Enamel matrix protein turnover during amelogenesis: Basic biochemical properties of short-lived sulfated enamel proteins

The formation and turnover of sulfated enamel proteins was investigated by SDS-PAGE, fluorography, and TCA-precipitations using freeze-dried incisors of rats injected intravenously with ³⁵S-sulfate (³⁵SO₄) and processed at various intervals from 1.6 minutes to 4 hours thereafter. Some rats were injected first with ³⁵SO₄ followed 5 minutes later by 0.3 mg of cycloheximide. This was done to terminate protein translation and allow events related to extracellular processing and degradation of the sulfated enamel proteins to be visualized more distinctly. Other rats were injected with cycloheximide followed at 0 minutes (simultaneous injection) to 30 minutes later by ³⁵SO₄. This was done to characterize the time required for proteins to travel from endoplasmic reticuium to Golgi apparatus, where they became sulfated. The results indicated that enamel organ cells (ameloblasts) rapidly incorporated ³⁵SO₄ into a major 65 kDa protein that was secreted into the enamel within 6–7.5 minutes. This parent protein appeared to be processed extracellularly within 15 minutes into major 49 kDa and 25 kDa fragments which themselves had apparent half-lives of about 1 and 2 hours, respectively. There were also many minor sulfated fragments varying in molecular weight (Mr) from 13–42 kDa, which appeared to originate from extracellular processing and/or degradation of the parent 65 kDa sulfated enamel protein or its major 49 kDa and 25 kDa fragments. Experiments with glycosidases further suggested that the majority of sulfate groups were attached to sugars N-linked by asparagine to the core of the 65 kDa sulfated enamel protein. The sulfated enamel proteins resemble acidic glycoproteins previously classified as enamelins. Unlike the enamelins, however, they are short-lived and do not appear to survive in enamel as it matures.     

Human heparanase: roles in invasion and metastasis of cancer

Heparanase, which is an extracellular matrix degradative enzyme, degrades heparan sulfate and heparan sulfate proteoglycans, which are chief components of extracellular matrix and vascular basement membrane. The gene structure of this enzyme was recently determined. The biological functions of this enzyme in vivo were as follows: 1) this enzyme accelerates cancer cell invasion and metastasis though the degradation of vascular basement membrane and extracellular matrix by cancer cells; 2) this enzyme releases and activates heparin-binding growth factors such as bFGF and VEGF from heparan sulfate proteoglycans, and induces angiogenesis; 3) the degradative products of heparan sulfate proteoglycans by this enzyme suppress the biological function of activated T-lymphocytes. Therefore, heparanase is thought to be a favorable molecule for acceleration of cancer invasion and metastasis. The expression of heparanase is strongly correlated with the metastasis of melanoma and fibrosarcoma. Thus, heparanase may play important roles in invasion and metastasis of cancer.     

Heat shock protein 27 overexpression in breast cancer lymph node metastasis

Background: Heat shock protein 27 (hsp-27) is overexpressed in 67% pure ductal carcinoma in situ (DCIS), in 50% DCIS associated with invasive ductal carcinoma (IDC), and in 25% IDC alone. If this decrease in hsp-27 expression has a role in the progression of malignancy in IDC, we postulate a further reduction in expression in nodal metastasis. Methods: To test this hypothesis, we evaluated the distribution of hsp-27 in primary IDC and in synchronous regional lymph node metastasis within the same patient by immunohistochemistry. Results: Nine of 30 primary IDCs (30%) and 22 of 30 lymph node metastases (73%) overexpressed hsp-27. Contrary to our hypothesis, of 21 IDCs with no or low hsp-27 expression, 13 (62%) had overexpression of this protein within nodal metastasis. Conclusions: hsp-27 appears to confer cytoprotection for metastatic cells, which may help explain why hsp-27 overexpression is associated with reduced disease-free survival in breast carcinomas.     

乙酰肝素酶在恶性肿瘤中的研究进展

乙酰肝素酶(HPSE)是目前在哺乳动物中发现的可以降解硫酸乙酰肝素蛋白聚糖(heparan sulfate proteoglycans,HSPGs)上的硫酸肝素(HS)糖链的一类β-D糖苷内切酶.其在肿瘤侵袭转移中起着关键作用.本文就HPSE分子结构、生物学特征、在恶性肿瘤中的表达、促进肿瘤转移机制及HPSE抑制剂的应...     

