Human thymic epithelial cells produce interleukin 1

Although the thymus plays a critical role in generation of immunocompetent T lymphocytes, the precise role of the epithelial component of the thymus in the induction of T cell proliferation and maturation remains unknown. Since interleukin 1 (IL 1) is required for mature T cell activation, we have determined whether human thymic epithelial (TE) cells produce IL 1. By using a system for longterm culture of human TE cells, we found that human TE cells produced an IL 1-like factor (TE-IL 1) that augmented the proliferation of C3H/HeJ mouse thymocytes to phytohemagglutinin. IL 1 activity (20 to 200 U/ml) was detected in supernatants of TE cultures from all individuals (2 to 13 yr old) tested. IL 1 activity was also detected in supernatants of TE cultures from a 17-wk fetus but not from a 10-wk fetus. Production of TE-IL 1 was dependent on TE cell density and time in culture with optimal TE-IL 1 activity observed at 10(6) TE cells/ml after 48 to 72 hr of culture. With the use of high performance liquid chromatography, TE-IL 1 chromatographed as a molecule of 18,000 to 20,000 relative molecular mass, and by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, TE-IL 1 migrated at 15,000 to 17,000 Mr. With the use of isoelectrofocusing gels, charge heterogeneity of TE-IL 1 was demonstrated with two major isoelectric points of 5.7 to 5.8 and 6.9 to 7.0. Polyclonal antibody to human monocyte IL 1 markedly inhibited the TE-IL 1 activity. In indirect immunofluorescence assay of frozen human thymic sections, rabbit anti-IL 1 antibody reacted with epithelial cells in human thymic cortex and medulla. Furthermore, high performance liquid chromatography-purified TE-IL 1 augmented human thymocyte proliferation to suboptimal concentrations of phytohemagglutinin. Thus, thymic epithelial cells are capable of providing an intrathymic source of IL 1-like cytokine (TE-IL 1), which affects thymocyte proliferation. We propose that TE-IL 1 may play an important role in intrathymic proliferation and differentiation of human thymocytes.     

Human thymocytes do not respond to interleukin-2 after removal of mature "bright" CD5 positive cells

Interleukin-2 receptors (IL-2R) are expressed on minor populations of immature and mature human thymocytes. These studies were designed to determine if immature T cells could respond to the mitogen phytohemagglutinin (PHA-P) plus IL-2 in vitro by increasing the expression of IL-2R and by proliferation. Using monoclonal antibodies to CD5 and magnetic immunobeads we were able to remove all mature, "bright" CD5+ cells from nylon wool-purified thymocytes and to obtain less mature cells which consisted almost completely of cells with the CD4+CD8+ phenotype. These immature cells were mostly "dim" CD5+ and less than 5% CD5- and a small percentage expressed the IL-2R. After culture in serum-free medium with PHA-P, these cells showed only a slight increase in the percentage of IL-2R+ cells and the addition of IL-2 did not increase the percentage of IL-2R+ cells and no proliferation was observed. Unseparated, nylon wool-purified thymocytes contained 14% bright CD5+ cells. These bright CD5+ cells had a mature phenotype of CD4+CD8- (52%) and CD4-CD8+ (27%) cells. A small percentage of these cells were IL-2R+. These bright CD5+IL-2R+ cells were predominantly mature CD4+CD8- cells as measured by three-color flow cytometry. After culture with PHA-P and IL-2, the percentage of IL-2R+ cells increased and they were now found not only on CD4+CD8- but also on CD4-CD8+ and on CD4+CD8+ cells. IL-2 plus PHA-P increased proliferation of these cells as compared to those cultured in medium with PHA-P without IL-2. Thus, we show that human immature thymocytes in contrast to mature thymocytes are not responsive to IL-2 as measured by a lack of IL-2R expression and proliferation. These data indicate that mature thymocytes can express a functional high affinity receptor for IL-2 and suggest that immature thymocytes may not possess a (functional) p75 chain of the IL-2R.     

Induction of interleukin 2 responsiveness in thymocytes by synergistic action of interleukin 1 and interleukin 2

Thymocyte cultures from C3H/HeJ mice were stimulated for proliferative responses with purified preparations of interleukin 1 (IL 1) and interleukin 2 (IL 2). Synergistic responses were obtained in the absence of mitogen. In the presence of excess IL 2, the thymocyte proliferation response was strictly dependent on the amount of IL 1 in the cultures. Antibodies to IL 1 inhibited the response in a dose-dependent manner. The combination of IL 1 plus IL 2 induced the appearance of IL 2 receptors on murine thymocytes as detected with a monoclonal antibody directed against the IL 2 receptor. Neither IL 1 nor IL 2 alone had this effect. The thymic subpopulation found to become IL 2 responsive upon IL 1 stimulus was the peanut agglutinin-negative (PNA-) medullary fraction.     

