Stimulatory and inhibitory effects of 1,25-dihydroxyvitamin D3 on thymocyte mitogenesis induced by phorbol ester and calcium ionophore.

Mouse medullary thymocytes have specific receptors for 1,25-dihydroxyvitamin D3 (1,25(OH)2D3). The mitogenic stimulation of these cells by phytohemagglutinin in the presence or absence of the phorbol ester TPA is inhibited by 1,25(OH)2D3. The calcium ionophore A23187 did not reverse the inhibition by 1,25(OH)2D3 of phytohemagglutinin. Stimulation of thymocytes with either TPA or A23187 alone did not result in proliferation. Co-stimulation of the thymocytes with TPA and A23187 induces cell proliferation. 1,25(OH)2D3 markedly enhanced the TPA and A23187-induced cell proliferation even when added 4 h after the initiation of the culture. In contrast, DNA synthesis by thymocytes incubated for 4 h in the presence of TPA and A23187 and then cultured in medium containing 1,25(OH)2D3 but in the absence of both TPA and A23187, was inhibited by 1,25(OH)2D3. The extent of inhibition was comparable to the inhibition of lectin-induced stimulation by the hormone. Using monoclonal antibodies to neutralize IL-2 and block IL-2 receptors we showed that 1,25(OH)2D3 enhanced the IL-2-independent component of the A23187- and TPA-induced mitogenesis. In conclusion: (1) The nature and presence of the mitogenic signal determines whether 1,25(OH)2D3 enhances or inhibits thymocyte stimulation. (2) Both stimulatory and inhibitory actions of 1,25(OH)2D3 seem to take place at points distal to the initial increase in intracellular calcium or activation of protein kinase C.     

Tumor promoters in conjunction with calcium ionophores mimic antigenic stimulation by reactivation of alloantigen-primed murine T lymphocytes

Antigen binding to its specific receptor on T cells initiates a series of intracellular events that result in cell differentiation, activation, and clonal expansion. However, the mechanism by which these antigen-occupied receptors induce the transmembrane signal transduction needs clarification. Because this mechanism appears to involve an increase in intracellular free Ca2+ concentration and activation of protein kinase C (PKC), we tested the effect of Ca2+ ionophores and PKC activators on alloantigen-specific primary mixed leukocyte culture cells. Both calcium ionophores, A23187 and ionomycin, in conjunction with 12-O-tetradecanoylphorbol 13-acetate (TPA) mimicked the effect of antigen or interleukin 2 (IL 2) by inducing strong proliferative and alloantigen-specific cytotoxic responses. In addition, Ca2+ ionophore and TPA induced IL 2 receptor expression and IL 2 secretion. The capacity of other phorbol esters or a non-phorbol ester tumor promoter (teleocidin) to replace TPA in induction of cell activation correlated with their ability to bind to and to activate PKC. In addition, the synergistic effect of Ca2+ ionophore and TPA was blocked by either a Ca2+ chelator (EGTA) or cAMP, which is thought to inhibit phosphatidylinositol metabolism. To determine whether the induction of this cytotoxic activity was mediated by a direct effect of Ca2+ ionophore and TPA on cytotoxic T (Tc) cells or was secondary to IL 2 secretion by activated helper T (Th) cells, we tested the effect of Ca2+ ionophore and TPA on isolated populations of cloned, alloantigen-specific Th and Tc cells. Both agents induced cell proliferation and IL 2 production by Th cells, but not by Tc cells. Activation of mixed clones of Th and Tc cells, but not of Tc cells alone, resulted in cytotoxic activity, an effect that could be blocked by anti-IL 2 receptor antibodies. The results thus demonstrate that an increased concentration of intracellular Ca2+ in conjunction with PKC activation can bypass the signal provided by antigen-receptor interaction on Th cells, but does not substitute for IL 2 in activating cytotoxicity by isolated Tc cells.     