The effect of verapamil on halothane-epinephrine or digitalis-induced ventricular dysrhythmias in dogs

The effect of verapamil on ventricular dysrhythmias was evaluated using two canine models. In one model, ventricular dysrhythmias were induced by 1% halothane-epinephrine (1.5 30µg/kg/min.) in 20 dogs (Group I). In the other model, ventricular dysrhythmias were induced by digoxin (0.1 0.2mg/kg) in 27 dogs (Group II). Verapamil (0.2 0.5mg/kg) was given to treat these ventricular dysrhythmias. When verapamil was ineffective, lidocaine (1 2mg/kg) was given following the administration of verapamil. In 7 dogs of group II, lidocaine alone was given. Verapamil was effective in 16 animals of group I, and in 10 animals of group II. Lidocaine was ineffective in the remaining 4 of group I, whereas effective in the remaining 17, including those given lidocaine alone of group II. From these findings, it was inferred that Ca²⁺ dependent abnormal automaticity and/or re-entry may be more closely related to the genesis of halothane-epinephrine-induced ventricular dysrhythmias refractory to lidocaine, whereas triggered activity may be more closely related to that of digitalis-induced ventricular dysrhythmias. In conclusion, verapamil was more effective against halothane-epinephrine-induced ventricular dysrhythmias than against digitalis-induced ventricular dysrhythmias.(Yoshizawa Y, Shimizu R, Kasuda H et al.: The effect of verapamil on halothan-epinephrine or digitalis-induced ventricular dysrhythmia in dogs. J Anesth 2: 28–35, 1988)     

Die klinische Relevanz der Sonographie in der Akutdiagnostik perforierter Gastroduodenalulcera

Zusammenfassung Anhand einer prospektiven, auslesefreien Serie von 22 Patienten mit perforierten Ulcera im Gastroduodenalbereich verweisen wir auf die begrenzte diagnostische Aussagekraft einfacher anamnestischer, klinischer und radiologischer Daten. Lediglich bei acht Patienten (36%) ergaben sich wesentliche Indizien aus Krankengeschichte und Aufnahmebefund. Bei 16 Patienten (73%) verriet die Thoraxübersichtsaufnahme durch Nachweis einer subphrenischen Luftsichel eine Perforation im Magen-Darm-Trakt. Die Ultraschalluntersuchung wird als bedeutungsvolle Ergänzungsmethode vor allem in der diagnostischen Abklärung radiologisch negativer Ulcusperforationen vorgestellt und diskutiert. Dabei werden die sonographisch faßbare flüssigkeitsbedingte Magendistension sowie das Magenwandödem als unspezifische Kriterien gewertet. Der Objektivierung einer pathologischen Magenkokarde kommt bei gleichzeitigem Nachweis freier Luft und/oder extraluminärer Ingesta sowie freier echoloser Transsudatflüssigkeit pathognomonische Beweiskraft zu. Selbige Ultraschallkriterien erlaubten im einschlägigen Krankengut bei 16 Patienten (73%) eine definitive Diagnose und erfaßte 4 Patienten mit negativem Röntgen. Die kombinierte Analyse radiologischer und sonographischer Befunde führte somit bei 20 Patienten (91 %) unverzüglich zur korrekten Diagnose.

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Perforated peptic ulcer: Diagnosis and assessment by sonography

Summary In a prospective unselected series of 22 patients with perforated gastroduodenal ulcers the diagnostic efficacy of clinical and radiologic data was modest. In eight patients only (36%), clinical data yielded sufficient evidence; in 16 patients (73%), plain X-ray demonstrated subphrenic gas. Sonography was proven to be a major advance, especially rewarding in the diagnosis of perforations with negative plain X-ray. Gastric distention and stomach wall edema are unspecifc sonographic criteria, whereas objectivation of a pathologic stomach cockade in the presence of free gas, extraluminary ingesta or echofree fluid in the peritoneal cavity are pathognomonic data. These criteria yielded a definite diagnosis in 16 patients (73%) including four patients with negative X-ray. The combined analysis of radiologic and sonographic findings yielded an immediate correct diagnosis in 20 patients (91%).

