Quantitation of Fc gamma RII mRNA in platelets and megakaryoblastic cell lines by a new method of in situ hybridization

An immunohistological study of anhydrous topical ascorbic acid compositions on ex vivo human skin

BACKGROUND

Ascorbic acid has numerous essential and beneficial functions in normal and photoaged skin. Ionisation of ascorbic acid in aqueous topical formulations leads to oxidative degradation. Ascorbic acid in an anhydrous vehicle would inherently have greater stability.

OBJECTIVE

The objective of this study was to observe the effects of two anhydrous formulations containing microfine particles of ascorbic acid on neocollagenesis and cytokeratin production in ex vivo human skin.

METHODS

Vitamin C preparations were applied topically onto the surface of freshly excised human abdominal skin. Following an exposure time of 48 h with appropriate controls, skin discs were cut into sections, placed on slides and assessed using immunohistochemical (antibodies: collagen type I, III, cytokeratin) staining. Analysis was performed using microscopy and descriptive rating.

RESULTS

Both formulations resulted in increased production of collagen types I and III and cytokeratin.

CONCLUSION

The application of anhydrous formulations containing microfine particles of ascorbic acid to ex vivo human skin in this study resulted in neocollagenesis and increased production of cytokeratin. This approach appears to enable biological effects of ascorbic acid in the skin using a vehicle which would provide it greater stability than an aqueous vehicle.

Proliferation and bone-related gene expression of osteoblasts grown on hydroxyapatite ceramics sintered at different temperature

Human osteoblast-like cells SaOS-2 (ATCC HTB85) were seeded onto three kinds of hydroxyapatite (HA) ceramics sintered at different temperature (1200 degrees C, 1000 degrees C and 800 degrees C). Scanning electron microscopy (SEM) was conducted to detect the surface microstructure. Cells were cultured on these substrates for 6 and 12 days and cell proliferation rate and mRNA expression for osteocalcin, osteonectin, type I collagen and alkaline phosphatase and protein production for osteocalcin, bone sialoprotein and osteonectin were detected with quantitative in situ hybridization and immunocytochemistry techniques. SEM revealed that crystal particle size was affected by sintering temperature. Result showed that cell proliferation rate on HA ceramics sintered at 1200 degrees C was the highest. Osteonectin and type I collagen mRNA expression was not altered by sintering temperature. After 12 days in culture, bone sialoprotein, osteocalcin and osteonectin proteins levels were significantly (p<0.05) higher when SaOS-2 cells were cultured on HA sintered at 1200 degrees C, compared to the other two surfaces, suggesting that HA sintered at high temperature may be a better candidate for in vivo implantation. This result provides valuable information concerning the clinic application of HA ceramics sintered at different temperature.

Phenotypic expression of bone-related genes in osteoblasts grown on calcium phosphate ceramics with different phase compositions

Calcium phosphate ceramics with different hydroxyapatite (HA) and tricalcium phosphate (TCP) ratios have different chemical properties. Does the difference in phase composition affect osteoblast behavior? In this study, osteoblasts were cultured on 4 kinds of calcium phosphate ceramics, i.e. pure (HA), HT1 (HA/TCP, 70/30), HT2 (HA/TCP, 35/65), and pure TCP. Cell proliferation of SaOS-2 cells together with bone-related genes' mRNA expression and protein production in osteoblasts cultured on different calcium phosphate ceramics were detected at different time points. Data suggested that cell proliferation rate on TCP ceramics was lower than that on the other substrates tested. Generally, mRNA expressions for osteonectin and osteocalcin were similar among the four kinds of ceramics in most circumstances, whereas at six days, alkaline phosphatase mRNA expression was higher on HA and HT1 surfaces than on the other two materials. Collagen I mRNA expression was also affected by the phase composition of substrates. Osteocalcin and bone sialoprotein production in SaOS-2 cells was very similar no matter which ceramic surface the cells were grown upon. This study revealed that calcium phosphate ceramics substrate could support osteoblast growth and bone-related gene expression and its gene expression pattern explained the basis of the biocompatibility and bioactivity for calcium phosphate ceramics.

