Prostaglandin-E2 regulation of tumor necrosis factor receptor release in human monocytic THP-1 cells

Recent in vitro studies indicate that tumor necrosis factor (TNF) production in human monocytic THP-1 cells is suppressed by action of arachidonic acid metabolite prostaglandin-E2 (PGE2). PGE2 stimulation of human monocytic cell line THP-1 demonstrates that PGE2 not only regulates TNF activity at production levels, but does so through the release of two soluble TNF receptors (BP-55, BP-75) as well. PGE2 can thus exert a regulatory effect on TNF biologic activity by interfering with its ability to reach cell membrane receptors. THP-1 cells were activated with PGE2 for either 2- or 6-hr time periods, and the supernatants subsequently tested by ELISA to quantitate the levels of soluble receptor released. In addition, we examined mechanisms of receptor shedding by investigating the rate of membrane internalization and the role of serine proteases. PGE2-stimulated THP-1 cells showed soluble 55- and 75-kDa TNF receptor release levels which exceeded that of spontaneous release at both 2- and 6-hr activation periods. The numbers of both membrane TNF receptors were significantly upregulated as well in PGE2-activated cells, whereas the levels of 55- and 75-kDa TNF receptor mRNA levels remained unchanged. Thus, PGE2 induces TNF receptor release primarily at posttranscriptional levels. Inhibition of serine proteases with Pefabloc, a phenylmethylsulfonyl fluoride analog, resulted in the inhibition of both spontaneous and PGE2-stimulated release. Treatment of THP-1 cells with N-ethylmaleimide and low-temperature incubation, both known to block membrane internalization, also blocked spontaneous and PGE2-induced release. Internalization and cleavage by protease are therefore critical factors in PGE2-induced release of soluble TNF receptor shedding.

Identification of the proteolytic enzyme which cleaves human p75 TNF receptor in vitro

The extracellular domains of the human 55 and 75 kD TNF receptors (p55 and p75 TNF-R) are proteolytically cleaved to produce 30 and 40 kD soluble fragments, respectively. In this study, the enzymatic activity involved in the cleavage of human p75 TNF-R, named TNF-R releasing enzyme (TRRE), was identified in the culture supernatant of PMA-stimulated THP-1 cells using an activity assay system established by our group. When THP-1 cells were stimulated with PMA, TRRE was released rapidly into the supernatant, reaching maximal activity within 3 hours. The release of TRRE into the culture supernatant depended on the concentration of PMA and FCS. TRRE activity was partially inhibited by chelating agents, suggesting that TRRE may be a metallo-protease-like enzyme. This is the first successful attempt to establish a stable TRRE source with a reliable assay system.

Host-tumor interaction in ovarian cancer. Spontaneous release of tumor necrosis factor and interleukin-1 inhibitors by purified cell populations from human ovarian carcinoma in vitro

The biological activity of tumor necrosis factor (TNF alpha/beta) and interleukin-1 beta (IL-1 beta) can be blocked by soluble, naturally occurring molecules--TNF alpha/beta binding proteins (BP-55 and BP-75), derived from the extracellular portion of the 55- and 75-kDa TNF alpha/beta membrane receptors, and IL-1 receptor antagonist (IL-1ra), respectively. We examined the levels of these cytokines and their inhibitors in cell-free ascites of 18 patients with advanced ovarian carcinoma by ELISA. Levels of both TNF BP and IL-1ra dramatically exceeded those of TNF and IL-1; thus, it is unlikely that these cytokines are active in ascites from patients with this disease. We then elutriated solid tumor samples from three additional patients, yielding pure populations of tumor cells, macrophages, and lymphocytes. Cells were cultured for up to 48 hr and the spontaneous production of TNF, IL-1, and their inhibitors was measured by ELISA. Tumor cells and macrophages both released inhibitors for TNF and IL-1. Tumor cells released IL-1ra and BP-55, while macrophages released IL-1ra and BP-75. Kinetic studies showed that both tumor cells and macrophages produced an initial burst of TNF alpha and IL-1 beta which was overtaken within 48 hr by a sustained production of TNF BP and IL-1ra. Lymphocytes released no TNF alpha or TNF beta, which alone suggests that tumor associated lymphocytes are locally quiescent in vivo. TNF and IL-1 inhibitors originate from tumor cells and tumor associated macrophages and probably block TNF and IL-1 activity locally and regionally in ovarian carcinoma patients. Whether this phenomenon contributes to the pathogenesis of this disease remains to be determined.