Lyt-2+ cells. Requirements for concanavalin A-induced proliferation and interleukin 2 production

The requirements for inducing Lyt-2+ T cell proliferation in response to concanavalin A (Con A) were examined. Purified Lyt-2+ or L3T4+ spleen cells of C57BL/6 origin were stimulated with Con A and syngeneic macrophages (MO) in the presence of monoclonal antibodies to T cell markers or to polymorphic determinants on major histocompatibility complex molecules, and assessed for the ability to proliferate and to produce interleukin (IL) 2. alpha I-Ab failed to inhibit the Con A response of Lyt-2+ cells at dilutions that significantly inhibited the response of L3T4+ cells. In contrast, alphaKb/Db or alpha Lyt-2.2 specifically inhibited the response of Lyt-2+ cells, but not L3T4+ cells. The ability of alpha Kb/Db and of alpha Lyt-2.2 to inhibit the response of Lyt-2+ cells was dependent upon the concentration of Con A. These data demonstrate that optimal triggering of T cell subsets to proliferate and to produce IL-2 in response to Con A requires interactions with the appropriate restricting major histocompatibility complex molecule. The role of accessory cells in Lyt-2+ Con A-induced proliferation and IL-2 production was also investigated. Purified Lyt-2+ cells and purified L3T4+ cells failed to respond to Con A in the absence of MO. IL-1 reconstituted the response when MO were limiting, but failed to restore the response of either Lyt-2+ or L3T4+ cells when T cells were rigorously purified to remove all MO. These results demonstrate that triggering Lyt-2+ T cells, like L3T4+ T cells, requires accessory cells, and that this does not merely reflect a requirement for IL-1 production. Thus, Con A-induced proliferation and IL-2 production by Lyt-2+ T cells requires intimate contact with accessory cells and interactions dependent upon the class I-restricting element.     

Functional status of interleukin 2 receptors expressed by immature (Lyt-2-/L3T4-) thymocytes

A subpopulation of phenotypically immature (Lyt-2-/L3T4-) thymocytes express receptors for the polypeptide hormone interleukin 2 (IL 2); however, these cells do not proliferate in vitro in response to IL 2. In investigating this phenomenon in greater detail, we observed that the IL 2 receptors (IL 2-R) on freshly isolated immature thymocytes bound IL 2 with about fivefold lower affinity (Kd approximately 100 pM) than IL 2-R on activated mature T cells and T cell lines (Kd approximately 20 pM). Furthermore, in contrast to activated T cells, Lyt-2-/L3T4- thymocytes did not endocytose bound IL 2. When stimulated in short-term culture with a combination of phorbol ester (PMA) and calcium ionophore, Lyt-2-/L3T4- thymocytes proliferated in a largely IL 2-dependent fashion. IL 2-R expression on these activated cells initially disappeared (at 24 hr) and subsequently reappeared (at 48 to 72 hr). Reexpressed IL 2-R on activated thymocytes resembled those on mature T lymphocytes in that they bound IL 2 with high affinity (Kd = 15 to 25 pM) and were capable of endocytosing IL 2. Taken together, these data place certain constraints on the putative physiologic role of IL 2 in intrathymic growth regulation.     

Assessment of interleukin 1 and interleukin 2 effects on cycling and noncycling murine thymocytes

When mouse thymocytes are stimulated with PHA, the proliferative response is very low, unless the culture medium is enriched with interleukin 1 (IL-1)- or interleukin 2 (IL-2)-containing supernatants. Cytofluorometric analyses show, however, that PHA stimulation generates a significant number of cells with increased RNA content (transition from the G₀ to G₁ phase of the cell cycle). If IL-2 is added to such cultures, the activated cells complete their process of RNA synthesis and then enter the S phase. The use of IL-2-containing culture medium thus permits one to obtain a high correlation between the number of g₁ cells and [³H]thymidine incorporation (r = 0.97). Enrichment with IL-1-containing supernatants also results in a statistically significant correlation (r = 0.68), but the regression lines are markedly different for the two interleukins (s = 20.3 for IL-2 and s = 9.2 for IL-1), when analyzed after 48 hr of incubation. These observations suggest that the G₁ phase must be divided into two subcompartments, G_1a and G_1b, the G_1a-G_1b transition being an IL-2-dependent event. If the number of G_1b cells is used to establish correlations with [³H]thymidine incorporation, all values fall on the same regression line, regardless of culture conditions and of the addition of interleukins. It is concluded that IL-2 regulates lymphocyte proliferation at the level of RNA synthesis (G_1a-G_1b transition) rather than that of DNA synthesis (G₁-S transition).     

