Cellular and molecular signals in T cell differentiation

The thymic epithelium exerts its differentiative effects through several mechanisms, involving direct contact with stem cells as well as secretion of various thymic hormones. Indirect and direct evidence suggests that the thymus produces chemotactic factors for the stem cells that colonize the thymus anlage. The epithelium also produces several maturational factors which act upon stem cells that have undergone primary differentiation by contact with thymic epithelium. The chemical characteristics of these hormones and their mode of action at the cellular level (high affinity receptors, metabolic effects, target cells) are partly known. Their relationship with T cell factors such as Interleukin-2--produced in the periphery and endowed with strong differentiative capacity--is intriguing, the more so because thymic hormones not only act within the thymus but also affect peripheral T cells after they have emigrated from the thymus.

Innate CD4+CD25+ regulatory T cells are required for oral tolerance and inhibition of CD8+ T cells mediating skin inflammation

To elucidate the role of CD4+CD25+ regulatory T cells in oral tolerance, we used the model of contact hypersensitivity (CHS) to 2,4-dinitrofluorobenzene (DNFB), which is mediated by CD8+ Tc1 effector cells independently of CD4+ T-cell help. Conversely to normal mice, invariant chain knock-out (KO) (Ii degrees / degrees ) mice, which are deficient in CD4+ T cells, cannot be orally tolerized and develop a chronic hapten-specific CHS response. Transfer of naive CD4+ T cells before hapten gavage into Ii degrees / degrees mice restores oral tolerance by a mechanism independent of interleukin-10 (IL-10) production by CD4+ T cells. That naturally occurring CD4+CD25+ T cells are critical for oral tolerance induction is demonstrated by the finding that (1) transfer of CD4+CD25+ but not CD4+CD25- T cells into Ii degrees / degrees recipients completely prevents the CHS response and skin infiltration by CD8+ T cells, by blocking development of hapten-specific CD8+ T cells; (2) in vivo depletion of CD4+CD25+ cells by antibody treatment in normal mice impairs oral tolerance; and (3) CD4+CD25+ T cells inhibit hapten-specific CD8+ T-cell proliferation and interferon gamma (IFN gamma) production, in vitro. These data show that naturally occurring CD4+CD25+ T cells are instrumental for orally induced tolerance and are key actors for the control of antigen-specific CD8+ T-cell effectors mediating skin inflammation.

A unique subpopulation of CD4+ regulatory T cells controls wasting disease, IL-10 secretion and T cell homeostasis

CD25(+)CD4(+) regulatory T cells have major roles in controlling immune responses, and use heterogeneous regulatory mechanisms. It is possible that these different activities are mediated by different subsets. Here we show that CD103(+)CD25(+)CD4(+) T cells (that control inflammatory bowel disease) are highly enriched in gut-associated lymphoid tissue and have unique functional properties. In vivo, only this subpopulation is able to control wasting disease and peripheral T cell homeostasis. In vitro, only this subpopulation is able to regulate IL-10 secretion, and it might also mediate infectious suppression. These results demonstrate that regulatory T cells can be divided into discrete subpopulations with defined functional properties and regulatory mechanisms.

Homeostasis of peripheral CD4+ T cells: IL-2R alpha and IL-2 shape a population of regulatory cells that controls CD4+ T cell numbers

We show that the lymphoid hyperplasia observed in IL-2Ralpha- and IL-2-deficient mice is due to the lack of a population of regulatory cells essential for CD4 T cell homeostasis. In chimeras reconstituted with bone marrow cells from IL-2Ralpha-deficient donors, restitution of a population of CD25(+)CD4(+) T cells prevents the chaotic accumulation of lymphoid cells, and rescues the mice from autoimmune disease and death. The reintroduction of IL-2-producing cells in IL-2-deficient chimeras establishes a population of CD25(+)CD4(+) T cells, and restores the peripheral lymphoid compartments to normal. The CD25(+)CD4(+) T cells regulated selectively the number of naive CD4(+) T cells transferred into T cell-deficient hosts. The CD25(+)CD4(+)/naive CD4 T cell ratio and the sequence of cell transfer determines the homeostatic plateau of CD4(+) T cells. Overall, our findings demonstrate that IL-2Ralpha is an absolute requirement for the development of the regulatory CD25(+)CD4(+) T cells that control peripheral CD4 T cell homeostasis, while IL-2 is required for establishing a sizeable population of these cells in the peripheral pools.

Modulation of Fas-dependent apoptosis: a dynamic process controlling both the persistence and death of CD4 regulatory T cells and effector T cells

We have previously shown that regulatory CD25(+)CD4(+) T cells are resistant to clonal deletion induced by viral superantigen in vivo. In this work we report that isolated CD25(+)CD4(+) T cells activated in vitro by anti-CD3 Ab are resistant to Fas-induced apoptosis, in contrast to their CD25(-)CD4(+) counterparts. Resistance of CD25(+)CD4(+) T cells to Fas-dependent activation-induced cell death is not linked to their inability to produce IL-2 or to their ability to produce IL-10. The sensitivity of both populations to Fas-induced apoptosis can be modulated in vitro by changing the CD25(+)CD4(+):CD25(-)CD4(+) T cell ratio. The sensitivity of CD25(-)CD4(+) T cells to apoptosis can be reduced, while the sensitivity of CD25(+)CD4(+) T cells can be enhanced. Modulation of Fas-dependent apoptosis is associated with changes in cytokine production. However, while CD25(-)CD4(+) T cell apoptosis is highly dependent on IL-2 (production of which is inhibited by CD25(+)CD4(+) T cells in coculture), modulation of CD25(+)CD4(+) T cell apoptosis is IL-2 independent. Taken together, these results suggest that CD25(+)CD4(+) and CD25(-)CD4(+) T cell sensitivity to Fas-dependent apoptosis is dynamically modulated during immune responses; this modulation appears to help maintain a permanent population of regulatory T cells required to control effector T cells.

Natural CD4 CD25(+) regulatory T cells control the burst of superantigen-induced cytokine production: the role of IL-10

In normal mice a subpopulation of CD4 T cells constitutively expresses the IL-2 receptor alpha chain (CD25). This natural CD4 CD25(+) subset is thymus-born, constitutively expresses IL-10 mRNA,does not produce IL-2 and is resistant to apoptosis. These cells behave as regulatory T cells in the control of self-tolerance, inflammatory reactions and T cell homeostasis. The mechanisms by which natural CD4 CD25(+) cells control the immune response is unclear. We examined CD25-deficient mice, which over-express various cytokines, including proinflammatory molecules, after bacterial superantigen stimulation in vivo. We observed that this abnormal cytokine production could be controlled by the injection of natural CD4 CD25(+) T cells and that IL-10 production is needed, as CD4 CD25(+) T cells from IL-10 knockout mice do not correct cytokine over-production in vivo. As the circulating IL-10 produced by CD25-deficient mice was ineffective, we deduced that the key source of IL-10 was the regulatory T cell population. IL-10 is also involved in the control of cytokine production by normal T cells. However, the target of IL-10 in this control is undefined. Whether it acts directly on the effector T cells or on the regulatory CD4 CD25(+) T cells themselves to induce their functional maturation has to be clarified.

Natural regulatory CD4 T cells expressing CD25

In normal mice, a subpopulation of CD4 T cells constitutively express CD25. These cells behave as regulatory T cells in autoimmune and inflammatory reactions, in tolerance to superantigens, and in peripheral T-cell homeostasis. They are unable to produce interleukin (IL)-2, and are dependent on IL-2 for growth in vitro and in vivo. CD4 CD25(+) T cells spontaneously secrete IL-10, which is involved in some of their regulatory functions. They are resistant to apoptosis, but can be tolerized by anergy.