Mechanism of release of soluble forms of tumor necrosis factor/lymphotoxin receptors by phorbol myristate acetate-stimulated human THP-1 cells in vitro

The mechanism involved in the release of the soluble forms of 55 and 75 kDa TNF and lymphotoxin (LT) membrane receptors was studied in a continuous human monocytic cell line, THP-1, in vitro. THP-1 cells were found to spontaneously release soluble forms of both 55 and 75 kDa TNF/LT receptors. Release was up-regulated by PMA, and optimal release was achieved at 10(-8) M PMA. Serine protease inhibitors such as PMSF,3,4 dichloroisocoumarin, N alpha-p-tosyl-L-lysine chloromethyl ketone (TLCK), and N-tosyl-L-phenylalanine chloromethyl ketone (TPCK) were found to inhibit the production of both soluble TNF/LT receptors. PMSF (2 mM) also blocked receptors shedding from paraformaldehyde-fixed THP-1 cells coincubated with conditioned media from PMA-stimulated THP-1 cells. Colchicine at 1 and 10 microM stimulated the production of both soluble TNF/LT receptors, but the PMA-induced release of both soluble TNF/LT receptors was inhibited. It appears that the PMA-induced release of soluble TNF/LT receptors involves serine proteases in the extracellular space where the soluble parts of the TNF/LT receptors are cleaved directly off the cell membrane.

Mechanism of release of soluble forms of tumor necrosis factor/lymphotoxin receptors by phorbol myristate acetate-stimulated human THP-1 cells in vitro

The mechanism involved in the release of the soluble forms of 55 and 75 kDa TNF and lymphotoxin (LT) membrane receptors was studied in a continuous human monocytic cell line, THP-1, in vitro. THP-1 cells were found to spontaneously release soluble forms of both 55 and 75 kDa TNF/LT receptors. Release was up-regulated by PMA, and optimal release was achieved at 10(-8) M PMA. Serine protease inhibitors such as PMSF,3,4 dichloroisocoumarin, N alpha-p-tosyl-L-lysine chloromethyl ketone (TLCK), and N-tosyl-L-phenylalanine chloromethyl ketone (TPCK) were found to inhibit the production of both soluble TNF/LT receptors. PMSF (2 mM) also blocked receptors shedding from paraformaldehyde-fixed THP-1 cells coincubated with conditioned media from PMA-stimulated THP-1 cells. Colchicine at 1 and 10 microM stimulated the production of both soluble TNF/LT receptors, but the PMA-induced release of both soluble TNF/LT receptors was inhibited. It appears that the PMA-induced release of soluble TNF/LT receptors involves serine proteases in the extracellular space where the soluble parts of the TNF/LT receptors are cleaved directly off the cell membrane.

The role of lymphotoxin in the IL-2-driven differentiation of human lymphokine-activated T-killer (T-LAK) cells in vitro

Brief stimulation of human peripheral blood mononuclear cells with PHA and subsequent coculture with IL-2 results by 5 days in cultures of human lymphokine-activated killer (T-LAK) cells. While IL-2 drives the proliferation of these cells in vitro, their maturation into functional effector cells capable of cytokine secretion and cell cytokines depends on the presence of other cytokines. The role of LT in the differentiation and proliferation of human T-LAK cells in vitro was investigated. Higher levels of LT than TNF were secreted by T-LAK cells during the first 5 days of the primary culture, then secretion levels dropped sharply. Human T-LAK cells cultivated with anti-LT rabbit antisera showed a slight reduction in growth compared to normal rabbit serum controls. In contrast, phenotypic analysis by FACS showed a decrease in CD4+ and an increase in CD8+ populations of T-LAK cells in the treated cultures. Addition of LT from the beginning of the T-LAK cell culture resulted in an increase in CD4+ and a decrease in CD8+ cell populations at day 7. In addition, the cytolytic activity of non-MHC-restricted cytotoxicity and NK-like activity of anti-LT cultured T-LAK cells was also effected. These data indicated that LT may have a role in differentiation of IL-2 stimulated human T-LAK cells in vitro.