Up-regulation of interleukin 4 receptor expression on immature (Lyt-2-/L3T4-) thymocytes

In this report, the effect of interleukin 4 (IL-4) on the growth and differentiation of Lyt-2-/L3T4-(2-4-) thymocytes was investigated. It was found that these thymocytes proliferated extensively when cultured in the presence of IL-4 + phorbol myristate acetate without apparent differentiation to Lyt-2+ or L3T4+ cells. We also demonstrated that 2-4- thymocytes constitutively express a high affinity (dissociation constant of 20 to 40 pM) receptor for IL-4. Freshly isolated 2-4- thymocytes expressed on average about 100 IL-4 receptors per cell, but the number of receptors increased approximately 8-fold within 3 days after activation by IL-4 + phorbol myristate acetate. These findings suggest that IL-4 may play an important role in T cell ontogeny by promoting self-renewal of stem cells within the thymus.     

UV-irradiated epidermal cells produce a specific inhibitor of interleukin 1 activity

UV irradiation of epidermal cells (EC) in vitro and in vivo leads to an enhanced synthesis of the immunostimulating cytokine interleukin 1 (IL 1). However, UV exposure in vivo also results in local as well as systemic immunosuppression. Therefore, it was tested whether UV-exposed murine EC in culture in addition to IL 1 release an inhibitor of IL 1 activity. Supernatants of UV-irradiated BALB/c EC and of a transformed keratinocyte cell line (Pam 212) were evaluated for their ability to suppress IL 1-mediated thymocyte proliferation. Crude supernatants derived from either UV-exposed or unirradiated EC did not interfere with IL 1 activity. When supernatants were subjected to HPLC gel filtration, fractions eluting at approximately 40 kD significantly blocked the activity of EC-derived IL 1 and murine recombinant IL 1. The release of this inhibitory cytokine (EC-derived contra-IL 1 [EC-contra-IL 1]) was confined to UV-exposed BALB/c or Pam 212 keratinocytes, since no inhibitory activity was detected in supernatants of unirradiated cells. EC-contra-IL 1 also blocked IL 1-induced fibroblast proliferation but did not suppress IL 2 or IL 3 activity. Moreover, EC-contra-IL 1 did not inhibit spontaneous proliferation of a variety of cell lines (Pam 212, P388D1, L 929, EL 4). With the use of chromatofocusing EC-contra-IL 1 exhibited a pI of 8.8, and upon reversed-phase chromatography it eluted within three distinct peaks. Therefore, murine UV-exposed EC, in addition to the production of immunoenhancing cytokines, also may release immunosuppressing mediators and thereby participate in UV-induced immunosuppression. These findings further support the notion that the epidermis may not only be considered as a simple barrier against harmful agents but represents an active element of the immune system.     

Role of interleukin 1 in early T cell development: Lyt-2-L3T4- thymocytes bind and respond in vitro to recombinant IL 1

Thymocytes that bear neither Lyt-2 nor L3T4 differentiation antigens (2-4- thymocytes) contain the precursors for mature L3T4+Lyt-2- and Lyt-2-L3T4+ T cells. In the present study we determined the capacity of 2-4- cells to respond to recombinant interleukin 1 (rIL 1) in vitro. The presence of rIL 1 enhanced IL 2-dependent proliferation to the lectins Con A and PHA by threefold to eightfold. In a second assay, rIL 1 enhanced proliferation and IL 2 production by 2-4- cells in response to phorbol myristate acetate (PMA) and the calcium ionophore, ionomycin. Using a direct IL 1 binding assay, we were able to detect both high-affinity (Kd approximately 5 pM) and low-affinity (Kd approximately 200 pM) classes of IL 1 receptors on freshly isolated 2-4- cells. Bound IL 1 was rapidly internalized, suggesting that such receptors were functional. These results are compatible with a role for interleukin 1 during thymocyte maturation.     

