SKYSCRAPER-02: Tiragolumab in Combination With Atezolizumab Plus Chemotherapy in Untreated Extensive-Stage Small-Cell Lung Cancer

PURPOSE

The phase III SKYSCRAPER-02 study determined whether the benefits of atezolizumab plus carboplatin and etoposide (CE) could be enhanced by the addition of tiragolumab in untreated extensive-stage small-cell lung cancer (ES-SCLC). We report final progression-free survival (PFS) and overall survival (OS) analyses.

METHODS

Patients received tiragolumab 600 mg/placebo, plus atezolizumab 1,200 mg and CE (four cycles), then maintenance tiragolumab/placebo plus atezolizumab. Primary end points were investigator-assessed PFS and OS in patients without history/presence of brain metastases (primary analysis set [PAS]). Additional end points included PFS and OS in all patients regardless of brain metastases status (full analysis set [FAS]), response, and safety.

RESULTS

Four hundred ninety patients were randomly assigned (FAS): 243 to tiragolumab arm and 247 to control arm. At the cutoff date (February 6, 2022; median duration of follow-up, 14.3 months [PAS] and 13.9 months [FAS]), final analysis of PFS in the PAS (n = 397) did not reach statistical significance (stratified hazard ratio [HR], 1.11; P = .3504; median, 5.4 months tiragolumab v 5.6 months control). At the cutoff date (September 6, 2022; median duration of follow-up, 21.2 months [FAS]), median OS in the PAS at final OS analysis was 13.1 months in both arms (stratified HR, 1.14; P = .2859). Median PFS and OS in the FAS were consistent with the PAS. The proportion of patients with immune-mediated adverse events (AEs) in the tiragolumab and control arms was 54.4% and 49.2%, respectively (grade 3/4: 7.9% and 7.7%). AEs leading to treatment withdrawal occurred in 8.4% and 9.3% of tiragolumab- and control-treated patients, respectively.

CONCLUSION

Tiragolumab did not provide additional benefit over atezolizumab and CE in untreated ES-SCLC. The combination was well tolerated with no new safety signals.

Contribution of FcRn binding to intestinal uptake of IgG in suckling rat pups and human FcRn-transgenic mice

Immunoglobulin G (IgG) is transcytosed across intestinal epithelial cells of suckling mammals by the neonatal Fc receptor (FcRn); however, the contribution of FcRn vs. FcRn-independent uptake to serum IgG levels had not been determined in either rat pups or human (h)FcRn-expressing mice (Tg276 and Tg32). In isoflurane-anesthetized rodents, serum levels were determined after regional intestinal delivery of human monoclonal antibodies (hIgG) with either wild-type (WT) Fc sequences or variants engineered for different FcRn binding affinities. Detection of full-length hIgG was by immunoassay; intestinal hFcRn and hIgG localization was by immunocytochemistry. High (μg/ml) serum levels of hIgG were detected after proximal intestinal delivery (0.1-10 mg/kg) in 2-wk-old rats. Human FcRn was visualized in epithelial cells of Tg276 mice, but low serum hIgG levels (<10 ng/ml) were obtained. In rat pups, intraintestinal hIgG1 WT administration resulted in dose-related and saturable uptake, whereas uptake of a low FcRn-binding affinity variant was nonsaturable. There were no differences in hIgG levels from systemic and hepatic portal vein serum samples, and intense hIgG immunostaining was noted in villi enterocytes and within lymphatic lacteal-like vessels. This study demonstrated that FcRn-mediated uptake in rat pups accounted for ~80% of serum hIgG levels and that IgG enters the circulation via the lymph and not the hepatic portal vein. The remaining uptake though the immature intestine is nonreceptor mediated. Intestinal epithelial cell hFcRn expression occurred in Tg276 mice, but receptor-mediated transport of IgG was not observed. The suckling rat pup intestine is a mechanistic model of FcRn-IgG-mediated transcytosis.

Expression of human tissue factor under the control of the mouse tissue factor promoter mediates normal hemostasis in knock-in mice

BACKGROUND

Tissue factor (TF) is expressed widely at the subluminal surface of blood vessels and serves as the primary cellular initiator of the extrinsic pathway of blood coagulation. Lack of TF in mice resulted in lethality in utero, but human TF (huTF) expressed at low levels from a human minigene rescued null mice from prenatal death. Although these low-TF expressing transgenic mice developed to term, they had a significantly shorter life span and exhibited hemorrhage and fibrosis in the heart.

