Lymphokine-mediated induction of antigen-presenting ability in thymic stromal cells

We have established and characterized long term thymic stromal cultures from BALB/c (H-2d) and CBA/J (H-2k) mice. All cultures contained multiple adherent cell types, whereas some also contained thymic macrophages (TM). Culture supernatants from all cultures tested contained macrophage colony-stimulating factor activity, whereas only cultures with TM had soluble or membrane-associated interleukin (IL)-1. However, a thymic epithelial cell line (3D . 1), cloned from one of these cultures, produced IL-1 bioactivity. Further analysis confirmed the production of IL-1 alpha mRNA by the epithelial cell. No IL-2 or IL-4 (formerly called B cell stimulatory factor 1) activity was detected in any of the cultures. Antigen-presenting (AP) ability was determined using the chicken ovalbumin (OVA)-specific, I-Ad-restricted T cell hybridoma 3DO-18.3. Harvested TM exhibited antigen-specific, Ia-restricted AP ability which was enhanced by IL-4 as well as interferon-gamma (IFN-gamma). In contrast, AP ability was detected in non-macrophage stromal cell cultures (NMSC) only after preincubation with IFN-gamma. AP by preinduced NMSC was also Ia-restricted and could be blocked by anti-I-Ad antibodies. Since the T cell receptor of 3DO-18.3 is known to recognize a peptide produced by CNBr degradation of OVA, these observations suggest that both TM and NMSC can process OVA to produce this peptide. Glutaraldehyde-fixation experiments confirmed that NMSC must process native OVA into antigenic peptides for successful AP. Assays using several cloned stromal cell lines of different lineages suggested that only epithelial cells could be induced with IFN-gamma to exhibit competent AP. Given the possible role for IFN-gamma in the maintenance of Ia in the thymus, we investigated whether IFN-gamma production could be ascribed to a subpopulation of thymocytes. Culture supernatants from calcium ionophore and phorbol ester-stimulated peanut agglutinin-negative, but not peanut agglutinin-positive, thymocytes induced AP ability in NMSC. Thus, some thymocytes can produce an Ia-inducing lymphokine (most likely IFN-gamma) which may play an important role in T cell ontogeny through its effects on both thymic macrophages and thymic epithelial cells.     

Induction of thymocyte proliferation by supernatants from a mouse thymic epithelial cell line

The thymic stroma plays a critical role in the generation of T lymphocytes by direct cell-to-cell contacts as well as by secreting growth factors or hormones. The thymic epithelial cells, responsible for thymic hormone secretion, include morphologically and antigenically distinct subpopulations that may exert different roles in thymocyte maturation. The recent development of thymic epithelial cell lines provided an interesting model for studying thymic epithelial influences on T cell differentiation. Treating mouse thymocytes by supernatants from one of TEC line (IT-76M1), we observed an induction of thymocyte proliferation and an increase in the percentages of CD4-/CD8- thymocytes. This proliferation was largely inhibited when thymocytes were incubated with IT-76M1 supernatants together with an anti-thymulin monoclonal antibody, but could be enhanced by pretreating growing epithelial cells by triiodothyronine. We suggest that among the target cells for thymulin within the thymus, some putative precursors of early phenotype might be included.     

The production of regulatory cytokines for thymocyte proliferation by murine thymic epithelium in vitro

Two separate cultures of pure, morphologically distinct thymic epithelial cells have been generated and maintained in culture for one year (A.C. Nieburgs et al., Cell. Immunol. 90, 439-450, 1985). Supernatants from one of these cell lines, TECs, were examined for functional activity on thymocytes in vitro. These supernatants contained three distinct intercellular mediators, each capable of modulating thymocyte responses to T-cell mitogens. Enhancement of thymocyte proliferation to suboptimal doses of mitogen was associated with a factor that eluted in the 97,000-Da region on molecular sieve chromatography and was functionally and physicochemically distinct from interleukin-1 and interleukin-2 (IL-1 and IL-2). Suppression of the thymocyte response to optimal doses of mitogen was mediated by a 1000- to 5000-Da factor. These two intercellular components have different susceptibilities to heat treatments and are trypsin insensitive. In addition, thymic epithelial cells produced significantly high levels of prostaglandin E2 (PGE2) which also suppressed thymocyte responses to mitogen, but only at high doses of supernatant. These epithelial cell-derived enhancing and inhibitory effects on thymocytes could play a role in regulating intrathymic events.     

