Interleukin-7 negatively regulates the development of mature T cells in fetal thymus organ cultures

Perturbed thymopoiesis in vitro in the absence of suppressor of cytokine signalling 1 and 3

Cytokine signals are central to the differentiation of thymocytes and their stepwise progression through defined developmental stages. The intensity and duration of cytokine signals are regulated by the suppressor of cytokine signalling (SOCS) proteins. A clear role for SOCS1 during the later stages of thymopoiesis has been established, but little is known about its role during early thymopoiesis, nor the function of its closest relative, SOCS3. Here, we find that both SOCS1 and SOCS3 are expressed during early thymopoiesis, with expression coincident during the double negative (DN)2 and DN3 stages. We examined thymocyte differentiation in vitro by co-culture of SOCS-deficient bone marrow cells with OP9 cells expressing the Notch ligand Delta-like1 (OP9-DL1). Cells lacking SOCS1 were retarded at the DN3:DN4 transition and appeared unable to differentiate into double positive (DP) thymocytes. Cells lacking both SOCS1 and SOCS3 were more severely affected, and displayed an earlier block in T cell differentiation at DN2, the stage at which expression of SOCS1 and SOCS3 coincides. This indicates that, in addition to their specific roles, SOCS1 and SOCS3 share overlapping roles during thymopoiesis. This is the first demonstration of functional redundancy within the SOCS family, and has uncovered a vital role for SOCS1 and SOCS3 during two important checkpoints in early T cell development.

Impaired thymopoiesis in interleukin-7 receptor transgenic mice is not corrected by Bcl-2

Murine thymocytes down-regulate IL-7 responsiveness following beta-selection and reacquire sensitivity after positive selection. To assess the potential consequences of IL-7 signaling during this phase of development, transgenic IL-7 receptor alpha (IL-7Ralpha) mice were evaluated for IL-7 responsiveness as gauged by STAT-5 phosphorylation. Transgenic IL-7Ralpha expression increased the percentage of thymocytes responsive to IL-7 yet resulted in a decrease in total thymic cellularity. Aberrant thymocyte development in transgenic mice was first manifested by a reduction of DN3 thymocytes that correlated with lower Bcl-2 expression. Surprisingly, transgenic restoration of Bcl-2 expression did not correct thymic hypocellularity induced by IL-7Ralpha overexpression. These findings demonstrate that failure to appropriately downregulate IL-7Ralpha expression interferes with thymocyte development past the pro-T stage resulting in significantly lower levels of mature thymocytes.

Alpha-dystroglycan is involved in positive selection of thymocytes by participating in immunological synapse formation

Alpha-dystroglycan has been proved to be involved in lymphocyte activation by participating in immunological synapse (IS) formation. Considering the existence of IS formation in thymic development, we questioned whether alpha-dystroglycan was expressed in thymus and influenced thymic development. In this study, we demonstrated that alpha-dystroglycan was expressed on fetal thymocytes, especially on double-positive (DP, CD4(+)CD8(+)) cells. Blocking alpha-dystroglycan by treatment of fetal thymus organ culture (FTOC) with anti-alpha-dystroglycan antibody IIH6C4 decreased the number of DP cells compared with nontreated or isotype antibody controls. Down-regulation of alpha-dystroglycan by retroviruses carrying antisense cDNA of dystroglycan in reaggregate thymus organ culture (RTOC) further confirmed these results. Enhanced apoptosis of DP cells was observed after blocking alpha-dystroglycan. Interestingly, we found that blocking alpha-dystroglycan reduced IS formation between DP cells and thymic epithelial cells. Furthermore, blocking alpha-dystroglycan up-regulated CD95/CD95L expression and reduced Bcl-2 expression on DP cells in the developing thymus. Finally, the increase in the apoptosis of DP cells was associated with a consequent decrease in the positive selection, as indicated by the reduction of both ERK phosphorylation in DP cells and single-positive (SP, CD4(+) or CD8(+)) cell outcome. Altogether, these results indicated that alpha-dystroglycan was involved in positive selection of thymocytes by participating in the IS formation.

Distinct roles of IL-7 and stem cell factor in the OP9-DL1 T-cell differentiation culture system

OBJECTIVE

The OP9-DL1 culture system is an in vitro model for T-cell development in which activation of the Notch pathway by Delta-like 1 promotes differentiation of mature T cells from progenitors. The roles of specific cytokines in this culture system have not been well defined, and controversy regarding the role of IL-7 has recently emerged. We examined the roles played by IL-7, Flt3 ligand, and stem cell factor (SCF) in differentiation of adult bone marrow cells in the OP9-DL1 culture system.