Activation of cord T lymphocytes. I. Evidence for a defective T cell mitogenesis induced through the CD2 molecule

A study was carried out on cord blood T cell activation via the CD2-mediated pathway. Despite similar percentages of circulating CD3+ and CD2+ cells in adult and cord blood, the proliferation of cord PBMC to the anti-CD3 mAb and cord T cells to anti-CD2 mAb were defective. The T cell CD3-surface structure was normally able to control CD2-mediated activation, as its modulation by a non-mitogenic anti-CD3 mAb blocked cord PBMC proliferation induced by anti-CD2 mAb. CD2-stimulated cord T cells did not proliferate and did not produce a significant amount of IL-2 in culture, although they expressed the IL-2R. This observation was confirmed by the optimal proliferation of CD2-induced cord T cells when rIL-2 was added. Despite the alternative T cell activation pathway is monocyte-independent in adults, the defective cord T cell activation via the CD2 molecule could also be bypassed by the addition of PMA, small amounts of either autologous or allogeneic adult and cord AC or simply rIL-1 alone. Our findings provide evidence for an intrinsic functional defect in cord CD2-mediated T cell activation, which is linked to an impaired increase of free cytoplasmic calcium, as confirmed by the effectiveness of calcium ionophore A23187 in restoring a good CD2-induced cord T cell proliferation and by measurement of cellular calcium uptake after activation via the CD2 molecule. The characteristics of cord T cells revealed by this study recall the thymocyte functional pattern and may represent functional expression of the previously described phenotypic immaturity of cord T cells.     

Protein kinase C-dependent and -independent mechanisms of cloned murine T cell proliferation. The role of protein kinase C translocation and protein kinase C activity

PMA can induce the proliferation of several CTL clones but not of several Th clones derived and tested in our laboratory. The PMA-stimulated proliferation of our CTL clones (which do not make IL-2 mRNA or protein) occurs independently of IL-2 and is not accompanied by lymphokine release. We now report, however, that protein kinase C (PKC) translocation is induced by PMA in CTL clones as well as in Th clones, which lack a proliferative response to PMA. These results suggest that PKC translocation itself is not a sufficient regulatory mechanism to account for cloned T cell proliferation. Moreover, IL-2 did not induce PKC translocation in a CTL clone, which proliferates when stimulated with IL-2. Thus, PKC translocation may not be necessary for activation of CTL proliferation. Nonetheless, cellular PKC activity appears to be required for the proliferative response of T cell clones after stimulation by PMA/PMA + calcium ionophore (A23187) or by triggering through the TCR: chronic PMA treatment, which depletes intracellular PKC activity, abrogates the proliferative response of T cell clones stimulated by PMA/PMA + A23187 or triggered through the TCR. T cell clones depleted of PKC activity, however, retain the ability to proliferate when challenged with IL-2. Murine T cell clones, therefore, possess PKC-dependent and PKC-independent pathways of proliferation that are not regulated by PKC translocation alone.     

Complete nucleotide sequence of the chromosomal gene for human IL-4 and its expression

We have isolated a chromosomal DNA segment of the human IL-4 gene based on homology with a human IL-4 cDNA sequence and determined its complete nucleotide sequence. The human IL-4 gene, which occurs as a single copy in the haploid genome, is mapped on chromosome 5. It is composed of four exons and three introns and is approximately 10 kilobase pairs in size. 5'-Flanking regions of human and mouse IL-4 genes share about 85% homology extending more than 500 base pairs upstream of a "TATA" like sequence. Several patches of sequences are found in the 5'-flanking region of the human IL-4 gene which are homologous to sequence in the 5'-flanking regions of the IL-2, IL-3, IL-5, and granulocyte-macrophage (GM)-CSF genes. The IL-4 gene is inducible after treatment of human T cell clone by phorbol-12-myristate-13-acetate (TPA) and calcium ionophore A23187. The 2.3-kb 5'-flanking region of the human IL-4 gene transiently transfected into Jurkat human T cell leukemia cells is activated efficiently in response to TPA and A23187 stimulation and, although less efficiently, by human T cell leukemia virus type I-encoded p40x or BPV-encoded E2 protein. Combination of TPA/A23187 and p40x or E2 protein further augmented the level of expression.     

CD8+ suppressor T cell clone capable of inhibiting the antigen- and anti-T cell receptor-induced proliferation of Th clones without cytolytic activity

A CD8+ Ts clone 13G2 was established from lymph node cells of bovine alpha s1-casein-primed C57BL/6 mice by in vitro antigenic stimulation followed by maintenance with IL-2-containing medium. The clone suppressed the Ag-induced proliferative responses of CD4+ Th cell clones without detectable cytotoxicity for both APC and responding T cells. The clone was able to suppress the in vitro proliferative response and antibody formation of Ag-primed lymph node cells. The suppression was Ag-nonspecific and not restricted to the MHC. The clone was able to suppress the proliferation of Th clones induced by an immobilized anti-TCR antibody in which APC was absent. The clone was, however, unable to suppress the proliferation of Th clones induced by anti-CD3 or IL-2. Thus, the mechanism of suppression by 13G2 was found to be due to a direct action on Th by inhibiting a consequence of signal transduction initiated through the TCR.     

