Apoptosis accompanies a change in the phenotypic distribution and functional capacity of murine bone marrow B-cells chronically exposed to prednisolone

Prednisolone (PD) is commonly used for the treatment of inflammation accompanying diseases such as arthritis, allergy, asthma, and autoimmunity. While it is well documented that PD induces apoptosis in immature T-cells of the mouse, the effects of PD on development of immature B-cells in normal bone marrow (BM) was not known. An implantation system was developed which chronically delivered PD at a rate of a few nanograms per milliliter of plasma to mice. Ten days of exposure to such levels of PD caused splenic and thymic atrophy, which was accompanied by a 50% decrease in the numbers of circulating lymphocytes. Flow cytometric analysis (FACS) of the effects of PD on the BM revealed a threefold decrease in the proportion of B220+IgM- pre-B-cells and immature IgM+IgD- B-cells. However, the mature IgM+IgD+ cells were reasonably resistant to the effects of PD. A 25% decrease in small nucleated cells presumed to be part of the lymphocyte compartment was also noted from the scatter profiles of the marrow of PD-treated mice. These marked changes in BM composition were also accompanied by significant reductions in capacity of the BM to respond to trinitrophenylated-lipopolysaccharide (TNP-LPS) after exposure to PD either in vivo or in vitro. Studies to ascertain whether apoptosis played a role in the decline in the number of developing B-cells of marrow exposed to PD were performed in vitro in order to reduce the possibility of phagocytosis of apoptotic cells. A recent modification of FACS cell cycle analysis, which is highly quantitative and allows rapid analysis of heterogeneous tissues such as the marrow, was used to detect the apoptotic cells. After 16 hr of culture in 10(-7) M PD, approximately 40% of IgM+ and B220+ cells of BM resided to the left of G0/G1 in a region associated with apoptotic cells previously termed the A0 or "hypodiploid" region. Thus, these data indicate that chronic exposure to low levels of PD significantly altered the B-cell compartment of the murine bone marrow both in vivo and in vitro, potentially inducing apoptosis in these cells.

Cytosolic and nuclear juvenile hormone-binding proteins from the brain of Diploptera punctata

Cytosol and crude nuclear extracts of brains from female Diploptera punctata were found to contain a juvenile hormone-binding protein. The binding kinetics of the cytosol and nuclear juvenile hormone-binding proteins toward racemic [3H]JH III were determined using the dextran-coated charcoal assay. Both proteins show reversible and saturable binding toward JH III. The mean dissociation constant (Kd) of the brain cytosol binding protein was 66.5 +/- 7.2 nM, whereas the nuclear-binding protein had a lower affinity, with a mean Kd of 170 +/- 22.1 nM. The specificity of both the cytosol and the nuclear binding proteins was determined by competitive displacement experiments with [3H]JH III. For the cytosol binding protein, the order of relative affinity was JH II > (10R)-JH III > (10RS)-JH III approximately JH I. For the nuclear binding protein the order of relative affinity was JH II > JH I > (10R)-JH III > (10RS)-JH III. The JH analog hydroprene (ZR 512) had > 100-fold lower affinity than JH III for both proteins.

Targeted overexpression of transforming growth factor alpha in the epidermis of transgenic mice elicits hyperplasia, hyperkeratosis, and spontaneous, squamous papillomas

To assess the effects of transforming growth factor alpha (TGF-alpha) on mammalian skin in vivo, we have targeted its expression to the epidermis of transgenic mice using a vector based on the human K1 (HK1) gene. Neonatal mice expressing the HK1.TGF-alpha transgene were often smaller than normal littermates and had precocious eyelid opening and wrinkled, scaly skin with diffuse alopecia. Juvenile transgenic mouse epidermis was uniformly hyperkeratotic, but this pattern was generally less pronounced in adult transgenic mice unless they expressed high levels of the HK1.TGF-alpha transgene. Spontaneous, squamous papillomas occurred at sites of wounding in adult mice expressing high levels of HK1.TGF-alpha; however, most were prone to regression. Immunoreactive TGF-alpha was 2-6 times higher in the epidermis of these HK1.TGF-alpha lines. Immunoreactive epidermal growth factor receptor had a normal pattern of expression in nonphenotypic adult epidermis, but a marked reduction in the receptor population was detected in hyperplastic newborn epidermis and phenotypic adult epidermis. Autoradiographic localization of 125I-epidermal growth factor showed a similar pattern of distribution, suggesting that the sites of increased TGF-alpha expression induced epidermal growth factor receptor down-regulation. These data demonstrate the in vivo effect of deregulated TGF-alpha expression on epidermal proliferation and differentiation and suggest a potential role for TGF-alpha in carcinogenesis and other hyperproliferative epidermal disorders.