METHODS

Human TF knock-in (TFKI) mice were generated by replacing the first two exons of the mouse (murine) TF (muTF) gene with the huTF complete coding sequence, thus placing it under the control of the endogenous muTF promoter.

RESULTS

Expression of huTF in the TFKI mice was similar to muTF in wild-type (wt) mice. The TFKI mice showed no microscopic evidence of spontaneous hemorrhage in the heart, nor cardiac fibrosis at up to 18 months of age. Immunohistochemistry showed that huTF was expressed in cells surrounding blood vessels in TFKI mice. Coagulation activity of brain homogenates from TFKI mice was comparable with that from wt brain. Cardiac hemorrhage similar to that of the low-TF transgenic mice occurred in the TFKI mice when huTF was blocked by a neutralizing anti-huTF monoclonal antibody.

CONCLUSION

We generated a transgenic mouse line that expresses huTF under the control of the endogenous muTF promoter at physiological levels. Our results suggest that huTF can fully reconstitute the murine coagulation system and mediate normal hemostasis.

Expression of human tissue factor under the control of the mouse tissue factor promoter mediates normal hemostasis in knock-in mice

BACKGROUND

Tissue factor (TF) is expressed widely at the subluminal surface of blood vessels and serves as the primary cellular initiator of the extrinsic pathway of blood coagulation. Lack of TF in mice resulted in lethality in utero, but human TF (huTF) expressed at low levels from a human minigene rescued null mice from prenatal death. Although these low-TF expressing transgenic mice developed to term, they had a significantly shorter life span and exhibited hemorrhage and fibrosis in the heart.

METHODS

Human TF knock-in (TFKI) mice were generated by replacing the first two exons of the mouse (murine) TF (muTF) gene with the huTF complete coding sequence, thus placing it under the control of the endogenous muTF promoter.

RESULTS

Expression of huTF in the TFKI mice was similar to muTF in wild-type (wt) mice. The TFKI mice showed no microscopic evidence of spontaneous hemorrhage in the heart, nor cardiac fibrosis at up to 18 months of age. Immunohistochemistry showed that huTF was expressed in cells surrounding blood vessels in TFKI mice. Coagulation activity of brain homogenates from TFKI mice was comparable with that from wt brain. Cardiac hemorrhage similar to that of the low-TF transgenic mice occurred in the TFKI mice when huTF was blocked by a neutralizing anti-huTF monoclonal antibody.

CONCLUSION

We generated a transgenic mouse line that expresses huTF under the control of the endogenous muTF promoter at physiological levels. Our results suggest that huTF can fully reconstitute the murine coagulation system and mediate normal hemostasis.

7E3 F(ab')2, an effective antagonist of rat alphaIIbbeta3 and alphavbeta3, blocks in vivo thrombus formation and in vitro angiogenesis

Abciximab (c7E3 Fab, ReoPro) blocks GPIIb/IIIa and alphavbeta3 and inhibits thrombotic and proliferative events only in humans and non-human primates. The bivalent F(ab')2 fragment is an effective anti-thrombotic agent in canine models. In the present study, 7E3 F(ab')2 was also found to bind to rat GPIIb/IIIa (KD = 27 +/- 4 microg/mL) and alphavbeta3 (KD = 9 +/- 8 microg/mL), to block in vitro rat platelet aggregation (IC50 = 16 +/- 6 microg/mL), and to inhibit alphavbeta3-mediated microvessel sprout formation in a rat aortic ring assay. Following administration of 7E3 F(ab')2 (4 mg/kg) to rats, platelet aggregation was completely blocked for up to 6 h and thrombus formation in response to a rat abdominal aorta double crush injury was prevented. Effective chronic dosing was achieved with 6 mg/kg daily I.P. injections. In vitro mixing experiments indicated that 7E3 F(ab')2 redistributed to unlabeled platelets in 2 h. Ex vivo, 7E3 F(ab')2 was detected on platelets for up to 4 days after a single 4-mg/kg injection. These data suggest that 7E3 F(ab')2 may be a useful agent to study the effects of GPIIb/IIIa and alphavbeta3 blockade in rat models of thrombosis and vascular disease.