Differential expression of osteogenic factors associated with osteoinductivity of human osteosarcoma cell lines

Differential expression of multiple osteogenic factors may be responsible for the different osteoinductivity of osteosarcoma cell lines. We compared in vivo osteoinductivity of human osteosarcoma cell lines (Saos-2 vs. U-2 OS) in nude mice, and their in vitro expression of various osteogenic factors of protein level by quantitative immunocytochemistry and mRNA level by RT-PCR and/or in situ hybridization. Saos-2 cells, but not U-2 OS, were osteoinductive in vivo. Significantly higher expression (independent t-test, all p < 0.005) of osteogenic factors were observed in Saos-2 cells compared with U-2 OS, which included bone morphogenetic proteins (particularly BMPs-2, 3, 4, and 7), transforming growth factor-beta (TGF-beta), BMP receptor (BMPR)-1A, receptor-regulated Smads (R-Smads), Smads 1, 2, and 5, and common-mediator Smad (Co-Smad), Smad 4. In contrast, U-2 OS cells expressed higher levels of inhibitory Smad 6 (I-Smad) protein than Saos-2 cells (p < 0.001). These results suggest that a combination of osteogenic factors (BMPs, TGF-beta, BMPRs, and R/Co-Smads) against I-Smad may play important roles in the Saos-2 cell osteoinductivity. This may have a clinical implication in selecting key osteogenic factors for combined therapy for bone defect diseases. The characterized cell lines can be used as positive and negative controls for the assessments of both in vitro and in vivo bone formation capabilities of designed tissues or biomaterials.

Prosthetic particles modify the expression of bone-related proteins by human osteoblastic cells in vitro

Loss of bone near joint prostheses is thought to be caused by activation of recruited osteoclasts by osteolytic mediators induced by wear particles. It is proposed that particles inhibit osteogenesis during bone remodelling causing a reduction in the levels of peri-implant bone. This study explores whether prosthetic particles modulate bone formation by affecting osteoblastic bone-related mRNAs (alkaline phosphatase, pro-collagen Ialpha1, osteopontin, osteonectin, osteocalcin, bone sialoprotein and thrombospondin) or their translated proteins using titanium alloy, commercially pure titanium, and cobalt-chrome particles. The direct effect of the particles revealed no change to the expression of the bone-related mRNAs in human bone-derived cells (HBDC) at the time points investigated; although non-collagenous translated proteins expressed by these HBDC were significantly effected (p<0.05). Different patterns of expression for bone-related proteins were induced by the different particles both directly and indirectly. Inflammatory mediators (interleukin-1beta, tumor necrosis factor alpha, interleukin-6, and prostaglandin E2) had similar effects on HBDC to the media obtained from monocytes incubated with particles. This study shows that prosthetic wear particles can significantly modify the expression of bone-related proteins by osteogenic cells in vitro. These alterations in osteogenic activity at the interface of the implant and bone may be an important factor in the failure of many orthopaedic implants.

Differentiation of human bone-derived cells grown on GRGDSP-peptide bound titanium surfaces

Various surface modifications have been applied to titanium alloy (Ti-6Al-4V) implants, in an attempt to enhance osseointegration; crucial for ideal prosthetic fixation. Despite the numerous studies demonstrating that peptide-modified surfaces influence in vitro cellular behavior, there is relatively little data reporting their effects on bone remodeling. The objective of this article was to examine the effects of chemically modifying Ti-6Al-4V surfaces with a common RGD sequence, a 15-residue peptide containing GRGDSP (glycine-arginine-glycine-aspartate-serine-proline), on the modulation of bone remodeling. The expression of proteins known to be associated with osseous matrix and bone resorption were studied during the growth of human bone-derived cells (HBDC) on these peptide-modified surfaces. HBDC grown for 7 days on RGD surfaces displayed significantly increased levels of osteocalcin, and pro-collagen Ialpha1 mRNAs, compared with the production by HBDC grown on the native Ti-6Al-4V. A pattern that was also reflected at the protein levels for osteocalcin, type I collagen, and bone sialoprotein. Moreover, HBDC grown for 7 and 14 days on RGD-modified Ti-6Al-4V expressed significantly higher level of osteoclast differentiation factors and lower levels of osteoprotegerin and IL-6 proteins compared with other surfaces tested. These results suggest that different chemical treatments of implant material (Ti-6Al-4V) surface result in differential bone responses, not only their ability to form bone but also to stimulate osteoclastic formation.