Chronic elevation of plasma corticosterone causes reductions in the number of cycling cells of the B lineage in murine bone marrow and induces apoptosis

Steroid-containing implants were used to ascertain the effects of chronic elevation of physiological levels of plasma corticosterone (CS) (30-100 micrograms/dl) on lymphopoietic processes in the bone marrow of the mouse. Phenotypic analysis of bone marrow B-lineage lymphocytes using flow cytometry (FACS) indicated a 50% decrease in bone marrow Ig+ cells, and a 70-80% decrease in B220+ cells had occurred 3 days after exposure to steroid. By day 5, the B220+ Ig- precursor B cells in the marrow of mice exposed to CS were nearly depleted, with many of the remaining B cells being B220bright IgM+IgDbright. To determine if the depletion of B cells was due to disruption in cell cycling and/or induction of apoptosis, phenotype-gated FACS cell cycle analysis was utilized. The proportion of B220+ cells in the S phase of the cell cycle declined 75% after 24 hr exposure to CS. A few hours after CS implantation, the appearance of a small but distinct population of B220+ and IgM+ cells in the 'hypodiploid' region of the cell cycle was also noted, which was previously termed the Ao region and corresponded to cells undergoing apoptosis. Thus, the chronic presence of modestly elevated levels of plasma CS analogous to that produced during malnutrition, stress and trauma caused rapid depletion of developing B-lineage cells in the marrow by reducing the number of cycling precursor B cells and inducing apoptosis.

Annexin-1 localization in human skin: possible association with cytoskeletal elements in keratinocytes of the stratum spinosum

Annexin-1 (also called lipocortin-1 or p35), a putative substrate of the epidermal growth factor/receptor kinase, protein kinase C, and transglutaminase, was immunolocalized in embryonic, neonatal, adult, and diseased human epidermis. In embryonic skin intense annexin-1 immunoreactivity was found in the periderm at 54 d estimated gestational age (EGA). Later (EGA = 91-143 d), annexin-1 immunoreactivity was restricted to basal keratinocytes. In neonatal skin, basal cells were often more heavily stained than were suprabasal keratinocytes, which were also stained. Only basal keratinocytes stained in adult plantar skin, but in thin skin annexin-1 was present in the basal, suprabasal, and sometimes even in the granular layers of the epidermis. Often, annexin-1 appeared concentrated around the perimeter of cells, especially tonofilament/desmosome-rich keratinocytes of the spinous-cell layer. At high magnification, annexin-1 appeared associated with distinct structures and was very granular in appearance in the intensely stained ductal keratinocytes of eccrine sweat glands, cells that are very highly enriched in keratin tonofilaments. This striking distribution in certain keratinocytes enriched in tonofilaments suggests a role for annexin-1 in cytoskeletal functions.

Glucocorticoids and irradiation-induced apoptosis in normal murine bone marrow B-lineage lymphocytes as determined by flow cytometry

A substantial proportion of murine bone marrow B220+ and IgM+ cells were induced to undergo apoptosis when exposed to glucocorticoids or ionizing radiation in vitro. Two-colour flow cytometric analysis of the cell cycle indicated that a distinct subpopulation of cells formed to the left of G0/G1 in the hypodiploid or Ao region previously shown to contain apoptotic cells with fragmented DNA. Indeed, 45-65% of all B220+ or IgM+ cells of the marrow were found in this apoptotic region 12 hr after treatment with dexamethasone (Dex) or exposure to 500 rads of irradiation. Zinc sulphate, a frequently cited inhibitor of apoptosis, prevented accumulation of cells exposed to glucocorticoids or ionizing radiation in the Ao region as did the glucocorticoid receptor antagonist RU 38486. Although Dex was more potent, corticosterone and cortisol also induced significant degrees of apoptosis in B220+ and IgM+ marrow cells at physiological concentrations. These results demonstrate that freshly isolated B-lineage cells of the murine bone marrow readily undergo apoptosis upon exposure to glucocorticoids and ionizing radiation and suggest that apoptosis may play a role in the regulation of lymphopoiesis. The data also show the value of flow cytometry to the study of apoptosis in subsets of cells within a heterogenous population such as the bone marrow which heretofore was exceedingly difficult to evaluate.

