Rat Thymus Cells Research Articles

CISPLATIN AND PROGESTERONE SEPARATE AND JOINT ACTION ON RAT THYMUS NUCLEAR PHOSPHOLIPIDS COMPOSITION AND CONTENT

The antitumor agent cisplatin and progesterone separate and joint action on content of total phospholipids and their individual fractions in nuclei from rat thymus cells were investigated. When used separately, cisplatin and progesterone exhibit its own distinctive properties. Cisplatin reduces, and progesterone, on the contrary, increases the content of total phospholipids. When used together, the effects of these drugs are summed up. Thus total phospholipid content of rat thymus cell nuclei increases by 17% after the joint injection of cisplatin and progesterone. These changes have different effects on quantities of individual phospholipid fractions. It is assumed that the effects of cisplatin and progesterone, both in their separate and joint action, can be mediated by quantitative changes in internuclear lipids that in turn can regulate the main functions of the cell nuclei.

Determination of the quantity and topography dynamics of the LCA+-dendritic cells in rat thymus in early postnatal period in norm and after the prenatal administration of the dexamethasone

Purpose was to study the dynamics of LCA + -dendritic thymus cells in newborn rats in the postnatal period after administration of dexamethasone in the intrauterine period. Materials and methods. The study was performed on 144 white nonlinear rats on the 1 st –3 rd , 5 th , 9 th , 14 th , 21 st , 30 th days after birth. There are 3 groups with 48 rats in each group: 1 – intact group; 2 – experimental group of rats, who received peritoneal injection of 0.05 ml of dexamethasone (in dilution 1:40) on the 18 th day of the prenatal period, and a control group of rats receiving 0.05 ml of 0.9 % NaCl according to M. A. Voloshyn. To study the dynamics of dendritic cells in the thymus carrying the α-D mannose receptors, LCA-lectin was used. Results . It has been established that the maximum content of LCA + -dendritic thymus cells in the intact (3.92 ± 0.65), experimental (3.41 ± 0.49) and control (4.07 ± 0.53) groups occurs on the 1 st day after birth. The minimum number of LCA + -dendritic cells was detected on the 5 th day in the intact (1.88 ± 0.37) and control (1.73 ± 0.34) groups, and their minimum in the experimental group (2.35 ± 0.48) was determined on the 30 th day after birth. In the experimental group from the 5 th day to the 9 th day a significantly higher number of LCA + -dendritic cells than in the comparison groups was detected. Conclusions. The introduction of dexamethasone in the prenatal period leads to the significant increase in the LCA + -dendritic cell content in the experimental group from the 5 th to the 9 th day of life, which indirectly coincides with the increase in the number of peripheral and mitotic dividing lymphocytes in the thymus of rats in the postnatal period.

Fluorescein-5-thiosemicarbazide (FTSC) labeling for fluorescent imaging of pectin-derived oligogalacturonic acid transported in living cells by confocal microscopy

Confocal microscopy is a powerful and effective tool for in vivo elucidation of the molecular basis of biological molecules. Chemical labeling of non-ultraviolet or non-fluorescent carbohydrate with fluorescent tag is an essential step that makes intra-cellular microscopic inspection possible. However, current labeling methods have proved unsuitable for functional pectin-derived oligogalacturonic acid due to the low fluorescence intensity and/or the formation of lactone byproducts. Here, fluorescein-5-thiosemicarbazide (FTSC) was employed for fluorescent labeling of pectin-derived oligogalacturonic acid under mild and neutral reaction conditions to avoid the generation of lactone byproducts. ESI–MS and LC–MS data displayed that pectin-derived pentagalacturonic acid, GalA5, could be quantitatively labeled with FTSC to afford thiosemicarbazones, without any lactone byproducts yielded. Moreover, GalA5-FTSC thiosemicarbazone showed good stability and desirable fluorescence characteristics and exhibited rather low levels of cytotoxicity against sensitive rat thymus cells. Additionally, we have achieved successful fluorescent imaging of GalA5-FTSC thiosemicarbazones transported in living SW-620 and HepG2 cells. The assay illustrates methodological significance in structure–activity elucidation study of functional carbohydrates especially acid oligo-/polysaccharides.

