion, our data show that in vivo administered IL-12 can profoundly upregulate a T helper I-type autoimmune response, resulting in severe joint disease in DBA/1 mice.

Co-activation of naive CD4+ T cells and bone marrow-derived mast cells results in the development of Th2 cells

Activation of naive dense CD4+ T cells by plate-bound anti-CD3 antibodies favors the development of Th1 cells which, upon re-stimulation, produce significant amounts of IFN-gamma but no IL-4. However, co-activation of such naive T cells in the presence of IgE [anti-dinitrophenyl (DNP)]-loaded bone marrow-derived mast cells (BMMC) on plates coated with anti-CD3 antibodies and DNP-BSA led to the development of IL-4-producing Th2 cells. The same result could be observed if irradiated (800 rad) BMMC were applied as co-stimulators. Moreover, BMMC could be replaced by the supernatant of IgE-activated BMMC suggesting that a soluble mediator, presumably IL-4, was responsible for this effect. This assumption was substantiated using neutralizing anti-IL-4 antibodies which abolished the BMMC-mediated Th2 development in all cases. Addition of IL-12, a cytokine that was shown to antagonize the Th2-promoting effect of IL-4 in vivo, could not inhibit the development of IL-4-producing T cells, but gave rise to a T cell population which produced relatively high amounts of IL-4 and IFN-gamma. Since BMMC represent the in vitro equivalent of mucosal mast cells these data suggest that IgE-activated mucosal mast cells can bias an emerging T cell dependent immune response towards a Th2 dominated reaction by the initial production of IL-4.

Interleukin-12 profoundly up-regulates the synthesis of antigen-specific complement-fixing IgG2a, IgG2b and IgG3 antibody subclasses in vivo

The influence of the cytokine interleukin-12 (IL-12) on humoral immune responses was studied in vivo. CBA/J mice immunized with protein antigens (keyhole limpet hemocyanin, phospholipase A2) adsorbed to aluminum hydroxide (Alum) develop a Th2-like immune response characterized by the production of large amounts of IgG1 as well as some IgE but little IgG2a, IgG2b and IgG3 antibodies. IL-12 is a cytokine that promotes the development and the activation of Th1 cells. Th1 cells are involved in the induction of cellular immunity, which is characterized by low or absent antibody production. On the other hand, some Th1-like immune responses are associated with a strong antibody production of the IgG2a, IgG2b and IgG3 subclasses. Thus, we investigated whether treatment with IL-12 would down-regulate the humoral immune response or stimulate antibody production of the IgG2a, IgG2b and IgG3 subclasses. We observed that: 1) administration of IL-12 to mice together with protein antigens adsorbed to Alum strongly enhanced the humoral immune response by increasing the synthesis of antigen-specific antibodies of the IgG2a, IgG2b and IgG3 subclasses 10- to 1000-fold. The synthesis of IgG1 was not or only slightly (2-5-fold) enhanced, whereas that of the IgE isotype was suppressed. 2) These effects of IL-12 were observed when high (10 micrograms, 100 micrograms) or low doses (0.1 microgram) of antigen were used for immunization. 3) Titration of IL-12 in vitro revealed that IgG2a is strongly up-regulated over a wide dose range of IL-12 (10 to 1000 ng/day). 4) The effects of IL-12 in vivo are at least partially interferon (IFN)-gamma-dependent because an anti-IFN-gamma mAb in combination with IL-12 prevented most of the enhanced IgG2a production. 5) Mice receiving IL-12 showed a strong up-regulation of IFN-gamma but no inhibition of IL-5 synthesis by spleen cells activated ex vivo with antigen. These results suggest that IL-12 is a potent adjuvant for enhancing humoral immunity to protein antigens adsorbed to Alum, primarily by inducing the synthesis of the complement-fixing IgG subclasses 2a, 2b and 3.

