Production of human immune interferon (Hu IFN-gamma) studied at the single cell level. Origin, evidence for spontaneous secretion and effect of cyclosporin A

Induction of antigen-specific tolerance by cyclosporin A

Recently, we reported preliminary evidence for the induction of tolerance in vivo by cyclosporin A (CSA) during a persistent virus infection in rats. In the present communication, those observations are verified and the findings extended to the functional level of cell-mediated immunity. Mice infected intracerebrally with lymphocytic choriomeningitis virus (LCMV) normally die from a fatal immune-mediated disease after 6-8 days but they do not succumb if treated intraperitoneally with 50 mg/kg/day of CSA. Immunosuppression initiated one day before infection and continued for at least two consecutive weeks resulted in the absence of immunopathologic disease of the brain and in the survival of mice, which were found to be persistently infected virus carriers. In these animals, no cytotoxic T cell activity could be detected. The effect of CSA was not due to a toxic effect on the immune response since immune reactivity was restored as early as 4 days after discontinuation of the drug in control animals. Neither secondary in vitro nor in vivo restimulation resulted in the generation of a cellular antiviral immune response. Cytotoxic T cell reactivity to third-party antigen, however, could be detected, although somewhat delayed. Additionally, spleen cells from CSA-treated mice did not clear the virus from LCMV-infected recipients upon adoptive transfer, whereas spleen cells from LCMV immune mice completely eliminated virus infection in carrier mice. However, mice immunosuppressed with CSA and infected with vesicular stomatitis virus (VSV) did not generate a primary immune response but were immunologically fully reactive to challenge infection, providing evidence for the absence of tolerance and the presence of antigen-specific temporal unresponsiveness. Thus, as exemplified by VSV infection in which the virus does not replicate to considerable titers in mice and viral antigens do not persist, the presence of the foreign antigen for prolonged periods of time could be shown to be a conditio sine qua non for CSA-induced tolerance.

Reverse ELISPOT assay for clonal analysis of cytokine production. II. Enumeration of interleukin-1-secreting cells by amplified (avidin-biotin anti-peroxidase) assay

Detection of cytokine-producing cells can be accomplished by reverse modifications of the ELISPOT assay using cytokine-specific unconjugated and enzyme-labelled antibodies as solid phase capture system and detecting reagents, respectively. However, in certain situations where the secreted cytokine is produced in minute amounts such as in the case of interleukin-1 (IL-1), the sensitivity of the indicator immunoenzyme system employed may be insufficient to permit detection of the corresponding secreting cells. We have developed a novel immunoenzyme amplification procedure that involves the use of a biotinylated secondary anti-enzyme antibody reagent to enhance the signal provided by the primary enzyme-labelled antibody conjugate. Following addition of enzyme-conjugated avidin, ELISPOT assay wells are developed with a suitable chromogen substrate yielding spots located at the former position of cells secreting the analyte under study. As a model system, the detection of IL-1 beta-secreting cells by human peripheral blood monocytes is described.

Functional evaluation of tumor-infiltrating mononuclear cells. Detection of endogenous interferon-gamma and tumor necrosis factor-alpha in human colorectal adenocarcinomas

Quantitative evaluation of the levels of interferon-gamma (IFN-gamma) and tumor necrosis factor-alpha (TNF-alpha) in the extracts of tumors and their corresponding normal tissues resected from 43 patients with colorectal adenocarcinoma was done using solid-phase, sandwich radioimmunoassay. The levels of both IFN-gamma and TNF-alpha detected in the tumor tissues were higher than those in the corresponding normal colorectal tissues obtained from each patient. A significant negative correlation was observed between the level of IFN-gamma and TNF-alpha in each tumor extract. The decrease of the level of IFN-gamma in the tumor correlated with the advance of clinical stage, and the levels of IFN-gamma of the patients with distant metastases were significantly lower than those of the patients without distant metastases. However, an increase in the level of TNF-alpha correlated not only with an enlarged diameter but also with the extent of the primary tumor. Immunohistochemical staining of IFN-gamma and TNF-alpha producing cells in tumor tissues showed that IFN-gamma was mainly produced by CD4+ CD8- T-lymphocytes and TNF-alpha was mainly produced by CD11c+ cells with macrophage-like morphology. These results suggest that CD4+ T-lymphocytes that produce IFN-gamma might play an important role in the antitumor response against cancer progression in human colorectal adenocarcinomas.