Effect of non-neuronal acetylcholine on proliferation and differenation of rat thymus cells

Objective To explore the effect of non-neuronal acetylcholine on the viability and differentiation of rat thymus cells.Methods The thymus cells were isolated from 10 rats,and cultured in vitro.The cells were divided into 4 groups:group A,control; group B,atropine; group C,vecuronium bromide; group D,atropine and vecuronium bromide.The viability was detected by using cell counting kit-8,and the differentiation by flow cytometry.Results The viability in group B was significantly lower than in the rest groups (P ＜ 0.05).The viability in group C was significantly higher than in the rest groups (P ＜ 0.05).There was no significant difference in the viability between group A and group D.The proportion of CD3 + CD4 + T cells and CD3 + CD8 + T cells showed no significant difference among the 4 groups (P ＞ 0.05).The ratio of CD3 + CD4 + T cells to CD3 + CD8 + T cells showed no significant difference among the 4 group (P ＞ 0.05).Conclusion ACh acts on the thymus cells in two opposite ways.It can enhance the viability of the cells,stimulating muscarinic receptors.In reverse,it can decrease the viability,stimulating nicotinic receptors.The experiment doesn' t show evidence that ACh have effect on the differen-tiation of neither CD3 + CD4 + T cells nor CD3 + CD8 + T cells. Key words: Non-neuronal acetylcholine; Thymocyte ; Cell viability; Cell differentiation

Immunomodulating Effects of Vilon and Its Analogue in the Culture of Human and Animal Thymus Cells

We studied molecular mechanisms of immunoprotective effects of two dipeptides, AB-O and R-1, on cultured human and rat thymic cells. Both dipeptides were shown to increase the expression of lymphocyte differentiation marker CD5 in thymic cells. Dipeptide AB-O induced T-cells precursor differentiation towards CD4(+)T-helpers and its effect was weaker than that of dipeptide R-1. Dipeptide R-1 stimulates differentiation of CD5(+) cells to mature T-helpers and cytotoxic CD8(+) T cells and hence can be considered as a bioactive substance possessing immunomodulator and antiallergic activity.

One-pot fluorescent labeling of saccharides with fluorescein-5-thiosemicarbazide for imaging polysaccharides transported in living cells

A simple and efficient procedure for the fluorescent labeling of saccharides is a prerequisite step for imaging the transport of polysaccharides in living cells. We report a one-pot strategy for the fluorescent labeling of saccharides with fluorescein-5-thiosemicarbazide (FTSC), which introduces the thiosemicarbazide group of FTSC to the aldehyde group at the reducing end of saccharides to form stable amino derivatives via reductive amination. The Glc-FTSC derivative was characterized by HPLC–MS, HRESIMS and NMR spectroscopy. Saccharides were quantitatively labeled with FTSC at 75 °C for 1 h under optimum reaction conditions. Fluorescence studies illustrated that the conjugation of FTSC to saccharides did not change its florescence properties ( λ ex = 495 nm, λ em = 517 nm), presenting desirable compatibility with commonly used fluorescence equipment. Polysaccharide AAG-FTSC derivatives exhibited rather low levels of cytotoxicity against rat thymus cells, and the fluorescent labeling procedure had slight impact on their anti-tumor activity. Results indicate that the assay neither introduces discernible cytotoxicity against living cells nor obviously alters the functional activities of polysaccharides, and provides a convenient, highly efficient fluorescent labeling approach for imaging the transport of polysaccharides in living cells.

Heroin-induced changes of CD34-positive rat thymus cells

Heroin is a well known opioid that causes al-terations to the immune system of a number of investigated animals. Only a few studies have explored the effect of heroin on the lymphocytes maturation. Thymocyte progenitors originate from haematopoietic stem cells in the bone marrow. The immature T-cells express neither CD4 nor CD8, and are therefore classed as double-negative (CD4-CD8-) cells. CD34 glycoprotein is the only defined marker of the immature T-lymphocytes. In this study we have investigated the changes induced in CD34+ rat thymocytes after heroin administration by immunofluores-cence in frozen rat thymus sections using the 4H11 monoclonal anti-CD34 antibody. There is a remarkable decrease in the number of CD34+ immature thymocytes when examined 1hour after last heroin injection and a small recover when examination took place 20 days after last injection. The above results suggest a major effect of heroin administration early in the ma- turation process of T lymphocytes probably by increasing the apoptotic cell death and the ne- gative consequences for the immune system responses.