Establishment of a mouse thymic epithelial cell line, IT-76MHC and a brief review on cultured thymic epithelial cells.

Interactions between thymocytes and thymic stromal cells are essential for thymocyte differentiation, but little evidence has been presented to directly show in vivo functions or interactions of the stromal cells. Among the stromal cells, the thymic epithelial cell has been considered to have profound effect on thymocyte differentiation and maturation. The calcium-depleted medium, originally developed for the culture of mouse epidermal cells, was applied for the culture of the mouse thymic epithelial cells, and successfully, an epithelial cell line, IT-76MHC was obtained from the mouse thymus. IT-76MHC cells were identified as distinct mouse thymic epithelial cells by 1/ mosaic-like arrangement, 2/ presence of well-developed desmosome and 3/ tonofilaments, 4/ positivity for cytokeratin, and 5/ induced expression of MHC class I and II by IFN-gamma treatment. IGF-1, IGF-2, oxytocin and vasopressin were also detected immunohistochemically in IT-76MHC cells. Furthermore, the IT-76MHC thymic epithelial cells, when injected intrathymically in the allogeneic mouse, prolonged the survival of skin graft from the same donor strain that IT-76MHC cells were derived. These results demonstrate that the thymic epithelial cell line IT-76MHC produces modest thymocyte survival factors as well as a growth suppressor, and that IT-76MHC cells have the ability to induce transplantation tolerance probably through their expression of MHC class I and II molecules. Taken altogether, the IT-76MHC thymic epithelial cells have been proved to be useful tools to better understand the in vivo functions of thymic epithelial cells, and to gain a deep insight into their involvement in the critical selection process of thymocytes which still remains obscure. Finally and additionally, literatures so far reported on thymic epithelial cells in culture, especially lines and clones, are reviewed and their identity as well as their functions are discussed.     

Thymic epithelium in vitro. V. Binding of thymocytes to cultured thymic epithelial cells

Direct contact between thymocytes and thymic stromal elements may be one of the mechanisms involved in thymocyte differentiation. Thymic lymphoepithelial complexes have been isolated in which thymocytes appear to be in direct association with cortical epithelial cells. We have previously reported the isolation and successful culture of two morphologically distinct types of murine thymic epithelial cells. We have utilized these to study the interactions of lymphoid and epithelial cells by means of an in vitro assay of the binding of radiolabeled thymocytes to monolayers of these cultured thymic epithelial cells. The percentage of bound cells increased rapidly during the first hour of incubation, reaching approximately 40% binding. Binding continued to increase slowly until plateau levels were reached at approximately 5 hr. Thymocyte binding to thymic epithelium, but not fibroblast monolayers, was trypsin-sensitive, suggesting that specific protein interactions may be involved. Binding of thymocytes to epithelium was temperature-dependent, involved formation of cytoplasmic projections, and was inhibited by cytochalasin B. We also found that cortical thymocytes (peanut agglutinin-positive (PNA+)cells) bound to cultured epithelium to a greater degree than medullary thymocytes (PNA- cells). This correlates with in vivo studies by others in which thymocytes associated with lymphoepithelial complexes have been found to have immature phenotypes. This system provides a means for a quantitative study of the role of cell to cell contact in the process of thymocyte selection and differentiation.     