Atypical dissemination of the highly neurotropic Borna disease virus during persistent infection in cyclosporine A-treated, immunosuppressed rats

In adult rats infected with Borna disease virus, the virus was found exclusively in the brain, whereas in cyclosporine A-treated rats, infectious virus was also detected in peripheral nerve fibers and, unexpectedly, in adjacent organ-specific cells. In contrast to untreated virus-infected rats, no major histocompatibility complex class II expression was found in the brain of cyclosporine A-treated animals.

In vivo effects of cyclosporin A on murine B-cells responding to type III pneumococcal polysaccharide

The effect of cyclosporin A (CSA) on the antibody response to pneumococcal polysaccharide type III (a T-independent class 2 antigen) was investigated in mice. A single oral CSA administration (50 mg/kg) was able to depress (40%) the primary antibody response evaluated as spleen plaque-forming cells. Repeated treatments (12-50 mg/kg x 5) resulted in a higher degree of inhibition (80%) of anti-SIII response. Both single and repeated CSA treatments were active only when administered concomitantly with or after immunization, whereas no effects were seen with drug pretreatment. Comparable inhibitions of anti-SIII response were observed in control and nude mice suggesting a direct effect of CSA on B-cells.

Enumeration of cytokine-secreting cells at the single-cell level

A sensitive assay utilizing enzyme-linked immunosorbent assay methodology has been developed for the quantitation of single cells secreting interferon (IFN)-gamma or tumor necrosis factor (TNF). Cloned T cells or cells from lymphoid organs were stimulated with antigen, concanavalin A, or phorbol myristate acetate and ionomycin in microwells coated with antibodies specific for IFN-gamma. Discrete "spots" overlying areas where cells secrete IFN-gamma were then developed by incubation with a second antibody to IFN-gamma, followed by an enzyme-labeled antibody conjugate and substrate. Similarly, using TNF-specific antibody reagents, TNF-secreting cells were detected and quantitated in cell populations obtained from normal lymphoid tissues, bone marrow and peripheral blood, following activation with phorbol myristate acetate and ionomycin. Provided specific antibodies are available, this method has the potential to measure the frequency of cells secreting any cytokine.

Effect of anticancer drugs on the release of interferon gamma in vitro

Some conventional and experimental anti-cancer drugs were tested for their effect on concanavalin-A-induced interferon gamma release from rat splenocytes in vitro. When 2.5 x 10(6) rat splenocytes/ml, stimulated with 1 microgram/ml concanavalin A, were incubated with various non-cytotoxic doses of the vinca alkaloid vincristine, there was an inhibition of the release of interferon gamma in culture supernatants. The antitumour antibiotics bleomycin and Adriamycin, alkylating agents 4-hydroperoxycyclophosphamide and mafosfamide, and the immunoactive peptides FK156 and FK565 did not affect the release of interferon gamma under similar conditions. However, cyclosporin A, in similar experiments, markedly inhibited the release of interferon gamma.

Reverse ELISPOT assay for clonal analysis of cytokine production. I. Enumeration of gamma-interferon-secreting cells

A reverse modification of ELISPOT assay using nitrocellulose membranes and epitope-specific monoclonal antibodies is described for the detection of single lymphokine-secreting cells. As a model, the production of gamma-interferon by mitogen stimulated human peripheral blood lymphocytes has been examined. The assay can also be modified to permit microscopic examination of spot-forming cells.

Hyperthermia in humans enhances interferon-gamma synthesis and alters the peripheral lymphocyte population

Induction of hyperthermia (39 degrees C) in human volunteers by immersion in warm water (41-45 degrees C) rapidly alters the cell populations in the peripheral blood. In addition to granulocytosis, there is an alteration of the normal ratios among T-lymphocyte subsets. Following in vitro mitogen stimulation, lymphocytes from hyperthermic individuals produce as much as 10-fold more interferon-gamma (IFN-gamma) than cells withdrawn at basal core temperatures from the same individuals. A temperature threshold of 39 degrees C for this response suggests potential relevance to fever. No change was noted in the activity of the macrophage population. The possible involvement of interleukin-2 (IL-2) in this enhanced production is discussed. No changes were noted in the circulating levels of IFN-gamma.