Natural CD4+ CD25+ regulatory T cells. Their role in the control of superantigen responses

CD4 regulatory T cells have a major role in controlling the immune response to self and foreign antigens. Natural CD4+ CD25+ T cells are a major component of the regulatory subset. Their absence is associated with the development of autoimmune and inflammatory diseases and with abnormal peripheral T-cell homeostasis. Two main characteristics discriminate natural CD4+ CD25+ T cells from their CD4+ CD25- counterparts, namely their cytokine production profile and their behavior during tolerance induction. Natural CD4+ CD25+ T cells produce interleukin (IL)-10, a cytokine that contributes to their regulatory role. They do not produce IL-2 and are dependent on exogenous IL-2 for proliferation in vitro and in vivo. Studies of their response to superantigen administration in vivo show that they are resistant to clonal deletion but can be tolerized by anergy. Their resistance to apoptosis may contribute to their continuous regulatory function, as it allows them to maintain permanent control over effector T cells.

In vivo elimination of viral superantigen-activated CD4+ T cells: apoptosis occurs at a distance from the activation site

Local injection of mouse mammary tumor virus (MMTV) induces a local immune response, with activation of viral superantigen (vSAG)-specific T cell subsets followed by their clonal deletion. We investigated the fate of vSAG-reactive T cells following footpad injection of MMTV(SW) to mice. Activated T cells accumulated in draining lymph nodes. However, we demonstrated that apoptosis did not occur at the activation site, on the contrary of what has been shown after bacterial SAG activation. Although activated T cells were already shown to have the capacity to migrate to the gut, the fate of gut homing cells remains unclear. We demonstrate that the number of vSAG-specific T cells activated in the periphery was increasing in the follicles of gut-associated lymphoid organs, together with the number of apoptotic cell clusters. These results strongly suggested that gut-associated lymphoid tissue was the specific graveyard for apoptotic vSAG-activated CD4 T cells.

Induction and inhibition of CD40-CD40 ligand interactions: a new strategy underlying host-virus relationships

Interaction between CD40 and the CD40 ligand (CD40L) is required for mouse mammary tumor virus (MMTV) propagation. We found that Fas was expressed on B cells and CD40L on a small subset of viral superantigen-cognate T cells 12 h after MMTV(SW) infection. CD40L and Fas were down-regulated after 24 h. All CD4 T cells then became resistant to anti-CD3-induced CD40L induction in vitro for 2 wk. Initiation of CD40L expression and its rapid shut-off was associated with IL-12 production and was controlled by IFN-gamma and shedding of soluble CD40. These results suggest that a rapid, transient CD40-CD40L interaction involving a small number of cells is sufficient for MMTV propagation. Modulation of CD40L expression may be a major mechanism regulating the balance between viral propagation and host defenses, allowing mutual survival.

Induction and Inhibition of CD40-CD40 Ligand Interactions: A New Strategy Underlying Host-Virus Relationships

Interaction between CD40 and the CD40 ligand (CD40L) is required for mouse mammary tumor virus (MMTV) propagation. We found that Fas was expressed on B cells and CD40L on a small subset of viral superantigen-cognate T cells 12 h after MMTV(SW) infection. CD40L and Fas were down-regulated after 24 h. All CD4 T cells then became resistant to anti-CD3-induced CD40L induction in vitro for 2 wk. Initiation of CD40L expression and its rapid shut-off was associated with IL-12 production and was controlled by IFN-γ and shedding of soluble CD40. These results suggest that a rapid, transient CD40-CD40L interaction involving a small number of cells is sufficient for MMTV propagation. Modulation of CD40L expression may be a major mechanism regulating the balance between viral propagation and host defenses, allowing mutual survival.

IL-4-producing NK T cells are biased towards IFN-gamma production by IL-12. Influence of the microenvironment on the functional capacities of NK T cells

NK T cells are an unusual T lymphocyte subset capable of promptly producing several cytokines after stimulation, in particular IL-4, thus suggesting their influence in Th2 lineage commitment. In this study we demonstrate that, according to the cytokines present in the microenvironment, NK T lymphocytes can preferentially produce either IL-4 or IFN-gamma. In agreement with our previous reports showing that their IL-4-producing capacity is strikingly dependent on IL-7, CD4-CD8-TCRalphabeta+ NK T lymphocytes, obtained after expansion with IL-1 plus granulocyte-macrophage colony-stimulating factor, produced almost undetectable amounts of IL-4 or IFN-gamma in response to TCR/CD3 cross-linking. However, the capacity of these T cells to produce IFN-gamma is strikingly enhanced when IL-12 is added either during their expansion or the anti-CD3 stimulation, while IL-4 secretion is always absent. A similar effect of IL-12 on IFN-gamma production was observed when NK T lymphocytes were obtained after expansion with IL-7. It is noteworthy that whatever cytokines are used for their expansion, IL-12 stimulation, in the absence of TCR/CD3 cross-linking, promotes consistent IFN-gamma secretion by NK T cells without detectable IL-4 production. Experiments in vivo demonstrated a significant upregulation of the capacity of NK T cells to produce IFN-gamma after anti-CD3 mAb injection when mice were previously treated with IL-12. In conclusion, we provide evidence that the functional capacities of NK T cells, which ultimately will determine their physiological roles, are strikingly dependent on the cytokines present in their microenvironment.

Regulatory CD4 T cells: expression of IL-2R alpha chain, resistance to clonal deletion and IL-2 dependency

We recently characterized a CD4+ T cell population expressing the IL-2R alpha chain (CD25), producing IL-10 and resisting clonal deletion induced by viral superantigen (vSAG) encoded by mouse mammary tumor virus [MMTV(SW)]. We now report that these apoptosis-resistant cells are generated in the thymus but not from the immature CD4+ CD8+ thymocytes. They migrate from the thymus and are found in the periphery from at least the 10th day of life, after which they expand with the same kinetics in normal and MMTV(SW)-infected mice. Their strong capacity for expansion in the periphery makes this population insensitive to thymectomy in adulthood. CD4+ CD25+ cells were totally dependent on exogenous IL-2 for growth in vitro and in vivo, and were missing in IL-2 knockout (KO) mice. The absence of this population and/or an inability to produce IL-10 may be the missing link between IL-2R alpha KO, IL-2 KO and IL-10 KO mice, which all die of inflammatory bowel disease.

Changes in 1,25-(OH)2D3 synthesis and its receptor expression in spleen cell subpopulations of mice infected with LPBM5 retrovirus

This study examines the influence of chronic retroviral infection of mice with a LPBM5 virus mixture on the paracrine system involving immune cells and 1,25-(OH)2D3 in the spleen. Plasma ionized calcium, 25-(OH)D and 1,25-(OH)2D of infected mice were unchanged. In contrast, the specific binding of 1,25-(OH)2D3 to spleen cytosol and the number of monocyte/macrophages expressing 1,25-(OH)2D3 receptors (VDR) were markedly increased. The retroviral infection also influenced the local production of 1,25-(OH)2D3 in the spleen. It did not alter this production in monocyte/macrophages but increased that in isolated T cells. Isolated B cells in control mice did not produce 1,25-(OH)2D3, but they increased the ability of isolated T cells to produce this metabolite during coculture incubations. Infection altered this cell interaction as 1,25-(OH)2D3 production in infected T cells decreased when these cells were cocultured with infected B cells. Thus, chronic retroviral infection alters both the local vitamin D metabolism and VDR expression by immune cells in mice. These findings suggest close local interactions between 1,25-(OH)2D3 and immune system activation during retroviral infection.

