Analysis of the microenvironment necessary for engraftment: role of the vascular smooth muscle-like stromal cells

This is a review of recent data concerning the phenotype of human and murine stroma, as grown in long-term cultures. Using data on cytoskeletal and extracellular matrix protein expression, a sequential model of differentiation from mesenchymal (stem) cells to vascular-smooth muscle-like stromal cells is proposed. This model would apply, at least in the mouse, to stromal cells generated from different sites of hematopoiesis (bone marrow, fetal liver, spleen, and yolk sac). The in vivo counterparts of vascular-smooth muscle-like stromal cells in the different sites of definitive hematopoiesis are discussed.

CD99 (E2) up-regulates alpha4beta1-dependent T cell adhesion to inflamed vascular endothelium under flow conditions

CD99/E2 is an integral transmembrane protein which forms, together with Xga, a distinct family whose genes are located in the pseudoautosomal region. The number of T cells that firmly bound to vascular endothelial cells under physiological shear stress increased 2-14-fold upon CD99 stimulation, and bound cells became much more resistant to detachment forces and spread. T cell arrest occurred within 1 min and was dependent on the alpha4beta1-VCAM-1 pathway. In contrast, the alphaLbeta2-ICAM-1 pathway remained unactivated. This was observed with T cell lines and with activated peripheral blood lymphocytes, and was limited within the resting peripheral CD4+ T cells to the memory subset, while virgin cells were unaffected. This discloses a stepwise regulation of the T cell extravasation cascade.

CD99 engagement: an effective therapeutic strategy for Ewing tumors

CD99 is a Mr 32,000 transmembrane molecule that shows a high level of expression on cells of the hemopoietic system as well as on Ewing tumor cells. Within the hematopoietic system, CD99 has been implicated in cell adhesion and cell death, participating in this way in the differentiation of T-cell precursors. In this study, we demonstrate that engagement of CD99 significantly inhibits the in vitro and in vivo growth ability of Ewing tumor cells by delivering an apoptotic stimulus and reducing the malignant potential of these cells. Moreover, we show that anti-CD99 monoclonal antibodies may be advantageously used in association with conventional anticancer agents. These results provide a novel entry site for therapeutic intervention, which may have application in the care of patients with Ewing tumor, and warrant additional studies to clarify the molecular mechanisms activated by CD99 engagement.

Caspase-independent commitment phase to apoptosis in activated blood T lymphocytes: reversibility at low apoptotic insult

Little is known about the mechanisms of programmed death triggered in T lymphocytes by stimuli that can bypass caspase activation. Anti-CD2 monoclonal antibody and staurosporine are such apoptosis inducers because they operate in the presence of broad-spectrum caspase inhibitors BOC-D.fmk and Z-VAD.fmk. A system was devised, based on the isolation according to density of activated blood T cells progressively engaged in the apoptotic process. This allowed definition of a sequence of caspase-dependent and caspase-independent apoptogenic events that are triggered by anti-CD2 and staurosporine. Thus, a commitment phase to apoptosis was defined that is entirely caspase independent and that is characterized by cell volume loss, partial chromatin condensation, and release into the cytosol and the nucleus of mitochondrial "apoptosis-inducing factor " (AIF). Committed cells were viable, displayed a high mitochondrial inner transmembrane potential (triangle upPsim), and lacked large-scale and oligonucleosomal DNA fragmentation. Mitochondrial release of AIF was selective because cytochrome c was retained in mitochondria of the very same cells. Mitochondrial release of cytochrome c occurred later, at the onset of the execution phase of apoptosis, concurrently with triangle upPsim collapse, poly (ADP-ribose) polymerase cleavage, and DNA fragmentation. The apoptogenic events of this commitment phase are reversible if the strength of the stimulus is low and of short duration.

[Integrins and cell cycle control by the environment]

Integrins insure cell adhesion to extra-cellular matrix components; they are thus involved in tissue architecture. They also can insure intercellular adhesions by binding to surface molecules from the immunoglobulin superfamily. Integrins binding to their ligands induce cytoskeleton reorganisation and, consequently, they gather into focal adhesion contacts. This greatly strenghthens mechanical forces. Nevertheless, integrins can also participate in cell locomotion and, moreover, tranduce within cells signals that can extensively influence cell metabolism, cell cycle and apoptosis. Doing so, they can interact with signals from other cellular receptors, such as soluble growth factors. They are therefore key molecules to integrate intrinsic and extrinsic events of the cellular behavior. They profoundly influence oncogenesis and the metastatic process.