An immunofluorescent method for identifying individual IFN-gamma-producing lymphocytes

An immunofluorescent staining method was developed for detecting human IFN-gamma-producing cells in single cell suspensions. Mononuclear leukocytes, stimulated in vitro to produce IFN-gamma, were fixed and made permeable. The cytoplasmic presence of IFN-gamma was visualized by indirect immunofluorescence using IFN-gamma-specific mouse monoclonal antibodies. The staining was found to be specific for IFN-gamma and allowed the detection of newly synthesized rather than internalized IFN-gamma molecules. The cytoplasmic fluorescence appeared locally in a polar, juxtanuclear position, which overlapped the Golgi apparatus, probably reflecting the glycosylation site of the newly formed IFN-gamma molecules. Two-colour staining experiments showed that the method is useful not only for the detection and enumeration but also for the phenotypic characterization of IFN-gamma-producing cells.

Differential effect of cyclosporin-A on the mixed-lymphocyte culture-induced proliferative and cytotoxic responses of T lymphocytes with different cell densities

We have analyzed the effect of cyclosporin-A (CsA) on the proliferative and cytotoxic responses induced by mixed-lymphocyte cultures (MLC-s) on low and high density T lymphocytes. Allogeneic stimulation had a different impact on the two subsets. Proliferative and cytotoxic responses were inversely correlated; i.e., high density cells proliferated but exerted low levels of cytotoxicity while the lytic activity of the low density subset was stronger and the proliferation was weak. CsA impaired the proliferative and cytotoxic responses of the high density T lymphocytes but influenced less markedly the response of the low density cells. In both subsets CsA inhibited the MLC-induced interleukin 2 (IL-2) production. The generation of specific cytotoxicity was markedly suppressed by CsA, whereas the generation of anomalous activity was less affected. Addition of exogenous IL-2 to the CsA-containing cultures fully restored the proliferation and the generation of nonspecific cytotoxicity. In contrast, addition of interferon gamma (IFN-gamma) restored neither of these responses. However, for complete restoration of the stimulation-specific cytotoxicity, addition of both lymphokines was required. Taken together these results suggest that the CsA-induced suppression of the lymphokine production has different consequences in the low and high density subsets; the expression of anomalous and specific cytotoxicities require different signals; CsA interferes with several steps in the T-cell activation.

The effects of cyclosporin A on the early responses of B and T cells to Epstein-Barr virus infection

The immunosuppressive properties of the fungal metabolite Cyclosporin A (CsA) on the human lymphocyte response to the polyclonal B cell activator Epstein-Barr virus (EBV) in vitro were assessed. CsA showed both stimulatory and inhibitory effects on EBV-induced IgM secretion, the net effect being dependent on the dose of virus used. T cells were required for both these effects to occur. A model is proposed to account for the complex interactions of CsA in this system.

Determination of the T-cell subset producing gamma-interferon in tuberculous pleural effusion

The percentage of cells of different T-cell subsets and their functions in tuberculous pleural effusion were investigated. The percentage of OKT4-positive cells was 65 +/- 2% (mean +/- SEM, n = 8) and that of OKT8-positive cells was 19 +/- 3% (n = 8). Pleural T lymphocytes of patients with tuberculous pleurisy responded well to stimulation with purified protein derivative of tuberculin, and deoxyribonucleic acid synthesis was observed along with gamma interferon (IFN-gamma) production. When pleural T lymphocytes of patients with tuberculous pleurisy were treated with OKT4 monoclonal antibody and complement, a significant decrease in IFN-gamma production was observed in all eight patients (P less than 0.005), whereas no definite decrease in IFN-gamma production was found after treatment with OKT8 monoclonal antibody and complement. These results suggest that at least the OKT4+/OKT8- T-cell subset is responsible for the antigen-specific IFN-gamma production in pleural T lymphocytes of patients with tuberculous pleurisy.

Effect of cyclosporin A on immunity to Listeria monocytogenes

The effect of the immunosuppressive drug cyclosporin A (CS-A) on immunity to the facultative intracellular bacterium Listeria monocytogenes was investigated in unprimed and primed mice. Different treatment protocols were followed to evaluate the time dependence of CS-A-mediated immune suppression and the effect of CS-A on immunological memory to L. monocytogenes. The effect of CS-A was observed only during and after activation of T cell-mediated immunity, whereas early resistance exerted by macrophages assessed 6 and 70 min after challenge remained unaffected. CS-A suppressed efficient elimination of L. monocytogenes even when given after day 3 of a primary infection. This contrasts with findings in other models, including viral infections, where CS-A must be administered very early in an immune response to suppress it. CS-A suppressed antibacterial resistance in mice primed at various times before challenge; suppression of protection was time dependent and was virtually complete in livers, whereas CS-A-resistant memory persisted in spleens for up to 10 months.