10.1007/s11827-008-1008-5

It is generally recognized nowadays that active lipid metabolism takes place in the nucleus of a mammalian cell. Experimental data testify to the biosynthesis of polyphosphoinositides and phosphatidylcholine and reveal corresponding enzymes within nuclei of mammalian cells. These findings suggest that lipidmediated signaling pathways in nuclei operate independently of lipid-mediated regulatory mechanisms functioning in membranes and cytosol. To explore the pathways of intranuclear lipid biosynthesis, we studied incorporation of 2-14C-acetate into lipids of cytosol and isolated nuclei of rat thymus cells after separate and combined incubation with the labeled precursor. The most efficient incorporation of 2-14C-acetate into lipids (cholesterol, free fatty acids, and phospholipids) was observed in a reaction mixture containing cytosol. When the reaction mixture contained only nuclei, incorporation of the radioactive precursor into lipids also took place, but specific radioactivity of the lipids was essentially lower than in the cytosol. In both cases, 2-14C-acetate incorporated into phosphatidylethanolamine, sphingomyelin, phosphatidylserine, phosphatidylinositol, and cardiolipin. Phosphatidylcholine, the most abundant membrane phospholipid, demonstrated the lowest radioactivity, which was significantly lower than that of phosphatidylethanolamine. Incorporation of newly synthesized free fatty acids in nuclear phospholipids was inhibited, if nuclei were incubated with cytosol. As a result, radioactive free fatty acids were accumulated in nuclei, while in cytosol they were efficiently incorporated into phospholipids. The levels of phospholipids and cholesterol remained constant regardless of incubation protocol, while the overall yield of free fatty acids decreased after combined incubation of nuclear and cytosolic fractions or after incubation of cytosol without nuclei. Putative mechanisms underlying the appearance of radioactive lipids in isolated nuclei of thymus cells are discussed.

Incorporation of 2-14C-acetate into isolated nuclei and cytosolic lipids of rat thymus cells

Incorporation of 2-14C-acetate into isolated nuclei and cytosolic lipids of rat thymus cells

A deletion mutation of the protein tyrosine phosphatase kappa (Ptprk) gene is responsible for T-helper immunodeficiency (thid) in the LEC rat

Bone marrow (BM)-derived T-cell progenitors differentiate into CD4 or CD8 single-positive (SP) cells in the thymus. We have previously reported that a single autosomal mutation, thid, causes a defect in the maturation of CD4 SP thymocytes and an abnormality of peripheral helper T cells in the LEC rat. In this study we attempted to identify a gene responsible for the thid mutation. We first performed genetic linkage analysis and mapped the thid locus between Myb and D1Rat392 on Chr 1. In this region we found an approximately 380-kb deletion from intron 3 of the Ptprk gene, which encodes a receptor-like protein tyrosine phosphatase type kappa (RPTPkappa) to intron 1 of the RGD1560849 predicted gene in the LEC rat genome. Reconstitution with syngenic BM cells transduced Ptprk but not the RGD1560849 predicted gene rescued development of CD4 SP cells in the LEC rat thymus. It is confirmed by this result that the Ptprk gene is responsible for the thid mutation in the LEC rat. Our results further suggest that RPTPkappa plays a critical role in the development of CD4 SP cells in the thymus.

Flavonoid Rutin Alters the Viability and Function of Mitogen-Stimulated Splenocytes and Thymocytes Compared with Non Stimulated Cells

Rutin is a flavonoid obtained from Dimorphandra mollis (Benth.), a medicinal Brazilian plant used as antioxidative, antihemorrhagic, and blood vessel protector. The present study has examined its effects on the viability and function of immune system cells in vitro. Rat spleen and thymus cells were cultured with 10 nM, 1 μM, and 10 μM of the drug in the presence or absence of PWM, LPS, or ConA mitogens. Cellular proliferation was analyzed by H3-thymidin uptake and IFN-γ and IL-10 were measured by ELISA after 48 and 72 hr. Viability was measured by flow cytometry using Annexin V and PI after 24 and 48 hr. The flavonoid rutin inhibited splenocytes and thymocytes proliferation under ConA stimulation observed by an increase on apoptosis levels of thymocytes stimulated with PWM in 24 hr and on splenocytes stimulated with PWM in 48 hr. Function studies showed a decrease on IFN-γ production by splenocytes and thymocytes stimulated with PWM or ConA. Spleen cells cultured with LPS and rutin showed a decrease on apoptosis after 24 hr and an increase on the IL-10 levels after 48 hr. There was no significant variation on the necrosis rate, viability, and function of cells treated with rutin in the absence of mitogenic stimulus.