The nitric oxide system and cortisol-induced hypertension in humans

1. The aim of the present study was to assess the role of the nitric oxide (NO) system in cortisol-induced hypertension in humans. 2. Plasma and urinary nitrate/nitrite concentrations and plasma concentrations of arginine and symmetric (SDMA) and asymmetric (ADMA) dimethyl arginine were measured in six subjects on a restricted nitrate diet who were treated with 80 mg/day cortisol and in subjects on an unrestricted nitrate diet who were treated with cortisol (80 mg/day, n = 6, or 200 mg/day, n = 10) for 5 days. 3. Cortisol significantly increased systolic and mean arterial pressure. Significant reductions in plasma nitrate/nitrite concentrations were observed in subjects on a restricted nitrate diet on days 3, 4 and 5 of cortisol treatment (to 11 +/- 1, 10 +/- 1, 11 +/- 1 pmol/L, respectively) compared with pretreatment (16 +/- 1 pmol/L; P < 0.01). There were no significant changes in plasma arginine, ADMA or SDMA concentrations. 4. Cortisol treatment significantly increased blood pressure and reduced plasma nitrate/nitrite concentrations. Reductions in plasma nitrate concentrations are not explained by changes in substrate availability or in endogenous nitric oxide synthase inhibitors. These data support a role for the NO system in cortisol-induced hypertension in humans.

Abciximab (ReoPro, chimeric 7E3 Fab) demonstrates equivalent affinity and functional blockade of glycoprotein IIb/IIIa and alpha(v)beta3 integrins

BACKGROUND

Large, randomized, and blinded clinical trials (EPIC, EPILOG, and CAPTURE) have demonstrated that abciximab (ReoPro, chimeric 7E3 Fab) markedly reduces thrombotic events associated with percutaneous transluminal coronary interventions. The marked early benefits at 30 days were sustained at 6 months and 3 years. Initially developed because of its efficacy in blocking GP IIb/IIIa (alphaIIb/beta3) receptors on platelets, abciximab also binds with equivalent affinity to alpha(v)beta3.

METHODS AND RESULTS

This study presents a detailed characterization of the alphavss3 interaction, including the ability of abciximab to (1) bind with comparable affinity to alpha(v)beta3 and GP IIb/IIIa, (2) inhibit alpha(v)beta3 and GP IIb/IIIa-mediated cell adhesion in vitro with IC50 values approximating binding KD values, and (3) redistribute between GP IIb/IIIa and alpha(v)beta3 integrins in vitro.

CONCLUSIONS

As an antagonist of not only GP IIb/IIIa but also alpha(v)beta3, abciximab may provide additional clinical benefit in preventing alpha(v)beta3-mediated effects such as thrombin generation, clot retraction, or smooth muscle cell migration and proliferation. Abciximab binds with equivalent affinity to both GP IIb/IIIa and alphavss3 and redistributes between the 2 integrin receptors in vitro. Abciximab has been previously shown to circulate on platelets for up to 2 weeks. Taken together, these findings suggest that abciximab may have the ability to inhibit both GP IIb/IIIa and alpha(v)beta3 for extended periods.

Neutralization of TNF by the antibody cA2 reveals differential regulation of adhesion molecule expression on TNF-activated endothelial cells

Upregulation of adhesion proteins plays an important role in mediating inflammation. The induction of adhesive molecules has been well studied, but the reversibility of their expression has not been well characterized. A neutralizing anti-TNF monoclonal antibody (cA2) was used to study the down regulation of TNF-induced E-selectin, vascular cell adhesion molecule-1 (VCAM-1) and intercellular adhesion molecule-1 (ICAM-1) on cultured human umbilical vein endothelial cells (HUVECs). Addition of cA2 following TNF stimulation of HUVECs enhanced the rate of E-selectin and VCAM-1 down-regulation from the cell surface and also reduced steady state E-selectin and VCAM-1 mRNA levels. The cA2-mediated disappearance of E-selectin, but not VCAM-1 protein was microtubule and not microfilament dependent. Neutralization of TNF only slightly reduced ICAM-1 cell surface levels following initial TNF stimulation, suggesting a slower turnover of ICAM-1 compared to E-selectin and VCAM-1. Microtubule inhibition during TNF stimulation partially inhibited E-selectin, VCAM-1 and ICAM-1 mRNA upregulation. VCAM-1 and ICAM-1 cell surface expression were similarly partially inhibited, however, E-selectin levels were unaffected, presumably due to the dual, opposing effect of inhibiting protein expression and inhibiting internalization. Microfilament inhibition during protein induction specifically inhibited the maximal expression of VCAM-1 protein and mRNA, without affecting E-selectin or ICAM-1. These data support the notion that E-selectin, VCAM-1, and ICAM-1 expression are differentially regulated on HUVECs and suggest that TNF neutralizing therapies may be effective because of their ability to reduce the levels of pre-existing adhesion proteins.

