Human keratinocyte-derived IL-12 affects LC-induced allogeneic T-cell responses.
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Allergens↗
Biomedical subjects
T Germann
Publications and source records attributed to T Germann.
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.
Animals↗
The IL-12 p40 homodimer as a specific antagonist of the IL-12 heterodimer.
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Animals↗
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.
Animals↗
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.
Animals↗
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.
Animals↗
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.
Animals↗
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.
Animals↗
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.
Animals↗
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.
Animals↗
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.
Antibodies, Monoclonal↗
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.
Animals↗
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.
Animals↗
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.
Animals↗
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.
Animals↗
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.
Antigen-Presenting Cells↗
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.
Amino Acid Sequence↗
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.
Animals↗