Thy-1-negative and Ly-1-negative variants of T cells produce interleukin 2 in response to mitogens

IL 2 production by T cell variants, which lack the Thy-1 or Ly-1 surface glycoproteins, was studied. Cross-linking of the Thy-1 molecule resulted in IL 2 production by the EL4 thymoma and by a T cell hybridoma, suggesting that Thy-1 may play a role in T lymphocyte triggering. To further study the functional role of this molecule, Thy-1-negative variants were selected and analyzed for IL 2 production in response to phorbol-12-myristate-13-acetate (PMA) or to Con A. It was demonstrated that in spite of their failure to express Thy-1, the Thy-1-negative clones were capable of IL 2 production. These results indicated that although Thy-1 cross-linking triggers cell activation, a signal provided by Thy-1 is not indispensable for cell activation by mitogens. The T cell tumor line LBRM331A5 responds synergistically to IL 1 and PHA by releasing IL 2. It was demonstrated that anti Ly-1 monoclonal antibodies and PHA co-stimulated LBRM331A5 cells, as did IL 1 plus PHA. Thus, anti Ly-1 antibodies mimic the effect of IL 1, suggesting a role for Ly-1 antigen in T cell activation, perhaps by serving as an IL 1 receptor or as an associated molecule. To further study the functional role of Ly-1 and its relation to IL 1 receptor, Ly-1-negative variants of the LBRM331A5 cell line were selected and analyzed for IL 2 production in response to PHA plus IL 1. It was demonstrated that the Ly-1-negative clones were capable of IL 2 production as efficiently as Ly-1-positive clones. These results indicate that the Ly-1 and IL 1 receptor are distinct molecules, which are involved in different activation pathways.     

Cultured human epidermis cells produce cell-associated interleukin 1-like prostaglandin E2- and collagenase-stimulating factors

In order to identify factors which may regulate the functions of dermal fibroblasts, cell lysates and conditioned media of cultured human epidermal cells were tested on dermal fibroblasts for the stimulation of prostaglandin E2- and collagenase-production. Both prostaglandin E2- and collagenase-stimulating activities appeared during epidermal cell culture: after 2 d they were detected in the cell lysate, and after 4 d of culture they were found also in the conditioned media. Molecular sieving chromatography of epidermal cell lysates led to the detection of two main peaks showing concomitant prostaglandin E2- and collagenase-stimulating activities at Mr approximately equal to 18 000 and Mr approximately equal to 10 000. A single peak of concomitant prostaglandin E2- and collagenase-stimulating activities were seen at Mr approximately equal to 10 000 in the epidermal cell conditioned media. This suggests that the cell-associated concomitant prostaglandin E2- and collagenase-stimulating activities are processed from a common precursor molecule and released. Collagenase-stimulating activity without accompanying prostaglandin E2 was also detected in the range of Mr approximately equal to 30 000-45 000.     

Regulation by interleukin 2 of interleukin 2 receptors and gamma-interferon synthesis by human thymocytes: augmentation of interleukin 2 receptors by interleukin 2

The role of interleukin 2 (IL 2) on the expression of IL 2 receptors and on the synthesis of gamma-interferon (gamma-IFN) by human thymocytes was investigated. Human thymocytes isolated from specimens obtained during cardiac surgery of infants and children were induced with one or all of the following agents: IL 2, concanavalin A (Con A), and 12-O-tetradecanoylphorbol 13-acetate (TPA). The expression of IL 2 receptors and gamma-IFN titers were determined. The results indicate that thymocytes cultured in complete medium do not express receptors for IL 2, nor did IL 2 by itself induce the expression of IL 2 receptors. Con A induced the expression of IL 2 receptors by a moderate number of the thymocyte population and induced the synthesis of low amounts of gamma-IFN. Preincubation of thymocytes with TPA increased the response to Con A; both the number of thymocytes expressing receptors and the synthesis of gamma-IFN were increased. Addition of IL 2 to these cultures further augmented the expression of IL 2 receptors and gamma-IFN synthesis and resulted in the optimal expression of IL 2 receptors and maximal gamma-IFN synthesis. The expression of IL 2 receptors could be detected within 24 hr and preceded the induction of proliferation; it was therefore probably not due to the clonal expansion of a population of receptor-bearing thymocytes. Conversely, inhibition of IL 2 synthesis with dexamethasone (Dex) by thymocytes activated with Con A, or inhibition of the function of IL 2 receptors by anti-Tac, resulted in a decrease in the number of IL 2 receptor-bearing thymocytes activated with Con A, or inhibition of the function of IL 2 receptors by anti-Tac, resulted in a decrease in the number of IL 2 receptor-bearing thymocytes and of gamma-IFN synthesis. Thymocytes activated with TPA and Con A were more resistant to the inhibitory effects of Dex on the expression of IL 2 receptors than thymocytes activated with Con A alone. Maximal inhibition of the expression of IL 2 receptors and of gamma-IFN synthesis was achieved as a result of the synergistic effect of anti-Tac with Dex. Therefore, when IL 2 was prevented from binding to the receptors, and IL 2 synthesis was inhibited, the number of thymocytes expressing IL 2 receptors was sharply reduced and gamma-IFN synthesis was markedly inhibited.(ABSTRACT TRUNCATED AT 400 WORDS)     