IL-9 production of naive CD4+ T cells depends on IL-2, is synergistically enhanced by a combination of TGF-beta and IL-4, and is inhibited by IFN-gamma

Dense CD4+ T cells isolated from naive mice produce only trace amounts of IL-9 when stimulated by immobilized anti-CD3 in combination with anti-CD28 Abs. In this situation, IL-9 production is significantly stimulated by TGF-beta and further enhanced by the addition of IL-4, which, by itself, has only a minimal influence. IFN-gamma was found to inhibit the enhancing effect of IL-4. However, increasing amounts of IL-4 in the presence of a constant concentration of IFN-gamma could overcome the inhibitory activity of IFN-gamma. The application of CD4+ T cells isolated from IL-2 knockout mice unequivocally revealed that IL-2 is essential for the production of IL-9 by T cells. In addition, the use of T cells from IL-4 knockout mice elucidated that the basic (IL-2 + TGF-beta) mediated IL-9 production is independent of IL-4. Therefore, our results demonstrate that optimal IL-9 production of naive dense CD4+ T cells is positively regulated at different levels: 1) by IL-2, which is essential for IL-9 secretion; 2) followed by TGF-beta, which promotes a considerable increase in IL-9 production above the level induced by IL-2; and 3) finally, by IL-4, which requires the presence of IL-2 and TGF-beta to strongly enhance the production of IL-9. IFN-gamma inhibits the production of IL-9 mainly at the level of IL-4 by neutralizing the effect of this cytokine.

Identification and induction of human keratinocyte-derived IL-12

Interleukin 12 is a heterodimeric molecule that serves as a potent co-stimulator enhancing the development of Th1 cells. As one of the classical Th1 cell-mediated responses is contact sensitivity in skin, we wondered whether IL-12 might be produced by epidermal cells and serve as a mediator of this immune response. Using a sensitive, quantitative PCR technique we demonstrate that p35 chain mRNA of IL-12 is produced constitutively by human epidermal cells, whereas p40 chain mRNA can only be detected in epidermis treated with contact allergen, but not epidermis exposed to irritants or tolerogens. Time course studies showed a dramatic induction of IL-12 p40 mRNA 4 h after in vivo allergen treatment reaching peak strength after 6 h. In cell depletion assays we show that epidermal keratinocytes are the major source of this cytokine in the epidermis. This was further supported by analysis of mRNA derived from the human keratinocyte cell line HaCat expressing IL-12 p35 and p40 mRNA upon stimulation. The presence of bioactive IL-12 in supernatants derived from allergen-stimulated epidermal cells was demonstrated by IL-12-specific bioassay. Additional evidence for the functional importance of IL-12 in primary immune reactions in skin was obtained in allogeneic proliferation assays using human haptenated epidermal cells containing Langerhans cells as APC and allogeneic CD4+ T cells as responders. Anti-IL-12 mAb inhibited the proliferation of T cells by approximately 50%. In aggregate our data demonstrate that nonlymphoid keratinocytes are capable of producing functional IL-12 and provide evidence for the functional significance of IL-12 in primary immune responses in skin.

IL-9 production of naive CD4+ T cells depends on IL-2, is synergistically enhanced by a combination of TGF-beta and IL-4, and is inhibited by IFN-gamma

Dense CD4+ T cells isolated from naive mice produce only trace amounts of IL-9 when stimulated by immobilized anti-CD3 in combination with anti-CD28 Abs. In this situation, IL-9 production is significantly stimulated by TGF-beta and further enhanced by the addition of IL-4, which, by itself, has only a minimal influence. IFN-gamma was found to inhibit the enhancing effect of IL-4. However, increasing amounts of IL-4 in the presence of a constant concentration of IFN-gamma could overcome the inhibitory activity of IFN-gamma. The application of CD4+ T cells isolated from IL-2 knockout mice unequivocally revealed that IL-2 is essential for the production of IL-9 by T cells. In addition, the use of T cells from IL-4 knockout mice elucidated that the basic (IL-2 + TGF-beta) mediated IL-9 production is independent of IL-4. Therefore, our results demonstrate that optimal IL-9 production of naive dense CD4+ T cells is positively regulated at different levels: 1) by IL-2, which is essential for IL-9 secretion; 2) followed by TGF-beta, which promotes a considerable increase in IL-9 production above the level induced by IL-2; and 3) finally, by IL-4, which requires the presence of IL-2 and TGF-beta to strongly enhance the production of IL-9. IFN-gamma inhibits the production of IL-9 mainly at the level of IL-4 by neutralizing the effect of this cytokine.