     

Magnetic resonance imaging of trabecular bone structure in the distal radius: Relationship with X-ray tomographic microscopy and biomechanics

The contribution of trabecular bone structure to bone strength is of considerable interest in the study of osteoporosis and other disorders characterized by changes in the skeletal system. Magnetic resonance (MR) imaging of trabecular bone has emerged as a promising technique for assessing trabecular bone structure. In this in vitro study we compare the measures of trabecular structure obtained using MR imaging and higher-resolution X-ray tomographic microscopy (XTM) imaging of cubes from human distal radii. The XTM image resolution is similar to that obtained from histomorphometric sections (18 µm isotropic), while the MR images are obtained at a resolution comparable to that achievable in vivo (156×156×300 µm). Standard histomorphometric measures, such as trabecular bone area fraction (synonymous with BV/TV), trabecular width, trabecular spacing and trabecular number, texture-related measures and three-dimensional connectivity (first Betti number/volume) of the trabecular network have been derived from these images. The variation in these parameters as a function of resolution, and the relationship between the structural parameters, bone mineral density and the elastic modulus are also examined. In MR images, because the resolution is comparable to the trabecular dimensions, partial volume effects occur, which complicate the segmentation of the image into bone and marrow phases. Using a standardized thresholding criterion for all images we find that there is an overestimation of trabecular bone area fraction (3 times), trabecular width (3 times), fractal dimension (1.4 times) and first Betti number/ volume (10 times), and an underestimation of trabecular spacing (1.6 times) in the MR images compared with the 18-µm XTM images. However, even for a factor of 9 difference in spatial resolution, the differences in the morphological trabecular structure measures ranged from a factor of 1.4 to 3.0. We have found that trabecular width, area fraction, number, fractal dimension and Betti number/volume measured from the XTM and MR images increases, while trabecular spacing decreases, as the bone mineral density and elastic modulus increase. A preliminary bivariate analysis showed that in addition to bone mineral density alone, the Betti number, trabecular number and spacing contributed to the prediction of the elastic modulus. This preliminary study indicates that measures of trabecular bone structure using MR imaging may play a role in the study of osteoporosis.     

Early attachment of platelets,leukocytes, and fibrinogen in endothelial cell seeded dacron grafts

Endothelial cell seeding has been advocated as a method for reducing the thrombogenicity of prosthetic grafts. Principally two different techniques for endothelial cell seeding can be used: immediate seeding of grafts followed by implantation or initial growth and establishment of an endothelial cell-covered surface before subsequent late implantation. This study was designed to determine whether the immediate seeding technique altered thrombogenicity directly after graft implantation. Carotid arteries from 19 sheep were replaced with Dacron interposition grafts; one side was seeded with endothelial cells and the other side was left-unseeded. The dynamics of thrombus formation involving radiolabeled platelets, leukocytes, and fibrinogen were studied for 4 hours with flow reduced to 35 ml/min. No difference in platelet uptake (6-fold increase compared to baseline values) was found between endothelial cell seeded and unseeded grafts. Likewise, there were no differences in leukocyte uptake (4-fold increase) or fibrinogen uptake (10- to 15-fold increase) between the two groups. No differences were demonstrated with regard to patency or thrombus weight. In this experimental investigation we were unable to verify any change in the uptake of platelets, white blood cells, or fibrinogen between endothelial cell seeded and unseeded Dacron grafts during the first 4 hours after graft placement. Immediate seeding does not affect the initial thrombogenicity of grafts.Supported by grants from the Swedish Heart and Lung foundation, the Swedish Medical Research Council (No. 00759), the Swedish Medical Society, the Faculty of Medicine, Lund University, and Malmö University Hospital.     

July 2003

The effect of the Prandtl number (Pr) and the Reynolds number (Re) on the behaviour of weak laminar axisymmetric and plane fountains has been studied using dimensional and scaling analyses and direct numerical simulation. For Fr 1.0 and assuming viscous effects are important, the analysis shows that for both the axisymmetric and plane fountains, y_mFrRe^–1/2, where Fr is the Froude number defined at the fountain source and y_m is the non-dimensionalized fountain height. These scalings are also valid for the non-dimensionalized fountain width. The analyses also shows _msFr², where _ms is the non-dimensionalized time scale for the fountain flow in the fountain core to reach steady state, and using this time scale y_TFr(RePr)^–1/2, where y_T is the non-dimensionalized thickness of the temperature layer on the symmetry axis over which the fountain fluid temperature changes from the inlet value to that of the ambient fluid. All these scalings have been quantified by the direct numerical simulations, hence confirming in certain ranges the phenomenological scaling obtained in this paper. The financial supports from the National Natural Science Foundation (grant numbers 19872059 and 10262003) and Yunnan Province of the People's Republic of China to W. Lin and the Australian Research Council are gratefully acknowledged.     