Release of soluble TNF/LT receptors from a human ovarian tumor cell line (PA-1) by stimulation with cytokines in vitro

We have demonstrated the presence of the 55- and 75-kDa receptor for tumor necrosis factor (TNF) and lymphotoxin (LT) (TNF-R) in serum, and ascites from women with ovarian cancer. The present studies were initiated to begin to examine the possible cellular source of these receptors in women with ovarian cancer. Human ovarian tumor cells (PA-1) were cocultured for 24-48 hr with various levels of recombinant human cytokines (IL-1 beta, IL-4, IFN-gamma) and the supernatants were assayed by ELISA for the soluble forms of each receptor. PA-1 cells spontaneously release the 55-kDa TNF-R and low levels of the 75-kDa TNF-R. The release of both 55- and 75-kDa TNF-R was stimulated when PA-1 cells were cultured with IL-1 beta and IFN-gamma but unaffected by IL-4. The level of 55-kDa TNF-R was elevated slightly over spontaneous release but the level of 75-kDa TNF-R increased dramatically. IFN-gamma was the most potent stimulator of receptor release particularly of the 75-kDa TNF-R. IFN-gamma also induced increased expression of cell membrane TNF-R measured by binding of 125I-labeled TNF. Membrane TNF-Rs which were induced by IFN-gamma were the 75-kDa type, because TNF binding was blocked by anti-75-kDa TNF-R antibody. These data suggest that IFN-gamma selectively induced release and expression of 75-kDa TNF-Rs.

Measurement of the soluble membrane receptors for tumor necrosis factor and lymphotoxin in the sera of patients with gynecologic malignancy

The shed portion of the 55 and 75 kDa membrane receptors for tumor necrosis factor (TNF) and lymphotoxin (LT) have been described in the serum of patients with cancer. This study was designed to determine whether serum levels of the 55 and 75 kDa soluble TNF/LT receptors (sTNFr) had clinical significance in patients with gynecologic malignancies. Serum samples from 79 patients with ovarian, endometrial, or cervical cancer were assayed for CA 125 levels by RIA and the 55 and 75 kDa sTNFr levels by ELISA. Receptor and CA 125 levels were also analyzed with respect to disease status and response to treatment in banked serum samples from 14 patients with epithelial ovarian cancer who had been followed clinically for 1-3 years. Patients resulted were compared to serum samples tested from normal donors. We found that serum levels of both sTNFr's were elevated in the 79 patients with various gynecologic malignancies [55 kDa of 3.07 +/- 3.79 ng/ml (P < 0.02) and 75 kDa of 2.93 +/- 1.27 ng/ml (P < 0.001)] compared to 16 normal controls (55 kDa of 0.65 +/- 0.22 ng/ml and 75 kDa of 1.62 +/- 0.37 ng/ml). Serum levels of 55 and 75 kDa TNF/LT receptors were a more sensitive indicator of active cancer and had greater predictive value for detecting tumor in patients with ovarian cancer than CA 125. The sTNFr's were also more sensitive than CA 125 in detecting persistent or recurrent tumor and measuring response to therapy. These preliminary results suggest that measurement of serum levels of 55 and 75 kDa sTNFr's, even though not tumor specific, may be a uniquely new method for identifying and monitoring patients with gynecologic malignancy.

Role of 55- and 75-kDa tumor necrosis factor membrane receptors in the regulation of intercellular adhesion molecules-1 expression by HL-60 human promyelocytic leukemia cells in vitro

Most human cells express two TNF and lymphotoxin (LT) membrane receptors (TNF-R), of 55 and 75 kDa. The regulatory effect of these two receptors on intercellular adhesion molecules (ICAM-1) expression was examined in various human cell lines in vitro, including human lymphokine-activated killer T cells (T-LAK) cells and HL-60 cells. Rabbit antihuman TNF-R antisera specific for each receptor were employed as probes to selectively stimulate 55- and 75-kDa TNF/LT membrane receptor production. These antisera compete with TNF/LT binding to each specific cell membrane receptor and have been found to bind to specific membrane receptors on various human cell lines in vitro. In the present study, we demonstrated biologic activity for anti-55-kDa TNF-R antiserum. For example, antibodies that bind to the 55-kDa TNF-R caused cytolysis of HeLa and ME-180 human cervical cancer cells and induced proliferation of MRC-5 human fibroblasts. In contrast, however, anti-75-kDa TNF-R antiserum demonstrated no bioactivity in these assays. In addition, no synergy or costimulation was observed when a combination of both anti-55- and anti-75-kDa TNF-R antisera were tested in these assay systems. Anti-55-kDa TNF-R antiserum up-regulated ICAM-1 expression on human HL-60, T-LAK, and THP-1 cells, whereas anti-75-kDa TNF-R antiserum had no effect. Unexpectedly, however, ICAM-1 expression was greatly enhanced by the addition of anti-75-kDa TNF-R to the anti-55-kDa TNF-R containing culture. This enhancing effect was also observed with human T-LAK cells and THP-1 monocytic leukemia cell, in vitro.