Differential expression of mRNA encoding interleukin-12 p35 and p40 subunits in situ

Interleukin-12 (IL-12) is a heterodimeric cytokine that plays an important role in the regulation of the immune response. For biological activity the expression of both subunits of IL-12, p35 and p40, is required. Moreover, in the mouse the p40 chain of IL-12 specifically inhibits the effects of the IL-12 heterodimer. In the present study we have analyzed by in situ hybridization the expression of the p35 and p40 mRNA in the spleens of BALB/c and mutant (SCID, nude, beige) mice, unstimulated and after in vivo stimulation with lipopolysaccharide (LPS) and with staphylococcal enterotoxin B (SEB). In unstimulated spleens of BALB/c mice p35 and p40 mRNA were only detectable in a few strongly stained single cells, p35 mRNA was expressed in addition weakly in the B cell areas. After injection of LPS or SEB, p40 mRNA was strongly induced in the T cell areas all over the spleen, whereas expression of p35 mRNA and its distribution pattern did not change. Surprisingly, most of the mRNA for p35 and p40 was localized in different areas of the spleen and was apparently produced by different cells. In macrophage-depleted spleens the increased expression of p40 mRNA in response to LPS was reduced but still detectable, demonstrating that other cells besides macrophages can up-regulate IL-12 p40 mRNA. Nude mice showed a stronger expression of p35 mRNA, SCID mice lacked the weak p35 staining of the B cell areas but showed a strong basal expression of both p35 and p40 mRNA and a focal response to LPS. The pattern of IL-12 mRNA expression in beige mice was the same as in normal mice. These data demonstrate a spatial dissociation of expression of the two chains of IL-12 and are compatible with a regulatory role of the isolated IL-12 p40 chain in vivo. In addition, they indicate that the demonstration of mRNA for both chains of IL-12 in whole tissues or cell mixtures is not necessarily indicative of functional IL-12.

T helper type 1 development of naive CD4+ T cells requires the coordinate action of interleukin-12 and interferon-gamma and is inhibited by transforming growth factor-beta

It was observed in vitro and in vivo that both interferon (IFN)-gamma and interleukin (IL)-12 can promote the development of T helper type 1 (TH1) cells. Since IL-12 was shown to be a costimulator for the production of IFN-gamma by T or natural killer (NK) cells, IL-12 might play only an indirect role in TH1 differentiation by providing IFN-gamma which represents the essential differentiation factor. Using anti-CD3 monoclonal antibody (mAb) for activation of naive CD4+ T cells in the absence of accessory cells we could demonstrate that costimulation by IFN-gamma alone results only in marginal TH1 development. Similarly, IL-12 in the absence of IFN-gamma is only a poor costimulator for inducing differentiation towards the TH1 phenotype. Our data indicate that both cytokines are required to allow optimal TH1 development and that IL-12 has a dual role, it promotes differentiation by direct costimulation of the T cells and also enhances the production of IFN-gamma which serves as a second costimulator by an autocrine mechanism. Another cytokine that was reported to favor TH1 differentiation in certain experimental systems is transforming growth factor (TGF)-beta. With naive CD4+ T cells employed in this study TGF-beta strongly inhibited the production of IFN-gamma triggered by IL-12 as well as the IL-12-induced TH1 development. When TGF-beta was combined with anti-IFN-gamma mAb for neutralization of endogenous IFN-gamma the TH1-inducing capacity of IL-12 was completely suppressed.

Differential effects of interleukin-12 on the development of naive mouse CD4+ T cells