Modulation of the expression of interleukin 2 receptors of human thymocytes by recombinant interleukin 2

The effect of purified recombinant interleukin 2 on the expression of the receptors for interleukin 2 by human thymocytes was examined. Interleukin 2 augmented the expression of interleukin 2 receptors and interferon-gamma synthesis by thymocytes activated with concanavalin A, and it was required to maintain the growth of thymocytes in vitro and the expression of interleukin 2 receptors. The increase observed in the number of receptor bearing thymocytes and in the density of receptors due to interleukin 2 occurred within the first 2 days of culture. Dexamethasone inhibited the expression of interleukin 2 receptors, the synthesis of interferon-gamma, and the early proliferation and protein synthesis of lectin-activated thymocytes during the first 2 days of culture. The inhibitory effect of dexamethasone on the expression of interleukin 2 receptors and on the synthesis of interferon-gamma was reversed by interleukin 2, whereas its effect on proliferation and on protein synthesis during the first two days of culture was not reversed by interleukin 2. Interleukin 2 induced the proliferation of thymocytes in vitro, even in the absence of activation by lectin; however, the number of cells displaying receptors which could be detected with anti-Tac remained low throughout the first week of culture and interferon-gamma synthesis was not observed. Nonetheless, interleukin 2-induced proliferation was inhibited by anti-Tac on a dose dependent manner. The results of the study document that recombinant interleukin 2, like purified natural interleukin 2, is required for the expression of interleukin 2 receptors, for interferon-gamma synthesis, and for the growth of thymocytes in vitro.     

Proliferation in vitro and interleukin production by 14 day fetal and adult Lyt-2-/L3T4- mouse thymocytes

When 14 day fetal mouse thymocytes, which are phenotypically Lyt-2-/L3T4-(2-4-), were stimulated in vitro with a combination of phorbol myristate acetate (PMA) and the calcium ionophore ionomycin, they proliferated without addition of exogenous interleukins and/or growth factors. Addition of exogenous IL 2 resulted in a slight enhancement of fetal thymocyte proliferation. By using factor-dependent indicator cell lines, this proliferation was shown to be accompanied by the production of IL 2 and IL 3. However, phenotypic analysis by using flow microfluorometry and monoclonal antibodies to Lyt-2 and L3T4 showed little differentiation among proliferating 2-4-fetal thymocytes. Interestingly, the in vitro growth of PMA + ionomycin-stimulated fetal thymocytes appeared to be IL 2 dependent in that it was inhibited by a monoclonal antibody to the IL 2 receptor. The results obtained with fetal thymocytes were compared with those obtained when using 2-4- thymocytes from adult mice.     

Positively selected Lyt-2+ and Lyt-2- mouse T lymphocytes are comparable, after Con A stimulation, in release of IL 2 and of lymphokines acting on B cells, macrophages, and mast cells, but differ in interferon production

Purified mouse T lymphocytes were separated into Lyt-2+ and Lyt-2- populations by the procedure of panning, in which a monoclonal rat anti-Lyt-2 antibody and dishes coated with affinity-purified mouse anti-rat Ig antibodies were used. The populations obtained were 95 to 99% pure as determined by immunofluorescence. Graded doses of these T cells were cultured with optimal mitogenic doses of concanavalin A and the 0 to 24 and 24 to 48-hr culture supernatants were collected. The dose-curve assays of the supernatants of Lyt-2+ and Lyt-2- cells showed comparable activity in interleukin 2 (IL 2) and T cell-replacing factor (TRF), assayed on antigen-stimulated culture of T-depleted spleen cells. Limiting dilution assays of IL 2-secreting precursor cells stimulated by Con A showed a high frequency of precursors in both populations, slightly higher among Lyt-2- cells. The supernatants also contained comparable levels of IPA (inducer of plasminogen activator production by the macrophages), MAF (macrophage-activating factor, assayed by induction of their cytolytic function), and MCGF (mast cells growth factor, assayed on a mast cell line). IPA and MAF were not produced with the same kinetics and in the same T cell concentration conditions as IL 2 and TRF. In contrast, interferon was principally produced by the Lyt-2+ cells.     