Massive mammary gland infection and pregnancy-dependent mammary tumor development in mice infected neonatally with mouse mammary tumor virus (SW) but not in mice infected as adults, despite a dramatic local response

Mouse mammary tumor virus (MMTV) (SW) caused a high incidence (65%) of pregnancy-dependent adenocarcinomas in BALB/c(SW) mice infected as newborns by suckling their mothers' milk. These tumors were type B adenocarcinomas which developed early, at about 1 year of age. Uninfected breeding females and those infected at an age of 8 weeks by injection of virus had the same low incidence of malignant tumors (13%), and the tumors developed later (at approx. 23-24 months). The low incidence of tumors in adult-infected mice was correlated with partial infection of the mammary glands, and delayed transmission of MMTV(SW) to the offspring. Although the virus was rapidly disseminated in both types of infection, the responses of neonatally infected and adult-infected mice to MMTV(SW) infection and viral superantigen (vSAG) presentation were different. Activation by and presentation of the vSAG was impaired in mice infected neonatally, and tolerance induction by clonal deletion was delayed. Local activation was dramatic in mice infected as adults and clonal deletion followed rapidly. Although interaction between B and T cells is needed for completion of the virus life cycle and viral amplification, the strong local immune response to MMTV(SW) in adult-infected mice limits mammary gland infection, and protects them against mammary tumor development.

T cell deletion induced by chronic infection with mouse mammary tumor virus spares a CD25-positive, IL-10-producing T cell population with infectious capacity

We found that T cells recognizing viral superantigen (vSAG) can be subdivided into two distinct functional subsets based on IL-2R alpha (CD25) expression. CD4+Vbeta6+CD25- and CD4+Vbeta6+CD25+ T cells were sensitive to vSAG activation. When obtained from BALB/c(SW) mice, both subsets were infected and capable to induce the tolerance process when transferred into noninfected recipients. However, in contrast to CD4+Vbeta6+CD25- cells, which were gradually deleted in MMTV(SW)-infected mice, the pool of CD4+Vbeta6+CD25+ lymphocytes was constant even at the end of the deletion process, and maintained a limited reactivity to vSAG-induced activation. The constant number of Vbeta6+CD25+ observed in infected mice could not be explained by their rapid turnover (deletion and renewal), as their proliferative rate measured by BrdU incorporation was similar in infected and naive mice, as well as in virus-nonspecific (Vbeta8.2+) cells. Neither was the Vbeta6+CD25+ subset dependent on vSAG activation since it was also present in MMTV-free mice and was not generated from Vbeta6+CD25- cells upon in vivo vSAG stimulation. Vbeta6+CD25+ T cells constitutively expressed IL-4 and IL-10 mRNA. IL-10 has been shown to be associated with viral, bacterial, and parasitic infections. This permanent CD25+ subpopulation may play a role in the control of viral infection and tolerance induction via vSAG recognition and IL-10 production.

T cell deletion induced by chronic infection with mouse mammary tumor virus spares a CD25-positive, IL-10-producing T cell population with infectious capacity

We found that T cells recognizing viral superantigen (vSAG) can be subdivided into two distinct functional subsets based on IL-2R alpha (CD25) expression. CD4+Vbeta6+CD25- and CD4+Vbeta6+CD25+ T cells were sensitive to vSAG activation. When obtained from BALB/c(SW) mice, both subsets were infected and capable to induce the tolerance process when transferred into noninfected recipients. However, in contrast to CD4+Vbeta6+CD25- cells, which were gradually deleted in MMTV(SW)-infected mice, the pool of CD4+Vbeta6+CD25+ lymphocytes was constant even at the end of the deletion process, and maintained a limited reactivity to vSAG-induced activation. The constant number of Vbeta6+CD25+ observed in infected mice could not be explained by their rapid turnover (deletion and renewal), as their proliferative rate measured by BrdU incorporation was similar in infected and naive mice, as well as in virus-nonspecific (Vbeta8.2+) cells. Neither was the Vbeta6+CD25+ subset dependent on vSAG activation since it was also present in MMTV-free mice and was not generated from Vbeta6+CD25- cells upon in vivo vSAG stimulation. Vbeta6+CD25+ T cells constitutively expressed IL-4 and IL-10 mRNA. IL-10 has been shown to be associated with viral, bacterial, and parasitic infections. This permanent CD25+ subpopulation may play a role in the control of viral infection and tolerance induction via vSAG recognition and IL-10 production.

Requirement of IL-7 for IL-4-producing potential of MHC class I-selected CD4-CD8-TCR alpha beta+ thymocytes

IL-7 plays an important role in the growth and differentiation of T cells. We have previously reported that IL-7 induces preferential expansion of MHC class I-selected CD4-CD8-TCR alpha beta+ thymocytes which express a skewed V beta repertoire and are potent IL-4 producers. In this report, we provide evidence that IL-1 in combination with granulocyte macrophage colony stimulating factor can also expand this population. Yet, these cells do not share the functional characteristics of those obtained in the presence of IL-7, i.e. cytotoxic activity and high IL-4 production. These functional capacities can be acquired by adding IL-7. In conclusion, our findings demonstrate that the capacity of MHC class I-selected CD4-CD8-TCR alpha beta+ thymocytes to produce IL-4 as well as to kill target cells is IL-7 dependent.

Mouse mammary tumor virus production by thymic epithelial cells in vivo

Exogenous mouse mammary tumor viruses (MMTV) replicate in the mammary glands of infected females, and so infect the suckling pups. We have previously shown that the virus is rapidly disseminated to all the lymphoid organs, including the thymus. The present electron microscope immunohistochemical study describes the viral production site in the thymus. Viral buds and viral proteins were restricted to the thymus medullary epithelial cells. MMTV-encoded proteins were identified on the free viral particles and on the budding ones, the ribosomes, the membrane of the endoplasmic reticulum, and on the membrane of the medullary type II epithelial cell vacuolar network. The thymus medullary epithelial cells can thus integrate the virus and allow viral replication. The results support earlier results indicating that in some experimental conditions, epithelial cells may be involved in MMTV-induced negative selection by showing that thymic epithelial cells do express MMTV-encoded proteins.

In vivo T cell response to viral superantigen. Selective migration rather than proliferation

Superantigens induce T cell activation and proliferation in vitro, and some also induce cell activation in vivo. MMTV(SW) is an infectious mouse mammary tumor virus (MMTV) encoding a superantigen with the same Vbeta specificity as MIs-1a (Mtv-7), which induces a strong local response in vivo. injection of MMTV(SW) into mouse footpads leads to accumulation of superantigen-reactive T cells (Vbeta6+CD4+) and B cells in the draining lymph nodes (LN). We investigated the kinetics of this cell accumulation by measuring cell activation (blastogenesis, CD25 and CD69 expression), cell migration (using syngenic FITC-labeled CD4+ cells and L-selectin detection), and cell proliferation (using in vivo labeling with bromodeoxyuridine). Specific T cells selectively migrated to the draining LN. Accumulating Vbeta6+CD4+ T cells were large CD69+ cells, but remained CD25 negative and showed down-regulated L-selectin expression. Their DNA synthesis rate, studied by pulse labeling and continuous administration of bromodeoxyuridine, was increased, but remained too low to explain the draining LN hyperplasia. These data show that the local T cell response to MMTV(SW) mainly consists of selective migration followed by local activation of reactive T cells, and that cell proliferation is only a minor component of the response. By contrast, the optimal dose of staphylococcal enterotoxin B that, nevertheless, leads to a lower reactive T cell accumulation in the draining LN induces a very high proliferation rate.