Reduction of anionic sites in the glomerular basement membrane by heparanase does not lead to proteinuria

Heparan sulfate in the glomerular basement membrane has been considered crucial for charge-selective filtration. In many proteinuric diseases, increased glomerular expression of heparanase is associated with decreased heparan sulfate. Here, we used mice overexpressing heparanase and evaluated the expression of different heparan sulfate domains in the kidney and other tissues measured with anti-heparan sulfate antibodies. Glycosaminoglycan-associated anionic sites were visualized by the cationic dye cupromeronic blue. Transgenic mice showed a differential loss of heparan sulfate domains in several tissues. An unmodified and a sulfated heparan sulfate domain resisted heparanase action in vivo and in vitro. Glycosaminoglycan-associated anionic sites were reduced about fivefold in the glomerular basement membrane of transgenic mice, whereas glomerular ultrastructure and renal function remained normal. Heparanase-resistant heparan sulfate domains may represent remnant chains or chains not susceptible to cleavage. Importantly, the strong reduction of glycosaminoglycan-associated anionic sites in the glomerular basement membrane without development of a clear renal phenotype questions the primary role of heparan sulfate in charge-selective filtration. We cannot, however, exclude that overexpression of heparanase and heparan sulfate loss in the basement membrane in glomerular diseases contributes to proteinuria.     

Duration of action of supplemental doses of vecuronium is related to the duration after the initial dose

Vecuronium was administered in an initial dose of 0.1 0.3mg·kg^–1 and in supplemental doses of 0.03mg·kg^–1 or 0.05mg·kg^–1 in 74 patients (ASA class 1 or 2) scheduled for abdominal surgery. The duration of the neuromuscular blockade provided by vecuronium after both the initial and supplemental doses was determined using the evoked integrated electromyographic device. A statistically significant positive correlation (correlation coefficient: 0.83 0.91) was found between the duration of action of the initial dose and that of the first to fourth supplemental doses.The regression lines of each of the first four supplemental doses to the initial dose were very similar to each other. These results suggest that, since the duration of action of supplemental doses of vecuronium was prolonged in patients showing a long duration of action of the initial dose, it would be wise to avoid blind adherence to a predetermined schedule for supplemental administration. Rather, anesthesiologists should take into account the patients response to the initial dose and then decide the most appropriate timing for supplemental doses. Moreover, since vecuronium shows little cumulative effect even after 4 supplemental administrations in clinical-range doses, it can be concluded that vecuronium can be safely used in a wide dose range.(Otagiri T, Narita M, Nishizawa M, et al.: Duration of action of supplemental doses of vecuronium is related to the duration after the initial dose. J Anesth 6: 138–144, 1992)     

Distribution of hsp-27 and HER-2/neu in in situ and invasive ductal breast carcinomas

Background: The overexpression of heat shock protein 27 (hsp-27) in early-stage breast cancer is associated with histopathologic features of poor prognosis and clinically with an increased probability of disease recurrence. Hsp-27 is overexpressed in 25% of invasive ductal carcinomas (IDC); however, its distribution in ductal carcinoma in situ (DCIS) and DCIS associated with IDC has not been investigated. We postulated that hsp-27 might be detected and variably expressed in DCIS and, like HER-2/neu oncoprotein expression, might be a tumor-specific marker worthy of future clinical investigation. Methods: To test these hypotheses, the distribution of hsp-27 in noncomedo and comedo DCIS, and DCIS associated with IDC, was evaluated by immunohistochemistry and compared with HER-2/neu expression within the same cancers. Results: Hsp-27 was overexpressed in 28 of 47 (60%) cases of DCIS; expression in pure DCIS was 16 of 24 (67%), and 12 of 23 (50%) in DCIS associated with IDC. Hsp-27 expression by in situ and invasive components of the same neoplasm were concordant in 22 of 23 (95%) cases tested. Comedo variants appeared to have somewhat higher hsp-27 expression than noncomedo DCIS, whether or not there was an associated IDC. These results are reminiscent of HER-2/neu oncoprotein expression in DCIS and DCIS associated with IDC observed by others. However, although 4 of 22 (18%) cancers containing DCIS + IDC expressed HER-2/neu, no relationship with hsp-27 expression in the same cancers was observed. Conclusions: We found a high incidence of hsp-27 overexpression in DCIS and in DCIS associated with IDC. This rate is twice that previously observed in IDC alone. Hsp-27 expression is independent of HER-2/neu expression.     