Direct induction of lymphocyte killing by calcium ionophore and phorbol ester treatment

To analyze transduction mechanisms in human lymphocyte killing, intracellular Ca2+ levels were increased by ionophore A23187 treatment and protein kinase C activated by phorbol ester 12-O-tetradecanoylphorbol-acetate (TPA). Drugs were tested either alone or in combinations on effector cells active in natural, antibody-dependent, and lectin-dependent killing. TPA suppressed killing in all systems at 100 ng/ml whereas A23187 was only suppressive for NK killing at concentrations higher than 0.1 microM. TPA combined with A23187, above 10 ng/ml and 0.5 microM, respectively, induced killing of all tested target cell lines with a slower kinetic than NK killing of K562 cells. Drug-induced killing did not increase optimal lectin and antibody-dependent killing and was demonstrated most easily on NK-resistant target cell lines. Fractionation of effector lymphocytes into NK cell-depleted, T3-positive and NK cell-enriched, T3-negative cells demonstrated that similar levels of TPA/A23187-dependent killing could be induced in both fractions. It is concluded that TPA/A23187 induce normal lymphocytes to nonselective killing of different target cells in similarity to the triggering effect these drugs have in many other cell systems. Whether the induced killing is representative of NK killing is discussed in relation to the presence of other potential effector cells and effector molecules in peripheral blood lymphocytes.     

Functional and biochemical characterization of a calcium-ionophore-induced state of unresponsiveness in a cytolytic T cell clone.

To characterize the requirements for the induction of an anergic state in immunocompetent cells we examined the effect of an increase in intracellular calcium concentration on the subsequent responsiveness of cytolytic T cells to antigenic stimulation in vitro. Pretreatment of a murine cytolytic T cell clone with the calcium-ionophore A23187 resulted in the induction of an anergic state characterized by a decrease in cytolytic activity and granule exocytosis upon Ag-specific stimulation. Furthermore, IFN-gamma synthesis declined whereas de novo synthesis of a yet unidentified protein with a molecular mass of 33 kDa as well as proliferative response of cells in response to exogenous IL-2 were unaffected. This state of partial unresponsiveness 1) could be prevented by concomitant pretreatment of cells with cyclosporin A or protein synthesis inhibitors and 2) was reversible within 48 h. Biochemical analysis of TCR-induced intracellular activation revealed a block in signal transduction before the activation of protein kinase C because cellular unresponsiveness could be bypassed by the phorbol ester PMA plus the calcium-ionophore A23187. However, phosphatidylinositol turnover was markedly inhibited in unresponsive cells that also did not show a calcium influx on stimulation with concanavalin A. We conclude that a rise in intracellular calcium in cytolytic T cells might not only be necessary for cellular activation but may also trigger the induction of a partial unresponsiveness to antigenic stimulation due to an inhibition in the early phase of signal transduction.     

Activation of human T cells with the physiological regulator of protein kinase C

We investigated whether sn-1,2-dioctanoylglycerol (diC8) activates highly purified human T cells. diC8's signaling activity was also compared with that of 12-O-tetradecanoylphorbol-13-acetate (TPA). diC8 and ionomycin were synergistic in promoting T-cell proliferation. The proliferative response was dependent upon an operational interleukin-2 (IL-2) system and exhibited a high degree of specificity; sn-1,2-diC8 was twice as active as racemic-1,2-diC8, and diC8 and TPA were not synergistic. diC8's signaling activity differed from that of TPA. diC8, unlike TPA, failed to elicit IL-2 receptors or proliferation, independently of ionomycin. diC8 also failed to promote the proliferation of T cells signaled with anti-CD3 or -CD2 monoclonal antibodies. Two different inhibitors of PKC, 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine or staurosporine, inhibited T-cell proliferation induced with diC8 and ionomycin, but not with TPA and ionomycin. These observations, in addition to demonstrating the differential activity of diC8 and TPA, document a signaling role for diacylglycerol in the activation of normal T cells.     

Freshly isolated, murine neonatal T cells produce IL-4 in response to anti-CD3 stimulation.