Perinuclear localization of biogenic amines (serotonin and histamine) in rat immune cells

EDAC fixation, which polymerizes biogenic amines, and clorgyline, which blocks monoamine oxydase, were used to demonstrate the perinuclear and nuclear localization of histamine and serotonin (5-HT) of immune cells in rat peritoneal fluid and thymus. For detection, an immunocytochemical method combined with confocal microscopy was used. In peritoneal cells (lymphocytes) 5-HT formed spots around the nucleus, while histamine was localized diffusely. In thymocytes, 5-HT was distributed in patches, while histamine formed a garland around the nuclear envelope. Clorgyline enhanced the fluorescence and increased the number of fluorescent positive cells only for histamine, and then histamine-positive nuclei were also present. The results show that: (1) EDAC fixation demonstrates the presence of biogenic amines in immune cells, (2) using this fixation, the perinuclear localization of biogenic amines can be demonstrated, (3) the localization of the two amines is different and (4) the nuclear localization of histamine in thymocytes can be surmised and in mast cells (described previously) is proven again. The potential importance of the nuclear and near-nuclear localization of biogenic amines is discussed.

Different Effects of Arborinine Alkaloid Obtained from Brazilian Erthela baihensis on Spleen and Thymus Cells Stimulated in Vitro with Different Mitogens

The present study has examined the effects of arborinine, an alkaloid obtained from Erthela bahiensis, a Brazilian plant popularly used as diuretic, antidiabetic, antithermic and expectorant, on the viability and function of immune system cells in vitro using a murine model. Rat spleen and thymus cells were cultured with 10nM, 1µM e 10µM of the drug in the presence or absence of pokeweed (PWM), lipopolysaccharide (LPS), or concanavallin (ConA) mitogens. Cellular proliferation was analyzed by H3‐thymidin uptake after 48 and 72 hr. Our results showed an inhibitory effect of arborinine on splenocytes proliferation under ConA or PWM stimulation and increased apoptosis on splenocytes and thymocytes stimulated with PWM in 24 hr. A decrease was observed on Interferon gamma (IFN-γ) production by ConA- or LPS-stimulated splenocytes in 48 hr and 72 hr and ConA- or PWM-stimulated thymocytes in 72 hr. In contrast, an increase on lymphoproliferation was observed on LPS-stimulated splenocytes and ConA- or PWM-stimulated thymocytes in 48 hr. On this period, apoptosis decreased on LPS- or PWM-stimulated splenocytes and IFN-γ production increased in PWM stimulated thymocytes. Arborinine also induced a decrease on Interleukin-10 production by splenocytes and thymocytes stimulated with ConA or PWM. There was no significant variation on the necrosis rate of the cells treated with arborinine or any change on their viability or function values in the absence of mitogenic stimulus.

Prolactin stimulates maturation and function of rat thymic dendritic cells

The current study analyses the effect of PRL, a hormone involved in numerous physiological processes, on dendritic cells (DC) of rat thymus. Most thymic DC express prolactin receptors (PRL-R) as demonstrated by both immunohistochemistry and flow cytometry. PRL administration during 2 or 6 days to fetal thymus organ cultures (FTOC) does not increase the proportions of DC in cultures but stimulates their differentiation. Furthermore, PRL-treated thymic DC exhibit increased allostimulatory capacity in mixed leukocyte reaction (MLR) assays in association with increased surface expression of both MHC antigens and the co-stimulatory molecule CD80. PRL-treated DC also produce increased amounts of pro-inflammatory cytokines, such as IL-12, TNFα and IL-1β, but not of IL6 or IL-10. Our data suggest a key role for IL-12 in the observed changes in the allostimulatory capacity of PRL-treated DC. Also, they permit us to hypothesize about the physiological role played by PRL in thymus ontogeny.

Synthesis of lipids in isolated nuclei from rat thymus and liver cells.