Role of 55- and 75-kDa tumor necrosis factor membrane receptors in the regulation of intercellular adhesion molecules-1 expression by HL-60 human promyelocytic leukemia cells in vitro

Most human cells express two TNF and lymphotoxin (LT) membrane receptors (TNF-R), of 55 and 75 kDa. The regulatory effect of these two receptors on intercellular adhesion molecules (ICAM-1) expression was examined in various human cell lines in vitro, including human lymphokine-activated killer T cells (T-LAK) cells and HL-60 cells. Rabbit antihuman TNF-R antisera specific for each receptor were employed as probes to selectively stimulate 55- and 75-kDa TNF/LT membrane receptor production. These antisera compete with TNF/LT binding to each specific cell membrane receptor and have been found to bind to specific membrane receptors on various human cell lines in vitro. In the present study, we demonstrated biologic activity for anti-55-kDa TNF-R antiserum. For example, antibodies that bind to the 55-kDa TNF-R caused cytolysis of HeLa and ME-180 human cervical cancer cells and induced proliferation of MRC-5 human fibroblasts. In contrast, however, anti-75-kDa TNF-R antiserum demonstrated no bioactivity in these assays. In addition, no synergy or costimulation was observed when a combination of both anti-55- and anti-75-kDa TNF-R antisera were tested in these assay systems. Anti-55-kDa TNF-R antiserum up-regulated ICAM-1 expression on human HL-60, T-LAK, and THP-1 cells, whereas anti-75-kDa TNF-R antiserum had no effect. Unexpectedly, however, ICAM-1 expression was greatly enhanced by the addition of anti-75-kDa TNF-R to the anti-55-kDa TNF-R containing culture. This enhancing effect was also observed with human T-LAK cells and THP-1 monocytic leukemia cell, in vitro.

Blocking factors (soluble membrane receptors) for tumor necrosis factor and lymphotoxin detected in ascites and released in short-term cultures obtained from ascites and solid tumors in women with gynecologic malignancy

Studies were conducted to identify and establish the cell and tissue source of blocking factors (BF), materials that inhibit the bioactivity of human TNF and LT in vitro. Ascites and samples of solid tumors were collected from women with various gynecologic malignancies. Supernatants were collected from cultures of tumor and ascites cells after 24, 48, and 72 h. Cell-free ascites (CFA) and culture supernatants were tested for their ability to block human recombinant TNF and LT-induced lysis of L929 cells in vitro. Levels of soluble forms of the 55- and 75-kDa TNF/LT receptors were measured by ELISA assay in the same samples. CFA and culture supernatants contained TNF/LT blocking factors and high levels of one or both soluble 55- and 75-kDa TNF/LT membrane receptors. Levels of BF bioactivity and receptors appeared rapidly, peaked at 24 h, and declined thereafter. Soluble TNF/LT receptors may be the active BF in these samples, and tumor tissues and ascitic cells may be a source of these receptors in the ascites fluid of these patients.

The autocrine role of tumor necrosis factor in the proliferation and functional differentiation of human lymphokine-activated T killer cells (T-LAK) in vitro

The autocrine role of tumor necrosis factor alpha (TNF) in the proliferation and functional differentiation of human lymphokine-activated T-killer cells (T-LAK) in vitro was investigated. Human peripheral blood lymphocytes initially stimulated with IL-2 and phytohemagglutinin-P (PHA) for 48 h will proliferate for long periods in vitro in the presence of IL-2. These T-LAK cells have been shown to be 95% CD3 positive. Employing ELISA techniques, greater than 500 pg/ml of TNF was found to be released in the supernatants of these cells during the first 5 days of culture. However, the levels dropped to 100-200 pg/ml by days 7-10. T-LAK cells grown from days 7 to 10 in the presence of IL-2 and rabbit anti-TNF were significantly growth inhibited (up to 23%). The cytolytic activity of T-LAK cells grown from days 0 to 7 in the presence of anti-TNF was also decreased (up to 75%). Phenotypic analysis of these anti-TNF treated T-LAK cells revealed a decrease in CD8 expression (up to 12%) and increase in CD4 expression (up to 27%) when compared with control cells. The data suggest that TNF has a regulatory role in the growth and functional differentiation of these human T-LAK cells.