Fertility after hysteroscopic myomectomy: effect of intramural myomas associated

OBJECTIVE

The aims of this retrospective study were to evaluate the subsequent fertility and outcome of pregnancies after hysteroscopic myomectomy according to (a) the characteristics of submucous myomas and (b) the association with intramural myomas.

MATERIALS AND METHODS

From July 1994 to June 1997, 119 patients had hysteroscopic myomectomy including 31 infertile women. Among these 31 patients, the mean number of removed myomas by hysteroscopy was 1.4 (range 1-4) and the mean diameter of fibroid was 20 mm (range 10 to 50).

RESULTS

Eleven out of 31 women (35.5%) became pregnant. Thirteen pregnancies were observed including nine term deliveries, three miscarriages and one premature labor at 24 weeks of amenorrhea. A difference in delivery rate was found between patients with one submucous myoma resected and those with two or more (p=0.02). No difference in pregnancy and in delivery rates was observed according to size and location of submucous myomas. In contrast, in patients without intramural myomas, the delivery rate (p<0.03) was significantly greater and the delay of conception (p=0.05) was significantly shorter than those found in patients with intramural myomas.

CONCLUSION

Our study suggest that fertility after hysteroscopic myomectomy depend on (a) the number of submucous myomas resected and (b) the association with intramural fibroids.

[Molecular pathways of T lymphocyte activation]

TRIGGERING T CELL ACTIVATION: A quiescent T cell must encounter an antigen presenting cell to become activated. The T receptor for the antigen delivers a first specific signal which alone is insufficient to trigger total T cell activation. A second signal independent of antigen recognition is furnished by the antigen presenting cell, leading to total activation. PATHWAYS CAPABLE OF DELIVERING THE SECOND SIGNAL: CD28 and CTLA-4 are two molecules capable of playing a powerful role in regulating immune response. The fate of B cells depends on the CD40 molecule they carry on their surface as well as on T cells which express its ligand. Adherence molecules CD11a/CD18 induce tight adherence between T cells and antigen presenting cells by binding their CD50 ligand (ICAM-3); this bond activates the T cell, completing the first signal. The CD2 pathway and its ligands, a constitutional expression on two partner cells, is a strong adherence pathway which also transmits two powerful activation signals for T cells. SEVERAL FUNCTIONAL STATES: These different activation pathways illustrate the subtle interaction between surface molecules, capable of inducing different functional states depending on the environment.

Vascular smooth muscle differentiation of murine stroma: a sequential model

Previous studies by our group showed that stromal cells from human long-term marrow cultures were mesenchymal cells following a vascular smooth muscle pathway. The present study using 58 immortalized stromal lines from different hematopoietic sites was conducted to verify whether this hypothesis also held true for murine stroma. Principal components analysis performed using cytoskeletal and extracellular matrix proteins allowed the segregation of five factors explaining more than 70% of the variance. Factor I, including osteopontin and vimentin, and factor II, laminins and fibronectins, were representative of the mesenchyme. The remaining three factors were representative of vascular smooth muscle: factor III, including alphaSM actin, SM alpha actinin, SM22alpha, EDa+ fibronectin, and thrombospondin-1; factor IV, metavinculin and h-caldesmon; and factor V, smooth muscle myosin SM1 and desmin. All lines expressed factors I and II; 53 lines expressed factor III, 35 lines expressed factor IV; and 11 lines expressed factor V. A second principal components analysis including membrane antigens indicated the cosegregration of vascular cell adhesion molecule-1 with osteopontin and that of Ly6A/E with vimentin, whereas CD34 and Thy-1 appeared to be independent factors. The heterogeneity of vascular smooth muscle markers expression suggests that harmonious maintenance of hematopoiesis depends on the cooperation between different stromal cell clones.

Chronic pulmonary paracoccidioidomycosis (South American blastomycosis): high-resolution CT findings in 41 patients

OBJECTIVE

To assess the pulmonary parenchymal findings on high-resolution CT in 41 patients with the chronic form of paracoccidioidomycosis (South American blastomycosis).

SUBJECTS AND METHODS

The study included 41 consecutive patients in whom chronic paracoccidioidomycosis had been proven. All patients underwent high-resolution CT (1-mm collimation, high-spatial-frequency reconstruction algorithm) at 12 equally spaced intervals through the chest. The images were analyzed by two radiologists, and each final decision was reached by consensus.