Transforming growth factor-β and prostate cancer

Summary This review focuses on the possible role of transforming growth factor- isoforms 1–3 (TGF) in prostate cancer. TGF1 appears to inhibit the cellular proliferation of normal prostate cells. Surprisingly, TGF1 is overexpressed in prostate cancer. To help explain this apparent paradox, it has been revealed that with tumor progression, prostate cancer cells acquire reduced sensitivity to the growth-inhibitory effects of TGF1. Aberrations of the TGF1 signaling pathway at the prereceptor, receptor, or postreceptor level may lead to prostate cancer cell resistance to TGF1 growth inhibition. Indirectly, elevated levels of TGF1 may induce host effects that may be beneficial to prostate tumor growth by suppressing the immune system, promoting angiogenesis and extracellular matrix formation, and enhancing metastatic potential. Consequently, TGF1 appears to be important in prostate carcinogenesis and tumorigenicity. TGF2 and TGF3 are only briefly presented as very little is known about their role in prostate cancer.     

An immunohistochemical study of glucagonoma conducted on the metastatic lymph nodes from a patient with recurrent metastatic glucagonoma: Report of a case

In this report, we briefly present the case of a 67-year-old woman who developed recurrent glucagonoma with lymph node metastasis. An immunohistochemical study of the metastatic tumor revealed immunoreactivity of glucagon and protein kinase C (PKC)-, -, and - in the tumor cells, two types of which were seen by electron microscopy. One type had abundant secretory granules and mitochondria, while the other had few granules and mitochondria. Some granules were similar to typical A cell granules and others were atypical. An immunoelectron microscopic demonstration revealed PKC-, -, and - immunostaining in the cytoplasm of all the tumor cells, while some secretory granules had PKC immunostaining, and others had no immunostaining. Thus, it appears that metastatic glucagonoma and its associated granules are composed of two types of mature and immature cells or granules. As immunoreactivity of PKC- and - was found in the tumor cells, but not in the normal A cells of the islets of Langerhans, the PKC subspecies and , which are not present in normal pancreatic A cells, may exist in human glucagonoma cells.     

Cyclins and breast cancer

Recent advances in the understanding of cell cycle control by cyclins and cyclin-dependent kinases provide a basis for delineating the molecular mechanisms of proliferation control by steroids and the development and progression of hormone-dependent cancers. Cyclin D1 is necessary, rate-limiting and sufficient for G₁ progression in breast cancer cells and regulation of cyclin D1 expression or function is an early response to steroid and steroid antagonist regulation of proliferation. The cyclin D1 gene is amplified in 15%, and its product overexpressed in 40–50%, of primary breast carcinomas. The strong evidence that cyclin D1 plays a major role in cell cycle control in breast epithelial cells suggests that its deregulated expression may have effects on disease progression and phenotype including sensitivity to endocrine therapies.     

Treatment outcomes and mortality of 94 patients with acromegaly

Summary Background. Due to new therapeutic modalities and modified therapeutic goals outcome of patients with acromegaly may change over time and differ by centre. We analysed treatment outcomes and mortality of our patients with acromegaly seen between 1971 and 2003.Method. The cohort consisted of 94 patients who had been followed for 0.3–31 years (mean 10.6 years). Remission criteria were a normalized IGF-I concentration, a nadir GH level during oral glucose load of <1.0µg/l and a random GH value of <2.5µg/l.Findings. Transsphenoidal surgery achieved remission in 80% of patients with micro-adenomas (<1cm), 65% with meso-adenomas (1cm to <2cm) and 27% with macro-adenomas (2cm). Patients with meso-adenomas operated on after 1995 tended to have a better outcome compared to those operated on before 1995 (Remission in 83% vs. 38%). Radiotherapy resulted in disease control in 22 of 47 patients (47%). Intramuscular depot formulation of octreotide (Sandostatin^® LAR^®) led to disease control in 17 of 26 patients (65%). After multimodal therapy persistent acromegalic activity remained in 18% of the patients; only one of them had an adenoma of <2cm. The standardized mortality ratio was 1.30 (95% CI 0.52–2.67) for patients in remission and 1.38 (95% CI 0.51–3.00) for patients with persistent acromegalic activity.Conclusions. Most patients with adenomas of <2cm can be expected to achieve remission by transsphenoidal surgery alone. Furthermore, virtually all patients with adenomas of <2cm and more than 80% of patients with adenomas of 2cm can be expected to achieve remission by adjuvant treatment. Aggressive multimodal therapy is critical in the management of acromegaly reducing mortality risk close to that of the general population.     