METHODS

Hematopoietic progenitor cells isolated from mouse bone marrow were cultured with OP9 or OP9-DL1 stromal cells and evaluated for T and B lymphocyte differentiation using immunofluorescent staining.

RESULTS

IL-7 provided both survival/proliferation and differentiation signals in a dose-dependent manner. T-cell development from the CD4/CD8 double-negative (DN) stage to the CD4/CD8 double-positive (DP) stage required IL-7 provided by the stromal cells, while differentiation from the DP to the CD8 single-positive (SP) stage required addition of exogenous IL-7. SCF favored the proliferation of DN lymphoid progenitors and inhibited differentiation to the DP stage in a dose-dependent manner. Conversely, blocking the function of SCF expressed endogenously by OP9-DL1 cells inhibited proliferation of lymphoid progenitors and accelerated T-lineage differentiation. Flt3 ligand promoted proliferation without affecting differentiation.

CONCLUSION

These results validate the OP9-DL1 model for the analysis of T-cell development from bone marrow-derived progenitor cells, and demonstrate specific roles of SCF, IL-7, and Flt3L in promoting efficient T-lineage differentiation.

TNF-α mediated modulation of T cell development and exacerbation of in vitro T1DM in fetal thymus organ culture

TNF-alpha is a pleiotropic cytokine that is constitutively expressed in the thymus. This cytokine has opposing effects on type 1 diabetes mellitus (T1DM) as non-obese diabetic (NOD) mice administered TNF-alpha early in life experience an acceleration in disease onset while TNF-alpha administered to adult NOD mice are rescued from disease entirely. Using fetal thymus organ culture (FTOC) as a model of T cell development and an associated in vitro T1DM model, we set out to reconcile the role of TNF-alpha in thymic development with its role in the pathogenesis of T1DM. Our data indicate that NOD derived FTOC produce a smaller percentage of double negative (CD4(-)/CD8(-)) thymocytes expressing TNF receptors compared to non-diabetic C57BL/6 (B6) derived FTOC. NOD FTOC produce more TNF-alpha than B6 FTOC during days 6-9 of culture, a time when negative selection of T cells is known to occur. Neutralization of this endogenous TNF-alpha production in NOD derived FTOC with soluble TNF receptor (sTNF R1) rescued insulin production in our in vitro T1DM model. Flow cytometric analysis of NOD FTOC treated with recombinant TNF-alpha (rTNF-alpha) or sTNF R1 demonstrated that the relative levels of TNF-alpha in the culture during the selection window (days 6-9) influence the ratio of immature vs. mature T cells that emerge from FTOC.

Propensity of adult lymphoid progenitors to progress to DN2/3 stage thymocytes with Notch receptor ligation

Notch family receptors control critical events in the production and replenishment of specialized cells in the immune system. However, it is unclear whether Notch signaling regulates abrupt binary lineage choices in homogeneous progenitors or has more gradual influence over multiple aspects of the process. A recently developed coculture system with Delta 1-transduced stromal cells is being extensively used to address such fundamental questions. Different from fetal progenitors, multiple types of adult marrow cells expanded indefinitely in murine Delta-like 1-transduced OP9 cell cocultures, progressed to a DN2/DN3 thymocyte stage, and slowly produced TCR(+) and NK cells. Long-term cultured cells of this kind retained some potential for T lymphopoiesis in vivo. Adult marrow progressed through double-positive and single-positive stages only when IL-7 concentrations were low and passages were infrequent. Lin(-)c-Kit(low)GFP(+)IL-7Ralpha(+/-) prolymphocytes were the most efficient of adult bone marrow cells in short-term cultures, but the assay does not necessarily reflect cells normally responsible for replenishing the adult thymus. Although marrow-derived progenitors with Ig D(H)-J(H) rearrangements acquired T lineage characteristics in this model, that was not the case for more B committed cells with V(H)-D(H)J(H) rearrangement products.

The role of Notch and IL-7 signaling in early thymocyte proliferation and differentiation

We have analyzed the roles of Notch and IL-7 signaling in the proliferation and differentiation of mouse progenitor thymocyte subpopulations cultured on Notch delta-like-1 ligand-expressing OP9 stromal cells. Using bulk and limiting dilution cultures, we show that DN1 and DN2 cells require both Notch and IL-7 signaling for efficient proliferation and differentiation into cytoplasmic TCRbeta and surface TCRalpha/beta and TCRgamma/delta expressing T cells. Selection for cytoplasmic TCRbeta-positive cells is dependent on preTalpha expression. Both gamma/delta and alpha/beta TCR expressing T cells arising in culture can be efficiently stimulated by anti-CD3 cross-linking, suggesting that they might be functional. The differentiation of adult, but not fetal, DN1 and DN2 thymocytes into CD4 and/or CD8 expressing cells is inhibited by IL-7. Finally, efficient proliferation and differentiation of DN3 cells requires Notch signaling and preTCR expression, but is independent of IL-7.