The influence of interleukin (IL)-12 and IL-4 on the differentiation of naive CD4+ T cells was studied in an accessory cell-free in vitro system. Dense CD4+ T cells were purified from unimmunized mice and activated using immobilized anti-CD3 monoclonal antibodies (mAb) in the presence of IL-4, IL-12, or a combination of both cytokines, and restimulated after 6 days by re-exposure to anti-CD3-coated culture wells. T cells initially activated in the presence of IL-4 produced substantial amounts of IL-4 and trace amounts of interferon (IFN)-gamma after restimulation at day 6 with plate-bound anti-CD3 mAb. By contrast, T cells primed in the presence of IL-12 produced high levels of IFN-gamma and only minimal amounts of IL-4, thus indicating that IL-12 and IL-4 by acting directly on stimulated naive CD4+ T cells support the development of TH1 and TH2 cells, respectively. When naive CD4+ T cells were stimulated in the presence of IL-12 together with IL-4 in comparable concentrations, the effect of IL-12 on TH1 differentiation was largely inhibited by IL-4. On the other hand, IL-12 exerted no inhibitory effect on IL-4-induced TH2 differentiation but rather enhanced the production of IL-4 after restimulation of the respective T cells. Decreasing amounts of IL-4 in combination with a high level of IL-12 led to an increasing production of IFN-gamma by the emerging T cells and, simultaneously, to a relatively high production of IL-4. These data were confirmed by time-course experiments which revealed that the delayed addition of IL-4 to IL-12-primed T cell cultures resulted in a gradual restoration of IFN-gamma production whereas in parallel the secretion of IL-4 was not reduced over a wide period of delay (6-72 h). These results, therefore, demonstrate that (a) IL-4 dominates the effect of IL-12, (b) IL-12 promotes the development of TH1 cells; however, in the presence of IL-12 and relatively high levels of IL-4 also the development of TH2-like cells is slightly but significantly enhanced by IL-12, and (c) high amounts of IL-12 in combination with relatively low levels of IL-4 give rise to a T cell population that upon rechallenge exhibited a cytokine profile resembling that of TH0 cells.

The interleukin-12 subunit p40 specifically inhibits effects of the interleukin-12 heterodimer

The recently discovered cytokine interleukin (IL)-12 is a heterodimeric protein of two disulfide-bonded subunits of 35 and 40 kDa. IL-12 has multiple effects on T cells and natural killer (NK) cells. In particular it appears to be a major factor for the development of cellular immunity. So far activity of the single subunits alone has not been described, however their expression is regulated independently. In this report we demonstrate for the first time that the mouse IL-12 subunit p40 (IL-12p40) specifically antagonizes the effects of the IL-12 heterodimer in different assay systems. The proliferation of mouse splenocytes activated by phorbol ester and IL-12 was inhibited by IL-12p40, whereas the proliferation induced by phorbol ester and IL-2 was not affected. Furthermore, the synthesis of interferon (IFN)-gamma by mouse splenocytes activated with IL-2 and IL-12 was suppressed by IL-12p40. Purified mouse splenic CD4+ T cells produced IFN-gamma upon activation with plate-bound anti-CD3 monoclonal antibody which was enhanced more than tenfold in the presence of IL-12. In this system IL-12p40 inhibited only the enhancement caused by IL-12 but not IFN-gamma synthesis of CD4+ T cells stimulated with anti-CD3 alone. Moreover, IL-12p40 inhibited the effects of IL-12 on differentiated T helper type 1 (Th1) cells. IFN-gamma production by Th1 cells induced in a T cell receptor-independent way by macrophages and IL-2 or macrophages and IL-12 was greatly reduced by IL-12p40 providing evidence for the endogenous synthesis of IL-12 in the Th1 cell, macrophage and IL-2 co-cultures. The specificity of inhibition was clearly demonstrated in the homotypic aggregation assay of Th1 cells. Incubation of Th1 cells with either IL-2 and IL-12 or IL-2 and tumor necrosis factor induces LFA-1/ICAM-1-dependent aggregation. Only IL-2 + IL-12 but not IL-2 + tumor necrosis factor-induced aggregation was inhibited in a dose-dependent manner by IL-12p40. Thus, the IL-12 subunit p40 appears to be a specific inhibitor for the IL-12 heterodimer.

Interleukin-12/T cell stimulating factor, a cytokine with multiple effects on T helper type 1 (Th1) but not on Th2 cells

At least two subsets of CD4+ T helper cell lymphocytes termed Th1 and Th2 exist in the mouse and probably in humans. They are characterized by the secretion of different lymphokines and by their functional behavior. Dysregulated expansion of one or the other subset may be one reason for the development of certain diseases. Thus, it is of importance to define the signals involved in the differentiation and activation of the two Th cell subsets. It is known and has been confirmed in this report that the cytokine interleukin (IL)-1 acts on Th2 cells but not on Th1 cells. We now report that a previously identified cytokine which was provisionally termed T cell stimulating factor is identical with IL-12 and exhibits a reciprocal behaviour to IL-1. IL-12 has several effects on Th1 cells. It can induce the proliferation of certain Th1 cells in combination with IL-2. Synthesis of interferon (IFN)-gamma by Th1 cells can be triggered by IL-2 plus IL-12. In contrast to the IFN-gamma production observed after T cell receptor (TcR) CD3 stimulation of Th1 cells with lectin Concanavalin A the IFN-gamma production induced by IL-12 + IL-2 is insensitive to the immunosuppressive drug cyclosporin A. Furthermore, IL-12 enhances the TcR/CD3-induced synthesis of IFN-gamma of several Th1 clones. Finally, IL-12 (+IL-2) induces homotypic cell aggregation of Th1 clones. This type of cell aggregation depends on the participation of LFA-1 and ICAM-1 molecules. In all activation systems with Th1 cells no effect of IL-1 was demonstrable. In contrast, only IL-1 but not IL-12 served as a co-stimulatory signal for several Th2 cell lines activated via the TcR/CD3 complex.