In previous studies of chimeric animals, we found that fetal intrathymic T cell precursors give rise to phenotypically abnormal peripheral T cell populations. Because most peripheral T lymphocytes in newborn mice are the progeny of fetal T cell precursors, this result led to the hypothesis that neonatal and adult T cells differ in their functional capacities. To investigate this issue, the responses of neonatal and adult T cells to anti-CD3 antibody and TCR-independent stimulation were compared. When stimulated with soluble anti-CD3 antibody in the presence of adult accessory cells, neonatal T cell proliferation was markedly decreased compared with that of adult T cells. This reduction in proliferation was associated with both quantitative and qualitative differences in lymphokine production. At 48 h of stimulation with anti-CD3 antibody, neonatal T cells produced at least 10-fold less IL-2 than adult T cells. This apparently accounted for their reduced proliferation because the addition of exogenous IL-2 restored their proliferation to the levels achieved by adult T cells. In striking contrast to adult T cells, neonatal T cells secreted large amounts of IL-4 upon primary stimulation in vitro. The differences between neonatal and adult T cells in proliferation and lymphokine production were shown to be specific for CD3-mediated stimulation. In the presence of phorbol ester and calcium ionophore, neonatal and adult T cells showed equivalent proliferation and IL-2 production. Under these conditions, IL-4 production by neonatal or adult T cells was essentially undetectable. Thus, in response to TCR-independent stimulation, freshly isolated neonatal and adult T cells show similar functional responses. However, when stimulation occurs via the CD3 components of the TCR, the responses of neonatal T cells resemble those of primed T cells from adult animals.     

Proliferative responses of epithelial cells to 8-bromo-cyclic AMP and to a phorbol ester change during breast Pathogenesis

We have explored the relationship of changes in proliferative responses of human mammary epithelial cells to a phorbol ester (TPA) and to 8-Br-cAMP, which modulate the activities of protein kinases A and C (PKA and PKC), with breast tumour progression. Treatment with TPA had no effect on nontumorigenic cell lines established from human fibrocystic biopsies and apparently normal tissue around a tumour. In contrast, TPA strongly inhibited the proliferation of numerous human tumorigenic breast cell lines. Treatment with 8-Br-cAMP decreased the proliferation of all studied nontumorigenic and tumorigenic cell lines. We have also studied the effect of TPA and 8-Br-cAMP on growth of epithelial cells in short-term culture obtained from surgical human mammary biopsies with different states of breast disease. Both drugs enhanced growth of normal breast cells but had no significant effects on cells from biopsies with benign breast disease. In contrast, all examined cuitures from breast cancer biopsies were strongly inhhited by 8-Br-cAMP. Otherwise, TPA had an inhibitory effect only in the case of invasive ductal carcinoma of grade III. Malignant Ha-ras-transformation of nontumorigenic TPA-insensitive breast HBL-100 cells induced an inhibitory effect of TPA. In addition, a TPA-insensitive MCF7 clone was much less tumorigenic in athymic mice than the parental strain shown to be inhibited by TPA. These data suggest that the two intracellular transduction pathways change at different stages of breast pathogenesis. Alterations in the PKA pathway are early events and are probably important to cell immortalization but do not necessarily lead to malignant development. In contrast, changes in PKC pathway are rather later events associated with advanced malignant transformation. © 1994 Wiley-Liss, Inc.     

Anthrax lethal toxin blocks MAPK kinase-dependent IL-2 production in CD4+ T cells

Anthrax lethal toxin (LT) is a critical virulence factor that cleaves and inactivates MAPK kinases (MAPKKs) in host cells and has been proposed as a therapeutic target in the treatment of human anthrax infections. Despite the potential use of anti-toxin agents in humans, the standard activity assays for anthrax LT are currently based on cytotoxic actions of anthrax LT that are cell-, strain-, and species-specific, which have not been demonstrated to occur in human cells. We now report that T cell proliferation and IL-2 production inversely correlate with anthrax LT levels in human cell assays. The model CD4+ T cell tumor line, Jurkat, is a susceptible target for the specific protease action of anthrax LT. Anthrax LT cleaves and inactivates MAPKKs in Jurkat cells, whereas not affecting proximal or parallel TCR signal transduction pathways. Moreover, anthrax LT specifically inhibits PMA/ionomycin- and anti-CD3-induced IL-2 production in Jurkat cells. An inhibitor of the protease activity of anthrax LT completely restores IL-2 production by anthrax LT-treated Jurkat cells. Anthrax LT acts on primary CD4+ T cells as well, cleaving MAPKKs and leading to a 95% reduction in anti-CD3-induced proliferation and IL-2 production. These findings not only will be useful in the development of new human cell-based bioassays for the activity of anthrax LT, but they also suggest new mechanisms that facilitate immune evasion by Bacillus anthracis. Specifically, anthrax LT inhibits IL-2 production and proliferative responses in CD4+ T cells, thereby blocking functions that are pivotal in the regulation of immune responses.     