Interleukin-7 deficiency in rheumatoid arthritis: consequences for therapy-induced lymphopenia

We previously demonstrated prolonged, profound CD4⁺ T-lymphopenia in rheumatoid arthritis (RA) patients following lymphocyte-depleting therapy. Poor reconstitution could result either from reduced de novo T-cell production through the thymus or from poor peripheral expansion of residual T-cells. Interleukin-7 (IL-7) is known to stimulate the thymus to produce new T-cells and to allow circulating mature T-cells to expand, thereby playing a critical role in T-cell homeostasis. In the present study we demonstrated reduced levels of circulating IL-7 in a cross-section of RA patients. IL-7 production by bone marrow stromal cell cultures was also compromised in RA. To investigate whether such an IL-7 deficiency could account for the prolonged lymphopenia observed in RA following therapeutic lymphodepletion, we compared RA patients and patients with solid cancers treated with high-dose chemotherapy and autologous progenitor cell rescue. Chemotherapy rendered all patients similarly lymphopenic, but this was sustained in RA patients at 12 months, as compared with the reconstitution that occurred in cancer patients by 3–4 months. Both cohorts produced naïve T-cells containing T-cell receptor excision circles. The main distinguishing feature between the groups was a failure to expand peripheral T-cells in RA, particularly memory cells during the first 3 months after treatment. Most importantly, there was no increase in serum IL-7 levels in RA, as compared with a fourfold rise in non-RA control individuals at the time of lymphopenia. Our data therefore suggest that RA patients are relatively IL-7 deficient and that this deficiency is likely to be an important contributing factor to poor early T-cell reconstitution in RA following therapeutic lymphodepletion. Furthermore, in RA patients with stable, well controlled disease, IL-7 levels were positively correlated with the T-cell receptor excision circle content of CD4⁺ T-cells, demonstrating a direct effect of IL-7 on thymic activity in this cohort.

IL-7 gene therapy in aging restores early thymopoiesis without reversing involution

Thymic involution begins early in life and continues throughout adulthood, resulting in a decreased population of naive T cells in the periphery and a reduced ability to fight off newly encountered infectious diseases. We have previously shown that the first step of thymopoiesis is specifically blocked in aging. This block at the DN1 to DN2 transition and the subsequent loss of thymic output in old age mirrors the changes seen in IL-7-deficient mice, and it is hypothesized that decreased intrathymic IL-7 is involved in age-related thymic involution. To separate the effect of IL-7 on thymic involution from its function as a peripheral lymphocyte growth cofactor, we injected IL-7-secreting stromal cells into the thymi of recipient mice. The increased local concentration of IL-7 maintained the first step of thymopoiesis at a level far higher than was seen in age-matched controls. However, despite this success, there was no decrease in thymic involution or increase in T cell output. The inability of IL-7 to prevent involution led us to the discovery of an additional age-sensitive step in thymopoiesis, proliferation of the DN4 population, which is unaffected by IL-7 expression.

Effects of nicotine exposure on T cell development in fetal thymus organ culture: arrest of T cell maturation

There is evidence for both physiological functions of the natural neurotransmitter, acetylcholine, and pharmacological actions of the plant alkaloid, nicotine, on the development and function of the immune system. The effects of continuous exposure to nicotine over a 12-day course of fetal thymus organ culture (FTOC) were studied, and thymocytes were analyzed by flow cytometry. In the presence of very low concentrations of nicotine many more immature T cells (defined by low or negative TCR expression) and fewer mature T cells (intermediate or high expression of TCR) were produced. In addition, the numbers of cells expressing CD69 and, to a lesser extent, CD95 (Fas) were increased. These effects took place when fetal thymus lobes from younger (13-14 days gestation) pups were used for FTOC. If FTOC were set up using tissue from older (15-16 days gestation pups), nicotine had little effect, suggesting that it may act only on immature T cell precursors. Consistent with an increase in immature cells, the expression of recombinase-activating genes was found to be elevated. Nicotine effects were partially blocked by the simultaneous addition of the nicotinic antagonist d-tubocurarine. Furthermore, d-tubocurarine alone blocked the development of both immature and mature murine thymocytes, suggesting the presence of an endogenous ligand that may engage nicotinic acetylcholine receptors on developing thymocytes and influence the course of normal thymic ontogeny.