Lymphocyte depletion in thymic nurse cells: a tool to identify in situ lympho-epithelial complexes having thymic nurse cell characteristics

Summary In situ pre-existing complexes of epithelial cells and thymocytes having thymic nurse cell characteristics were visualized in the murine thymus cortex using dexamethasone as a potent killer of cortisone-sensitive thymocytes. The degradation and subsequent depletion of cortisone-sensitive thymocytes enclosed within cortical epithelial cells appeared to be paralleled by thymocyte degradation and depletion in thymic nurse cells isolated from thymic tissue fragments from dexamethasone-treated animals. This suggests that thymic nurse cells are derived from pre-existing sealed complexes of cortical epithelial cells and thymocytes. Not all thymocytes situated within in situ epithelial or thymic nurse cells complexes appear to be cortisone-sensitive: a minority of 1–2 thymocytes per complex survives the dexamethasone-treatment, thus constituting a minor subset of cortical cortisone-resistant thymocytes predominantly localized within cortical epithelial cells in situ and within thymic nurse cells derived from such structures. Cortisone resistance in thymocytes thus seems to be acquired within the cortical epithelial cell microenvironment. Cortisone-resistant thymocytes in thymic nurse cells express the phenotype of mature precursors of the T helper lineage, indicating that the in situ correlates of thymic nurse cells may play an important role in T cell maturation and selection.     

A novel cell surface antigen involved in thymocyte and thymic epithelial cell adhesion.

A murine mAb, 7D3, was produced by fusion of spleen cells obtained from mice immunized with a rat thymic epithelial cell line, Tu-D3 and NS/1 myeloma cells. 7D3 antibody reacted with approximately 95% thymocytes, 17% spleen cells, less than 9% of mesenteric lymph node cells and 32% of bone marrow cells of rat origin. 7D3 also reacted with two rat thymic epithelial cell lines but not with a rat fibroblastic cell line. Immunochemical analysis demonstrated that 7D3 antibody recognized a single polypeptide with molecular weight of 80,000 in FTE cells and 80,000 to 96,000 in thymocytes. 7D3 antibody strongly inhibited the thymocyte binding to thymic epithelial cells. In addition, 7D3 antibody inhibited TPA-induced thymocyte aggregation. 7D3 negative rat thymic lymphoma cells bound to 7D3 positive thymic epithelial cells and this binding was inhibited by 7D3 antibody, indicating that a part of thymocyte-thymic epithelial cell binding was mediated by the interaction of 7D3 Ag and undefined ligand to 7D3.     

Defective thymic T cell activation by concanavalin A and anti-CD3 in autoimmune nonobese diabetic mice. Evidence for thymic T cell anergy that correlates with the onset of insulitis.

In nonobese diabetic (NOD) mice, T cells play a major role in mediating autoimmunity against pancreatic islet beta-cells. We and others previously reported that age-related alterations in the thymic and peripheral T cell repertoire and function occur in prediabetic NOD mice. To study the mechanism responsible for these T cell alterations, we examined whether a defect exists in the thymus of NOD mice at the level of TCR-mediated signaling after activation by Con A and anti-CD3. We found that thymocytes from NOD mice respond weakly to Con A- and anti-CD3-induced proliferation, compared with thymocytes from control BALB/c, BALB.B, (BALB.B x BALB.K)F1, C57BL/6, and nonobese non-diabetic mice. This defect correlates with the onset of insulitis, because it can be detected at 7 to 8 weeks of age, whereas younger mice displayed a normal T cell responsiveness. Thymic T cells from (NOD x BALB/c)F1 mice, which are insulitis- and diabetes-free, exhibit an intermediate stage of unresponsiveness. This T cell defect is not due to a difference in the level of CD3 and IL-2R expression by NOD and BALB/c thymocytes, and both NOD CD4+ CD8- and CD4- CD8+ mature thymic T cells respond poorly to Con A. BALB/c but not NOD thymic T cells respond to Con A in the presence of either BALB/c or NOD thymic APC, suggesting that the thymic T cell defect in NOD mice is intrinsic to NOD thymic T cells and is not due to an inability of NOD APC to provide a costimulatory signal. The defect can be partially reversed by the addition of rIL-2 to NOD thymocytes. To determine whether a defect in signal transduction mediates this NOD thymic T cell unresponsiveness, we tested whether these cells elevate their intracellular free Ca2+ ion concentration in response to Con A. An equivalent Con A-induced increase in Ca2+ ion concentration in both NOD and BALB/c thymocytes was observed, suggesting a normal coupling between the CD3 complex and phospholipase C in NOD thymocytes. In contrast to their low proliferative response to Con A or anti-CD3, NOD thymocytes respond normally (i.e., as do BALB/c thymocytes) to the combinations of PMA plus the Ca2+ ionophore ionomycin and PMA plus Con A but weakly to Con A plus ionomycin. Our data suggest that the age-related NOD thymocyte unresponsiveness to Con A and anti-CD3 results from a defect in the signaling pathway of T cell activation that occurs upstream of protein kinase C activation.     