Resistance to murine AIDS in offspring of mice infected with LP-BM5. Role of CD8 T cells

The murine-acquired immunodeficiency syndrome (MAIDS) is caused by a mixture of murine leukemia viruses (LP-BM5 MuLV). The influence of perinatal contact with retroviruses or their Ags on the response to infection was tested by infecting with LP-BM5 (MuLV) the adult offspring of mice with MAIDS. These offspring were resistant to disease after virus challenge. Most of them were free of defective viral DNA, and even those with molecular evidence of infection had lymphoid cells with a lower infectious capacity to cause MAIDS in naive recipients. No ecotropic, xenotropic, or mink cell focus-forming (MCF) virus expression was found at the age of 5 wk, which is the time of LP-BM5 (MuLV) challenge. However, at 22 wk of age, one-half of the offspring from MAIDS mothers had ecotropic virus-expressing cells in their spleens. At the time of suckling, offspring from infected mothers had enhanced percentages of B cells and CD4 and CD8 T cells in the spleen, possibly followed by a slight persistent splenomegaly. These results suggest that immune reactivity, rather than tolerance to the virus, is responsible for resistance to disease after challenge. The offspring of MAIDS mice could clear the virus after challenge. This clearance was mediated by CD8 T cells, as continuous CD8 T cell depletion initiated at the time of viral challenge abrogated the resistance of these mice to MAIDS.

Resistance to murine AIDS in offspring of mice infected with LP-BM5. Role of CD8 T cells

The murine-acquired immunodeficiency syndrome (MAIDS) is caused by a mixture of murine leukemia viruses (LP-BM5 MuLV). The influence of perinatal contact with retroviruses or their Ags on the response to infection was tested by infecting with LP-BM5 (MuLV) the adult offspring of mice with MAIDS. These offspring were resistant to disease after virus challenge. Most of them were free of defective viral DNA, and even those with molecular evidence of infection had lymphoid cells with a lower infectious capacity to cause MAIDS in naive recipients. No ecotropic, xenotropic, or mink cell focus-forming (MCF) virus expression was found at the age of 5 wk, which is the time of LP-BM5 (MuLV) challenge. However, at 22 wk of age, one-half of the offspring from MAIDS mothers had ecotropic virus-expressing cells in their spleens. At the time of suckling, offspring from infected mothers had enhanced percentages of B cells and CD4 and CD8 T cells in the spleen, possibly followed by a slight persistent splenomegaly. These results suggest that immune reactivity, rather than tolerance to the virus, is responsible for resistance to disease after challenge. The offspring of MAIDS mice could clear the virus after challenge. This clearance was mediated by CD8 T cells, as continuous CD8 T cell depletion initiated at the time of viral challenge abrogated the resistance of these mice to MAIDS.

In vivo T cell response to viral superantigen. Selective migration rather than proliferation

Superantigens induce T cell activation and proliferation in vitro, and some also induce cell activation in vivo. MMTV(SW) is an infectious mouse mammary tumor virus (MMTV) encoding a superantigen with the same Vbeta specificity as MIs-1a (Mtv-7), which induces a strong local response in vivo. injection of MMTV(SW) into mouse footpads leads to accumulation of superantigen-reactive T cells (Vbeta6+CD4+) and B cells in the draining lymph nodes (LN). We investigated the kinetics of this cell accumulation by measuring cell activation (blastogenesis, CD25 and CD69 expression), cell migration (using syngenic FITC-labeled CD4+ cells and L-selectin detection), and cell proliferation (using in vivo labeling with bromodeoxyuridine). Specific T cells selectively migrated to the draining LN. Accumulating Vbeta6+CD4+ T cells were large CD69+ cells, but remained CD25 negative and showed down-regulated L-selectin expression. Their DNA synthesis rate, studied by pulse labeling and continuous administration of bromodeoxyuridine, was increased, but remained too low to explain the draining LN hyperplasia. These data show that the local T cell response to MMTV(SW) mainly consists of selective migration followed by local activation of reactive T cells, and that cell proliferation is only a minor component of the response. By contrast, the optimal dose of staphylococcal enterotoxin B that, nevertheless, leads to a lower reactive T cell accumulation in the draining LN induces a very high proliferation rate.

Neonatal impaired response to viral superantigen encoded by MMTV(SW) and Mtv-7

MMTV(SW) is an exogenous mouse mammary tumor virus that codes for a superantigen sharing the same V beta specificity as Mtv-7 (Mis-1a). Neonatal mice infected by suckling-infected milk show a deletion of the CD4+ V beta 6+ T cell subset within 8 weeks. In contrast, adult mice infected by injection of the virus in the footpad have a much faster deletion, which occurs within 2 weeks. In the present work, we investigated possible mechanisms for the different kinetics of deletion in the adult and newborn mice. To find out if the route of infection could be responsible for this discrepancy, we infected 5-day-old and adult mice by injection in the footpad. Our results demonstrate that the route of infection is not responsible for the delayed kinetics of reactive T cell deletion since newborn mice injected with the virus show similar kinetics to neonates infected by maternal milk. To exclude differences in viral spreading between the two models, we used a PCR assay to detect proviral DNA. Spreading of the virus was shown to occur at a similar rate or even more rapidly in neonates than in adults. We also compared the activation induced by MMTV(SW) or Mis-1a spleen cells in the draining lymph node in neonatal and adult mice and showed that a poor local activation is induced in neonates compared with adults. In vitro, neonatal T cell reactivity to anti-V beta 6 antibody was also impaired. Thus, the delay in clonal deletion could be linked to impaired expression, presentation and/or response to the viral superantigen. Our results suggest that the initial response to MMTV(SW) could be of importance for the kinetics of reactive T cell deletion.

MHC class I-selected CD4-CD8-TCR-alpha beta+ T cells are a potential source of IL-4 during primary immune response

Differentiation of naive CD4+ lymphocytes into either Th1 or Th2 cells is influenced by the cytokine present during initial Ag priming. IL-4 is the critical element in the induction of Th2 response; however, its origin during a primary immune response is not well defined. In the present study, we characterized a novel potential source of IL-4, the class I-selected CD4-CD8-TCR-alpha beta+ T cells. In a first set of experiments, we demonstrated that CD4-CD8-TCR-alpha beta+ thymocytes produce a large amount of IL-4 after in vitro anti-CD3 stimulation. This phenomenon was not observed in class I-deficient mice, demonstrating that among these cells, the class I-selected subset was predominantly responsible for IL-4 production. Further studies focused on the in vivo IL-4-producing capacity of peripheral CD4-CD8-TCR-alpha beta+ T cells. To this end, a single injection of anti-CD3 mAb, which promptly induces IL-4 mRNA expression, was used. Peripheral CD4-CD8-TCR-alpha beta+ T cells express high levels of IL-4 mRNA in response to in vivo anti-CD3 challenge. Furthermore, analysis performed in mice lacking MHC class I or class II molecules demonstrates that both the class I-selected subset of CD4-CD8-TCR+ and CD4+ peripheral T lymphocytes are the major IL-4 producers after in vivo anti-CD3 stimulation. These findings suggest that class I-selected CD4-CD8-TCR-alpha beta+ and CD4+ T cell populations are important sources of IL-4 probably implicated in the development of specific Th2 immune responses.