RESULTS

Thirty-eight (93%) of the 41 patients had CT scans with abnormal findings. The findings included interlobular septal thickening in 36 patients (88%), 1-25 mm diameter nodules in 34 (83%), peribronchovascular interstitial thickening in 32 (78%), centrilobular opacities in 26 (63%), intralobular lines in 24 (59%), ground-glass opacities in 14 (34%), cavitation in seven (17%), air-space consolidation in five (12%), traction bronchiectasis in 34 (83%), and paracicatricial emphysema in 28 (68%). In approximately 90% of patients, the abnormalities were bilateral and symmetrical and involved all lung zones.

CONCLUSION

High-resolution CT findings of paracoccidioidomycosis consist predominantly of interstitial abnormalities and nodules associated with traction bronchiectasis and paracicatricial emphysema in a bilaterally symmetrical distribution.

[Mechanical and metabolic complications of hysteroscopic surgery: report of a retrospective study of 352 procedures]

Our objective was to determine, in a retrospective study of 352 operative hysteroscopies: (a) the rates and the types of complications and (b) the risk factors of peroperative perforations. The most important complications represented 1.7% including two haemorrhage, one symptomatic metabolic abnormalities and three uterine perforations with bowel injuries (0.8%). Furthermore, minor complications were observed in 9.3% including non symptomatic metabolic abnormalities (5.5%) and uterine perforations without visceral injury. Among mechanical complications, the majority were uterine perforations (4%). No relation was found between menopausal status of the patients and the occurrence of uterine perforation. In contrast, the perforation rate was statistically greater in patients treated for synechia than those found for myoma (p < 0.0001). Furthermore, the perforation rate was statistically higher for resection of myomas as compared with endometrial resection (p < 0.0001) or polyp resection (p < 0.0008). Moreover, in our experience, the perforation rate depended on hysteroscopic experience of surgical operators.

[Laparoscopy-assisted hysterectomy and laparoscopic preparation. Apropos of a series of 177 cases]

Our objective was to determine the limits of laparoscopic-assisted vaginal hysterectomy (LAVH) and the value of a preoperative scoring system to determine the operative approach to hysterectomy. Between January 1991 and December 1996, 152 out of 177 patients had LAVH and 25 had laparoconversion. The mean operating time was 163 min. The overall postoperative complication rate was 8.4%. The hospital stay was 4.8 days for LAVH versus 6.2 days for laparoconversion (p < 0.01). For each patient, a preoperative scoring system was established according to uterine size, previous laparotomy, uterine mobility, pelvic adhesions and endometriosis stage. The laparoconversion rate increased according to the score, as it was 7.8% for a score < or = 7 and 80% for a score > 7. LAVH offers a technique to convert some abdominal hysterectomies into vaginal hysterectomies. The use of the preoperative scoring system may help to determine patients who may benefit from the laparoscopic route and those with a high risk of laparoconversion.

End-tidal carbon dioxide monitoring for weaning patients: a pilot study

This research pilot study evaluates the usefulness of capnography for patients being weaned from mechanical ventilation in a medical intensive care unit (MICU). The hypothesis that capnography would allow for more rapid weaning from mechanical ventilation, and require fewer arterial blood gases (ABGs) during the process, was found to be untrue. Several implications for critical care nursing practices were derived from the literature review and findings of this study.

How to use articles about harm: the relationship between high tidal volumes, ventilating pressures, and ventilator-induced lung injury

BACKGROUND

Intensivists commonly encounter patients who may be inadvertently harmed by critical care interventions. This article is designed to guide clinicians in the evaluations of an individual article assessing a question of harm, as well as the sum of multiple pieces of evidence.

OBJECTIVES

To assess the vaidity of a group of articles about the relationship between high tidal volumes and ventilating pressures on ventilator-induced lung injury; to interpret the results of these studies; and to consider whether they apply in practice.

DATA SOURCES

Issues of harm are sometimes measured in randomized trials, but are evaluated more often in myriad observational studies.

DATA EXTRACTION

We use critical appraisal guides for experimental studies (e.g., randomized trials) and observational studies (e.g., cohort studies, case-control studies and case series) that evaluate the potentially harmful exposure of high tidal volumes and ventilating pressures. This involves assessing the validity of the research, then determining the strength of association between the putative harmful exposure and adverse outcomes. These study designs and their interpretation using relative risks and odds ratios are reviewed. Finally, the relevance of this information (or lack thereof) to clinical practice needs to be determined.