Effects of cyclosporin A on humoral immune response and resistance against vesicular stomatitis virus in mice

The effect of cyclosporin A (CS-A) on the antiviral humoral response was studied by using vesicular stomatitis virus (VSV); VSV provided the opportunity to simultaneously assess both T-independent and T-dependent antibody responses. The T-independent anti-VSV immunoglobulin M (IgM) response was virtually unaffected, whereas the T-dependent primary anti-VSV IgG response was suppressed by CS-A; in contrast, the secondary IgG response was highly resistant to CS-A. Moreover, once the switch from IgM to IgG had occurred, the primary response also became refractory to suppression by CS-A. We concluded that the effect of CS-A on the primary anti-VSV antibody response was mediated via impairment of a T-dependent mechanism; in contrast, memory T cells or memory B cells or both were quite resistant to the suppressive effects of CS-A. CS-A treatment rendered mice highly susceptible to VSV infection; under CS-A treatment, mortality was 100% after infection via footpads, whereas immunocompetent mice survived. Since CS-A does not impair induction of early T-independent anti-VSV IgM neutralizing antibodies, this high mortality in CS-A treated mice illustrates the crucial role of CS-A-sensitive cells in resistance against VSV.

Immunopharmacological study of CCA (Lobenzarit disodium), an anti-arthritis agent--I. Abrogation of IL 1 secretion by LPS-stimulated human monocytes and induction of gamma-interferon production with CCA

In vitro effects of CCA, an anti-arthritis agent, were studied upon autologous mixed lymphocyte reaction (AMLR), lymphocyte mitogenesis, IL 1 and IL 2 production, immunoglobulin production and gamma-interferon (IFN) production. CCA at 50 micrograms/ml, which was not toxic to cells, blocked AMLR, IL 1 production and immunoglobulin production (IgM and IgG) significantly, while CCA at the same dose did not affect IL 2 production and lymphocyte mitogenic responses to Staphylococcus aureus Cowan I(SAC) and pokeweed mitogen(PWM). CCA at both 20 ng/ml and 20 micrograms/ml induced human gamma/IFN. Addition of IL 1 and/or IL 2 reversed inhibitory effect of CCA on AMLR. These data suggest that CCA exerts its actions by mainly affecting T cells and monocytes and can be used as an immunomodulator.

Interferon regulation of DR antigen expression and alloantigen-presenting capabilities of the promyelocytic cell line HL60

Our research suggests that interferon may have an immunoregulatory role in the initiation phase of immune responses. Recent evidence has demonstrated that lymphokines regulate monocyte cell surface expression of DR antigens and, consequently, the ability of monocytes to activate T lymphocytes in an antigen-specific manner. In this report cloned interferons and a homogeneous cell line were used to demonstrate that interferon possesses these immunoregulatory functions. Cells of the promyelocytic cell line HL60, when incubated in vitro with recombinant gamma (IFN-gamma) and with alpha interferons (IFN-alpha), expressed enhanced levels of DR antigen as determined by both cytotoxicity and fluorescence-activated cell sorter analyses. Lower concentrations of IFN-gamma than IFN-alpha were needed to induce DR expression, and a higher percentage of monocytes were induced to express DR antigen by IFN-gamma than IFN-alpha. HL60 cells preincubated with lymphocyte-derived lymphokines or IFN-alpha also stimulated a significantly better in vitro allogeneic response in the mixed leukocyte reaction than untreated HL60 cells. Thus, interferon both the phenotypic expression of DR antigens of HL60 cells and their functional ability to initiate T-lymphocyte responses to an alloantigen.