Trafficking of syngeneic murine lymphokine activated killer T cells following intraperitoneal administration in normal and tumor bearing mice

Nongenetically restricted T cells may be important host effector cells in women with ovarian cancer receiving intraperitoneal (ip) IL-2 therapy. We developed an in vitro technique to produce murine lymphokine-activated killer T cells. Murine splenocytes were cultured in the presence of 1000 U/ml IL-2 for 10 to 15 days. Phenotypical analysis showed 95% of total cells to express the pan T phenotype Thy 1.2 and no NK cell phenotypes by Day 7 in culture. These cells were labeled with 51Cr and their trafficking pattern after ip administration into normal and M5067 tumor bearing mice was examined. Various organs and tissues were collected at different timepoints and monitored for radioactivity. Within 4 hr., about 60% of the counts were associated with the bowel, peritoneum, and omentum of both normal and tumor bearing mice. About 15% of counts were associated with the blood, lung, kidney, spleen, and liver of both normal and tumor bearing mice.

Growth of the endometrial adenocarcinoma cell line AN3 CA is modulated by tumor necrosis factor and its receptor is up-regulated by estrogen in vitro

We demonstrate that tumor necrosis factor (TNF) has a biphasic effect on the growth of the human endometrial adenocarcinoma cell line AN3 CA in vitro. Low levels (0.2-5 pg/ml) of TNF were moderately growth stimulatory (up to 20% enhancement), while levels over 100 pg were growth inhibitory (up to 45% inhibition). Northern blot analysis showed expression of the 75-kilodalton (kDa) TNF receptor mRNA, but no expression of the 55-kDa TNF receptor mRNA or TNF mRNA. The growth of these cells was not directly affected by physiological concentrations (10(-7)-10(-9) M) of 17 beta-estradiol (E2). However, [125I]TNF binding studies and Scatchard analysis showed that 18-h coculture with 10(-8) M E2 increased the number of TNF receptors expressed on these cells 3-fold. Quantitative mRNA analysis confirmed that 75-kDa TNF receptor mRNA expression increased within 4 h of incubation with E2. These observations suggest an interaction between the endocrine and the immune systems, with an important implication for the homeostasis of endometrial tissues.

The regulation of TNF receptor mRNA synthesis, membrane expression, and release by PMA- and LPS-stimulated human monocytic THP-1 cells in vitro

The regulation of the 55-kDa TNF receptor (TNF-R) mRNA synthesis, membrane expression, and TNF binding factor (BF) release was examined in resting and activated human monocytic THP-1 and human promyelocytic leukemia HL-60 cells in vitro. Cells were activated with phorbol myristate acetate (PMA) and bacterial lipopolysaccharide (LPS). TNF alpha cytolytic activity in the supernatant of THP-1 cells stimulated by PMA began to appear at 4 hr, reached a peak at 8 hr, and declined by 12 hr. For THP-1 cells stimulated with LPS, the peak of TNF alpha activity appeared at 4 hr and then declined. TNF alpha-binding sites on the cell membrane were down-regulated within 1 hr after PMA and LPS treatment and then reappeared 12 hr later. Fifty-five-kilodalton TNF-R mRNA expression during this time period did not correlate with the level of membrane TNF-binding site expression. Additional studies indicated the presence of a 30-kDa TNF-BF in the supernatants which appeared after 24 hr. These data suggest that activated THP-1 and HL-60 cells are capable of releasing TNF-BF into the supernatant and this material may be involved in the control of secreted TNF alpha activities.