The role of platelet α-granular proteins in the regulation of thrombopoietin messenger RNA expression in human bone marrow stromal cells

Thrombopoietin (TPO), the specific cytokine that regulates platelet production, is expressed in human bone marrow (BM), kidney, and liver. There appears to be no regulation of TPO in the kidney and liver, but TPO messenger RNA (mRNA) expression can be modulated in the stromal cells of the BM. In this study, we used primary human BM stromal cells as a model to study the regulation of TPO mRNA expression in response to various platelet -granular proteins. We showed that platelet-derived growth factor (PDGF) BB and fibroblast growth factor (FGF) 2 stimulated TPO mRNA expression in both a dose-dependent and time-dependent manner. The addition of 50 ng/mL of PDGF and 20 ng/mL of FGF resulted in maximal induction of TPO mRNA expression in 4 hours. We also found that platelet factor 4 (PF4), thrombospondin (TSP), and transforming growth factor-beta (TGF-β) are negative modulators of megakaryocytopoiesis. We observed suppression in TPO mRNA expression with 1 μg/mL of both PF4 and TSP and 50 ng/mL of TGF-β, with maximal suppression occurring 4 hours after the addition of these proteins. Finally, the addition of whole-platelet lysate produced a dose-dependent inhibition of TPO expression. On the basis of these findings, we propose that the platelet -granular proteins studied may regulate TPO gene expression in BM stromal cells by means of a feedback mechanism.

The role of platelet α-granular proteins in the regulation of thrombopoietin messenger RNA expression in human bone marrow stromal cells

Thrombopoietin (TPO), the specific cytokine that regulates platelet production, is expressed in human bone marrow (BM), kidney, and liver. There appears to be no regulation of TPO in the kidney and liver, but TPO messenger RNA (mRNA) expression can be modulated in the stromal cells of the BM. In this study, we used primary human BM stromal cells as a model to study the regulation of TPO mRNA expression in response to various platelet -granular proteins. We showed that platelet-derived growth factor (PDGF) BB and fibroblast growth factor (FGF) 2 stimulated TPO mRNA expression in both a dose-dependent and time-dependent manner. The addition of 50 ng/mL of PDGF and 20 ng/mL of FGF resulted in maximal induction of TPO mRNA expression in 4 hours. We also found that platelet factor 4 (PF4), thrombospondin (TSP), and transforming growth factor-beta (TGF-β) are negative modulators of megakaryocytopoiesis. We observed suppression in TPO mRNA expression with 1 μg/mL of both PF4 and TSP and 50 ng/mL of TGF-β, with maximal suppression occurring 4 hours after the addition of these proteins. Finally, the addition of whole-platelet lysate produced a dose-dependent inhibition of TPO expression. On the basis of these findings, we propose that the platelet -granular proteins studied may regulate TPO gene expression in BM stromal cells by means of a feedback mechanism.

Relationship between expression of the KAI1 metastasis suppressor and other markers of advanced bladder cancer

Expression of a newly described inhibitor of tumour metastasis, KAI1, was examined in bladder cancer progression and compared with the expression of p53 and pRb, which are markers of advanced disease. KAI1 mRNA (by in situ hybridization) and protein levels (by immunohistochemistry) were examined in 135 paraffin-embedded bladder tissue sections. Significant decreases in KAI1 mRNA and protein levels were detected between normal and tumour tissue (p<0.001 and p=0.026, respectively), and between non-invasive and invasive tumours (p=0.046 and p<0.001, respectively). Loss of KAI1 protein expression was accompanied by a shift in staining pattern from a uniform distribution to a weaker, membranous or heterogeneous pattern. Normal tissue and low-grade tumours showed little p53 protein staining. High level staining (indicative of mutant p53) was associated with increased grade in non-invasive tumours (p=0.031) but was not significantly higher in invasive tumours. Whilst p53 protein staining increased with malignant progression and KAI1 mRNA expression decreased, there was no significant correlation between the two patterns (p=0.33, adjusted for group, p=0.18) or when only cancer samples were analysed (p=0.065, adjusted for group, p=0.26), even when taking into account overexpression of MDM-2 protein as a pathway for inactivation of p53. There was no correlation between loss of KAI1 mRNA expression and gain of abnormal pRb staining (p=0. 30, or adjusted for tumour samples only, p=0.59). These results suggest that loss of KAI1 expression is associated with invasive bladder cancer, but is not related to mutation of p53 or to loss of normal pRb expression.