Effects of diet on measurement of nitric oxide metabolites

1. The present study investigated whether a low nitrate/nitrite diet could minimize variability in the measurement of endogenous plasma and urine nitric oxide (NO) metabolites, nitrate and nitrite (NOx) in normal subjects. 2. Nitrate and nitrite concentrations were measured in plasma and urine as indicators of NO production in six subjects during a free diet and then during a low nitrate/nitrite diet for 6 days. 3. The plasma concentration and 24 h urine NOx/creatinine ratio were significantly lower on the low nitrate/nitrite diet than on the free diet (P < 0.01). Nitric oxide production appeared to vary greatly within and between subjects, but these variations were substantially decreased by the fourth day of a low nitrate/nitrite diet. 4. Human plasma and urine NOx measurements should be determined after a low nitrate/nitrite diet for at least 4 days.

Reflex sympathetic function in cortisol-induced hypertension in humans

Nine healthy male subjects underwent measurement of reflex sympathetic function, pressor responsiveness and baroreflex sensitivity to phenylephrine (PE) and glyceryltrinitrate (GTN) before (C1) and following six days of treatment (E6) with cortisol (F), 200 mg/day. Seven subjects had washout studies (W) performed at least two weeks following the end of treatment. The BP responses to head tilt, isometric exercise and mental arithmetic were unaltered by F, however, there was a significant diminution of the diastolic BP response to cold pressor stimulus (delta DBP: 19 +/- 3 vs 25 +/- 5 vs 27 +/- 5 mmHg; E6 vs C1 vs W, p < 0.05 C1 vs E6 and W). Baroreflex sensitivity to PE was increased (28 +/- 3 vs 19 +/- 2 ms/mmHg, E6 vs C1, p = 0.03). These data demonstrate that increased BP during F treatment is not attributable to increased SNS activity, and suggest that SNS activity may be decreased by F.

The role of the hypothalamic-pituitary-adrenal (HPA) axis in the regulation of blood pressure

The role of the HPA axis in blood pressure regulation was examined in 6 normal male volunteers by comparing haemodynamic and hormonal effects of placebo, captopril, and dexamethasone given in random order for two days. The average 24-hour systolic and mean arterial pressures on placebo (135 +/- 6 and 93 +/- 2 mmHg respectively) were significantly higher than on captopril (118 +/- 1 and 85 +/- 1 mmHg respectively, p < 0.05) but there were no significant changes on dexamethasone compared with placebo (128 +/- 3 and 89 +/- 3 mmHg respectively). There were no differences in the average 24-hour diastolic blood pressures or heart rates, nor the day-night differences, night:day ratios or percentage changes in blood pressure and heart rate between treatments. Captopril significantly increased active plasma renin concentration, whilst dexamethasone decreased cortisol concentration. These results confirm the role of the renin-angiotensin system in the regulation of blood pressure in normal subjects but suggest that the HPA axis does not play a major role in determining ambulatory blood pressure or day-night variability in the short term.

Autonomic blockade amplifies cortisol-induced hypertension in man

1. We investigated the role of the autonomic nervous system (ANS) in cortisol induced hypertension using the technique of total autonomic blockade (AB). 2. Four healthy young males were given 50 mg cortisol 6 hourly for 6 days. On the day prior to, and the last day of, cortisol treatment, AB was produced using oral prazosin 1 mg, intravenous clonidine 300 micrograms, propranolol 0.2 mg/kg and atropine 2 mg. The adequacy of blockade was assessed using the haemodynamic response to Valsalva manoeuvre. 3. Cortisol produced a significant rise in systolic blood pressure (130 +/- 2 vs 110 +/- 1 mmHg, pre vs post cortisol; P < 0.01). On the final treatment day, AB augmented the increase in diastolic blood pressure (delta DBP), mean arterial pressure (delta MAP) and heart rate (delta HR) compared to the pretreatment day, delta DBP: 43 +/- 6 vs 17 +/- 4 mmHg, post vs pre cortisol, P < 0.005, delta MAP: 39 +/- 4 vs 14 +/- 4 mmHg, P < 0.001, delta HR: 45 +/- 5 vs 26 +/- 4 b.p.m., P < 0.05. The change in systolic blood pressure (delta SBP) was not statistically significant (32 +/- 4 vs 7 +/- 3 mmHg, P = 0.065). 4. These results suggest that the ANS exerts a modulating influence on the hypertensive effect of cortisol.