Heparin-associated thrombocytopenia and thrombosis (HATT) presenting with livedo reticularis

BACKGROUND

Heparin-associated thrombocytopenia and thrombosis (HATT) is an infrequently encountered syndrome characterized by ischemic necrosis of soft tissue and vital organs following anticoagulation with heparin. The syndrome is thought to be due to heparin-dependent platelet aggregation and thrombosis, which is mediated by pathologic immunoglobulins.

CASE REPORT

A 60-year-old man developed truncal livedo reticularis and ischemic necrosis of the left foot associated with thrombocytopenia, disseminated intravascular coagulopathy (DIC), and microangiopathic hemolytic anemia during intravenous heparin therapy. Skin biopsy from an area of livedo reticularis revealed fibrin thrombi in dermal blood vessels, which is characteristic of HATT. The diagnosis of HATT promoted discontinuation of heparin and a resulting rapid resolution of the livedo reticularis and hematologic abnormalities. No other potential causes of DIC were identified, and, other than stopping heparin, no specific therapy was employed.

CONCLUSIONS

Periodic monitoring of platelets should be performed on all patients receiving treatment with heparin, as early detection of heparin-induced thrombocytopenia followed by discontinuation of the drug may prevent life threatening thrombotic complications. HATT should be included in the differential diagnosis of patients with livedo reticularis that occurs during heparin therapy.

Transgenic mice as a model to study the role of TGF-beta-related molecules in hair follicles

There is increasing evidence that members of the TGF-beta superfamily are important regulators of epithelial growth and differentiation in vivo. Here, transgenic mice have been used to study the role of the TGF-beta-related growth factors BMP-2 and BMP-4 in hair and whisker development. In the mature hair follicle, BMP-2 transcripts are normally seen only in precortex cells at the base of the hair shaft. In the transgenic mice reported here, BMP-4, a closely related molecule, has been ectopically expressed in the outer root sheath of hair and whisker follicles using an expression vector based on the bovine cytokeratin IV* promoter. In response to transgene expression, both outer root sheath cells below the stem cell compartment and hair matrix cells around the dermal papilla cease proliferation. In addition, the expression pattern of cytokeratin markers is disturbed in some transgenic hair follicles. These results support a model in which members of the TGF-beta superfamily play an active role in the inhibiton of cell proliferation and the onset of expression of trichocyte-specific genes that take place when cells leave the matrix of the follicle and differentiate into shaft cells.

Detergent solubilization is a prerequisite for aggregation-induced stimulation of epidermal growth factor (EGF) receptor kinase

Unlike EGF, concanavalin A and wheat germ agglutinin do not increase EGF receptor-kinase activity in intact A-431 membranes. However, they increase both autophosphorylation and phosphorylation of exogenous substrates about twice as much as EGF if the membranes are solubilized in detergent. Following solubilization, autophosphorylation due to the combined presence of a lectin and EGF is additive suggesting that each increases kinase activity by a different mechanism. These different mechanisms were studied by autophosphorylating membranes at increasing detergent concentrations after they had been permeabilized to [gamma-32P]ATP with alamethicin. As the detergent concentration increased, EGF stimulated autophosphorylation decreased 3-fold and 6-fold for the native 170 kDa receptor and for a protease-generated 150 kDa receptor form, respectively. However, in the presence of either lectin the same increase in detergent concentration only slightly altered the autophosphorylation rates which never exceeded the rate measured in the absence of EGF and detergent. Hence, the lectins increase kinase activity in solubilized membranes by preventing the adverse effects of detergent on the receptor-kinase and may not be useful models for how EGF activates its receptor.

Focal-adhesion components are enriched in ventral membranes isolated from transformed keratinocytes in culture

The lack of procedures for isolating or reconstituting intact focal adhesions has hindered studies of how focal adhesions are organized and of how their assembly/disassembly is controlled. A method for isolating large quantities of the ventral portion of plasma membranes from transformed keratinocytes (A-431 cells) in culture is described. Plasma membranes are stabilized using Zn2+ and the ventral portion isolated attached to the culture substratum after the body of the cell has been sheared away. Compared with complete plasma membranes isolated from cells scraped from the dish, these ventral-membrane preparations are enriched 18.5-fold and 5.1-fold in the focal-adhesion components talin and vinculin respectively. While the epidermal-growth-factor receptor-kinase is less abundant in preparations of ventral membranes, a recently described tyrosine kinase which localizes to focal adhesions in mouse fibroblasts is enriched 19.9-fold. Extracellular matrix components, as well as their integrin receptors, are also enriched in these preparations of the ventral portion of plasma membranes compared with preparations of complete plasma membranes.