A 20 amino acid synthetic peptide of a region from the 55 kDa human TNF receptor inhibits cytolytic and binding activities of recombinant human tumour necrosis factor in vitro

Tumour Necrosis Factor (TNF) and Lymphotoxin (LT) can exert a wide range of effects on cells and tissues and they are important effector molecules in cell mediated immunity. All these effects are induced subsequent to the binding of these cytokines to specific membrane receptors. Recently, two of these membrane receptors of 55 and 75 kDa, have been identified which share some amino acid (AA) homology in their N-terminal extracellular domains but differ in their intracellular domains. We synthesized two synthetic 20 AA peptides from hydrophilic regions of the N-terminal extracellular domains of the 55 kDa receptor; peptide A shares homology with both 55 and 75 kDa receptors, peptide B is unique. We found peptide B inhibits both the binding and cytolytic activity of recombinant human TNF when tested on murine L929 cells in vitro. Polyclonal antiserum generated against peptide B will block binding of 125I-labelled TNF to these cells in vitro. However, peptide A and antiserum prepared against peptide A are without effect in these same assay systems. These data suggest that the 20 AA sequences from AA 175 to 194 in the N-terminal extracellular domain of the 55 kDa TNF receptor are expressed on the cell surface and are involved in the binding of TNF.

Identification of tumor necrosis factor and lymphotoxin blocking factor(s) in the ascites of patients with advanced and recurrent ovarian cancer

Ascites obtained from human ovarian cancer patients contains material(s) that inhibit the cytolytic activity of tumor necrosis factor (TNF) and lymphotoxin (LT) in vitro. These inhibitor(s) are found in ascites from ovarian cancer patients and are detected in very low amounts in the ascites from patients with nonmalignant hepatic disease. These ascites TNF/LT blocking factors are heat sensitive and heterogeneous with respect to molecular weight. Kinetic studies indicate these factors inhibited cytolysis at the stage of TNF/LT interaction with membrane receptors on L929 cells. Because TNF and LT are key cytokines in host cell-mediated antitumor mechanisms, factor(s) that inhibit these cytokines could have a profound effect on the tumor host interaction and their presence in the ascitic fluid, should be considered before designing clinical trials that employ intraperitoneal administration of TNF or LT for immunotherapy of ovarian cancer.

Enhancement of lymphokine-activated T killer cell tumor necrosis factor receptor mRNA transcription, tumor necrosis factor receptor membrane expression, and tumor necrosis factor/lymphotoxin release by IL-1 beta, IL-4, and IL-6 in vitro

Co-culture with IL-2 can induce human T lymphocytes to proliferate and become nongenetically restricted, lymphokine-activated killer (LAK) cells in vitro. Our studies were conducted with long term cultured, human T-LAK cells from peripheral blood, which are 95 to 99% CD3+. We found that proliferating 7 to 10-day human T-LAK cells express TNFR, by using a 125I-TNF binding assay. Additional treatment of these cells with the cytokines IL-1 beta, IL-4, or IL-6 rapidly up-regulated 55-kDa TNFR mRNA transcription and doubled TNFR membrane expression. Further studies revealed that these cytokines also increased the release of TNF and lymphotoxin (LT). Antibody neutralization studies indicated that IL-1 induces release of both TNF and LT; however, IL-4 and IL-6 induce primarily LT release. These results further support the concept that these cytokines are involved in the regulation of TNF/LT release, TNFR synthesis, and TNFR membrane expression. It is apparent that cytokines and their membrane receptors are involved in the autocrine/paracrine control of T cell proliferation, differentiation, and expression of functional activity after IL-2 stimulation in vitro.

Enhancement of lymphokine-activated T killer cell tumor necrosis factor receptor mRNA transcription, tumor necrosis factor receptor membrane expression, and tumor necrosis factor/lymphotoxin release by IL-1 beta, IL-4, and IL-6 in vitro

Co-culture with IL-2 can induce human T lymphocytes to proliferate and become nongenetically restricted, lymphokine-activated killer (LAK) cells in vitro. Our studies were conducted with long term cultured, human T-LAK cells from peripheral blood, which are 95 to 99% CD3+. We found that proliferating 7 to 10-day human T-LAK cells express TNFR, by using a 125I-TNF binding assay. Additional treatment of these cells with the cytokines IL-1 beta, IL-4, or IL-6 rapidly up-regulated 55-kDa TNFR mRNA transcription and doubled TNFR membrane expression. Further studies revealed that these cytokines also increased the release of TNF and lymphotoxin (LT). Antibody neutralization studies indicated that IL-1 induces release of both TNF and LT; however, IL-4 and IL-6 induce primarily LT release. These results further support the concept that these cytokines are involved in the regulation of TNF/LT release, TNFR synthesis, and TNFR membrane expression. It is apparent that cytokines and their membrane receptors are involved in the autocrine/paracrine control of T cell proliferation, differentiation, and expression of functional activity after IL-2 stimulation in vitro.