Biosynthesis and maturation of the Lyt-2/3 molecular complex in mouse thymocytes

The biosynthesis and maturation of the three subunits alpha (Mr = 37,000), beta (Mr = 32,000), and gamma (Mr = 27,000) of the mouse Lyt-2/3 antigenic complex have been studied by using two monoclonal antibodies directed against a monomorphic determinant of the Lyt-2 antigen. Short time-pulse labeling of thymocytes reveals three different high mannose intermediates that give rise upon endo-beta-N-acetyl-glucosaminidase H digestion to three distinct precursor polypeptides of Mr = 22,000 (alpha P), Mr = 18,000 (beta P), and Mr = 19,500 (gamma P). Pulse-chase analysis indicates rapid posttranslational processing, because mature forms already appear after 10 min of chase. The half-life of the endo-H-sensitive early forms are in the range of 20 to 30 min. Both the alpha and beta subunits are suggested to contain three N-asparagine-linked oligosaccharides, one of which is of the high mannose type. In contrast, the gamma-chain contains only one such glycan unit of the complex type. Moreover, the results presented show that all three chains undergo additional posttranslational modifications. Finally, the data suggest that the cytoplasmic domains of these chains are of different size.     

Characterization of interleukin 2-dependent cytotoxic T-cell clones. IV. Production of alpha, beta and gamma interferons and interleukin 2 by Lyt-2+ T cells

The production of alpha, beta and gamma interferons (IFN) and interleukin 2 (IL-2) by Lyt-2+-dependent cytotoxic T-cell lines/clones was investigated. Cloned and uncloned T-cell lines specific for H-2Dd or the unique RL male 1 leukemia antigen were studied. After infection with Sendai virus (SV) or Newcastle disease virus (NDV) all cell lines produced IFN-alpha and -beta. Induction of IFN-gamma was attempted with the mitogens Con A, PHA, PWM, SEA, and SEB, with poly(I:C), with antibodies Lyt-1.2, -2.2, and Thy-1.2, or with the target cells Meth A (H-2Dd+) and RL male 1. All mitogens were effective inducers. However, the antibodies and poly(I:C) were not. One uncloned RL male 1-specific cell line CTLL-RP, produced IFN-gamma after induction with RL male 1. Production of IFN-alpha, beta depended on IL-2, whereas production of IFN-gamma did not, although addition of highly purified IL-2 increased IFN-gamma production even in the absence of other inducers. Crude IL-2 inhibited the production of IFN-gamma but not IFN-alpha, beta. In response to mitogens, some T-cell clones also produced IL-2. The results demonstrate that Lyt-2+ cells can produce a broad spectrum of lymphokine activities after appropriate stimulation. Their availability now affords us the opportunity to study the regulation of lymphokine production at the clonal level.     

Antigen-presenting cells for Lyt-2+ cells. I. Stimulation of unprimed Lyt-2+ cells by H-2 different Thy-1-Ia- cells prepared from spleen and bone marrow

In agreement with previous studies on Ia- tumor cells, evidence is presented that primary MLR of purified Lyt-2+ T cells to class I alloantigens can be elicited by a minor population of Thy 1- Ia- cells present in normal spleen, bone marrow, and day-13 fetal liver; these cells are non-stimulatory for L3T4+ T cells. The data strengthen the view that primary responses of Lyt-2+ cells do not require the presence of Ia+ cells.     

Effect of interleukin 2 on fetal thymocytes in organ cultures: generation of lymphokine-activated killer cells

Cells with cytolytic activity can be detected in mouse fetal thymic lobes cultured in the presence of interleukin 2 for 6 days. The lymphokine-activated killer cells from 14-day fetal thymic lobes are relatively resistant to treatment with anti-Ly-2 antibody and complement (CD8-) but sensitive to anti-Thy-1 and complement treatment (Thy-1+). They display major histocompatibility complex-unrestricted killing, lysing both syngeneic and allogeneic tumor cells, but will not lyse human xenogenic target cells. Low levels of cytotoxic activity can be detected in thymic lobes from Day 12-13 embryos and this activity increases with embryonic age. While the events which lead to the inhibition of normal maturation of fetal thymocytes by inclusion of IL-2 in fetal thymus organ cultures are unknown, the appearance of cytotoxic cells raises the question of whether they are involved in the normal intrathymic cell death process.