Case Report: Intracerebral Hemorrhage After γ-Knife Surgery for Metastatic Brain Tumor

The authors report a case of a 69-year-old man with metastatic brain tumors who died of spontaneous intracerebral hemorrhage 3 days after -knife surgery. He had been suffering from lung cancer with multiple systemic metastasis. Preoperative magnetic resonance images showed two well-defined round lesions with intratumoral hemorrhage in the left frontal and right occipital lobe. There was no bleeding tendency in the hematological examination and the patient was normotensive. -Knife surgery was performed on both lesions in a single session. However, the patient died of massive intracerebral hemorrhage from the left frontal lesion 3 days after the surgery. There have been no previous reports of mortality resulting from spontaneous intracerebral hemorrhage after -knife surgery in metastatic brain tumors documented in the literature. It is likely that the two events, -knife surgery and spontaneous intracerebral hemorrhage, occurred separately and were not associated. However, it is worth noting that there is a possibility of bleeding after -knife surgery, especially in a metastatic brain tumor with preexisting intratumoral hemorrhage as in our case.     

Anastomotic Healing in a Small Bowel Transplantation Model in the Rat

Anastomotic healing is impaired after intestinal surgery because of ischemia and reperfusion injury (IRI), which can result in intestinal leaks leading to increased mortality. The objective of this study was to determine the effects of transplant IRI and immune mechanisms on intestinal graft anastomotic healing. Orthotopic intestinal transplantations (OIT) were performed in rats. The experimental design consisted of six groups A–F (n = 5/group): A, allogeneic OIT treated with tacrolimus (1mg/kg/day); B, syngeneic OIT treated with tacrolimus; C, syngeneic OIT; D, allogeneic OIT; E, proximal and distal anastomoses performed in nontransplanted animals; F, same as in group E but treated with tacrolimus. Anastomotic bursting pressure (ABP), hydroxyproline content (HPC), and mucosal inflammatory infiltrate (MII) were determined at the anastomotic sites (proximal and distal) and compared between groups. ABP was significantly (p < 0.001) reduced in OIT groups A, B, C, and D compared to control groups E and F at both the proximal and distal anastomotic sites. HPC was 1 g/mg of tissue in groups A, B, C, and D, and 5g/mg of tissue in groups E and F. This demonstrates a significant (p < 0.001) reduction in HPC after OIT. MII was significantly (p < 0.001) increased in OIT groups when compared to nontransplanted control groups. MII was also significantly (p < 0.05) increased in allogeneic OIT groups A and D compared to syngeneic OIT groups B and C. Generally, ABP and HPC were inversely proportional to MII in both nontransplanted control and OIT groups. Reduced anastomotic strength was demonstrated in both syngeneic and allogeneic OIT anastomotic sites irrespective of immunosuppressive therapy, and is probably related to IRI.     

A study on OX39, a murine anti-rat interleukin 2 receptor antibody

OX39, a murine IgG1 monoclonal antibody (MoAb) that recognizes the 55 kDa alpha chain of the rat interleukin 2 receptor (R-IL2), was studied in vitro for its ability to interfere with IL2 binding and IL2-induced proliferation on rat concanavalin A (ConA) blasts and in vivo in a model of rat heart allografts. In vitro studies indicated that OX39 MoAb interacts with a single class of sites on the alpha chain of the rat R-IL2 with a high affinity (K_D=0.8 nm) and competes with IL2 binding on this chain (K_I=0.53 nm). In contrast, OX39 MoAb was found to be 10–20 times less efficient in competing with IL2 binding to the high-affinity R-IL2 (K_I10 nm). It is proposed that the epitope recognized by OX39 on the alpha chain (low-affinity R-IL2) is modified on (or buried in) the high-affinity R-IL2 configuration. Accordingly, OX39 was found to be a weak inhibitor in vitro on IL2-induced proliferation and in vivo on allograft rejection. Allograft survival was unaffected by doses of OX39 of 20 and 50 g/rat for 9 days; only a borderline effect was noted when doses as high as 250 g/rat were used. A significant, but restricted, effect of OX39 could be further detected when combined with low doses of cyclosporine A (1.5 mg/kg), which were ineffective by themselves. Together, our data suggest that in order to be efficient in vivo, anti-R-IL2 MoAbs must bind with high affinity to epitopes involved in the high-affinity IL2 binding site.