DATA SYNTHESIS

Examining these studies individually and in totality, there appears to be a relationship between high tidal volumes and ventilating pressures, although the strength of inference from this research is limited by design issues and sample sizes.

CONCLUSIONS

Critically appraising a body of literature is more challenging than evaluating a single study, but often gives a broader view of the available evidence. Future large, rigorous, randomized trials of different approaches to mechanical ventilation will help to advance our understanding and to better inform our practice.

The efficacy and acceptability of amineptine versus fluoxetine in major depression

The temporal dimension, particularly anticipation, appears to be a very important component in the understanding of depressed patients. In a 90-day multicentre study, the efficacy and acceptability of amineptine and fluoxetine were compared in 169 patients with major depression. Comparison of the two antidepressants was based on double-blind methods, after random allocation of the treatments between two parallel groups. The two drugs did not differ over the whole course of the study, but the improvement in scores on day 4 was globally more marked in the amineptine than the fluoxetine group. Intragroup analysis showed that amineptine was significantly superior to fluoxetine on the retardation pole of the mood, anxiety, retardation, danger scale. The positive effect of amineptine on anticipation may enable the depressed patient to make plans for the future. Anticipation may be a key dimension to be more precisely explored in specific psychopharmacological protocols with antidepressants.

Apoptosis of immature thymocytes mediated by E2/CD99

E2/CD99 is a 32-kDa transmembrane molecule that does not belong to any known family of proteins. It appears to regulate adhesion properties of T cells as previously reported, in particular, the induction of homotypic adhesion in CD4+ CD8+ thymocytes. Apoptosis induced via E2/CD99 displays characteristic morphologic features, but includes early mitochondrial alterations and phosphatidylserine exposure at the outer leaflet of the plasma membrane. It is not followed by detectable DNA fragmentation, and its time course is much longer than apoptosis induced via the Fas/CD95 pathway. It requires 18 h for completion. E2/CD99-induced apoptosis does not require any RNA or protein synthesis and still occurs following blockage of the Fas pathway. It is, however, dependent on CPP32 and IL-1beta-converting enzyme-type cysteine proteases, as shown by blockade with their respective specific inhibitors. This effect is restricted to double-positive thymocytes carrying an intermediate density of CD3 and including all CD69+ cells. Thus, E2/CD99 apears to mediate a distinctive apoptotic signal at a critical stage of thymocyte differentiation, i.e., when positive selection is known to occur.

Apoptosis of immature thymocytes mediated by E2/CD99

E2/CD99 is a 32-kDa transmembrane molecule that does not belong to any known family of proteins. It appears to regulate adhesion properties of T cells as previously reported, in particular, the induction of homotypic adhesion in CD4+ CD8+ thymocytes. Apoptosis induced via E2/CD99 displays characteristic morphologic features, but includes early mitochondrial alterations and phosphatidylserine exposure at the outer leaflet of the plasma membrane. It is not followed by detectable DNA fragmentation, and its time course is much longer than apoptosis induced via the Fas/CD95 pathway. It requires 18 h for completion. E2/CD99-induced apoptosis does not require any RNA or protein synthesis and still occurs following blockage of the Fas pathway. It is, however, dependent on CPP32 and IL-1beta-converting enzyme-type cysteine proteases, as shown by blockade with their respective specific inhibitors. This effect is restricted to double-positive thymocytes carrying an intermediate density of CD3 and including all CD69+ cells. Thus, E2/CD99 apears to mediate a distinctive apoptotic signal at a critical stage of thymocyte differentiation, i.e., when positive selection is known to occur.

Integrin-associated protein (CD47) is a comitogenic molecule on CD3-activated human T cells

IAP is a glycoprotein functionally and physically associated with some integrins, i.e., the leukocyte response integrin and the beta3 integrin chain on placenta, platelets, and polymorphonuclear cells. IAP may act as a transducer element in activation mediated via these integrins. Since IAP is present at high density on peripheral T lymphocytes we have investigated its involvement in T cell activation. We tested three mAbs against IAP, namely B6H12, BRIC126, and 2D3, which recognize two distinct epitopes. IAP cross-linking with B6H12 or BRIC126, but not 2D3, transduces costimulatory signals within highly purified CD3-activated T lymphocytes, i.e., enhancement of proliferation, CD25 expression, and IL-2 secretion, while no effect was observed upon CD2 stimulation. However, we could not observe any functional association between IAP and integrins on peripheral T cells. In an attempt to explore further the activation signal delivered by IAP, we show here that IAP cross-linking with the comitogenic B6H12 mAb induces the phosphorylation on tyrosine of several proteins, one of which is identified as p56(lck) protein tyrosine kinase. Moreover, we observed that IAP is associated with p56(lck) on PMA-activated, but not on resting, T cells. These data suggest that on T cells, IAP may be involved directly via a specific ligand in cell-matrix or cell-cell interactions. Such interactions could trigger protein tyrosine phosphorylation pathways, which play an important role in both maturation and activation of T cells.

