Fine tuning T lymphocytes: A role for the lipid phosphatase SHIP-1

Imaging of biphasic signalosomes constructed by checkpoint receptor 2B4 in conventional and chimeric antigen receptor-T cells

A co-signaling receptor, 2B4, has dual effects in immune cells, but its actual functions in T cells remain elusive. Here, using super-resolution imaging technology with an immunological synapse model, we showed that 2B4 forms "2B4 microclusters" immediately after 2B4-CD48 binding. A lipid phosphatase, SHIP-1, subsequently combined with 2B4 to form coinhibitory signalosomes, leading to the suppression of cytokine production. An activating adapter, SLAM-associated protein (SAP), attenuated the clustering of SHIP-1 and recruited a kinase, Fyn, enhancing the Vav1 signaling pathway as costimulatory signalosomes. Furthermore, we found that a chimeric antigen receptor with a 2B4 tail (2B4-CAR) retained the original signal transduction mechanism of 2B4. With endogenous levels of SAP expression, 2B4-CAR-T cells exposed sufficient antitumor efficacy in vivo without excess cytokine production. Our results may help explain the biphasic feature of 2B4 in T cell responses from the viewpoint of the signalosome and provide a new candidate for CAR development.

SHIP-1 affects herpetic simplex keratitis prognosis by mediating CD4+ T lymphocytes migration through PI3K signaling and transcription factor KLF2 in the cornea

Herpetic simplex keratitis (HSK) mainly represents an immune cell-mediated, and more specifically, CD4⁺ T cell-orchestrated inflammatory response to virus invasion. The virus in infected corneas could be easily inhibited or hidden in the trigeminal ganglion using antiviral drugs, but the immune-related inflammation will last for a long time and lead to significant complications. In the present study, we found that the subconjunctival injection of SHIP-1 activator AQX1125 in mouse HSK model alleviated the corneal inflammatory and angiogenic responses, as well as promoted quicker recovery of the cornea, with significantly fewer infiltration of CD4⁺ T lymphocytes. Furthermore, using primary CD4⁺ T lymphocytes, we observed that by modulating PI3K signaling and the expression of transcription factors KLF2 and CCR7, SHIP-1 could significantly influence the migration of lymphocytes toward CCL19 and 21, which are the "exit cues" for cells to emigrate from inflammatory sites. Thus, we propose that the pharmacological SHIP-1 activation represents a new potential therapeutic approach to control HSK lesions, and its function on the CCR7-CCL19/21 biological axis may be a novel underlying mechanism for its anti-inflammatory action.

Oral Immune-Related Adverse Events Caused by Immune Checkpoint Inhibitors: Salivary Gland Dysfunction and Mucosal Diseases

Conventional chemotherapy and targeted therapies have limited efficacy against advanced head and neck squamous cell carcinoma (HNSCC). The immune checkpoint inhibitors (ICIs) such as antibodies against CTLA-4, PD-1, and PD-L1 interrupt the co-inhibitory pathway of T cells and enhance the ability of CD8+ T cells to destroy tumors. Even in advanced HNSCC patients with recurrent diseases and distant metastasis, ICI therapy shows efficiency and become an effective alternative to conventional chemotherapy. However, as this therapy releases the immune tolerance state, cytotoxic CD8+ T cells can also attack organs and tissues expressing self-antigens that cross-react with tumor antigens and induce immune-related adverse events (irAEs). When patients with HNSCC are treated with ICIs, autoimmune diseases occur in multiple organs including the skin, digestive tract, endocrine system, liver, and respiratory tract. Treatment of various malignancies, including HNSCC, with ICIs may result in the appearance of oral irAEs. In the oral cavity, an oral lichenoid reaction (OLR) and pemphigoid develop. Sicca syndrome also occurs in association with ICIs, affecting the salivary glands to induce xerostomia. It is necessary to elucidate the pathogenic mechanisms of these intractable diseases that are not seen with conventional therapy. Early diagnosis and appropriate approaches to irAEs are needed for efficient treatment of advanced HNSCC by ICIs.