Titanium substrata composition influences osteoblastic phenotype: In vitro study

In spite of observed differences at the interface between boon and either commercially pure titanium [Ti(cpi)] or titanium alloy (Ti-6Al-4V), the mechanism of such a response is ill understood. This prompted further investigation of the influence of similar metals on human bone-derived cells (HBDCs). This study investigated the influence of Ti(cpi) and its alloy on osteoblastic proteins formed by HBDCs grown for 5, 7, 10, and 14 days on these metals and compared them to cells grown on tissue culture polystyrene plates. Messenger RNA and translated proteins that form an array of osteogenic parameters were determined: alkaline phosphatase (ALP), thrombospondin, osteopontin, osteocalcin (OC), osteonectin (ON/SPARC), type I collagen (Col I) and bone sialoprotein (BSP). At the four predetermined time points, cells grown on either Ti(cpi) or Ti-6Al-4V generally expressed similar mRNA levels, while levels of their respective proteins differed. Cells on Ti(cpi) had peak levels for most proteins at day 7, whereas those on Ti-6Al-4V peaked at either day 5 and/or day 7. At day 5 cells grown on Ti-6Al-4V had higher levels of ALP, Col I, ON/SPARC, OC, and BSP than those in Ti(cpi); this difference was not maintained at later time points in culture. The differential regulation of proteins occurring between cells from the same patient grown on titanium and its alloy implies that HBDCs respond to small differences in the surface chemistry and/or microcrystallinity.

Effect of surface chemical modification of bioceramic on phenotype of human bone-derived cells

In the search for methods to improve the biocompatibility of prosthetic materials, attention has recently been directed toward the potential use of surface chemical modification and its influence on cellular behavior. This in vitro study investigates the effect of surface chemistry modification of bioceramics on human bone-derived cells (HBDCs) grown on biomaterial surfaces for 2 weeks. Cells were cultured on either alumina (Al2O3), alumina doped with magnesium ions ([Mg]-Al2O3), or hydroxyapatite (HAP), as well as tissue culture polystyrene (TCPS). Expression of alkaline phosphatase (ALP), thrombospondin (Tsp), osteopontin (OP), osteocalcin (OC), osteonectin (ON/SPARC), type I collagen (Col I), and bone sialoprotein (BSP) were determined in terms of mRNAs and proteins. Protein levels for ALP, OP, OC, and BSP were significantly (p < 0. 05) greater at day 5 in HBDCs cultured on [Mg]-Al2O3 compared to those cells grown on Al2O3. At day 14 the levels of ALP, Tsp, Col I, OP, ON/SPARC, and BSP rose significantly (p < 0.05) above those occurring in HBDCs grown on Al2O3, HAP, and TCPS. This suggests that HBDCs from the same patient respond to differences in the surface chemical groups. This study confirms that the chemistry of a substratum, which facilitates cellular adhesion, will enhance cellular differentiation.

Quantitative aspects of an in situ hybridization procedure for detecting mRNAs in cells using 96-well microplates

The universal quantitation of the DNA hybridization reaction has been a goal sought by many researchers. Part of this search has been the need to develop a rapid, sensitive, easy-to-perform, and quantitative method to measure the abundance of specific mRNAs directly within cells. Conventionally mRNA detection can be done by advanced quantitative in situ hybridization (ISH) using either image analysis or fluorescence in situ hybridization (FISH), or indirectly by extraction of mRNA from cells or tissue and using Northern blot or quantitative polymerase chain reaction (PCR). We examined the quantitative nature of probe binding to intracellular mRNA in a sensitive and easy-to-use nonisotopic method of ISH previously developed in our laboratories. The method is applicable to isolated primary cells or cells in culture. The procedural details are very simple, with cells being centrifuged into 96-well microplates, fixed with formalin, and pretreated with Triton X-100 and Nonidet P-40 before photobiotin-labeled cDNA probes are applied. Biotin from the hybridization of probe to target is detected using multiple applications of streptavidin and biotinylated alkaline phosphatase and visualized by the p-nitrophenyl phosphate conversion method. The quantitative parameters of the ISH procedure were determined by measuring the levels of expression of erythropoietin (EPO) mRNA and its translated protein in transfected COS-7 cells. There is a log-linear relationship between the levels of signal obtained in the ISH reaction in 96-well microplates and the EPO protein levels measured by enzyme-linked immunosorbent assay (ELISA). This demonstrated relationship is important in the standardization and use of these procedures to measure quantitatively mRNAs within cells.