The thymic stromal cell line MTSC4 induced thymocyte apoptosis in a non-MHC-restricted manner

Mouse thymic stromal cell line 4 (MTSC4) is one of the stromal cell lines established in our laboratory. While losing the characteristics of epithelial cells, they express some surface markers shared with thymic dendritic cells (TDCs). To further study the biological functions of these cells, we compared the capability of MTSC4 with TDCs in the induction of thymocyte apoptosis, using thymic reaggregation culture system. Apoptosis of thymocytes induced by MTSC4 and TDCs was measured by Annexin V and PI staining and analyzed by flow cytometry. We found that MTSC4 selectively augmented the apoptosis of CD4^ 8^ (DP) thymocytes. This effect was Fas/FasL independent and could not be blocked by antibodies to MHC class I and class II molecules. In addition, MTSC4 enhanced the apoptosis of DP thymocytes from different strains of mice, which implies that MTSC4-induced thymocyte apoptosis is not mediated by the TCR recognition of self peptide/MHC molecules. In contrast to MTSC4, thymocyte apoptosis induced by TDCs was MHC-restricted. Thus, MHC-independent fashion of stromal-DP thymocyte interaction may be one of the ways to induce thymocyte apoptosis in thymus. Our study has also shown that the interaction of MTSC4 stromal cells and thymocytes is required for the induction of thymocyte apoptosis.     

A chemotactic factor for rat thymocytes may regulate T-lymphocyte migration toward the thymic microenvironment

Using a modified Boyden chamber assay, extracts or culture supernatants of rat thymic stromal cells, or thymocytes were examined by chemotactic activity to rat leukocytes. Rat thymocytes responded chemotactically to the aqueous extract as well as to culture supernatants of thymic stromal cells. However, neither the extract and culture medium from concanavalin A-stimulated thymocytes nor any component of rat serum has shown such an activity. The thymic extract was fractionated into three molecular species with chemotactic activity for thymocytes. The thymocyte chemotactic factor(s) (TCFs) in the extract was distinct from known lymphocytic chemotactic factors, such as interleukin-1 (IL-1), IL-2, C5a, and the culture supernatant of stimulated thymocytes. In vitro, TCFs could attract, in addition to thymocytes, bone marrow cells, fetal liver cells, and nylon-wool nonadherent lymphocytes from peripheral blood and spleen. Lymph node cells, neutrophils, macrophages, and B cells from peripheral blood could not respond to TCFs. Thymocytes also responded to the extract of splenic stromal cells. Unlike the thymic extract, however, the splenic extract was chemotactically active for lymphocytes from lymph nodes but not for bone marrow cells. These results indicate that thymic stromal cells secrete a chemotactic factor(s) for a relatively immature type of T-lineage cells, which may by a thymus-homing progenitor T cell, while spleen may contain an attractant for a relatively mature type of T-lineage cells.     

Accessory function of human thymic dendritic cells in Con A-induced proliferation of autologous thymocyte subsets.