Synthesis of lipids was studied in isolated nuclei from rat thymus and liver cells. On incubation of the isolated nuclei with [2-14C]acetate and [1-14C]glycerol, the label was intensively incorporated into phospholipids and with a significantly lower intensity into fatty acids and cholesterol. Only trace amounts of radioactivity were detected in the lipids of chromatin prepared from isolated thymus nuclei after their incubation, and this suggested that lipids were mainly synthesized on the nuclear membrane. On the preincubation of thymus tissue homogenate with [2-14C]acetate and the subsequent isolation of the nuclei and chromatin, the radioactivity of chromatin lipids was comparable to the radioactivity of nuclear lipids. The findings suggested that in the isolated nuclei the newly synthesized lipids were not transported into chromatin from the nuclear membrane. The specific radioactivities of individual phospholipids and fatty acids were different in the isolated nuclei and in nuclei obtained from preincubated homogenate. Mechanisms of lipid synthesis in isolated nuclei and causes of the different radioactivities of lipids in the isolated nuclei and in the nuclei obtained from the preincubated homogenate are discussed.

Cell death in the rat thymus: a minireview.

During the last decades, the literature has clearly established the fundamental role of the thymus in the development of an effective immune system. During thymocyte development and maturation, potentially autoreactive thymocytes are eliminated by a process known as apoptosis or programmed cell death responsible for the negative selection occurring within the thymus. This process is in sharp contrast to other types of cell death referred to as necrosis. Actually, three different types of cell death have been recently observed morphologically in the rat thymus, i.e. necrosis, apoptosis and clustered cell death. Moreover, among the numerous factors influencing thymocyte cell death, particular attention has been paid to hormones, chemicals, biological compounds and physical agents that may influence the type and/or the extent of cell death. Finally, a brief overview has been devoted to the contribution of mitochondria, nitric oxide, glutathione and intracellular levels of cations in addition to the activity of genes as cdk2, p53, Fas and members' of the Bcl2 family in modulating rat thymus cell death.

MRNA expression profiles for corticotrophin-releasing factor (CRF), urocortin, CRF receptors and CRF-binding protein in peripheral rat tissues.

The expression and activities of corticotrophin-releasing factor (CRF), urocortin (UCN), the CRF-binding protein (CRF-BP) and CRF receptors in rat brain have been well documented; however, information regarding their peripheral distributions remains incomplete. Given the multiple immunomodulatory effects of peripherally administered CRF and UCN and the high levels of CRF receptor type 2 (CRF-R2) mRNA and protein expressed in the heart, the lymphoid organs and heart have become targets for some of the latest CRF-related research. Here we demonstrate the presence of UCN mRNA in both the rat spleen and human Jurkat T-lymphoma cells using 3'-RACE (rapid amplication of cDNA ends) PCR. Following on from these initial results, we used semi-quantitative RT-PCR to carry out a comprehensive study assessing the relative amounts of CRF, UCN, CRF-R1, CRF-R2 and CRF-BP mRNAs in the brain, thymus, spleen and heart of normal, untreated rats. The rank orders of mRNA abundance in each of the tissue types were as follows: for CRF, brain>>thymus=spleen=heart; for UCN, heart>/=brain>thymus>spleen; for CRFR1, brain>>thymus>spleen (absent in heart); for CRF-R2, brain=heart>thymus>spleen; and CRF-BP was only detectable in the brain. We have provided evidence for the existence of CRF, UCN, CRF-R1 and CRF-R2 expression in resting immune cells, with UCN expression being particularly predominant in the rat thymus and human Jurkat cells. Additionally, the high levels of UCN mRNA detected in heart corresponded to the high expression of CRF-R2 mRNA, suggesting an important role for UCN/CRF-R2 coupling in this tissue.

Differentiation of rat thymic myoid progenitor cell line established by coculture with human T-lymphotropic virus type-I producing human T cells.