Localization and Regulation of Thrombopoietin mRNA Expression in Human Kidney, Liver, Bone Marrow, and Spleen Using In Situ Hybridization

Thrombopoietin (TPO) is the primary hematopoietic growth factor involved in the regulation of platelet production. Although the kidney, liver, bone marrow (BM), and spleen have been identified as the major sources of TPO production, the precise cellular location of TPO mRNA expression in these tissues remains unknown. We have identified the cells expressing TPO mRNA in the human kidney, liver, and BM using an in situ hybridization assay. In the BM of individuals with normal platelet counts, the hybridization signal was too weak to allow identification of the TPO mRNA expressing cells. However, in thrombocytopenic subjects with aplastic anemia, postchemotherapy marrow aplasia, and immune thrombocytopenia, the stromal cells showed strong TPO mRNA expression. In the human subjects with normal platelet counts, the cells of the proximal convoluted tubules of the kidney showed consistent positive staining whereas the signal in the cells of the distal convoluted tubules was less consistent. Strong hybridization signal was also evident in the hepatocytes. The hybridization signal in the spleen, even in thrombocytopenic subjects, was too weak to allow confident identification of the cells expressing TPO mRNA. In all subjects, the interstitial cells and endothelial cells of the liver and spleen, the renal peritubular cells, and the hematopoietic precursor cells of the BM showed no TPO mRNA expression. Our data suggest that TPO mRNA expression in the human BM may be modulated by platelet mass.

Localization and Regulation of Thrombopoietin mRNA Expression in Human Kidney, Liver, Bone Marrow, and Spleen Using In Situ Hybridization

Thrombopoietin (TPO) is the primary hematopoietic growth factor involved in the regulation of platelet production. Although the kidney, liver, bone marrow (BM), and spleen have been identified as the major sources of TPO production, the precise cellular location of TPO mRNA expression in these tissues remains unknown. We have identified the cells expressing TPO mRNA in the human kidney, liver, and BM using an in situ hybridization assay. In the BM of individuals with normal platelet counts, the hybridization signal was too weak to allow identification of the TPO mRNA expressing cells. However, in thrombocytopenic subjects with aplastic anemia, postchemotherapy marrow aplasia, and immune thrombocytopenia, the stromal cells showed strong TPO mRNA expression. In the human subjects with normal platelet counts, the cells of the proximal convoluted tubules of the kidney showed consistent positive staining whereas the signal in the cells of the distal convoluted tubules was less consistent. Strong hybridization signal was also evident in the hepatocytes. The hybridization signal in the spleen, even in thrombocytopenic subjects, was too weak to allow confident identification of the cells expressing TPO mRNA. In all subjects, the interstitial cells and endothelial cells of the liver and spleen, the renal peritubular cells, and the hematopoietic precursor cells of the BM showed no TPO mRNA expression. Our data suggest that TPO mRNA expression in the human BM may be modulated by platelet mass.

Fibroblast growth factor and heparin protect endothelial cells from the effects of interleukin 1

Vascular endothelium is involved in both active and passive processes in haemostasis, but inflammatory cytokines such as interleukin 1 (IL-1) and tumour necrosis factor (TNF) have been reported to convert the comparatively inert endothelial cell to an inflammatory state. Acidic fibroblast growth factor (aFGF) in the presence of heparin has effects opposite to IL-1 on cultured human umbilical vein endothelial cells (HUVEC); therefore, we have investigated the modulation of IL-1-induced effects by the c combination of aFGF and heparin (aFGF/heparin). First passage HUVEC were cultured for 6 days in the presence of 20% human serum with and without the addition of 625 pM human recombinant aFGF (hr aFGF) and 7 microM heparin. On day 5, recombinant IL-1 beta was included for 24 h. The following day the cells were washed and measurements made of the release of prostacyclin, von Willebrand factor, plasminogen activator inhibitor type 1, and thrombospondin, both in the resting state and following stimulation for 60 min with 1 U/ml thrombin. Tissue-type plasminogen activator was assayed in HUVEC lysates. Similar experiments were performed to assess effects on the expression of vascular adhesion molecule, intracellular adhesion molecule, and E-selectin using an ELISA on cells in situ. This study indicates that aFGF/heparin in the culture medium of HUVEC abrogates the measured responses to IL-1. These data imply that routine endothelial cell culture with aFGF/heparin may cause artefacts, the effects of FGF and Il-1 may involve common pathways, and FGF/heparin may offer an approach to design therapeutics to counter the adverse effects of IL-1.