Extracorporeal photopheresis for the treatment of cutaneous T-cell lymphoma

BACKGROUND

Many regimens are used for cutaneous T-cell lymphoma (CTCL), but with advanced disease response rates and patient survival are not adequate with any current therapy. Recently extracorporeal photochemotherapy (ECP) was proposed as an alternative therapy.

OBJECTIVE

Our purpose is to present the results of ECP in patients with CTCL refractory to other treatments.

METHODS

Patients with CTCL received ECP at 3- to 5-week intervals for at least 6 months. All patients except one were in stage T2 (patch/plaque) or higher. Eight patients had extracutaneous disease involving lymph nodes (six patients), bone marrow (five), or Sézary cells (six). The interval between initial symptoms and diagnosis was 5.9 +/- 1.9 years (mean +/- standard error of the mean) and the interval between diagnosis and ECP was 2.2 +/- 0.4 years.

RESULTS

A complete response (disappearance of all lesions) was obtained in five patients (25%) and a partial response (disappearance of at least 50% of lesions) in six patients (30%). Four patients (20%) showed stabilization of their disease and five progressed (25%). The only variable that predicted responders versus nonresponders was the number of ECP sessions (p < 0.05 by multivariate logistic regression). In contrast, no separate beneficial effect of adjunctive chemotherapy (p > 0.5) or electron beam therapy (p > 0.1) was found.

CONCLUSION

Long-term ECP may be an effective alternative treatment for CTCL refractory to other therapies and is likely to be even more useful when combined with other modalities.

Quantitative determination of EGF-R during epidermal wound healing

Little is known about the intrinsic regulation of growth factors of cytokines during the normal epidermal wound-healing processes in skin. A simplified model of wounding (tape stripping to remove the stratum corneum) was used to study the role of epidermal growth factor receptors (EGF-R) in this process. Although the dynamics of EGF-R in epidermal wound healing have not been determined, the immunoreactive EGF-R that are present presumably play an active role. Prior studies show that 1) EGF-R are present in increased numbers in proliferative skin diseases; 2) a hypertrophic epidermis, closely resembling normal wound healing, is induced in mouse skin by EGF injections; and 3) exogenous topical EGF potentiates wound healing. The number of immunoreactive receptors as measured by an enzyme-linked immunosorbent assay (ELISA) and histologic methods increased prior to an increase in epidermal thickness, total protein, and DNA content. This early increase in the levels of EGF-R was followed by a sharp decline in EGF-R and subsequent decline in epidermal thickness (hypertrophy), total protein, and DNA levels. Alterations in the temporal sequence in these parameters indicate that the EGF-R-mediated signaling systems play an active role in epidermal wound repair.

Increased epidermal growth factor receptors in melanocytic lesions

BACKGROUND

Epidermal growth factor receptors (EGF/R) have been reported to be absent in melanomas or, in contrast, to be markers for potential malignancy in melanocytic lesions.

OBJECTIVE

Our purpose was to evaluate the literature discrepancies regarding the presence of EGF/R in melanocytic lesions and to determine whether EGF/R presence correlates with the potential for malignancy of melanocytic lesions.

METHODS

An EGF/R-specific polyclonal antibody was used to study melanomas, dysplastic nevi, congenital nevi, and nevocellular nevi.

RESULTS

All melanocytic cell types (nevus and melanoma cells) in the lesions studied had immunoreactive EGF/R. EGF/R immunoreactivity was also observed throughout the epidermal basal to granular cell layers overlying the melanocytic lesions, although dermal fibroblasts were negative.

CONCLUSION

The pattern of increased immunoreactive EGF/R in both benign and malignant nevocellular lesions suggests that although EGF/R are not a specific marker of potential malignancy in melanocytic lesions, they may mediate or coordinate growth of keratinocytes and nevus cells.

