A model that explains how maintenance of normal homeostasis in human epidermis is achieved describes a heterogeneous cell population of stem cells (SC) and transit amplifying cells (TAC). There must be a tightly regulated balance between SC self-renewal and the generation of TAC that undergo a limited number of divisions before giving rise to postmitotic, terminally differentiated cells. To investigate whether this balance is disturbed in psoriatic epidermis, we have characterized flow sorted enriched SC and TAC using immunocytochemistry, flow cytometry, and real-time quantitative PCR. Our data demonstrate phenotypical and functional differences in SC (beta(1)-integrin bright) and TAC (beta(1)-integrin dim) enriched fractions between normal and psoriatic keratinocytes. Some of these were expected, such as mRNA levels of keratins 6 and 10 and of the Ki-67 antigen. Most remarkable were differences in phenotype of the psoriatic TAC compared with TAC from normal skin. These subpopulations also displayed striking differences following culture. Downregulation of markers associated with the regenerative phenotype (psoriasin, elafin, psoriasis-associated fatty acid binding protein) in cultured psoriatic dim cells in the absence of hyperproliferation suggests that proliferation and regenerative maturation are coupled. From these results, in combination with our earlier findings, we propose a model for epidermal growth control in which TAC play a crucial role.

BackgroundTo study the apoptotic process in time, we used the following flow cytometric (FCM) techniques: phosphatidylserine (PS) translocation by Annexin‐V (AnV), DNA fragmentation by in situ end labeling (ISEL), and propidium iodide (PI) staining. Because PS translocation is assumed to be an early feature of programmed cell death (PCD), we questioned if AnV positivity implies inevitable cell death.MethodsApoptosis was induced in Jurkat cells by γ‐irradiation, incubation with camptothecin (CPT), or cytosine β‐D‐arabinofuranoside (Ara‐C). At different time intervals, PCD was quantified by AnV/PI and ISEL. To analyze the influence of cell handling procedures on PCD, we applied these three FCM techniques on CD34+ bone marrow (BM) stem cells after selection and after a freeze‐thaw procedure. Various AnV/PI− CD34+ fractions were cultured in a single‐cell single‐well (SCSW) assay.ResultsJurkat cells under three different detrimental conditions showed essentially the same pattern of apoptosis in time. Initially developed AnV+/PI− cells subsequently (within 1 h) showed ISEL positivity, after which they turned into AnV+/PI++ cells with even higher levels of ISEL positivity (80–90%). Eventually, they lost some of their PI and ISEL positivity and formed the AnV+/PI+ fraction. Cell handling of CD34+ cells caused high and variable AnV+/PI− fractions (overall range 23–62%). Within total AnV+ and AnV+/PI− populations, only a minority of CD34+ cells showed ISEL positivity (range 4–8% and 0.8–6%, respectively). Different fractions of AnV+/PI− CD34+ cells did have clonogenic capacity.ConclusionsPCD of cell suspensions in vitro can be followed accurately in time by these three FCM techniques. PS translocation is followed rapidly (within 1 h) by oligo‐nucleosomal DNA fragmentation, after which cell (and nuclear) membrane leakage occurs. Detection of PS asymmetry by AnV‐fluorescein isothiocyanate (FITC) is not always associated with (inevitable) apoptosis, as can be concluded from the proliferative capacity of AnV+ /PI− CD34+ cells in the SCSW assay. Cytometry 47:24–31, 2002. © 2001 Wiley‐Liss, Inc.

Summary.Pgp is expressed on normal haemopoietic progenitor cells. The significance of the efflux pump in protecting normal progenitors for anthracycline toxicity is not defined and is the subject of this study. Pgp was measured in CD34 þ progenitors with a rhodamine efflux assay. A high efflux, modulated by verapamil, was only found in a distinct subpopulation (20-30%). Pgp measured by the monoclonal antibody antibody (MoAb) MRK-16 was low in the rhodamine dull, but significantly (P < 0 . 04) higher than in the rhodamine bright cells. Reverse transcriptase polymerase chain reaction (RT-PCR) of MDR1 mRNA showed a very weak signal in both populations. In a single-cell clonogenic assay, rhodamine dull cells appeared less sensitive to anthracyclines (IC 50 daunorubicin 0 . 005 mg/ml; adriamycin 0 . 03 mg/ml) compared to rhodamine bright cells (IC 50 daunorubicin 0 . 0025 mg/ml; adriamycin 0 . 01 mg/ml). Furthermore, verapamil significantly (P < 0 . 05) potentiated anthracycline toxicity only in the rhodamine dull cells, proving its Pgp-specific modulating effect. Rhodamine dull cells gave larger and more mixed colonies compatible with a more primitive origin. Although detection with MoAbs and RT-PCR revealed a low Pgp level, functionally this Pgp appeared to be very important in protecting primitive progenitors against anthracycline toxicity. This protection can be jeopardized by administration of Pgp modulators.

PCD of cell suspensions in vitro can be followed accurately in time by these three FCM techniques. PS translocation is followed rapidly (within 1 h) by oligo-nucleosomal DNA fragmentation, after which cell (and nuclear) membrane leakage occurs. Detection of PS asymmetry by AnV-fluorescein isothiocyanate (FITC) is not always associated with (inevitable) apoptosis, as can be concluded from the proliferative capacity of AnV+ /PI- CD34+ cells in the SCSW assay.

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