The thymus is the primary organ responsible for de novo generation of immunocompetent T cells that have a diverse repertoire of antigen recognition. During the developmental process, 98% of thymocytes die by apoptosis. Thus apoptosis is a dominant process in the thymus and occurs through either death by neglect or negative selection or through induction by stress/aging. Caspase activation is an essential part of the general apoptosis mechanism, and data suggest that caspases may have a role in negative selection; however, it seems more probable that caspase-8 activation is involved in death by neglect, particularly in glucocorticoid-induced thymocyte apoptosis. Caspase-8 is active in double-positive (DP) thymocytes in vivo and can be activated in vitro in DP thymocytes by T-cell receptor (TCR) crosslinking to induce apoptosis. Caspase-8 is a proapoptotic member of the caspase family and is considered an initiator caspase, which is activated upon stimulation of a death receptor (e.g., Fas), recruitment of the adaptor molecule FADD, and recruitment and subsequent processing of procaspase-8. The main role of caspase-8 seems to be pro-apoptotic and, in this review, we will discuss about the involvement of caspase-8 in (1) TCR-triggered thymic apoptosis; (2) death receptor-mediated thymic apoptosis; and (3) glucocorticoid-induced thymic apoptosis. Regarding TCR triggering, caspase-8 is active in medullary, semi-mature heat-stable antigen hi (HAS hi SP) thymocytes as a consequence of strong TCR stimulation. The death receptors Fas, FADD, and FLIP are involved upstream of caspase-8 activation in apoptosis; whereas, Bid and HDAC7 are involved downstream of caspase-8. Finally, caspase-8 is involved in glucocortocoid-induced thymocyte apoptosis through an activation loop with the protein GILZ. GILZ activates caspase-8, promoting GILZ sumoylation and its protection from proteasomal degradation.

The leaves of Artocarpus tonkinensis are used in Vietnamese traditional medicine for treatment of arthritis, and the compound maesopsin 4-O-β-D-glucoside (TAT-2), isolated from them, inhibits the proliferation of activated T cells. Our goal was to test the anti-proliferative activity of TAT-2 on the T-cell leukemia, Jurkat, and on the acute myeloid leukemia, OCI-AML. TAT-2 inhibited the growth of OCI-AML (and additional acute myeloid leukemia cells) but not Jurkat cells. Growth inhibition was shown to be due to inhibition of proliferation rather than increase in cell death. Analysis of cytokine release showed that TAT-2 stimulated the release of TGF-β, yet TGF-β neutralization did not reverse the maesopsin-dependent effect. Gene expression profiling determined that maesopsin modulated 19 identifiable genes. Transcription factor CP2 was the gene most significantly modulated. Real-time PCR validated that up-regulation of sulphiredoxin 1 homolog (SRXN1), hemeoxygenase 1 (HMOX1), and breast carcinoma amplified sequence 3 (BCAS3) were consistently modulated.

In humans, T-cell development takes place in the thymus, which contains an external cortical region and an inner medulla. The skeleton of the thymus consists of stromal cells and is filled with thymocytes in different stages of differentiation. Thymocytes undergo to a development process before becoming mature T lymphocytes ready for export to the peripheral lymphoid organs. Classically, T-cell development has been reported to occur in four steps. First, bone marrow derived thymocytes that express neither CD4 nor CD8 surface antigens (double negative [DN] thymocytes) undergo an extensive phase of proliferation and differentiation and begin to express CD4 and CD8 (step 2: double positive [DP] thymocytes). During a subsequent negative selection process, approximately 5% of these DP cells undergo apoptosis. If these cells are not eliminated, they could differentiate into autoreactive lymphocytes, leading to the development of peripheral autoimmune diseases. In the thymus, a particular population of T regulatory (Treg) cells also develops. These Treg cells migrate to the periphery and are capable of suppressing autoreactive lymphocytes that may have escaped from the negative selection process. Autoimmune diseases are generally the result of insufficient negative selection of autoreactive cells in the thymus or a deficiency in Treg cell production or function. Future therapeutic strategies for autoimmune diseases should exploit manipulations in the negative selection process and/or the differentiation of Treg cells in the thymus.

In this study, we evaluated the possible cross-talk between glucocorticoid (GC)-induced leucine zipper (Gilz) and caspase-8 in dexamethasone (Dex)-treated thymocytes. We determined that expression of Dex-induced Gilz protein was reduced when caspase-8 activity was inhibited, and this effect was not partially due to altered Gilz mRNA expression. Inhibition of the proteasome abrogated this reduction in Gilz expression, suggesting that Dex-induced caspase-8 activation protects Gilz from degradation. We hypothesized that the caspase-8-dependent protection of Gilz could be due to caspase-8-driven sumoylation. As a putative small ubiquitin-like modifier (SUMO)-binding site was identified in the Gilz sequence, we assessed whether SUMO-1 interacted with Gilz. We identified a 30-kDa protein that was compatible with the size of a Gilz-SUMO-1 complex and was recognized by the anti-SUMO-1 and anti-Gilz antibodies. In addition, Gilz bound to SUMO ubiquitin-conjugating (E2)-conjugating enzyme Ube21 (Ubc9), the specific SUMO-1 E2-conjugating enzyme, in vitro and coimmunoprecipitated with Ubc9 in vivo. Furthermore, Gilz coimmunoprecipitated with SUMO-1 both in vitro and in vivo, and this interaction depended on caspase-8 activation. This requirement for caspase-8 was further evaluated in caspase-8-deficient thymocytes and lymphocytes in which Gilz expression was reduced. In summary, our results suggest that caspase-8 activation protects Gilz from proteasomal degradation and induces its binding to SUMO-1 in GC-treated thymocytes.

We used transgenic mice to investigate the effect of IL-2 stimulation on T lymphocyte functions of GILZ-overexpressing splenic T cells. When compared to their controls, T cells from transgenic mice underwent normal activation after stimulation with anti-CD3 plus anti-CD28 monoclonal antibodies, as evaluated by CD25 expression, CD2 up-regulation and proliferation. IL-10, IL-13 and IFN-gamma increased more consistently in CD3/CD28-triggered TG compared to WT splenic CD4(+)cells. Analysis of the CD4(+)and CD8(+)T cells demonstrated a decreased CD4(+)/CD8(+)T-cell ratio (1:1 instead of 1:2) in response to IL-2 stimulation, possibly due to an unresponsiveness of IL-2 receptor beta and/or gamma chains. Finally, the total number of T cells was significantly increased in aged mice and this was due to the augmentation of CD4(+)T cells. These results support the hypothesis that GILZ regulates, at least in part, peripheral T-cell functions by influencing their responsiveness to IL-2.

The thymus is a lymphoid organ that governs the development of a diverse T-cell repertoire capable of defending against nonselfantigens and avoiding autoimmunity. However, the thymus can also succumb to different diseases. Hypertrophic diseases, such as thymomas, are typically associated with impairment of negative selection, which leads to autoimmune disease, or disruption of positive selection, which results in immunodeficiency. Hypotrophic diseases of the thymus can manifest during acute infections, cancer, allogeneic bone marrow transplantation, or with aging. This condition leads to decreased immune function and can be treated by either replacing lost thymic tissue or by preventing thymic tissue death. Studies have demonstrated the critical role of caspase-8 in regulating apoptosis in the thymus. In this review, we discuss how pharmacological activation and inhibition of caspase-8 can be used to treat hypertrophic and hypotrophic diseases of the thymus, respectively, to improve its function.