Integrin-associated protein (CD47) is a comitogenic molecule on CD3-activated human T cells

IAP is a glycoprotein functionally and physically associated with some integrins, i.e., the leukocyte response integrin and the beta3 integrin chain on placenta, platelets, and polymorphonuclear cells. IAP may act as a transducer element in activation mediated via these integrins. Since IAP is present at high density on peripheral T lymphocytes we have investigated its involvement in T cell activation. We tested three mAbs against IAP, namely B6H12, BRIC126, and 2D3, which recognize two distinct epitopes. IAP cross-linking with B6H12 or BRIC126, but not 2D3, transduces costimulatory signals within highly purified CD3-activated T lymphocytes, i.e., enhancement of proliferation, CD25 expression, and IL-2 secretion, while no effect was observed upon CD2 stimulation. However, we could not observe any functional association between IAP and integrins on peripheral T cells. In an attempt to explore further the activation signal delivered by IAP, we show here that IAP cross-linking with the comitogenic B6H12 mAb induces the phosphorylation on tyrosine of several proteins, one of which is identified as p56(lck) protein tyrosine kinase. Moreover, we observed that IAP is associated with p56(lck) on PMA-activated, but not on resting, T cells. These data suggest that on T cells, IAP may be involved directly via a specific ligand in cell-matrix or cell-cell interactions. Such interactions could trigger protein tyrosine phosphorylation pathways, which play an important role in both maturation and activation of T cells.

Role of the type I interferons in allograft rejection

Type I interferons are potent immuno-modulatory cytokines that enhance expression of the major histocompatibility complex (MHC) class I antigens, T-cell cytotoxicity, and natural killer (NK) cell activity, all of which are implicated in graft rejection. A monoclonal antibody (mAb) directed against the extracellular domain of the human interferon gamma (IFN-gamma) receptor (IFN-alpha R), which inhibits both the binding and biological activity of all the type I IFNs tested, exerted a dose-dependent inhibition of the mixed lymphocyte reaction and induced permanent survival of skin allografts in MHC-divergent Cynomologus monkeys treated with a subeffective dose of cyclosporin A. Marked differences were observed in the composition of T lymphocyte subpopulations in anti-IFN-alpha R mAb-treated animals relative to the various control groups. Skin biopsies from animals treated with anti-IFN-R Mab + cyclosporin A revealed very low levels of MHC class I and class II antigen expression and the absence of histological signs of rejection, whereas skin biopsies from control animals exhibited high levels of MHC antigen expression and the histological signs of acute rejection, including a pronounced lymphocytic infiltrate, edema, and necrosis. No monkey antibodies (IgG) to the mouse anti-human IFN-alpha R mAb were detected in the serum of any of the animals treated with the anti-IFN-alpha R mAb either alone or together with cyclosporin A. Treatment of lethally irradiated Cynomologus monkeys with the anti-IFN-alpha R mAb together with a subeffective dose of cyclosporin A was also found to markedly enhance the survival of animals grafted with allogeneic bone marrow cells from donors differing in both MHC class I and class II antigens. These results show that selective and lasting immunosuppression can be obtained by the short-term administration of an IFN-alpha antagonist together with a subeffective dose of cyclosporin A, and may have important implications for the therapy of human allograft rejection.

A controlled study of cognitive behaviour therapy with buspirone or placebo in panic disorder with agoraphobia

BACKGROUND

This multicentre study compared a 16-week buspirone treatment with placebo in patients presenting with panic disorder with agoraphobia and also receiving cognitive behaviour therapy (CBT).

METHOD

Double-blind testing was maintained until week 68, but not tested; 91 patients were included; 14 placebo-responders excluded; 77 patients randomised; 48 reached week 16 and 41 reached week 68.