Inhibition of lipid phosphatase SHIP1 expands myeloid-derived suppressor cells and attenuates rheumatoid arthritis in mice

Rheumatoid arthritis (RA) is a debilitating autoimmune disease of unknown cause, characterized by infiltration and accumulation of activated immune cells in the synovial joints where cartilage and bone destructions occur. Myeloid-derived suppressor cells (MDSCs) are of myeloid origin and are able to suppress T cell responses. Src homology 2 domain containing inositol polyphosphate 5-phosphatase 1 (SHIP1) was shown to be involved in the regulation of MDSC differentiation. The purpose of the present study was to investigate the effect of inhibition of SHIP1 on expansion of MDSCs in RA using a collagen-induced inflammatory arthritis (CIA) mouse model. In DBA/1 mice treatment with a small molecule specific SHIP1 inhibitor 3α-aminocholestane (3AC) induced a marked expansion of MDSCs in vivo. Both pre-treatment with 3AC of DBA/1 mice prior to CIA induction and intervention with 3AC during CIA progression significantly reduced disease incidence and severity. Adoptive transfer of MDSCs isolated from 3AC-treated mice, but not naïve MDSCs from normal mice, into CIA mice significantly reduced disease incidence and severity, indicating that the 3AC-induced MDSCs were the cellular mediators of the observed amelioration of the disease. In conclusion, inhibition of SHIP1 expands MDSCs in vivo and attenuates development of CIA in mice. Small molecule specific inhibition of SHIP1 may therefore offer therapeutic benefit to patients with RA and other autoimmune diseases.

Beyond the Cell Surface: Targeting Intracellular Negative Regulators to Enhance T cell Anti-Tumor Activity

It is well established that extracellular proteins that negatively regulate T cell function, such as Cytotoxic T-Lymphocyte-Associated protein 4 (CTLA-4) and Programmed Cell Death protein 1 (PD-1), can be effectively targeted to enhance cancer immunotherapies and Chimeric Antigen Receptor T cells (CAR-T cells). Intracellular proteins that inhibit T cell receptor (TCR) signal transduction, though less well studied, are also potentially useful therapeutic targets to enhance T cell activity against tumor. Four major classes of enzymes that attenuate TCR signaling include E3 ubiquitin kinases such as the Casitas B-lineage lymphoma proteins (Cbl-b and c-Cbl), and Itchy (Itch), inhibitory tyrosine phosphatases, such as Src homology region 2 domain-containing phosphatases (SHP-1 and SHP-2), inhibitory protein kinases, such as C-terminal Src kinase (Csk), and inhibitory lipid kinases such as Src homology 2 (SH2) domain-containing inositol polyphosphate 5-phosphatase (SHIP) and Diacylglycerol kinases (DGKs). This review describes the mechanism of action of eighteen intracellular inhibitory regulatory proteins in T cells within these four classes, and assesses their potential value as clinical targets to enhance the anti-tumor activity of endogenous T cells and CAR-T cells.

Intracellular B Lymphocyte Signalling and the Regulation of Humoral Immunity and Autoimmunity

B lymphocytes are critical for effective immunity; they produce antibodies and cytokines, present antigens to T lymphocytes and regulate immune responses. However, because of the inherent randomness in the process of generating their vast repertoire of antigen-specific receptors, B cells can also cause diseases through recognizing and reacting to self. Therefore, B lymphocyte selection and responses require tight regulation at multiple levels and at all stages of their development and activation to avoid diseases. Indeed, newly generated B lymphocytes undergo rigorous tolerance mechanisms in the bone marrow and, subsequently, in the periphery after their migration. Furthermore, activation of mature B cells is regulated through controlled expression of co-stimulatory receptors and intracellular signalling thresholds. All these regulatory events determine whether and how B lymphocytes respond to antigens, by undergoing apoptosis or proliferation. However, defects that alter regulated co-stimulatory receptor expression or intracellular signalling thresholds can lead to diseases. For example, autoimmune diseases can result from altered regulation of B cell responses leading to the emergence of high-affinity autoreactive B cells, autoantibody production and tissue damage. The exact cause(s) of defective B cell responses in autoimmune diseases remains unknown. However, there is evidence that defects or mutations in genes that encode individual intracellular signalling proteins lead to autoimmune diseases, thus confirming that defects in intracellular pathways mediate autoimmune diseases. This review provides a synopsis of current knowledge of signalling proteins and pathways that regulate B lymphocyte responses and how defects in these could promote autoimmune diseases. Most of the evidence comes from studies of mouse models of disease and from genetically engineered mice. Some, however, also come from studying B lymphocytes from patients and from genome-wide association studies. Defining proteins and signalling pathways that underpin atypical B cell response in diseases will help in understanding disease mechanisms and provide new therapeutic avenues for precision therapy.

Purinergic regulation of the immune system

Cellular stress or apoptosis triggers the release of ATP, ADP and other nucleotides into the extracellular space. Extracellular nucleotides function as autocrine and paracrine signalling molecules by activating cell-surface P2 purinergic receptors that elicit pro-inflammatory immune responses. Over time, extracellular nucleotides are metabolized to adenosine, leading to reduced P2 signalling and increased signalling through anti-inflammatory adenosine (P1 purinergic) receptors. Here, we review how local purinergic signalling changes over time during tissue responses to injury or disease, and we discuss the potential of targeting purinergic signalling pathways for the immunotherapeutic treatment of ischaemia, organ transplantation, autoimmunity or cancer.