Flow cytometric analysis of the expression of murine B and T surface markers from birth to adulthood

The proportion of nucleated splenocytes bearing B-lymphocyte markers B220, surface IgM (sIgM) and sIgD, as well as the T-lymphocyte markers Thy 1.2, CD5, CD8a and CD4 were quantitated by flow cytometric analysis (FACS) throughout postpartum development in the A/J mouse. Full expression of B lymphocyte markers was achieved much sooner than expression of T lymphocyte markers. This was especially true for B220, which was found on 8% of all splenocytes at day 5 and reached adult levels (47-50%) by weaning at day 22. Expression of sIgM and sIgD were 13% and 9%, respectively, of all splenocytes at day 5 with mature levels not expressed until day 35 postpartum (approximately 36% of cells were positive for these markers). T lymphocyte markers, on the other hand, did not reach full expression until sexual maturity. For example, Thy 1.2 expression was 8% on day 5 and did not reach mature levels (28-30%) until day 56. CD5 closely paralleled Thy 1.2 expression rising from only 2% on day 5 to 27% by day 56. Likewise, CD8a and CD4 marker development paralleled one another with CD8a rising from 1% on day 5 to 10% by day 56 and CD4 rising from 5% on day 5 to 19% by day 56. These data demonstrate the variability in the time of appearance and rate of maturation of the various lymphocyte cell surface markers during postpartum development. They also serve as a reference to identify alterations in lymphocyte development created by immunodeficiency diseases.

Comparative evaluation of several DNA binding dyes in the detection of apoptosis-associated chromatin degradation by flow cytometry

Mouse thymocytes readily undergo apoptosis-associated DNA degradation upon exposure to glucocorticoids or ionizing radiation. It has been previously shown that flow cytometric cell cycle analysis of propidium iodide-stained apoptotic thymocytes results in the appearance of a distinct cell cycle region (the A0 region) below the G0/G1 region. Cells in this region were shown to be undergoing apoptosis, and determination of apoptosis by flow cytometric analysis was proposed as a superior method for evaluating thymocyte apoptosis. In this study, a variety of DNA binding dyes with diverse primary binding mechanisms were evaluated for their ability to detect glucocorticoid and ionizing radiation-induced apoptosis in mouse thymocytes. Apoptotic thymocytes stained with DNA binding dyes from the phenanthridinium, acridine, actinomycin, chromomycinone, anthracycline, and bisbenzimidazole groups all demonstrated clearly defined A0 regions with percentages comparable to those obtained for propidium iodide. These results indicate that the appearance of the A0 region is not dependent on a particular dye binding characteristic and may be the consequence of extensive changes in chromatin structure resulting in a significant degree of dye exclusion.

Epidermal growth factor receptors in idiopathic and virally induced skin diseases

The altered distribution of epidermal growth factor receptors (EGF-R) in hyperproliferative skin lesions such as psoriasis vulgaris, seborrheic keratoses, acanthosis nigricans, ichthyosis, and others implies aberrant control of growth/proliferation by epidermal growth factor (EGF), transforming growth factor alpha (TGF alpha), and other growth factors/cytokines. Whether overexpression of EGF-R: 1) correlates with epidermal proliferation, 2) serves as a hallmark of specific dermatoses, or 3) is due to modulation by multiple growth factors remains unclear. To correlate distributions of EGF-R with in vivo proliferative status, two benign diseases of unknown etiology, seborrheic keratoses and acrochordons (skin tags), were examined using EGF-R immunolocalization and 125I-EGF binding techniques. Lesions documented as growing by clinical criteria or 5-bromodeoxyuridine incorporation (a measure of cell proliferation) were compared to nongrowing lesions of the same type. To correlate distributions of EGF-R to specific dermatoses, skin diseases of viral etiology (verruca vulgaris and molluscum contagiosum) were also probed by EGF-R immunolocalization and 125I-EGF binding. Elevated immunostaining for EGF-R and 125I-EGF binding sites were associated with actively growing seborrheic keratoses and skin tags whereas normal patterns of immunostaining and 125I-EGF binding were seen in nongrowing seborrheic keratoses and skin tags. Viral diseases showed unique patterns. No EGF-R were detected in verruca vulgaris. Molluscum contagiosum lesions showed intense EGF-R in basal keratinocytes and no EGF-R in virally infected cells. Thus elevations in EGF-R show a positive in vivo correlation with proliferation in at least two differing benign diseases of the epidermis. The decreased levels of EGF-R in virally infected lesions suggests that EGF-R may show unique patterns for specific dermatoses and are not universally elevated in benign hyperproliferative skin disorders.