MHC class I-selected CD4-CD8-TCR-alpha beta+ T cells are a potential source of IL-4 during primary immune response

Differentiation of naive CD4+ lymphocytes into either Th1 or Th2 cells is influenced by the cytokine present during initial Ag priming. IL-4 is the critical element in the induction of Th2 response; however, its origin during a primary immune response is not well defined. In the present study, we characterized a novel potential source of IL-4, the class I-selected CD4-CD8-TCR-alpha beta+ T cells. In a first set of experiments, we demonstrated that CD4-CD8-TCR-alpha beta+ thymocytes produce a large amount of IL-4 after in vitro anti-CD3 stimulation. This phenomenon was not observed in class I-deficient mice, demonstrating that among these cells, the class I-selected subset was predominantly responsible for IL-4 production. Further studies focused on the in vivo IL-4-producing capacity of peripheral CD4-CD8-TCR-alpha beta+ T cells. To this end, a single injection of anti-CD3 mAb, which promptly induces IL-4 mRNA expression, was used. Peripheral CD4-CD8-TCR-alpha beta+ T cells express high levels of IL-4 mRNA in response to in vivo anti-CD3 challenge. Furthermore, analysis performed in mice lacking MHC class I or class II molecules demonstrates that both the class I-selected subset of CD4-CD8-TCR+ and CD4+ peripheral T lymphocytes are the major IL-4 producers after in vivo anti-CD3 stimulation. These findings suggest that class I-selected CD4-CD8-TCR-alpha beta+ and CD4+ T cell populations are important sources of IL-4 probably implicated in the development of specific Th2 immune responses.

Non-exclusive Fas control and age dependence of viral superantigen-induced clonal deletion in lupus-prone mice

To investigate the role of Fas in the induction of tolerance by viral superantigen (SAG), we infected MRL-+/+ and MRL-lpr (Fas mutant) mice with mouse mammary tumor virus (MMTV) (SW), a virus encoding an SAG with the same specificity as endogenous Mtv-7-SAG. In normal mice, this infection has two distinct consequences on specific V beta 6+CD4+ T cells, consisting of activation followed by clonal deletion. MMTV (SW)-SAG-induced activation in vivo was identical in MRL-+/+ and MRL-lpr mice. In contrast, clonal deletion showed age-dependent impairment. Early infection (5 weeks) led to identical clonal deletion of specific T cells in blood lymphocytes from MRL-+/+ and MRL-lpr mice, although clonal deletion was slightly impaired in the MRL-lpr lymph nodes. Late infection (10 weeks) of MRL-lpr mice led to markedly delayed and reduced clonal deletion. V beta 6+CD4+ T cells which escaped clonal deletion in aging MRL-lpr mice were not anergized by interaction with SAG. These results show that peripheral clonal deletion induced by viral SAG in adult mice is controlled by Fas, but not exclusively so.

Endogenous granulocyte-macrophage colony-stimulating factor is involved in IL-1- and IL-7-induced murine thymocyte proliferation

We have reported previously that IL-1 induces murine thymocyte proliferation in the absence of artificial comitogens, provided that the cells are cultured at high densities. In the present study, we show that, in these conditions, TdR uptake in response to IL-1 is diminished significantly by anti-granulocyte-macrophage colony-stimulating factor (GM-CSF) Abs. Indeed, a substantial production of this growth factor occurs when thymocytes are cultured in the presence of IL-1. Maximal GM-CSF levels are attained within 3 days of culture, and mRNA expression is detected after a 48-h stimulation. Both GM-CSF production and IL-1-induced thymocyte proliferation are decreased considerably by the depletion of I-A+ Mac-1+ accessory cells. Yet, addition of exogenous GM-CSF to accessory cell-depleted thymocytes does not restore the proliferative response to IL-1 alone, suggesting the implication of another accessory cell-derived mediator. Our data design IL-7 as the endogenous factor required in our culture system because: 1) GM-CSF can reverse the decrease in the proliferation after accessory cell depletion when IL-7 is provided together with IL-1, and 2) the proliferative response to IL-1 plus IL-7 is diminished as much by neutralization of GM-CSF by its specific Abs as by accessory cell removal (approximately 30%). Finally, the cells responding to IL-1 + IL-7 were identified as mature CD4-CD8-TCR+ thymocytes by the use of bromodeoxyuridine (BrdUrd), suggesting that the GM-CSF produced by thymic accessory cells in response to IL-1 participates in IL-7-dependent, intrathymic expansion of the CD4-CD8-TCR+ compartment.

Endogenous granulocyte-macrophage colony-stimulating factor is involved in IL-1- and IL-7-induced murine thymocyte proliferation

We have reported previously that IL-1 induces murine thymocyte proliferation in the absence of artificial comitogens, provided that the cells are cultured at high densities. In the present study, we show that, in these conditions, TdR uptake in response to IL-1 is diminished significantly by anti-granulocyte-macrophage colony-stimulating factor (GM-CSF) Abs. Indeed, a substantial production of this growth factor occurs when thymocytes are cultured in the presence of IL-1. Maximal GM-CSF levels are attained within 3 days of culture, and mRNA expression is detected after a 48-h stimulation. Both GM-CSF production and IL-1-induced thymocyte proliferation are decreased considerably by the depletion of I-A+ Mac-1+ accessory cells. Yet, addition of exogenous GM-CSF to accessory cell-depleted thymocytes does not restore the proliferative response to IL-1 alone, suggesting the implication of another accessory cell-derived mediator. Our data design IL-7 as the endogenous factor required in our culture system because: 1) GM-CSF can reverse the decrease in the proliferation after accessory cell depletion when IL-7 is provided together with IL-1, and 2) the proliferative response to IL-1 plus IL-7 is diminished as much by neutralization of GM-CSF by its specific Abs as by accessory cell removal (approximately 30%). Finally, the cells responding to IL-1 + IL-7 were identified as mature CD4-CD8-TCR+ thymocytes by the use of bromodeoxyuridine (BrdUrd), suggesting that the GM-CSF produced by thymic accessory cells in response to IL-1 participates in IL-7-dependent, intrathymic expansion of the CD4-CD8-TCR+ compartment.

Interleukin 7 induces preferential expansion of V beta 8.2+CD4-8- and V beta 8.2+CD4+8- murine thymocytes positively selected by class I molecules

We analyzed the phenotype and V beta-T cell receptor (TCR) repertoire, together with interleukin 7 receptor (IL-7R) expression in unfractionated thymocytes stimulated in vitro with IL-7. This culture system results in a specific proliferation of mature thymocytes belonging to the CD3+CD4-, CD4+8-, and CD4-8+ subsets. IL-7 induced a preferential expansion of V beta 8.2+CD4-8- and V beta 8.2+CD4-8- thymocytes. This phenomenon is not observed in beta 2-microglobulin-deficient mice, showing that a fraction of CD4+8- thymocytes, enriched in V beta 8.2+ cells, is selected by class I molecules in normal mice, as are a large proportion of CD4-8- alpha beta TCR+ thymocytes. Our findings also establish that IL-7 plays a major role in the expansion of rare thymocyte subsets, which could exert important functions in inflammatory and immune responses.

MTS-32 monoclonal antibody defines CD4+8- thymocyte subsets that differ in their maturation level, lymphokine secretion, and selection patterns

We have previously described MTS-32 as identifying an Ag on both thymic stromal cells and thymocytes. In contrast with CD4+8+ and CD4-8+ thymocytes, of which the vast majority are MTS-32+, a notable subset of CD4+8- thymocytes is MTS-32-. Here we show that with regard to heat-stable Ags, Qa-2, and CD69 expression CD4+8- MTS-32- thymocytes are phenotypically enriched in mature cells when compared with their MTS-32+ counterparts. Moreover, sorted CD4+8- MTS-32+ thymocytes are unable to respond to anti-CD3 cross-linking, whereas MTS-32- CD4+8- thymocytes respond to the same stimulus by producing IL-4, IL-5, IL-10, IFN-gamma, and trace amounts of IL-2. In addition, MTS-32- CD4+8- and CD4-8- TCR-alpha beta+ thymocytes differ in their TCR V beta repertoire on a Mls-1a selecting background. We therefore suggest that the MTS-32 ligand is involved in signals consecutive with TCR recognition in the thymus, i.e., selection, activation, and lymphokine production.

