Naive T cells encountering their cognate antigen become activated and acquire the ability to proliferate in response to cytokines. Stat5 is an essential component in this response. We demonstrate that Stat5 cannot access DNA in naive T cells and acquires this ability only after T-cell receptor (TCR) engagement. The transition is not associated with changes in DNA methylation or global histone modification but rather chromatin decondensation. Condensation occurs during thymocyte development and proper condensation is dependent on kleisin-b of the condensin II complex. Our findings suggest that this unique chromatin condensation, which can affect interpretations of chromatin accessibility assays, is required for proper T-cell development and maintenance of the quiescent state. This mechanism ensures that cytokine driven proliferation can only occur in the context of TCR stimulation.

Monoubiquitination of core histone 2A (H2A-K119u) has a critical role in gene regulation in hematopoietic differentiation and other developmental processes. To explore the interplay of histone H2A deubiquitinase Myb-like SWIRM and MPN domain containing1 (2A-DUB/Mysm1) with the p53 axis in the sequential differentiation of mature lymphocytes from progenitors, we systematically analyzed hematopoiesis and early T-cell development using Mysm1−/− and p53−/−Mysm1−/− mice. Mysm1−/− thymi were severely hypoplastic with <10% of wild-type cell numbers as a result of a reduction of early thymocyte progenitors in context with defective hematopoietic stem cells, a partial block at the double-negative (DN)1–DN2 transition and increased apoptosis of double-positive thymocytes. Increased rates of apoptosis were also detected in other tissues affected by Mysm1 deficiency, including the developing brain and the skin. By quantitative PCR and chromatin immunoprecipitation analyses, we identified p19ARF, an important regulator of p53 tumor suppressor protein levels, as a potential Mysm1 target gene. In newly generated p53−/−Mysm1−/− double-deficient mice, anomalies of Mysm1−/− mice including reduction of lymphoid-primed multipotent progenitors, reduced thymocyte numbers and viability, and interestingly defective B-cell development, growth retardation, neurological defects, skin atrophy, and tail malformation were almost completely restored as well, substantiating the involvement of the p53 pathway in the alterations caused by Mysm1 deficiency. In conclusion, this investigation uncovers a novel link between H2A deubiquitinase 2A-DUB/Mysm1 and suppression of p53-mediated apoptotic programs during early lymphoid development and other developmental processes.

Defective development and function of CD4+CD25high+Foxp3+ regulatory T cells (Tregs) contribute to the pathogenesis of psoriasis and other autoimmune diseases. Little is known about the influence of adhesions molecules on the differentiation of Foxp3+ Tregs into proinflammatory Th17 cells occurring in lesional skin and blood of psoriasis patients. In the CD18hypo PL/J mouse model of psoriasis, reduced expression of CD18/β2 integrin to 2–16% of wild-type levels is associated with progressive loss of Tregs, impaired cell–cell contact between Tregs and dendritic cells (DCs), as well as Treg dysfunction as reported earlier. In the present investigation, Tregs derived from CD18hypo PL/J mice were analyzed for their propensity to differentiate into IL-17–producing Th17 cells in vivo and in in vitro Treg–DC cocultures. Adoptively transferred CD18hypo PL/J Tregs were more inclined toward conversion into IL-17–producing Th17 cells in vivo in an inflammatory as well as noninflammatory environment compared with CD18wt PL/J Tregs. Addition of neutralizing Ab against CD18 to Treg–DC cocultures in vitro promoted conversion of CD18wt PL/J Tregs to Th17 cells in a dose-dependent manner similar to conversion rates of CD18hypo PL/J Tregs. Reduced thymic output of naturally occurring Tregs and peripheral conversion of Tregs into Th17 cells therefore both contribute to the loss of Tregs and the psoriasiform dermatitis observed in CD18hypo PL/J mice. Our data overall indicate that CD18 expression levels impact Treg development as well as Treg plasticity and that differentiation of Tregs into IL-17–producing Th17 cells is distinctly facilitated by a subtotal deficiency of CD18.

Clonal expansion of T cells is vital to adaptive immunity, yet this process must be tightly controlled to prevent autoimmune disease. The serine/threonine kinase death-associated protein kinase-related apoptosis-inducing kinase 2 (DRAK2) is a negative regulator of TCR signaling and sets the threshold for the activation of naive and memory T cells and selected thymocytes. Despite enhanced T cell activation, Drak2 ؊/؊ mice are resistant to experimental autoimmune encephalomyelitis, an autoimmune demyelinating disease that resembles multiple sclerosis. However, the basis for this autoimmune resistance is currently unknown. In this study, we show that, in the absence of DRAK2 signaling, T cells require greater tonic signaling for maintenance during clonal expansion. Following stimulation, Drak2 ؊/؊ T cells were more sensitive to an intrinsic form of apoptosis that was prevented by CD28 ligation, homeostatic cytokines, or enforced Bcl-x L expression. T cell-specific Bcl-x L expression also restored the susceptibility of Drak2 S everal parameters regulate a T cell's decision to proliferate, including amount and avidity of TCR stimulation, presence of costimulation, and duration of T cell:APC interactions (1-4). Peripheral T cell tolerance to self is, in part, mediated through low-avidity interactions between the TCR and self-Ag presented by nonactivated APCs that lack sufficient expression of costimulatory molecules, resulting in T cell anergy, clonal deletion, or T cell suppression (4 -7). When these signals are disrupted, the threshold for T cell activation is altered, often allowing inappropriate initiation of T cell responses to self-Ag and ultimately resulting in autoimmune disease. Consistent with this, exacerbated autoimmune susceptibility is a common phenotype observed in mice lacking negative regulatory molecules such as Cbl, GRAIL, MGAT5,.After an infection is cleared, most T cells generated during the clonal expansion phase are removed through either of two apoptotic pathways, termed activation-induced cell death (AICD) 4 and activated cell autonomous death (ACAD) (13). AICD is thought to depend on an extrinsic form of apoptosis induced by the ligation of death receptors such as Fas on the surface of the T cell (14). Patients with mutations in Fas display increased survival of lymphocytes and develop a disease termed autoimmune lymphoproliferative syndrome (15). Also, mice with mutations in Fas (lpr) or Fas ligand (FasL) (gld) develop lymphadenopathies and an accumulation of autoreactive T cells in the periphery, indicating the importance of Fas-dependent AICD in lymphocyte homeostasis as well as peripheral tolerance (16,17).Although many of the molecular mechanisms behind AICD have been elucidated, much less is known about ACAD. ACAD is thought to depend on an intrinsic form of cell death similar to that induced by cytokine withdrawal. This form of death by neglect is regulated by the balance of pro (Bim, Bad, Bax, Bak)-and anti-apoptotic (Bcl-2, Bclx L , and inhibitor of apoptosis (IAP) family of proteins...