Characterization of IgG Fc receptors on CD34 antigen-expressing cell lines (KG-1 and KG-1a)

Although Fc gamma receptors (Fc gamma R) on mature blood cells have been extensively studied, there are only limited data on Fc gamma R expression in the early haematopoietic progenitor cells. In this study, we used the stem cell antigen (CD34)-expressing cell line (KG-1) and its less differentiated subline (KG-1a) as a model for the study of Fc gamma R in the early haematopoietic progenitors. Flow cytometry and immunoprecipitation studies on KG-1 and KG-1a cells with anti-Fc gamma R mAb showed that Fc gamma RII is the only Fc gamma R expressed on the cell surface. Analysis of the steady-state levels of Fc gamma R mRNA in KG-1 and KG-1a cells using a quantitative in situ hybridization assay revealed the presence of only Fc gamma RII mRNA. On further analysis Fc gamma RIIA mRNA but no Fc gamma RIIB or Fc gamma RIIC transcripts were found in these cells; Fc gamma RIIA transcripts with and without the transmembrane exon were present in approximately equal amounts. These findings are surprisingly similar to those observed previously with Fc gamma R in platelets and megakaryocytic cells but different from those found with Fc gamma R in cells of other lineages. These data suggest that the Fc gamma R transcript distribution pattern observed in the early haematopoietic progenitors (KG-1 cells) is retained in later stages of haematopoietic differentiation only in cells of megakaryocytic lineage.

A novel technique for quantitative detection of mRNA expression in human bone derived cells cultured on biomaterials

A nonisotopic and quantitative in situ hybridization technique was adapted to investigate the effect of biomaterials on the cellular expression of mRNA from human bone derived cells (HBD cells). HBD cells were cultured for 24 or 48 h on tissue culture plastic, alumina, and ion modified alumina. Osteocalcin, osteopontin, alkaline phosphatase, type I collagen alpha 1, and type I collagen alpha 2 mRNAs were quantified. Protein expression for collagen types I, III, and V, and for anti-human macrophages CD68 (DAKO-CD68, KP1) and CD68 (PG-M1), and anti-human myeloid/histiocyte antigen (DAKO-MAC 387) were determined immunohistochemically using monoclonal antibodies. At 24 and 48 h, levels of mRNA for alkaline phosphatase and osteonectin were greater than mRNA levels for osteopontin, osteocalcin, collagen type I alpha 1, and collagen type I alpha 2 for cells grown on the three substrata. However, at 48 h mRNA levels for alkaline phosphatase and osteonectin were significantly higher on the modified ceramic substrata relative to the native alumina. HBD cells appear to express CD68-KP1 when cultured for 24 h. The techniques provide a sensitive and reproducible assay to evaluate gene and protein expression of cells grown on different substrata.

Quantitation of soluble and membrane-bound Fc gamma RIIA (CD32A) mRNA in platelets and megakaryoblastic cell line (Meg-01)

Fc gamma receptors (Fc gamma Rs) are glycoproteins on platelet surface that bind IgG-containing immune complexes. However, excessive binding of immune complexes leads to platelet activation and thrombosis or increased platelet clearance and thrombocytopenia. In this study, Fc gamma R transcripts in platelets and megakaryoblastic cell line (Meg-01) were investigated using specifically designed oligonucleotides and a new quantitative in situ hybridization assay. Platelets and Meg-01 cells were found to express only Fc gamma RII transcripts. Of Fc gamma RIIA mRNA isoforms (Fc gamma RIIA, B and C), Fc gamma RIIA mRNA predominates in these cells. Platelets and Meg-01 cells contain both alternative spliced forms of Fc gamma RIIA mRNA, those with and without the transmembrane (TM) exon and both forms were present in near equal amounts. In contrast, Fc gamma RIIA transcript with the TM exon predominates in neutrophils and monocytes, suggesting that the splicing of the TM exon is under lineage-specific control.