RESULTS

At week 16, within-group analysis showed significant improvements in agoraphobia, panic attacks, and depression in both groups. Generalised anxiety improved only in CBT+buspirone. Between-group comparisons showed buspirone to have an effect on generalised anxiety and agoraphobia. Changes in degree of agoraphobia and depression were correlated in subjects on CBT+buspirone only. A significantly higher proportion of women, and of subjects showing high avoidance dropped out. Positive expectations regarding medication predicted success in both groups. At week 68, improvement was retained without significant buspirone effect.

CONCLUSION

Buspirone enhanced the effects of cognitive behaviour therapy on generalised anxiety and agoraphobia in the short term.

Detection of potato leafroll virus in single aphids by the reverse transcription polymerase chain reaction and its potential epidemiological application

A reverse transcription and polymerase chain reaction (RT-PCR) system was developed using two 20-mer primers located in the potato leafroll virus (PLRV) capsid gene. A 336-bp PCR product was detected from aphids (Myzus persicae) which had been fed on PLRV-infected plants. The PCR band was specific to PLRV as determined by Southern blots and detection by a PLRV-specific probe. As little as 5 min exposure of aphids to PLRV-infected leaves resulted in the presence of PLRV-specific bands in 13% of aphids. However, the percentage of PLRV-positive aphids increased with longer exposure to infected sources and reached 90% after 3-4 days of feeding. PLRV can be detected from a single viruliferous aphid or a single viruliferous aphid combined with up to 29 non-viruliferous aphids. PLRV can be detected from freshly collected aphids, those stored at -70 degrees C, or those stored in 70% ethanol at room temperature for extended periods. This method is applicable to assess the viruliferous nature of aphids caught in yellow-pan traps during the growing season or stored for over a year.

Sphingosine, oleylamine and stearylamine inhibit both CD11a/CD18-dependent and -independent homotypic aggregation: demonstration by cytofluorimetry

An CD11a/CD18-dependent homotypic aggregation pathway is induced by triggering CD45 molecules on human thymocytes. By contrast, a CD11a/CD18-independent homotypic aggregation process is induced by triggering the CD99 molecule (E2, MIC2 gene product) expressed at the surface of either Jurkat T cells or human thymocytes. A new quantitative method based on FACS analysis of aggregated cells was used and allowed to show that both types of aggregation (CD11a/CD18-dependent and CD11a/CD18-independent) were inhibited with sphingosine, oleylamine or stearylamine. These three compounds had no effect on the expression of CD99, CD45, CD11a, CD18 or other [correction of others] known integrins expressed at the surface of the cells studied.

[Mid-term failure of balloon dilatation treatment of antral stenosis induced by caustics]

We report the case of an antral stricture following lye ingestion. The patient was treated by 3 dilations using a through-the-scope balloon dilator, initially with good results. One year later, the recurrence of the symptoms led to 2 other sessions of dilation without success and a partial gastrectomy was performed. The intensity of the gastric wall fibrosis on the surgical specimen, probably responsible for major motor impairment, accounts for the discordance between the good endoscopic result and the clinical failure. Endoscopic dilation of lye-induced gastric strictures could be a temporary alternative to surgical resection because the gastric wall fibrosis blemishes the long-term functional result.

Comitogenic effects of very late activation antigens on CD3-stimulated human thymocytes. Involvement of various tyrosine kinase pathways

Thymocytes display several integrins that are involved in cell-extracellular matrix interactions and differentiation processes. We have examined the role of very late activation Ag (VLA) on human thymocyte stimulation. VLA-4, VLA-5, and VLA-6 activated with either mAbs or their natural ligands (fibronectin, laminin, and vascular cell adhesion molecule-1) are able to transduce costimulatory signals in thymocytes activated via the CD3 pathway, i.e., enhancement of thymocyte proliferation, CD25 and CD69 expression, and IL-2 secretion. In contrast, activation of thymocytes with a mitogenic pair of CD2 mAb was not modified by VLA molecules. Cross-linking of both beta 1- and alpha 5-chains induced tyrosine phosphorylation of several proteins, whereas the cross-linking of the alpha 4- and alpha 6-chains did not. Moreover, a different pattern of tyrosine phosphorylation was observed when thymocytes were activated via either beta 1- or alpha 5-chains. These results suggest that VLA molecules activate tyrosine kinase pathways in thymocytes, and that different pathways would be implicated during thymocyte interactions with extracellular matrix or accessory cells, which are likely to play a role in thymocyte differentiation.