MTS-32 monoclonal antibody defines CD4+8- thymocyte subsets that differ in their maturation level, lymphokine secretion, and selection patterns

We have previously described MTS-32 as identifying an Ag on both thymic stromal cells and thymocytes. In contrast with CD4+8+ and CD4-8+ thymocytes, of which the vast majority are MTS-32+, a notable subset of CD4+8- thymocytes is MTS-32-. Here we show that with regard to heat-stable Ags, Qa-2, and CD69 expression CD4+8- MTS-32- thymocytes are phenotypically enriched in mature cells when compared with their MTS-32+ counterparts. Moreover, sorted CD4+8- MTS-32+ thymocytes are unable to respond to anti-CD3 cross-linking, whereas MTS-32- CD4+8- thymocytes respond to the same stimulus by producing IL-4, IL-5, IL-10, IFN-gamma, and trace amounts of IL-2. In addition, MTS-32- CD4+8- and CD4-8- TCR-alpha beta+ thymocytes differ in their TCR V beta repertoire on a Mls-1a selecting background. We therefore suggest that the MTS-32 ligand is involved in signals consecutive with TCR recognition in the thymus, i.e., selection, activation, and lymphokine production.

A kinetic study on the deletion of thymic, peripheral, and gut-associated V beta 6+ T cells in an Mls-1b BALB/c colony infected with an exogenous mouse mammary tumor virus

Mls-1a expression results in the deletion of T cells bearing V beta 6 chains of the TCR. However, V beta 6+ T cells are also deleted in Mls-1b BALB/c mice that have been infected with an exogenous mouse mammary tumor virus (swiss mice) via maternal milk intake, and whose open reading frame region is markedly similar to that of the provirus Mtv-7. In this report we describe the kinetics of V beta 6+ T cell deletion in the thymus, spleen, lymph nodes, and gut-associated lymphoid populations of these BALB/c mice from the early weeks of life to 6 mo of age. Deletion of V beta 6+ T cells within the CD4+ T cell population was more obvious in the thymus than in the spleen at 8 wk of age. However, the earliest incidence of deletion was observed in the gut intraepithelial lymphocyte population at 5 wk of age. Furthermore Mtv-7 (SW) transcripts were only found in the gut in the first wk of life, whereafter they could be detected in the thymus, spleen, and lymph nodes. This report suggests that after entering the intestinal tract of host mice, mouse mammary tumor virus (swiss mice) is subsequently transferred to the thymus and peripheral lymphoid organs resulting in the deletion of CD4+V beta 6+ T cells in that order.

A kinetic study on the deletion of thymic, peripheral, and gut-associated V beta 6+ T cells in an Mls-1b BALB/c colony infected with an exogenous mouse mammary tumor virus

Mls-1a expression results in the deletion of T cells bearing V beta 6 chains of the TCR. However, V beta 6+ T cells are also deleted in Mls-1b BALB/c mice that have been infected with an exogenous mouse mammary tumor virus (swiss mice) via maternal milk intake, and whose open reading frame region is markedly similar to that of the provirus Mtv-7. In this report we describe the kinetics of V beta 6+ T cell deletion in the thymus, spleen, lymph nodes, and gut-associated lymphoid populations of these BALB/c mice from the early weeks of life to 6 mo of age. Deletion of V beta 6+ T cells within the CD4+ T cell population was more obvious in the thymus than in the spleen at 8 wk of age. However, the earliest incidence of deletion was observed in the gut intraepithelial lymphocyte population at 5 wk of age. Furthermore Mtv-7 (SW) transcripts were only found in the gut in the first wk of life, whereafter they could be detected in the thymus, spleen, and lymph nodes. This report suggests that after entering the intestinal tract of host mice, mouse mammary tumor virus (swiss mice) is subsequently transferred to the thymus and peripheral lymphoid organs resulting in the deletion of CD4+V beta 6+ T cells in that order.

Multiple effects caused by anti-IL-4 mAb inoculation in the thymus and spleen of adult mice

We investigated the physiological role of IL-4 in the thymus and spleen by administering a neutralizing monoclonal anti-IL-4 antibody (11B11 mAb) into adult mice for 7 or 14 days. After this treatment the thymus decreased in size, this was mainly attributed to a decrease in the CD4+CD8+ subset after 7 days and both the CD4+CD8+ and the CD4+CD8- subsets after 14 days. These data suggest that IL-4 has a role in CD4+CD8+ thymocyte development and can differentially modulate the maturation of CD4+CD8- thymocytes in adult mice. Conversely, anti-IL-4 inoculation induced an increase in spleen size. After 7 days of treatment this enlargement appeared to be due to a increase in number of immature cells, the majority of which were CD4-CD8-alpha beta TCR-B220-slg-Ia-Mac-1-Pgp1 positive. After 14 days, an expanded spleen size was mainly due to inflated numbers of mature CD4+CD8-, B and Mac-1+ cells. In addition, we showed that anti-IL4 mAb in vivo enhanced the CD4-CD8-alpha beta TCRlow subset within the spleen after 7 days which is also observed at 14 days of treatment. Finally, we demonstrated that anti-IL-4 mAb treatment is highly stimulatory for hemopoietic activity in the adult spleen. Taken together, these results support the notion that IL-4 acts in vivo, directly or indirectly, on T cell differentiation in the thymus and T subsets homeostasis as well as on other cell types in the spleen of adult mice.

Tissue distribution of Mtv-7-like exogenous retroviral transcripts and clonal deletion of V beta 6+ T cells in Mls-1b BALB/c mice

We have previously described an Mls-1a-like clonal deletion of mature CD4+ T cells which express V beta 6 and V beta 8.1 chains of the TCR in half of the mice of a BALB/c, Mls-1b colony (BALB/c IC). This occurs in the absence of the Mtv-7 provirus which is responsible for the clonal deletion in Mls-1a mice. We developed a polymerase chain reaction assay in order to study the presence of retroviral transcripts homologous to the viral superantigen gene (vSAG) of Mtv-7 and Mtv-6 in various tissues. Mtv-7 homologous transcripts were present in the mammary glands of lactating BALB/c IC mice and in the thymuses and/or spleens of BALB/c IC virgin mice with deletion of V beta 6+ lymph node T cells, and not in BALB/c IC with normal V beta 6 expression. These results indicate that this BALB/c colony is infected with an exogenous mouse mammary tumour virus (MMTV) whose vSAG is similar to Mtv-7, as recently reported. Thymectomies performed at 4-5 weeks of age (at least 4 weeks before detection of clonal deletion), did not affect the occurrence of clonal deletion in peripheral lymph nodes when tested 20 weeks later. This suggests that clonal deletion can be achieved without further intrathymic contact with the antigen. Since MMTV is transmitted through milk and is likely to be present in the gut, we evaluated the percentage of V beta 6+CD4+ T cells within the gut intraepithelial lymphocyte (IEL) population. Mice with normal V beta 6 expression in lymph nodes may show partial deletion of V beta 6+CD4+ IEL.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of thymic cell subpopulations involved in IL-1- or GM-CSF-induced IL-6 production