A thymus-derived myoid precursor cell line (ST1), which differentiates to myoid cells in the growth arrest condition, was established by the cocultivation of F344 rat thymic cells with human T-lymphotropic virus type-I (HTLV-I)-producing human lymphoid cells. No integration of HTLV-I was detected in ST1 cells by Southern blot hybridization. In a differentiation culture condition such as confluent culture or serum starvation, ST1 cells began to fuse, creating multinuclear giant cells, with the induced expression of MyoD1 and various muscle-specific antigens, including alpha-sarcomeric actin, skeletal muscle myosin, myoglobin, desmin, and acetylcholine receptor. Ultrastructural investigation revealed that differentiated ST1B cells created aggregates of thick and thin filaments with Z-band-like composition, then formed sarcomeric structures and tubular honeycomb arrays. Finally, these cells spontaneously contracted with a frequency of 0.5-2.0 Hz and synchronized with adjoining cells. Transplantation of ST1B cells into nude mice produced a small tumor nodule, showing clear differentiation to skeletal muscle cells. ST1B cells did not indicate any colony-forming activities in soft agar, demonstrating that ST1B cells retain some of the physiologically normal phenotypes. This rare cell line is promising for use in various physiological and pathological investigations including functional research of thymic myoid cells and the pathological role in autoimmune diseases, as well as animal model experiments of cell therapy related to muscular degenerative disorders or regeneration of injured muscles.

Myoid cell differentiation of rat thymus-derived progenitor cell line established by coculture with HTLV-I-producing human T cells

P042 Thymus-derived myoid precursor cell line (ST1), which differentiates to myoid cells in the growth arrest condition, was established by the co-cultivating F344 rat thymic cells with human T-lymphotropic virus type-I (HTLV-I)-producing human lymphoid cells. No integration of HTLV-I was detected in ST1 cells by Southern blot hybridization. In the serum starvation condition as the trigger of differentiation, ST1 cells began to fuse creating multinuclear giant cells, and formed a myotube-like structure. Finally, these cells started to contract spontaneously. Under the serum starvation condition, ST1B cells sub-cloned from ST1 cells became to express muscle specific proteins including MyoD1, α-sarcomeric actin, skeletal muscle myosin, myoglobin, desmin and acetylcholine receptor. From the ultrastructural investigation, it was observed that ST1B cells created the aggregates of thick and thin filaments with Z-band-like composition, then forming sarcomeric structures. Transplantation of ST1B cells into nude mice produced small tumor nodules showing clear differentiation to skeletal muscle cells. This cell line did not have colony forming activities in soft agar. These results demonstrate that ST1B cells retain some physiologically normal phenotypes of thymic myoid cells. ST1B cells were also found to specifically adhere to freshly isolated thymocytes in vitro, suggesting some specific cross talking with thymocytes. This system would be a good system to investigate the functional role of thymic myoid cells in terms of differentiation, selection and maturation of thymocytes as well as pathogenesis of HTLV-I associated polymyositis (PM).

Expression of parathyroid hormone-related protein and the parathyroid hormone/parathyroid hormone-related protein receptor in rat thymic epithelial cells.

Thymic epithelial cells are an important source of cytokines and other regulatory peptides which guide thymocyte proliferation and maturation. Parathyroid hormone-related protein (PTHrP), a cytokine-like peptide, has been reported to affect the proliferation of lymphocytes in vitro. The studies presented here were undertaken to test the hypotheses that PTHrP is produced locally within the thymus where it could influence thymocyte maturation and, more specifically, that thymic epithelial cells (TEC) could be the intrathymic source of PTHrP expression. To this end, immunohistochemical studies were performed to localise PTHrP and the PTH/PTHrP receptor within the adult rat thymus. Antibodies directed against 2 different PTHrP epitopes, PTHrP(1-34) and PTHrP(34-53), demonstrated prominent specific PTHrP immunoreactivity in both subcapsular and medullary TEC. In addition, faint but specific staining for PTHrP was seen in the cortex, interdigitating between cortical lymphocytes while sparing epithelial-free subcapsular areas, thus suggesting that cortical TEC could also be a source of PTHrP immunoreactivity. In contrast, PTH/PTHrP receptor immunoreactivity was only seen in medullary and occasional septal TEC; no evidence of cortical or lymphocytic PTH/PTHrP receptor immunoreactivity was detected. Immunohistochemical studies of cultured cytokeratin-positive rat TEC confirmed the results of these in situ studies as cultured TEC were immunoreactive both for PTHrP and the PTH/PTHrP receptor. Thus these results demonstrate that PTHrP is produced by the epithelial cells of the mature rat thymus. This suggests that PTHrP, a peptide with known cytokine, growth factor and neuroendocrine actions, could exert important intrathymic effects mediated by direct interactions with TEC, or indirect effects on PTH/PTHrP receptor-negative thymocytes.