Proliferative potential of murine peritoneal mast cells after degranulation induced by compound 48/80, substance P, tetradecanoylphorbol acetate, or calcium ionophore A23187.

Proliferative potential of degranulated mast cells was investigated. Mast cells were collected from the peritoneal cavity of mice, and degranulation was induced by compound 48/80, substance P, 12-O-tetradecanoylphorbol 13-acetate (TPA), or calcium ionophore A23187. The potentiality of colony formation in methylcellulose was not reduced by treatment of various concentrations of compound 48/80, substance P and TPA. When degranulation was induced by compound 48/80, substance P or TPA, proportion of highly degranulated mast cells containing less than five granules was rather small. In contrast, considerable proportion of highly degranulated mast cells was obtained after the treatment with the low concentration (0.1 microgram/ml) of A23187. These highly degranulated mast cells, which were individually picked up by the micromanipulator, proliferated not only in methylcellulose but also in the skin of mast cell-deficient WBB6F1-W/Wv mice. Inasmuch as we have already shown the proliferation of IgE-sensitized and Ag-stimulated mast cells, degranulated mast cells appear to retain the proliferative potential in general.     

Selective modulation of MAP kinase in embryonic palate cells

Murine embryonic palate mesenchyme (MEPM) cells are responsive to a number of endogenous factors found in the local embryonic tissue environment. Recently, it was shown that activation of the cyclic AMP (cAMP) or the transforming growth factor β (TGFβ) signal transduction pathways modulates the proliferative response of MEPM cells to epidermal growth factor (EGF). Since the mitogen-activated protein kinase (MAPK) cascade is a signal transduction pathway that mediates cellular responsiveness to EGF, we examined the possibility that several signaling pathways which abrogate EGF-stimulated proliferation do so via the p42/p44 MAPK signaling pathway. We demonstrate that EGF stimulates MAPK phosphorylation and activity in MEPM cells maximally at 5 minutes. Tyrosine phosphorylation and activation of MAPK was unaffected by treatment of MEPM cells with TGFβ or cholera toxin. Similarly, TGFβ altered neither EGF-induced MAPK tyrosine phosphorylation nor activity. However, the calcium ionophore, A23187, significantly increased MAPK phosphorylation which was further increased in the presence of EGF, although calcium mobilization reduced EGF-induced proliferation. Despite the increase in phosphorylation, we could not demonstrate induction of MAPK activity by A23187. Like EGF, phorbol ester, under conditions which activate PKC isozymes in MEPM cells, increased MAPK phosphorylation and activity but was also growth inhibitory to MEPM cells. The MEK inhibitor, PD098059, only partially abrogated EGF-induced phosphorylation. Likewise, depletion of PKC isozymes partially abrogated EGF-induced MAPK phosphorylation. Inhibition of both MEK and PKC isozymes resulted in a marked decrease in MAPK activity, confirming that EGF uses multiple pathways to stimulate MAPK activity. These data indicate that the MAPK cascade does not mediate signal transduction of several agents that inhibit growth in MEPM cells, and that there is a dissociation of the proliferative response and MAP kinase activation. Furthermore, other signaling pathways known to play significant roles in differentiation of palatal tissue converge with the MAPK cascade and may use this pathway in the regulation of alternative cellular processes. J. Cell. Physiol. 176:266–280, 1998. © 1998 Wiley-Liss, Inc.     