Human thymic dendritic cells (DC) have previously been shown to be intimately associated with thymocytes in situ and in culture. We report that thymic DC express LFA-3 and ICAM-1 adhesion molecules and may spontaneously associate with autologous thymocytes within mitogen-independent clusters. Moreover, the accessory activity of isolated human thymic DC was investigated in Con A-stimulation assays. By proliferation experiments, measured as [3H]TdR incorporation, we demonstrated that irradiated thymic DC strongly increase the mitogen-induced activation of autologous PBL as well as of unfractionated thymocytes. More interestingly, in coculture assays performed with purified thymocyte subsets, we have found that thymic DC greatly enhance the Con A proliferation of CD1- CD3bright thymocytes whereas the accessory activity toward the CD1+ CD3- thymocytes was very weak. Inhibition experiments demonstrated that the DC accessory activity is inhibited by anti-DR-related and anti-IL-2R mAb. However, blocking assays with anti-CD11b, anti-CD11c, anti-LFA-3, and anti-ICAM1 mAb showed that the accessory function obtained is similar to that with untreated cultures. We conclude that isolated human thymic DC may present potent DR- and IL-2-dependent accessory activity mainly directed toward the CD1- CD3bright thymocyte subpopulation, suggesting that thymic DC may be involved in the in vivo proliferation of mature thymocytes.     

Human thymic epithelial cells function as accessory cells for autologous mature thymocyte activation

Using human thymocytes and autologous thymic epithelial (TE) cells grown in vitro in long-term culture, we have found TE cells can function as accessory cells for mitogen-induced mature thymocyte activation. Tritiated thymidine incorporation, blast formation, and protein synthesis were all induced in accessory cell-depleted thymocytes by autologous TE cells in the presence of suboptimal concentrations of PHA. After 3 days of mitogen stimulation of thymocyte-TE cell cocultures in vitro, thymocyte blasts bound to TE cells and 77 +/- 4% (mean +/- SEM) of TE cells acquired expression of major histocompatibility complex (MHC) class II (DR) antigen. TE accessory cell function for thymocyte activation was dependent on the number of TE cells added to thymocyte cultures, was not dependent on TE cell division, but did require TE cell protein synthesis. In thymocyte separation experiments, the predominant cell type responding to PHA in the presence of TE cells was T6- mature (stage III) thymocytes. Thus, human TE cells are capable of providing signals that lead to mature thymocyte activation.     

Functional maturation of mouse CD4~+CD8~- thymocytes induced by medullary-type thymus epithelial cells

Murine CD4+CD8- (CD4SP) thymocyte subset is a heterogeneous population, in which the Qa-2- cells are less functional, whereas the Qa-2+ cells are fully functional. Evidence is provided here that the transition from Qa-2- to Qa-2+ CD4SP thymocytes is an intrathymic process of differentiation induced by thymic medullary-type epithelial cells. The separated Qa-2-CD4SP could be induced to express Qa-2 molecules up to 84%- 89% of the total viable celb after cocultured for 3d with MTEC1 cells, a murine thymic medullary type epithelial cell line established in our laboratory. Kinetic study showed that both the percentage of Qa-2+ cells and the density of the expressed Qa-2 molecules on CD4SP thymocytes induced by MTEC1 were progressively increasing in 72-h cultures. The MTECl-induced Qa-2+CD4SP thymocytes were fully functional, which exhibited capabilities of proliferation and cytokine secretion in response to Con A stimulation as high as those of freshly isolated Qa-2+CD4SP thymocytes. The profile of cytokine     

Thymic epithelium in vitro. III. Cytokine production by a thymic epithelial subset

