Myeloproliferative disorders are clonal haematopoietic stem cell malignancies characterized by independency or hypersensitivity of haematopoietic progenitors to numerous cytokines(1,2). The molecular basis of most myeloproliferative disorders is unknown. On the basis of the model of chronic myeloid leukaemia, it is expected that a constitutive tyrosine kinase activity could be at the origin of these diseases. Polycythaemia vera is an acquired myeloproliferative disorder, characterized by the presence of polycythaemia diversely associated with thrombocytosis, leukocytosis and splenomegaly(3). Polycythaemia vera progenitors are hypersensitive to erythropoietin and other cytokines(4,5). Here, we describe a clonal and recurrent mutation in the JH2 pseudo-kinase domain of the Janus kinase 2 (JAK2) gene in most (>80%) polycythaemia vera patients. The mutation, a valine-to-phenylalanine substitution at amino acid position 617, leads to constitutive tyrosine phosphorylation activity that promotes cytokine hypersensitivity and induces erythrocytosis in a mouse model. As this mutation is also found in other myeloproliferative disorders, this unique mutation will permit a new molecular classification of these disorders and novel therapeutical approaches.

A unique clonal JAK2 mutation leading to constitutive signalling causes polycythaemia vera

Naturally arising CD25+CD4+ regulatory T cells actively maintain immunological self-tolerance. Deficiency in or dysfunction of these cells can be a cause of autoimmune disease. A reduction in their number or function can also elicit tumor immunity, whereas their antigen-specific population expansion can establish transplantation tolerance. They are therefore a good target for designing ways to induce or abrogate immunological tolerance to self and non-self antigens.

Naturally arising Foxp3-expressing CD25+CD4+ regulatory T cells in immunological tolerance to self and non-self.

Socializing with the Neighbors: Stem Cells and Their Niche

Apoptosis in the immune system is a fundamental process regulating lymphocyte maturation, receptor repertoire selection and homeostasis Thus, death by apoptosis is as essential for the function of lymphocytes as growth and differentiation This article focuses on death receptor-associated apoptosis and the role of CD95 (Apo-1/Fas)-mediated signalling in T-cell and B-cell development and during the course of an immune response Gaining an insight into these processes improves our understanding of the pathogenesis of diseases such as cancer, autoimmunity and AIDS, and opens new approaches to rational treatment strategies

CD95's deadly mission in the immune system

Protein–protein interactions have a key role in most biological processes, and offer attractive opportunities for therapeutic intervention. Developing small molecules that modulate protein–protein interactions is difficult, owing to issues such as the lack of well-defined binding pockets. Nevertheless, there has been important progress in this endeavour in recent years. Here, we use illustrative examples to discuss general strategies for addressing the challenges inherent in the discovery and characterization of small-molecule inhibitors of protein–protein interactions.

Small-molecule inhibitors of protein–protein interactions: progressing towards the dream

Studies of the biology of the IL-2 receptor have played a major part in establishing several of the fundamental principles that govern our current understanding of immunology. Chief among these is the contribution made by lymphokines to regulation of the interactions among vast numbers of lymphocytes, comprising a number of functionally distinct lineages. These soluble mediators likely act locally, within the context of the microanatomic organization of the primary and secondary lymphoid organs, where, in combination with signals generated by direct membrane-membrane interactions, a wide spectrum of cell fate decisions is influenced. The properties of IL-2 as a T-cell growth factor spawned the view that IL-2 worked in vivo to promote clonal T-cell expansion during immune responses. Over time, this singular view has suffered from increasing appreciation that the biologic effects of IL-2R signals are much more complex than simply mediating T-cell growth: depending on the set of conditions, IL-2R signals may also promote cell survival, effector function, and apoptosis. These sometimes contradictory effects underscore the fact that a diversity of intracellular signaling pathways are potentially activated by IL-2R. Furthermore, cell fate decisions are based on the integration of multiple signals received by a lymphocyte from the environment; IL-2R signals can thus be regarded as one input to this integration process. In part because IL-2 was first identified as a T-cell growth factor, the major focus of investigation in IL-R2 signaling has been on the mechanism of mitogenic effects in cultured cell lines. Three critical events have been identified in the generation of the IL-2R signal for cell cycle progression, including heterodimerization of the cytoplasmic domains of the IL-2R beta and gamma(c) chains, activation of the tyrosine kinase Jak3, and phosphorylation of tyrosine residues on the IL-2R beta chain. These proximal events led to the creation of an activated receptor complex, to which various cytoplasmic signaling molecules are recruited and become substrates for regulatory enzymes (especially tyrosine kinases) that are associated with the receptor. One intriguing outcome of the IL-2R signaling studies performed in cell lines is the apparent functional redundancy of the A and H regions of IL-2R beta, and their corresponding downstream pathways, with respect to the proliferative response. Why should the receptor complex induce cell proliferation through more than one mechanism or pathway? One possibility is that this redundancy is an unusual property of cultured cell lines and that primary lymphocytes require signals from both the A and the H regions of IL-2R beta for optimal proliferative responses in vivo. An alternative possibility is that the A and H regions of IL-2R beta are only redundant with respect to proliferation and that each region plays a unique and essential role in regulating other aspects of lymphocyte physiology. As examples, the A or H region could prove to be important for regulating the sensitivity of lymphocytes to AICD or for promoting the development of NK cells. These issues may be resolved by reconstituting IL-2R beta-/-mice with A-and H-deleted forms of the receptor chain and analyzing the effect on lymphocyte development and function in vivo. In addition to the redundant nature of the A and H regions, there remains a large number of biochemical activities mediated by the IL-2R for which no clear physiological role has been identified. Therefore, the circumstances are ripe for discovering new connections between molecular signaling events activated by the IL-2R and the regulation of immune physiology. Translating biochemical studies of Il-2R function into an understanding of how these signals regulate the immune system has been facilitated by the identification of natural mutations in IL-2R components in humans with immunodeficiency and by the generation of mice with targeted mutations in these gen