IL-6 has been demonstrated by in vitro studies to be a cytokine involved in thymocyte activation We show herein that thymocytes cultured at high concentrations in the absence of comitogen respond to IL-1 and, to a lesser degree, to GM-CSF, by producing IL-6. This phenomenon disappears rapidly with decreasing cell densities, suggesting the involvement of a minor cellular component of the thymus which may be solely responsible for or cooperate in IL-6 production. We have analysed several thymic subpopulations for IL-6 production and show that accessory cells, and eventually their precursors, are the major if not exclusive, producers of this cytokine. Mature steroid-resistant thymocytes do not secrete IL-6. Production of IL-6 by total CD4-CD8- thymic cells is largely reduced by the depletion of mature accessory cells which express I-A and Mac-1 antigens. As shown previously, accessory cell precursors within the CD4-CD8- compartment are induced to differentiate into M phi and DC in response to IL-1 and GM-CSF. We provide evidence that this maturation is associated with IL-6 production. Thymic DC and phagocytic cells of the thymic reticulum (P-TR) in vitro produce high levels of IL-6 which are enhanced by GM-CSF or IL-1. These factors have a synergistic effect on IL-6 production by total thymocytes, and on CD4-CD8- cells that are not depleted for mature I-A+ Mac-1+ accessory cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Acquired Mls-1a-like clonal deletion in Mls-1b mice

BALB/c mice (H-2d, Mls-1b) from one colony progressively modify their T cell repertoire during aging, by deleting T cells that express products of the V beta 6 and V beta 8.1 genes of the T cell receptor. Clonal deletion occurs only in 50% of mice between 27 and 43 wk of age, affecting thymus, spleen, and lymph node T cells. The phenomenon is progressive and seems to affect nearly all thymuses between 14 and 19 wk of age. CD4+CD8- mature T cells are more affected than CD4-CD8+ cells. In the thymus, deletion occurs at the stage of immature J11d+ cells expressing a high level of V beta 6, while J11d+V beta 6low- expressing cells are not modified. Clonal deletion is thus an early phenomenon that deletes cells of the immature generative compartment in the thymus. This Mls-1a-like clonal deletion is associated neither with the expression of an Mls-1a-like antigen that could be identified in mixed lymphocyte reaction in vitro, nor with the presence of Mtv-7, the endogenous mouse mammary tumor virus (MMTV) proviral loci. Spleen cells, bone marrow cells, and total thymocytes injected into newborn thymuses cannot induce V beta 6+ cell deletion. However, newborn thymuses grafted into old BALB/c mice behave like their recipients, suggesting that a new antigen, present in these old BALB/c mice, is indeed able to induce an Mls-1a-like clonal deletion. As other BALB/c colonies tested do not behave in same way, the hypothesis of a new exogenous deleting factor rather than a genetic transmission is likely. This may suggest that acquired clonal deletion is a more common phenomenon than expected, and may be the spontaneous reaction of the immune system to the introduction of a new retrovirus or other superantigen.

Thymic abnormalities in fetuses aborted from human immunodeficiency virus type 1 seropositive women

Pathological abnormalities of the thymus were found in 3 of 37 fetuses aborted from human immunodeficiency virus (HIV)-infected mothers. These lesions were located predominantly in the thymic cortex, which contains mostly immature lymphocytes. Areas of focal lymphocyte depletion were infiltrated with CD4+ macrophages and were associated with abnormalities of the epithelial stromal network. No evidence of extensive HIV infection in any of the 37 thymuses was detected by either immunofluorescence or in situ hybridization techniques, although rare cells that expressed HIV antigens were found in 3 fetuses. Although less extensive, this thymic fetopathy was similar to that described in postnatal acquired immunodeficiency syndrome thymuses, strongly suggesting that the lesions were related to HIV infection. Thymic fetopathy might represent the initial injury to the lymphoid system in HIV-infected infants in whom early and severe immunosuppression develops.

IL-7 is requisite for IL-1-induced thymocyte proliferation. Involvement of IL-7 in the synergistic effects of granulocyte-macrophage colony-stimulating factor or tumor necrosis factor with IL-1

In the absence of artificial comitogens murine thymocytes proliferate significantly in response to IL-1 at high but not at low cell densities. This observation has led us to examine a possible indirect mechanism requiring other thymocyte-growth factors, such as IL-2, IL-4, IL-6, and IL-7, in this phenomenon. Our data provide evidence that IL-7 is requisite for the IL-1-induced proliferative response because on the one hand the growth-promoting activity of IL-1 is completely inhibited by an anti-IL-7 mAb, and on the other hand IL-7 synergizes with IL-1 on thymocyte growth. This synergy is observed even at concentrations at which IL-7 is not detected in the pre-B cell proliferation assay, and results, at optimal doses, in TdR incorporation levels similar to those attained in response to IL-1 + IL-2. The anti-IL-7 mAb acts in a dose-dependent manner and does not affect other activities of IL-1, such as its capacity to sustain the growth of the U373 astrocytoma cell line. It is also noteworthy that this mAb does not significantly impair thymocyte growth in response to IL-2 and that the growth-promoting activity of IL-1 is not affected by neutralizing mAb against IL-2, IL-4, and IL-6. In addition, we show that the potentiating effect of granulocyte-macrophage (GM)-CSF and TNF-alpha on IL-1-induced thymocyte growth is dependent on IL-7 because i) the anti-IL-7 mAb abrogates the respective synergistic interactions and ii) both factors potentiate the proliferative response to IL-7. Finally, depletion of thymocyte suspensions for Ia+ Mac-1+ accessory cells results in a considerable decrease in IL-1- and IL-1 + GM-CSF-induced TdR uptake, whereas IL-7-induced growth remains unchanged. Taken together, these results support the notion that, in the absence of artificial comitogens, thymocyte proliferation in response to IL-1 alone or in combination with GM-CSF is dependent on accessory cell-derived IL-7.

IL-7 is requisite for IL-1-induced thymocyte proliferation. Involvement of IL-7 in the synergistic effects of granulocyte-macrophage colony-stimulating factor or tumor necrosis factor with IL-1

In the absence of artificial comitogens murine thymocytes proliferate significantly in response to IL-1 at high but not at low cell densities. This observation has led us to examine a possible indirect mechanism requiring other thymocyte-growth factors, such as IL-2, IL-4, IL-6, and IL-7, in this phenomenon. Our data provide evidence that IL-7 is requisite for the IL-1-induced proliferative response because on the one hand the growth-promoting activity of IL-1 is completely inhibited by an anti-IL-7 mAb, and on the other hand IL-7 synergizes with IL-1 on thymocyte growth. This synergy is observed even at concentrations at which IL-7 is not detected in the pre-B cell proliferation assay, and results, at optimal doses, in TdR incorporation levels similar to those attained in response to IL-1 + IL-2. The anti-IL-7 mAb acts in a dose-dependent manner and does not affect other activities of IL-1, such as its capacity to sustain the growth of the U373 astrocytoma cell line. It is also noteworthy that this mAb does not significantly impair thymocyte growth in response to IL-2 and that the growth-promoting activity of IL-1 is not affected by neutralizing mAb against IL-2, IL-4, and IL-6. In addition, we show that the potentiating effect of granulocyte-macrophage (GM)-CSF and TNF-alpha on IL-1-induced thymocyte growth is dependent on IL-7 because i) the anti-IL-7 mAb abrogates the respective synergistic interactions and ii) both factors potentiate the proliferative response to IL-7. Finally, depletion of thymocyte suspensions for Ia+ Mac-1+ accessory cells results in a considerable decrease in IL-1- and IL-1 + GM-CSF-induced TdR uptake, whereas IL-7-induced growth remains unchanged. Taken together, these results support the notion that, in the absence of artificial comitogens, thymocyte proliferation in response to IL-1 alone or in combination with GM-CSF is dependent on accessory cell-derived IL-7.