We have previously shown that two populations of thymic epithelium can be separated in culture on the basis of their differential growth rates and their adherence to the culture substratum, and maintained as long-term, morphologically distinct cell cultures, TECs and TECL. We have also described the effects of supernatants from the small epithelial cell (TESs) on the proliferative responses of thymocytes cocultured with mitogen and TESs over a 72-hr period. We now describe the effects in thymic epithelial supernatants (TESL) of soluble factors produced by TECL (the large epithelial cell) on thymocytes costimulated with mitogen and compare their effects to those derived from TECs. Both TESL and TESs suppress optimally stimulated thymocytes and enhance the proliferative responses of suboptimally stimulated thymocytes over a 72-hr period. The suppressive activities produced by TECL and TECs appear distinct, based upon markedly different molecular weights, but have similar sensitivities to heat treatment. The enhancing activities are of similar molecular weight, but have different sensitivities to heat treatment. In addition, TECL synthesize four- to fivefold less PGE2 than TECs. These results provide additional distinctions between the two cell types, and taken in conjunction with data on the anatomic distribution of similar cells, suggest that although they have similar functional effects in vitro, they may prove to have separable roles in vivo.     

Human thymic epithelial cells produce interleukin 1

Although the thymus plays a critical role in generation of immunocompetent T lymphocytes, the precise role of the epithelial component of the thymus in the induction of T cell proliferation and maturation remains unknown. Since interleukin 1 (IL 1) is required for mature T cell activation, we have determined whether human thymic epithelial (TE) cells produce IL 1. By using a system for longterm culture of human TE cells, we found that human TE cells produced an IL 1-like factor (TE-IL 1) that augmented the proliferation of C3H/HeJ mouse thymocytes to phytohemagglutinin. IL 1 activity (20 to 200 U/ml) was detected in supernatants of TE cultures from all individuals (2 to 13 yr old) tested. IL 1 activity was also detected in supernatants of TE cultures from a 17-wk fetus but not from a 10-wk fetus. Production of TE-IL 1 was dependent on TE cell density and time in culture with optimal TE-IL 1 activity observed at 10(6) TE cells/ml after 48 to 72 hr of culture. With the use of high performance liquid chromatography, TE-IL 1 chromatographed as a molecule of 18,000 to 20,000 relative molecular mass, and by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, TE-IL 1 migrated at 15,000 to 17,000 Mr. With the use of isoelectrofocusing gels, charge heterogeneity of TE-IL 1 was demonstrated with two major isoelectric points of 5.7 to 5.8 and 6.9 to 7.0. Polyclonal antibody to human monocyte IL 1 markedly inhibited the TE-IL 1 activity. In indirect immunofluorescence assay of frozen human thymic sections, rabbit anti-IL 1 antibody reacted with epithelial cells in human thymic cortex and medulla. Furthermore, high performance liquid chromatography-purified TE-IL 1 augmented human thymocyte proliferation to suboptimal concentrations of phytohemagglutinin. Thus, thymic epithelial cells are capable of providing an intrathymic source of IL 1-like cytokine (TE-IL 1), which affects thymocyte proliferation. We propose that TE-IL 1 may play an important role in intrathymic proliferation and differentiation of human thymocytes.     

Functional maturation of mouse CD4+CD8- thymocytes induced by medullary-type thymus epithelial cells

Murine CD4⁺CD8^- (CD4SP) thymocyte subset is a heterogeneous population, in which the Qa-2^- cells are less functional, whereas the Qa-2⁺ cells are fully functional. Evidence is provided here that the transition from Qa-2^- to Qa-2⁺ CD4SP thymocytes is an intrathymic process of differentiation induced by thymic medullary-type epithelial cells. The separated Qa-2^-CD4SP could be induced to express Qa-2 molecules up to 84%- 89% of the total viable celb after cocultured for 3d with MTEC1 cells, a murine thymic medullary type epithelial cell line established in our laboratory. Kinetic study showed that both the percentage of Qa-2⁺ cells and the density of the expressed Qa-2 molecules on CD4SP thymocytes induced by MTEC1 were progressively increasing in 72-h cultures. The MTECl-induced Qa-2⁺CD4SP thymocytes were fully functional, which exhibited capabilities of proliferation and cytokine secretion in response to Con A stimulation as high as those of freshly isolated Qa-2⁺CD4SP thymocytes. The profile of cytokines secreted by MTECl-induced Qa-2⁺CD4SP thymocytes was Thy 0 type specified by the production of IL-2, IL-4 and IL-6. The results suggest that Qa-2^-CD4SP thymocytes may give rise to the Qa-2⁺CD4SP thymocytes, and acquire fully functional competence in thymic medulla under the foster of local epithelial cells.     