Structure-function studies on synthetic peptides derived from the 109-118 lectin domain of selectins

Previously, we identified several peptides corresponding to amino acid sequences within the lectin domains of selectins that inhibit neutrophil (PMN) adhesion to P-selectin. Here we focused on one of the active regions, 109-118, which contains residues that have been identified as critical for E-selectin binding to the sialyl Lexis X (sLex) counter receptor. Analogues were synthesized and examined for their inhibitory effect on PMN binding to P-selectin and E-selectin immunoglobulin fusion proteins (P-IgG, E-IgG) and also on P-IgG and E-IgG binding to sLex coated surfaces. Peptide sequences which inhibited PMN binding to the fusion proteins were not necessarily those that inhibited fusion protein binding to sLex. In addition, various amino acid substitutions could be tolerated at the 111 and 113 positions without altering inhibitory activity. Modeling suggests that structural conformations of peptide analogues could explain the differences in biological activity of peptide analogues compared to mutants of the native protein.

Determination of the core sequence of an antagonist of selectin-dependent leukocyte adhesion and correlation of its structure with molecular modeling studies

The sequence 36-50 from the lectin domain of human P-selectin has been previously identified as a weak inhibitor of selectin-dependent leukocyte adhesion. A series of C- and N-terminally truncated peptides was synthesized to determine the limits of the active core region within the parent sequence. Deletions from both the N- and C-termini gave significant increases in inhibitory activity and identified 41-50 or 36-49 as minimum active sequences, but surprisingly not the common 41-49 peptide. All peptides tested showed parallel inhibition of both P- and E-selectin-dependent adhesion. A molecular model of the lectin domain was constructed using homology modeling. Examination of this model suggests one hypothesis to explain the increase in activity on deletion of Asp36.

Antibiotics enhance binding by human lipid A-reactive monoclonal antibody HA-1A to smooth gram-negative bacteria

The effect of antibiotic exposure of phenotypically smooth gram-negative bacteria on binding by the human lipid A-reactive monoclonal antibody HA-1A (trademark of Centocor, Inc.) was examined by liquid-phase immunoassay and by dual-parameter flow cytometry (fluorescence-activated cell sorter [FACS]) analysis. HA-1A exhibited dose-dependent binding to untreated rough gram-negative bacteria such as the Escherichia coli D21F2 Re chemotype strain but little binding to untreated smooth strains such as E. coli O111:B4, or to gram-positive bacteria. However, overnight incubation of E. coli O111:B4 with inhibitory concentrations of ceftazidime produced dose-dependent enhancement of HA-1A binding. Similar augmentation of HA-1A binding was observed when other smooth strains were exposed to cell wall-active agents. Dual-parameter FACS analysis of E. coli O111:B4 exposed overnight to two times the MIC of ceftazidime revealed a decrease in forward light scatter, indicating a reduction in average cell size or bacterial fragmentation, accompanied by a striking increase in lipid A-inhibitable HA-1A binding. Moreover, ceftriaxone, but not gentamicin, produced a marked increase in propidium iodide uptake, indicating an increase in bacterial cell permeability, and a corresponding enhancement of HA-1A binding. Antibiotic-induced enhancement of HA-1A binding to smooth strains of gram-negative bacteria thus appears related to specific alterations in bacterial cell morphology resulting in exposure of the epitope recognized by HA-1A.

Labeling and stability of radiolabeled antibody fragments by a direct 99mTc-labeling method

The in vitro labeling and stability of 99mTc-labeled antibody Fab' fragments prepared by a direct labeling technique were evaluated. Eight antibody fragments derived from murine IgG1 (N = 5), IgG2a (N = 2) and IgG3 (N = 1) isotypes were labeled with a preformed 99mTc-D-glucarate complex. No loss of radioactivity incorporation was observed for all the 99mTc-labeled antibody fragments after 24 h incubation at 37 degrees C. The 99mTc-labeled antibody fragments (IgG1, N = 2; IgG2a, N = 2; IgG3, N = 1) were stable upon challenge with DTPA, EDTA or acidic pH. Furthermore, using the affinity chromatography technique, two of the 99mTc-labeled antibody fragments displayed no loss of immunoreactivity after prolonged incubation in phosphate buffer up to 24 h at 37 degrees C. The bonding between 99mTc and antibody fragments was elucidated by challenging with a diamide ditholate (N2S2) compound. The Fab' with IgG2a isotype displayed tighter binding to 99mTc in comparison to the Fab' from IgG1 and IgG3 isotype in N2S2 challenge and incubation with human plasma. The in vivo biodistribution of five 99mTc-labeled fragments were evaluated in normal mice. In conclusion, the direct labeling method allows stable 99mTc labeling of antibody fragments from three of the major murine isotypes.