Effect of cyclosporin-A (CsA) on the ability of T lymphocyte subsets to inhibit the proliferation of autologous EBV-transformed B cells

We have analyzed the effect of Cyclosporin-A (CsA) on the in vitro suppression of Epstein-Barr virus (EBV)-induced B-cell proliferations separated on the basis of cell density. CsA abolished the growth-inhibitory capacity of high-density T cells but influenced only marginally the activity of low-density lymphocytes; this suggested that different mechanisms mediate suppression in the two subsets. Stimulation with irradiated EBV-transformed cells had a different impact on the activity of low- and high-density lymphocytes. Proliferative and cytotoxic responses were inversely correlated, i.e. high-density cells proliferated but exerted low cytotoxicity, while the lytic activity of the low-density subset was stronger in the absence of significant cell proliferation. Proliferation and generation of cytotoxicity were abrogated by CsA in both subsets. The activities could be restored by addition of exogenous IL-2, suggesting that the drug may interfere with the cascade of lymphokine--cell interactions which leads to activation of immune responses. From the analysis of athe CsA effects on the two subsets it seems that the high-density one contains specific memory T cells which are activated and proliferate upon encounter with EBV-infected cells. On the other hand, low-density lymphocytes are induced to release soluble factors with antiproliferative activity. The secretory function was resistant to the suppressive effect of CsA.

Cyclosporin A inhibits antigen- and lectin-induced but not constitutive production of interleukin 3

Cyclosporin A (CyA) strongly inhibited production of interleukin 3 (IL3) by normal mouse spleen cells and by the T lymphoma LBRM-33 clone 1A5 induced by alloantigens and by the lectins concanavalin A or phytohemagglutinin. At the same concentrations at which CyA exerted this suppressor activity, it did not affect the growth of the Ea3.15 pre-B cell clone supported by recombinant mouse IL3 (the read-out for IL3 activity used in this study) nor did it show significant inhibitory effect on the spontaneous secretion of IL3 by WEHI-3 cells and by the LD1 T helper line. Taken together, these observations indicate that CyA does not interfere with translation, synthesis or secretion of IL3. CyA most likely interferes with the process by which antigens and lectins induce transcription of genes coding for lymphokines.

Effects of treatment with azathioprine and cyclosporin A on interferon-gamma production by peripheral blood leukocytes of renal allograft recipients

We have studied the concanavalin A (ConA)-induced interferon gamma (IFN-gamma) production of peripheral blood mononuclear cells (PBMC) of renal allograft recipients. Both under immunosuppressive treatment with azathioprine and with cyclosporin A (CsA) the PBMC of these patients proved deficient for IFN-gamma production when compared to those of healthy controls. After conversion from conventional azathioprine to CsA medication the ConA-induced IFN-gamma production increased.

Effect of cyclosporin and interleukin-2 on the restoration of in vitro immune responses to cytomegalovirus

Previous studies have shown that cyclosporin (CSA) inhibits lymphoproliferation to cytomegalovirus (CMV)-infected, glutaraldehyde-fixed, and irradiated fibroblasts (CMVFFx) in vitro. Generation of cytotoxic cell activity is impaired in cultures with CSA, but the induction of suppressor cells is not. In the present studies we tested the ability of interleukin-2 (IL-2) and supernatants of lymphocytes stimulated by CMVFFx with or without CSA (1 microgram/ml) to restore functional activities of lymphocytes from primary cultures treated or not treated with CSA. IL-2 significantly enhanced lymphoproliferation, cell-mediated cytotoxicity to CMV-infected fibroblasts (CMVF), natural killer cell activity, and the activity of cells capable of suppressing the response of fresh autologous cells to CMVFFx of cells derived from control and CSA-treated primary cultures. IL-2 was found in day-2 supernatants of control cultures but not CSA-treated cultures. Day-2 control supernatants were capable of significantly enhancing proliferation and suppressor cell activity but were less efficient at restoring cytotoxic cell function. Day-2 supernatants from CSA-treated cultures were not able to enhance lymphoproliferation or cytotoxic cell function but did induce significant levels of suppressor cell activity. The results indicate the presence of different functional mediators in the culture supernatants. The ability of IL-2 to restore lymphocyte effector functions against a clinically important virus may have important therapeutic implications in the treatment of this viral infection in immunodeficiency diseases and in the restoration of immune competence after transplantation.