Thymic stroma-derived T cell growth factor (TSTGF). IV. Capacity of TSTGF to promote the growth of L3T4- Lyt-2- thymocytes by synergy with phorbol myristate acetate or various IL

The present study investigates the role of thymic stroma-derived T cell growth factor (TSTGF) in promoting the growth of L3T4- Lyt2- (double-negative) thymocytes. Partially purified TSTGF samples were prepared from the culture supernatant of a newly established thymic stromal cell line, MRL104.8a. The TSTGF alone induced only marginal proliferation of double-negative thymocytes, whereas this factor exerted a potent growth-promoting effect on these cells in combination with PMA. Because such an enhanced proliferation was not inhibited by anti-IL-4 or anti-IL-2R antibody, this was not due to the stimulation of an autocrine mechanism involving the production and utilization of IL-4 or IL-2. In scrutinizing PMA-equivalent physiologic substance(s), IL-1 was revealed to be capable of replacing the role of PMA in the above co-stimulation cultures and including enhanced proliferation of double-negative thymocytes in combination with TSTGF. Although TSTGF plus IL-2 or IL-4 also exhibited an appreciable or moderate synergistic effect on the growth of double-negative thymocytes, its magnitude was weaker compared with that obtained by TSTGF plus IL-1. More important, the strikingly enhanced proliferation was induced in the combinations of TSTGF, IL-1, and IL-2 or IL-4 under conditions in which the proliferation induced by IL-1 plus IL-4 or IL-1 plus IL-2 was marginal or slight. Furthermore, such strongly enhanced proliferation was also observed in the double-negative thymocyte population which was additionally depleted of T3+ cells (namely, the L3T4- Lyt-2- T3- or dull population). These results indicate the crucial role of TSTGF in the proliferation of immature thymocytes by synergy with various cytokines.     

New thymocyte growth factor from thymic epithelial cell line

Three polypeptide fractions were separated from the culture supernatant of a thymic epithelial cell line, TAD3, by high-pressure liquid chromatography (HPLC) equipped with gel-filtration column (GFC). One (estimated molecular weight: 10 kD) of the polypeptide fractions possessed the capacity to induce thymocyte proliferation. The sensitive cells for the growth factor in the fraction seem to be immature thymocytes which exist in the outer-cortical or the subcapsular area of thymic lobule. Furthermore, the mechanism to proliferate the thymocytes appears to differ from that of other cytokines. Thus, the fraction might possibly contain a previously unidentified thymocyte growth factor.     

Monoclonal antibodies to CD2 and lymphocyte function-associated antigen 3 inhibit human thymic epithelial cell-dependent mature thymocyte activation

Recent study of human thymocyte-thymic epithelial (TE) cell interactions has demonstrated that thymocytes bind to TE cells, and a consequence of this binding is the provision of accessory cell signals by TE cells for phytohemagglutinin (PHA)-induced mature thymocyte activation. In this paper we report on studies of the molecules involved in TE cell-dependent mature thymocyte activation. TE-thymocyte interactions necessary for PHA-induced thymocyte activation were inhibited by monoclonal antibodies against the cluster of differentiation (CD)2 antigen on thymocytes and lymphocyte function-associated (LFA)-3 antigen on TE cells. Inhibition of TE accessory cell signals by antibodies against CD2 (alpha CD2) and LFA-3 (alpha LFA-3) antigens occurred early on during thymocyte activation and prevented thymocyte interleukin 2 receptor expression. Further, alpha CD2 and alpha LFA-3 inhibited PHA-induced thymocyte activation in whole thymic explant cultures suggesting a significant role of the CD2 and LFA-3 antigens in thymocyte activation when accessory cell signals for PHA-induced thymocyte triggering were delivered by cells within an intact thymic microenvironment.