Evaluation of the 323/A3 monoclonal antibody and the use of technetium-99m-labeled 323/A3 Fab' for the detection of pan adenocarcinoma

The 323/A3 murine monoclonal antibody, initially described as reactive to breast carcinomas, is found by immunohistological analyses to have broad cross reactivity with adenocarcinomas of diverse histologic origin. The 323/A3 antigen is similar to the tumor-associated 17-1A antigen as revealed by immunoblot and cross-competition cell binding studies. We have investigated the potential use of the 323/A3 monoclonal antibody for tumor imaging as a Fab' molecule labeled with 99mTc. In vitro studies demonstrate that 323/A3 Fab' has high affinity (2-3 x 10(9) M-1) with no significant loss of immunoreactivity compared to the intact IgG. In vivo studies demonstrate that 99mTc 323/A3 Fab' can rapidly detect human breast and colon tumor xenografts growing in athymic nude mice. Distinct breast tumor visualization is observed as early as 1 h post intravenous administration with the 99mTc 323/A3 Fab'. Distinct colon tumor visualization is observed by 3 h (the earliest time point imaged). Tumor-to-blood ratios are higher for 99mTc 323/A3 Fab' than with a 99mTc-labeled nonspecific isotype-matched Fab' antibody. These results suggest that 99mTc 323/A3 Fab' can detect 17-1A antigen and may have potential clinical utility for the rapid diagnostic imaging of adenocarcinomas.

Expression in Escherichia coli of open reading frame gene segments of HTLV-III

Human T-cell lymphotropic virus type III (HTLV-III), the causative agent of the acquired immune deficiency syndrome (AIDS), was recently isolated and its genomic structure analyzed by DNA cloning methods. In the studies reported here a combined cloning and expression system was used to identify HTLV-III encoded peptides that react immunologically with antibodies in sera from AIDS patients. Cloned HTLV-III DNA was sheared into approximately 500-base-pair fragments and inserted into an "open reading frame" expression vector, pMR100. The inserted DNA was expressed in Escherichia coli transformants as a polypeptide fused to the lambda CI protein at its amino terminus and to beta-galactosidase at its carboxyl terminus. Sera from AIDS patients containing antibodies to HTLV-III were then used to screen for immunoreactive fusion proteins. Twenty clones, each specifying a fusion protein strongly reactive with AIDS serum, were identified. DNA sequence analysis indicated that the HTLV-III fragments were derived from the open reading frame DNA segments corresponding to the gag and pol gene coding regions and also the large open reading frame region (env-lor) located near the 3' end of the viral genome.

A cDNA clone encoding a product of activated human T lymphocytes

A cDNA cloning approach was used to study the regulation of gene expression in human T lymphocytes upon mitogen stimulation. Poly(A)+ mRNA was prepared from phytohemagglutinin A (PHA) and 12-O-tetradecanoyl phorbol 13-acetate (TPA) activated human T cells and a cDNA clone library was constructed. After screening by colony hybridization with [32P]cDNA probes made from resting and activated T cell mRNA, several clones whose mRNA increased at least 10 to 20-fold upon stimulation were isolated. Northern blot analysis of the mRNA from various cell types using these cDNA clones as probes revealed that one of the cDNA clones, pNC5A, encoded a gene expressed only in PHA and TPA-stimulated human T lymphocytes and in a human neoplastic T cell line HUT102-SH9. Less than 20 copies of this mRNA species per cell was detected in resting human T lymphocytes, B lymphocytes and monocytes and in two other T cell lymphoma lines (CEM and MOLT4), two B lymphoblastoid cell lines (WIL2-729-HF2 and HFB-1), a myeloid cell line (HL60) and a human embryonic lung fibroblast cell line (MRC-5). Hybrid selection translation and sodium dodecyl sulfate/polyacrylamide gel electrophoretic analysis of the translated product indicated that a polypeptide of 30,000 to 32,000 Mr is encoded by this particular cDNA clone. Thus, this cDNA clone may define a novel gene that is expressed only in activated human T cells.