Differentiation driven by granulocyte-macrophage colony-stimulating factor endows microglia with interferon-gamma-independent antigen presentation function

The antigen presentation function of microglial cells was analyzed after differentiation in neonatal mouse brain cell cultures supplemented either with macrophage (M) or granulocyte/macrophage (GM) colony-stimulating factor (CSF). The cells separated from concomitant astrocytes in both culture systems turned out to exhibit cytological characteristics of macrophages and bore MAC-1 and F4/80 markers in a similar way. When comparatively tested for accessory cell function, only microglia developed with GM-CSF were able to efficiently induce antigen-directed proliferation of a series of helper T cell lines representing both the TH1 and TH2 subtype. Antigenic T cell activation by this microglia population was performed without prior stimulation and exceeded that of M-CSF-dependently grown microglial cells, even if those had been pretreated with interferon-gamma (IFN-gamma). In contrast to such difference in function, low cell surface expression of MHC class II or intercellular adhesion molecule-1 determinants proved to coincide in both populations. Correlating with the capacity for antigen presentation, expression of membrane-bound interleukin-1 (IL1)--a costimulatory signal for TH2 cells--was augmented significantly in GM-CSF-grown microglia. In parallel, the interaction only of this microglia population with a selected TH1 cell line was accompanied by maximal release of T cell-stimulating factor, a cytokine recently identified as an IL1-analogous second signal for TH1 cells. Thus, a developmental process is suggested which produces a form of microglia specialized in antigen presentation and thereby acting uncoupled from IFN-gamma.

Different accessory function for TH1 cells of bone marrow derived macrophages cultured in granulocyte macrophage colony stimulating factor or macrophage colony stimulating factor

The ability of macrophages to stimulate immune responses is heterogeneous and may have influence on the type of the developing immune response. Therefore, in an attempt to define different functional states of mouse macrophages, we made use of the two macrophage growth factors: macrophage colony stimulating factor (M-CSF) and granulocyte macrophage colony stimulating factor (GM-CSF). Generation of macrophages from freshly isolated bone marrow cells in the presence of GM-CSF results in a population expressing profound antigen presenting function for mouse TH1 cells, resulting in strong lymphokine production and proliferation of the T cells. Furthermore, high amounts of a novel soluble cytokine active on mouse TH1 cells are generated during the interaction of TH1 cells with macrophages elicited with GM-CSF. In contrast, macrophages grown from bone marrow cells for at least 14 days in the presence of M-CSF express only minimal antigen-presenting function for TH1 cells. Treatment of such macrophages for 24 h with either IFN-gamma or GM-CSF allows the distinction between two further functional states. Those treated with IFN-gamma efficiently presented antigen towards TH1 cells. The T cells produced large amounts of lymphokines and proliferate well. However, synthesis of the novel soluble cytokine (active on TH1 cells) was not detectable. The generation of this mediator requires a short-term treatment with GM-CSF of macrophages developed in the presence of M-CSF prior to their interaction with TH1 cells.

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.