Altered distribution of phospholipase C-gamma 1 in benign hyperproliferative epidermal diseases

Phospholipase C-gamma 1 (PLC-gamma 1) is a well characterized substrate for the epidermal growth factor receptor tyrosine kinase and has been implicated in the intracellular biochemical signaling cascade which occurs following stimulation of cells with epidermal growth factor. The in vivo localization of PLC-gamma 1 was examined by immunohistochemistry in sections of normal human skin and in skin sections from a diverse series of hyperproliferative epidermal conditions (psoriasis, seborrheic keratoses, acrochordons, and margins near second-degree burns). Immunoreactive PLC-gamma 1 was detected only in the basal compartment of normal skin but was readily detectable in both the basal and outer epidermal compartment in hyperproliferative skin conditions. In addition, immunoreactive PLC-gamma 1 colocalizes with immunoreactive epidermal growth factor receptor in both normal and hyperproliferative epidermis.

Acneiform eruption due to eruptive vellus hair cysts

We have reported a case of eruptive vellus hair cysts, a rare type of acneiform eruption. Patients with unusual or refractory acneiform eruptions may have any of a variety of non-acne-vulgaris lesions. For this reason, biopsy and culture of these lesions may be helpful in establishing a correct diagnosis and determining proper treatment.

Modulation of epidermal growth factor receptors in psoriatic lesions during treatment with topical EGF

Active psoriatic lesions have increased EGF/TGF alpha receptors, historically known as the EGF-R. This increase is due to their persistence into the outer parakeratotic layers as measured by autoradiography, immunohistochemistry, and mRNA assays. When psoriatic lesions in patients resolve due to therapy with different modalities, the EGF-R persistently expressed in the outer layers of the epidermis either disappear or resume a basal location presumably due to receptor downregulation. To test whether EGF could downregulate EGF-R and biologically affect psoriatic epidermis, split-thickness skin grafts of active psoriatic lesions were sutured onto the dorsal surface of nude mice. After 3 weeks, the mice were treated daily for a 6-week period with placebo, or 10 or 50 micrograms/ml EGF. Immunostaining showed persistent EGF-R in all epidermal layers in the untreated, placebo-, and 10 micrograms/ml EGF-treated groups. Those grafts receiving a high dose of EGF (50 micrograms/ml) showed either no immunoreactive EGF-R or faint basilar staining. As an additional check for functional activity of the EGF-R, an abundant substrate for this receptor, PLC-gamma 1 was also evaluated following EGF treatment. A similar distribution and modulation pattern following treatment were observed in the grafts immunostained for PLC-gamma 1, suggesting that exogenous EGF treatment affected metabolic pathways subsequent to ligand receptor binding. Morphologic alterations characteristic of a regressing psoriatic phenotype (a decrease in acanthosis, thickness, and the resumption of the orthokeratotic mode of differentiation) were noted in those lesions receiving the 50 micrograms/ml EGF treatment. This study indicates that persistent EGF-R in psoriasis vulgaris are biologically active in vivo and may serve a pivotal role in the regulation of psoriatic lesions.

Comparative evaluation of several DNA binding dyes in the detection of apoptosis-associated chromatin degradation by flow cytometry

Mouse thymocytes readily undergo apoptosis-associated DNA degradation upon exposure to glucocorticoids or ionizing radiation. It has been previously shown that flow cytometric cell cycle analysis of propidium iodide-stained apoptotic thymocytes results in the appearance of a distinct cell cycle region (the A0 region) below the G0/G1 region. Cells in this region were shown to be undergoing apoptosis, and determination of apoptosis by flow cytometric analysis was proposed as a superior method for evaluating thymocyte apoptosis. In this study, a variety of DNA binding dyes with diverse primary binding mechanisms were evaluated for their ability to detect glucocorticoid and ionizing radiation-induced apoptosis in mouse thymocytes. Apoptotic thymocytes stained with DNA binding dyes from the phenanthridinium, acridine, actinomycin, chromomycinone, anthracycline, and bisbenzimidazole groups all demonstrated clearly defined A0 regions with percentages comparable to those obtained for propidium iodide. These results indicate that the appearance of the A0 region is not dependent on a particular dye binding characteristic and may be the consequence of extensive changes in chromatin structure resulting in a significant degree of dye exclusion.