Biology of the interleukin-2 receptor.

Absence of Ku80 results in increased sensitivity to ionizing radiation, defective lymphocyte development, early onset of an age-related phenotype, and premature replicative senescence. Here we investigate the role of p53 on the phenotype of ku80-mutant mice and cells. Reducing levels of p53 increased the cancer incidence for ku80−/− mice. About 20% of ku80−/− p53+/− mice developed a broad spectrum of cancer by 40 weeks and all ku80−/− p53−/− mice developed pro-B-cell lymphoma by 16 weeks. Reducing levels of p53 rescued populations of ku80−/− cells from replicative senescence by enabling spontaneous immortalization. The double-mutant cells are impaired for the G1/S checkpoint due to the p53 mutation and are hypersensitive to γ-radiation and reactive oxygen species due to the Ku80 mutation. These data show that replicative senescence is caused by a p53-dependent cell cycle response to damaged DNA in ku80−/− cells and that p53 is essential for preventing very early onset of pro-B-cell lymphoma in ku80−/− mice.

Analysis of ku80-Mutant Mice and Cells with Deficient Levels of p53

TCR stimulation of T lymphocytes that are activated and cycling in the presence of IL-2 leads to programmed cell death. We now show that this effect is at least partly attributable to the ability of IL-2 to dramatically increase the expression of mRNAs encoding ligands and receptors that mediate apoptosis. We also found that cyclosporin was not able to fully inhibit the TCR induction of death molecule mRNAs or TCR-induced apoptosis, although it could completely turn off IL-2 expression. The effect growth cytokines was further explored in T cells derived from mice bearing a homozygous deficiency of the IL-2R alpha-chain. We found that IL-2Ralpha-/- cells were resistant to death if IL-2 was used to induce apoptosis susceptibility, but that large amounts of other T cell growth cytokines, such as IL-4 and IL-7, could induce cell cycle progression and promote TCR-induced apoptosis. However, our findings suggest that autoimmunity and lymphoproliferation in IL-2Ralpha-/- mice can result from the loss of IL-2 stimulated feedback apoptosis and that other growth cytokines are not produced at levels sufficient to compensate for this deficit.

T Cell Growth Cytokines Cause the Superinduction of Molecules Mediating Antigen-Induced T Lymphocyte Death

Regulated genomic instability and neoplasia in the lymphoid lineage.

High-affinity IL-2R signals are required for peripheral lymphoid homeostasis in vivo. We found that CD25 was required for regulation of peripheral T cells in mice bearing either the DO11.10 MHC class II-restricted TCR transgene or an Iaβ-null mutation, suggesting that MHC class I- and class II-dependent T cell subsets are regulated independently by IL-2R signals. In contrast, deregulation of serum IgG1 levels in CD25 −/− mice was dependent on CD4 + T cells. T cell expansion in DO11.10 CD25 −/− mice was not preferential for cells escaping allelic exclusion by the TCR transgene, but was suppressed by a Rag-2-null mutation. Together, these findings suggest that endogenous TCR are required to trigger T cell expansion, but that CD25 regulates T cells activated by low-specificity signals. Expansion of DO11.10 T cells in response to cognate Ag was modestly reduced in CD25 −/− T cells transferred into the normal lymphoid compartments of BALB/c mice. Moreover, activation-induced clonal contraction and apoptosis in vivo were intact in the absence of CD25. These data indicate that the regulatory role of high-affinity IL-2R signals extends beyond the control of Ag-specific responses and suggest a role for these signals in control of bystander T cell activation.

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Dennis M. Willerford

Papers

Biology of the interleukin-2 receptor.

Analysis of ku80-Mutant Mice and Cells with Deficient Levels of p53

T Cell Growth Cytokines Cause the Superinduction of Molecules Mediating Antigen-Induced T Lymphocyte Death

Regulated genomic instability and neoplasia in the lymphoid lineage.

Regulation of Lymphoid Homeostasis by IL-2 Receptor Signals In Vivo