Leflunomide (HWA 486), a novel immunomodulating compound for the treatment of autoimmune disorders and reactions leading to transplantation rejection

Leflunomide has been shown to be very effective in preventing and curing several autoimmune animal diseases. Further, this agent is as effective as cyclosporin A in preventing the rejection of skin and kidney transplants in rats. Preliminary results from patients suffering from severe cases of rheumatoid arthritis demonstrated that clinical and immunological parameters could be improved with leflunomide therapy. Mode of action studies revealed that this substance antagonizes the proliferation inducing activity of several cytokines and is cytostatic for certain cell types. In this light, we could show that tyrosine phosphorylation of the RR-SRC peptide substrate and the autophosphorylation of the epidermal growth factor (EGF) receptor were, dose dependently, inhibited by leflunomide. EGF activates the intrinsic tyrosine kinase of its receptor, which stimulates the phosphorylation of a variety of peptides, the amino acid residue in all cases is tyrosine. These results indicate that much of leflunomide's activity could be due to the inhibition of tyrosine-kinase(s), which is an important general mechanism for the proliferation of various cell types. Thus, leflunomide, which is effective against autoimmune diseases and reactions leading to graft rejection, would seem to have a mode of action separating it from known immunosuppressive drugs.

Requirements for the growth of TH1 lymphocyte clones

Besides the signal generated in a T lymphocyte after triggering the T cell receptor (TcR), most lymphocytes need a "second signal" to become fully activated. The necessity and nature of the "second signal" differs between different types of T cells. At the level of CD4-positive T helper lymphocytes interleukin 1 (IL 1) serves as "second signal" for those of the TH2 subtype (IL4, 5, 6 producer) but not for those of the TH1 subtype (IL 2, IFN-gamma producer). This correlates with the absence of the IL 1 receptor at the surface of TH1 clones. We report herein the further purification of T cell stimulating factor (TSF), a soluble mediator involved in the proliferation of TH1 lymphocytes. A preparation free of detectable IL 1, 2, 4 and IL 6 activity could act as "second signal" required for the growth of TH1 lymphocytes in a TcR-mediated, as well as a TcR-independent activation system. In addition, we suggest that IL 1 can influence the proliferation of TH1 clones in an indirect way, probably via the induction of TSF in accessory cells.

Involvement of soluble mediator(s) different from interleukin (IL) 1 in the antigen-induced IL 2 receptor expression and proliferation of L3T4+ (CD4+) T lymphocytes

Proliferation of T lymphocytes (T cells) requires the interaction of interleukin 2 (IL 2) with the high affinity form of the IL 2 receptor (IL 2R). IL 2 production as well as IL 2R expression are generally induced simultaneously in T cells by the recognition of specific antigen displayed on the surface of syngeneic antigen-presenting cells. The experiments described herein show that the expression of IL 2R has different requirements than the production of IL 2 (and other lymphokines). Stimulation of antigen-specific L3T4+ T cell lines with antigen-pulsed spleen cells (SC) treated with ultraviolet (UV) light results in efficient IL 2 production but only minimal proliferation due to reduced IL 2R expression, as compared to T cells stimulated by antigen and SC without UV light treatment. The reduced IL 2R expression/proliferation correlates with the absence of a soluble mediator(s) termed T cell-stimulating factor (TSF) in the supernatants of T cells stimulated with antigen-pulsed, UV light-irradiated SC. Addition of TSF of these T cells could at least partially restore the proliferative response or enhance IL 2R expression. Because TSF is present in the supernatant of T cells triggered by antigen and SC but is absent when the latter are UV light treated, we suggest that TSF is a product of metabolically active splenic antigen-presenting cells. The macrophage products interleukin 1 alpha + beta as well as some other cytokines show no TSF activity.

An antigen-independent physiological activation pathway for L3T4+ T lymphocytes

The data presented in this report describe an antigen-independent activation pathway leading to reinduction of proliferation of class II major histocompatibility complex (MHC)-restricted murine T cell lines that after previous antigen-specific stimulation reverted to a resting state. Antigen-independent proliferation and interleukin 2 (IL2)-receptor expression occur in the presence of splenic accessory cells, exogenous IL2 and a soluble factor(s) provisionally termed T cell-stimulating factor(s) (TSF). Each of these components is essential for inducing growth. TSF is found in the supernatant of an autoreactive T cell line upon stimulation with syngeneic accessory cells. Neither TSF nor accessory cells can be replaced by IL1 and by some other cytokines. Monoclonal antibodies against class II MHC molecules, the T cell receptor and L3T4 do not block this antigen-independent stimulation. This demonstrates that the function of the accessory cell in this system is not MHC restricted and that the T cell receptor is also not involved. Furthermore, it is suggested that the blocking of L3T4 molecules by antibody will mediate a negative signal only if T cells are triggered via their antigen receptors.