Effects of TPA, A23187, and prostaglandin F2 alpha on progesterone synthesis by dispersed ovine luteal cells

The possible roles of protein kinase C, intracellular calcium, and oxygen environment in luteal progesterone (P4) production and their interaction with prostaglandin (PGF2 alpha) were investigated in dispersed ovine luteal cells. The following experiments were performed: 1) dose response to TPA and A23187, 2) interactions between the phorbol ester TPA and PGF2 alpha at 5% or 18% O2, 3) effect of TPA and PGF2 alpha on basal and luteinizing hormone (LH)-stimulated P4 secretion, 4) interaction of submaximal inhibitory concentrations of TPA with PGF2 alpha and the effect of indomethacin (IN) on the TPA response. Day 9 (Day 0 = first day of estrus) corpora lutea (CL) from ewes exhibiting estrous cycles of normal duration (15-17 days) were dispersed and 50,000-150,000 cells were cultured for 4 h in Dulbecco's Modified Eagle Medium. The proportion of luteal cells greater than 22 microns in diameter in these preparations averaged 17.8 +/- 2.1%. P4 in medium and cells was measured by radioimmunoassay. Both TPA and A23187 inhibited basal P4 accumulation in a dose-dependent manner. Maximum inhibition (500 nM) by TPA was greater than by A23187 at the same concentration (66.4 +/- 3.4 and 83.2 +/- 7.2% of controls, respectively; p less than 0.05), and the two were not additive in their effects. Reducing O2 did not affect P4 accumulation with or without TPA, PGF2 alpha, or both. Basal P4 accumulation was inhibited 30% by TPA and 10% by PGF2 alpha, but no additivity was seen.(ABSTRACT TRUNCATED AT 250 WORDS)     

LFA-1/ICAM-1在ConA诱导的小鼠胸腺细胞活化中的作用

用抗LFA-1/ICAM-1粘附分子单克隆抗体和ConA联合刺激小鼠胸腺细胞,初步研究了该膜分子在经TCR/CD3介导的胸腺细胞活化信号传导以及胸腺细胞亚群选择中的作用。在ConA刺激系统中,抗ALFA-1/ICAM-1单抗均能抑制胸腺细胞的增殖应答,且以抗LFA-1单抗的作用更为显著;而在PMA加钙离子载体A23187刺激体系中,抗LFA-1单抗却表现出明显的促活化效应。当加入IL-2 时,抗LFA——1/ICAM-1单抗便不能抑制ConA刺激的胸腺细胞活化。此外,抗体对已活化的胸腺母细胞增殖也无影响。FACS分析的结果还显示,抗LFA-1单抗可明显降低CD4~-CD8~ 胸腺细胞亚群的比例,而抗ICAM-1单抗对此无影响。表明胸腺细胞表面粘附分子LFA-1具有直接参与TCR/CD3途径介导的跨膜信号传导的功能,并对CD4~-CD8~ 胸腺细胞亚群的功能分化与成熟可能起重要作用。     

Interferon-gamma is produced by activated immature mouse thymocytes and inhibits the interleukin 4-induced proliferation of immature thymocytes

We have recently shown that interleukin 4 (IL-4) (formerly called BSF-1) is a potent stimulator of fetal and adult immature thymocyte proliferation and that adult L3T4-/Lyt-2-thymocytes can be stimulated by calcium ionophore (A23187) and phorbol ester to secrete IL-4 (Zlotnik, A., J. Ransom, G. Frank, M. Fischer, and M. Howard. 1987. Proc. Natl. Acad. Sci. (USA) 84:3856). This report shows that fetal thymocytes (day 15 of gestation) can also be activated to produce IL-4 suggesting that IL-4 may be a mediator of fetal as well as adult immature thymocyte proliferation. Furthermore, we demonstrate that interferon-gamma (IFN-gamma) inhibits the IL-4-mediated proliferation of both fetal and adult L3T4-/Lyt-2-thymocytes. The inhibition of proliferation is blocked by anti-IFN-gamma antibody and is unaffected by indomethacin suggesting that IFN-gamma directly inhibits immature thymocyte proliferation. IFN-gamma does not block the IL-4/phorbol myristate acetate-mediated proliferation of an adult thymocyte population, which is enriched for L3T4-/Lyt-2+ and L3T4+/Lyt-2- cells, suggesting that the inhibitory effect of IFN-gamma is limited to the immature thymocyte population. Both fetal (day 15) and adult L3T4-/Lyt-2--thymocytes can be activated to secrete an IFN-gamma like activity. This activity is neutralized by a monoclonal anti-IFN-gamma antibody indicating that the activity is due to IFN-gamma. mRNA analysis of adult L3T4-/Lyt-2- thymocytes stimulated with A23187 and phorbol myristate acetate confirms that mRNA for both IL-4 and IFN-gamma is induced in adult L3T4-/Lyt-2- thymocytes. These results indicate that IL-4 and IFN-gamma can regulate immature thymocyte proliferation.