Effect of cytokines on thymic hematopoietic precursors. Phenotypic and electron-microscopic study

We have previously shown that the interaction of thymocytes with thymic accessory cells (macrophages and/or interdigitating cells) is one of the factors required for thymocyte activation. Precursors of both thymic accessory cells and thymocytes are included in the CD4- CD8- Mac-1- Ia- subpopulation, and their respective maturation and/or activation may be modulated by granulocyte-macrophage colony-stimulating factor, interleukin 1 and interleukin 2. When CD4- CD8- thymic cells are activated with granulocyte-macrophage colony-stimulating factor plus interleukin 2, both macrophages and interdigitating-like cells are present, as shown by electron microscopy. When activated with interleukin 1 plus interleukin 2, the interdigitating-like cell is the only accessory cell present. In both culture conditions, large clusters are formed between interdigitating cells and lymphoid cells. These results have led us to propose two-step signals for thymocyte proliferation: first, the maturation of macrophages under granulocyte-macrophage colony-stimulating factor control and the production of interleukin 1, and secondly, the maturation of interdigitating cells under interleukin 1 control, their clustering with thymocytes which are then activated.

Persistence of stem cell activity within the murine thymus after transfer of a bone marrow fraction enriched in CFU-S

Mechanisms involved in prothymocyte migration, differentiation and self-commitment were investigated. We used a murine bone marrow fraction isolated on a discontinuous Ficoll gradient and enriched 10-20 times in CFU-S activity, and studied its fate after intrathymic transfer over a period of 200 days. In order to assess their hemopoietic activity, chimeric thymuses were intravenously transferred to secondary lethally irradiated hosts and both day 8 and day 12 spleen colonies were evaluated. The results show that transfer of a stem cell enriched fraction leads to long-term repopulation of the thymuses and that the input of progenitors is regulated by the size of the intrathymic precursor pool. Furthermore, stem cells can locate within the irradiated thymus and remain in a primitive stage for several months.

In vivo and in vitro repertoire of CD3+CD4-CD8- thymocytes

CD4-CD8- thymocytes contain a cell subset which expresses the complete CD3-TCR complex (alpha/beta or gamma/delta) of which the ontogeny and filiation are unknown. One of the questions is whether this population can be intrathymically selected, an obligatory step for the mature CD4+ and CD8+ cell differentiation pathway, or if the absence of CD4 an CD8 allows them to escape thymic selection. The repertoire of CD3+CD4-CD8- (CD3+ DN) thymocytes was analyzed in different strains of mice with different combinations of H-2 and Mls expression. The expression of V beta 8.1 in freshly isolated CD3+ DN cells is the highest in Mls-1b mice and the lowest in Mls-1a and F1 mice, suggesting that selection against this specificity can be achieved in vivo. By contrast, a low percentage of V beta 6+ cells is found in all the strains, with no correlation according to Mls expression. In vitro culture of DN thymocytes with IL-1 and IL-2 leads to the proliferation of CD3+ DN cells. In vitro culture amplifies the in vivo pattern of V beta 8.1 expression, while V beta 6+ cells only expand in DN cells of B6 and B10D2 Mls-1b mice with the same genetic background.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo dynamics of CD4-8- thymocytes. Proliferation, renewal and differentiation of different cell subsets studied by DNA biosynthetic labeling and surface antigen detection

The proliferative status of CD4-8- thymocyte subsets was determined by in vivo or in vitro labeling with bromodeoxyuridine (BrdUrd), a nonreutilized thymidine analogue detectable with a monoclonal antibody, simultaneously with relevant surface proteins. An actively cycling subset [J11d+, interleukin 2 receptor-positive (IL 2R+)] was defined, besides a relatively resting one (J11d-, Pgp1+, T cell receptor-positive). Continuous per os administration of BrdUrd showed that 85% only of CD4-8- thymocytes were labeled in 6 days confirming the existence of a relatively long-term resting subset. By contrast, CD4+8+ thymocytes were all labeled in 3-4 days. Observation of labeled CD4-8- cells after pulse labeling showed an immediate decrease of their absolute number per thymus, confirming their low autorenewal capacity. However, a small number of labeled cells which were hydroxyurea or colchicine resistant remained CD4-8- for several days and progressively acquired surface expression of IL 2R. IL 2R expression by cycling CD4-8- cells during thymus regeneration after antimitogenic drug treatment was rapid, but very transient. According to these results most CD4-8- thymocytes appear as largely engaged in a proliferation-dependent differentiative process, and do not behave as true stem cells. Consequently, this subset is principally renewed by thymic immigration of exogenously produced resting cells. However, a tenfold expansion of CD4-8- cells was found in the fetal and regenerating thymus, suggesting two proliferative phases during intrathymic CD4-8- cell maturation, the first one yielding to cell expansion and the second to cell differentiation. A tentative evaluation of daily cell immigration is proposed starting with the determination of the number of cells beginning DNA synthesis each day. A global model is finally discussed by confronting our kinetic results with the known reconstitution capacities of CD4-8- thymocyte subsets.

Phenotype of thymic stromal cells. An immunoelectron microscopic study with anti-IA, anti-MAC-1, and anti-MAC-2 antibodies

To better comprehend the thymic microenvironment, it is necessary to identify the antigenic profile of cells forming the thymic reticulum which are involved in intrathymic T cell differentiation. These cells are of three types: epithelial cells, macrophages, and interdigitating cells (IDC). Although several studies have been done on thymus section in light microscopy, identification of the positive cells, and mainly the antigenic equipment of the macrophages and IDC has not been clearly analyzed. Morphology, in electron microscopy is so far the best method to identify the different types of cells, and immunoelectron microscopy on thymic sections may be the best method to define clearly stromal cell phenotypes. In the present paper, we analyzed two antigens which classically define the macrophage family, Mac-1 and Mac-2, as well as major histocompatibility complex class II antigen which is present on epithelial cells and on bone marrow derived stromal cells. We show that epithelial cells are Ia+ Mac-1-, Mac-2-; macrophages are all Mac-1+, Mac-2+ but only half are Ia+; IDC are Ia+, Mac-1+, Mac-2+. These results show that IDC and macrophages both express antigens which were originally described as macrophage-specific.

Repopulation potential of thymocytes forming rosettes with phagocytic cells of the thymic reticulum

Thymocytes binding in vitro to phagocytic cells of the thymic reticulum (P-TR), termed 'rosetting thymocytes', were injected intravenously into irradiated congenic mice and their migration patterns were compared with those that do not bind to P-TR, called 'non-rosetting thymocytes', similarly transferred. Donor cells, C57BL/Ka Thy 1.2, were distinguished from recipient cells, C57BL/Ka Thy 1.1 by a direct immunofluorescence technique using an anti-Thy 1.2 monoclonal antibody. The results demonstrate that the rosetting thymocytes have a greater capacity for homing back to the thymus and for populating the mesenteric lymph node and the spleen. Intrathymic transfer assay revealed that the donor-derived cells detected in the peripheral organs were of thymic origin.

Synergistic effect of colony-stimulating factors and IL-2 on prothymocyte proliferation linked to the maturation of macrophage/dendritic cells within L3T4-Lyt-2-Ia-Mac- cells

Bone marrow-derived precursors colonize the thymus, where they constitute the minor L3T4-Lyt2- subset which can give rise to all thymocyte subpopulations. We show in the present paper that L3T4-Lyt2- population depleted of Ia+, Mac-1+ cells contain pluripotent hemopoietic stem cells (CFU-S) and granulocyte-macrophage colony-forming cells (GM-CFC). Addition of GM-CSF to the culture medium leads to the production of adherent and nonadherent cells of the macrophage-monocyte lineage. L3T4-Lyt2- cells poorly respond to IL-2 in vitro, but the addition of either rIL-3 or rGM-CSF allows the IL-2 response of L3T4-Lyt2- cells. This response is at least partly mediated by maturation of double-negative cells for L3T4 and Lyt-2 Ag into cells able to produce IL-1.