Decrease of HLA-DR antigen expression by human monocytes during cultivation in absence of exogenous or endogenous interferon-gamma

The kinetics of HLA-DR antigen expression by human peripheral blood monocytes during cultivation in vitro was studied. Immediately after separation by glass adherence, about 60% of monocytes expressed DR antigens as judged by indirect fluorescence staining with a monoclonal antibody (BL-DR/1). Monocytes carefully depleted of lymphocytes, gradually lost their DR antigens during cultivation in the absence of exogenous interferon-gamma (IFN-gamma). However, addition of a few lymphocytes to the adherent cells prevented the decrease of DR antigen expression. Furthermore, it was shown that doses as low as 1 IU/ml of IFN-gamma are sufficient to induce DR antigen expression by cultured monocytes. Experiments with cyclosporin A suggest that lymphocytes contaminating the monocyte preparations can produce spontaneously sufficient amounts of IFN-gamma for maintenance of the DR antigens on monocytes.

Phenotypic and functional heterogeneity of human T cell clones producing gamma-interferon

Human T cell clones were derived from peripheral blood and screened for their ability to release gamma-interferon (gamma-IF) following PHA stimulation. Nine clones producing more than 20 U/ml of gamma-IF were expanded and analyzed for cytolytic activity in a lectin-dependent assay and for the ability to release interleukin-2 (IL-2). In addition, clones were analyzed for T4 and T8 antigen expression. Five out of nine clones had cytolytic T lymphocyte activity, while four released relatively large amounts of IL-2. Five clones were T4+ while the remaining expressed the T8+ phenotype. These results clearly indicate that gamma-IF production is not restricted to T cell subsets defined according to either functional or phenotypic criteria.

Accessory cells and T cell activation. The relationship between two components of macrophage accessory cell function: I-A and IL1

Accessory cells are required for the activation of helper T cells. We have examined two characteristics of accessory cells, their expression of I-A, and their ability to release IL1. We provide evidence that these two properties are related, and postulate that membrane I-A molecules participate in the pathway leading to IL1-release. Experimental results are described relating I-A to IL1-release as follows: 1. In vitro-educated Ly1 T cells stimulate IL1-release from M phi; this process is H-2-restricted and blocked by anti-I-A antibodies. 2. H-2-restriction between T cells and M phi is overcome in the presence of ConA, but this unrestricted interaction is also blocked by anti-I-A. 3. LPS stimulation of IL1-release is blocked by anti-I-A. These findings suggested an active role for I-A molecules on IL1-producing cells. We next describe a series of experiments designed to assess the requirements for I-A versus IL1 during T cell activation. In a number of experimental systems, T cells demonstrated a requirement for I-A-recognition, but none that could not also be satisfied by IL1: 1. Generation of helper T cells in allogeneic chimeras. 2. Proliferation of KLH-primed lymph node cells. 3. Proliferation of KLH-primed lymph node cells from chronically anti-I-A-suppressed mice. 4. Proliferation of GAT-primed lymph node cells from nonresponder mice. These findings suggest that for many kinds of T cells (not necessarily all) the apparent requirement for I-A-recognition is primarily involved in stimulating IL1-release from accessory cells.

Rapid loss of sensitivity of mitogen-induced lymphocyte activation to inhibition by cyclosporin A

The ability of the immunosuppressive drug cyclosporin A (CS-A) to inhibit the activation of lymphocytes by phytohaemagglutinin (PHA) and concanavalin A (Con A) is progressively lost over the 8-hr period following mitogen addition. This process is dependent on the presence of Ca2+ in the culture medium and is complete at a time when activation still requires the continued presence of the mitogen. While inhibition by CS-A is reduced to some extent by lymphokines produced by mitogen-activated cultures, the initial loss of sensitivity to CS-A is too complete and too rapid to be accounted for in this way. We conclude that CS-A inhibits an early Ca2+-dependent step in mitogen-induced activation that is not in itself sufficient to commit the cells to initiate proliferation, but is required for later steps in the activation process, including lymphokine production.

Activation of T lymphocytes by 12-O-tetradecanoylphorbol-13-acetate is resistant to inhibition by cyclosporin A

Cultured T lymphocytes from pig blood can be activated by the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA). Activation is additive with that induced by the mitogenic lectin phytohaemagglutinin (PHA). Activation by TPA differs from that induced by PHA or other mitogenic lectins in that it is not inhibited even by high concentrations of the immunosuppressive drug cyclosporin A (CS-A). Neither co-culture of lymphocytes with PHA and TPA nor addition of culture supernatants from TPA-stimulated cultures affected the sensitivity to CS-A of the response to PHA.