Identification of SHIP-1 and SHIP-2 homologs in channel catfish, Ictalurus punctatus

Src homology domain 2 (SH2) domain-containing inositol 5'-phosphatases (SHIP) proteins have diverse roles in signal transduction. SHIP-1 and SHIP-2 homologs were identified in channel catfish, Ictalurus punctatus, based on sequence homology to murine and human SHIP sequences. Full-length cDNAs for catfish SHIP-1 and SHIP-2 (IpSHIP-1 and IpSHIP-2) were obtained using 5' and 3' RACE protocols. Catfish SHIP molecules share a high degree of sequence identity to their respective SHIP sequences from diverse taxa and both are encoded by single copy genes. IpSHIP-1 and IpSHIP-2 transcripts were expressed in all catfish tissues analyzed except for skin, and IpSHIP-1 message was more abundant than IpSHIP-2 message in lymphoid tissues. Catfish clonal B, cytotoxic T, and macrophage cell lines also expressed message for both molecules. IpSHIP-1 and IpSHIP-2 SH2 domains were expressed as recombinant proteins and were both found to be bound by cross-reacting rabbit anti-mouse SHIP-1 pAb. The anti-mouse SHIP-1 pAb also reacted with cell lysates from the cytotoxic T cell lines, macrophages and stimulated PBL. SHIP-1 is also phosphorylated at a conserved tyrosine residue, as shown by immunoprecipitation studies.

Signaling in lymphocyte activation

The fate of T and B lymphocytes, the key cells that direct the adaptive immune response, is regulated by a diverse network of signal transduction pathways. The T- and B-cell antigen receptors are coupled to intracellular tyrosine kinases and adaptor molecules to control the metabolism of inositol phospholipids and calcium release. The production of inositol polyphosphates and lipid second messengers directs the activity of downstream guanine-nucleotide-binding proteins and protein and lipid kinases/phosphatases that control lymphocyte transcriptional and metabolic programs. Lymphocyte activation is modulated by costimulatory molecules and cytokines that elicit intracellular signaling that is integrated with the antigen-receptor-controlled pathways.

PTEN-inhibition by zinc ions augments interleukin-2-mediated Akt phosphorylation

Free zinc ions (Zn(2+)) participate in several signaling pathways. The aim of the present study was to investigate a potential involvement of Zn(2+) in the PI3K/Akt pathway of interleukin (IL)-2 signaling in T-cells. The IL-2 receptor triggers three major pathways, ERK1/2, JAK/STAT5, and PI3K/Akt. We have previously shown that an IL-2-mediated release of lysosomal Zn(2+) into the cytoplasm activates ERK1/2, but not STAT5. In the present study, Akt phosphorylation in response to IL-2 was abrogated by the Zn(2+) chelator N,N,N',N'-tetrakis-2(pyridyl-methyl)ethylenediamine, and was induced by treatment with Zn(2+) and the ionophore pyrithione. The latter were ineffective in cells that were treated with siRNA against the phosphatase and tensin homolog deleted on chromosome 10 (PTEN), a phosphatase that degrades the lipid second messenger PI(3,4,5)P3, which is produced by PI3K and leads to activation of Akt. Inhibition of recombinant PTEN by Zn(2+)in vitro yielded an IC50 of 0.59 nM. Considering a resting free cytoplasmic Zn(2+) level of 0.2 nM in the T-cell line CTLL-2, this seems ideally suited for dynamic regulation by cellular Zn(2+). Oxidation with H2O2 and supplementation with Zn(2+) led to similar changes in the CD spectrum of PTEN. Moreover, Zn(2+) partially prevented the oxidation of cysteines 71 and 124. Hence, we hypothesize that zinc signals affect the IL-2-dependent PI3K/Akt pathway by inhibiting the negative regulator PTEN through binding with a sub-nanomolar affinity to cysteine residues that are essential for its catalytic activity.

Molecular control of PtdIns(3,4,5)P3 signaling in neutrophils

Neutrophils play critical roles in innate immunity and host defense. However, excessive neutrophil accumulation or hyper-responsiveness of neutrophils can be detrimental to the host system. Thus, the response of neutrophils to inflammatory stimuli needs to be tightly controlled. Many cellular processes in neutrophils are mediated by localized formation of an inositol phospholipid, phosphatidylinositol (3,4,5)-trisphosphate (PtdIns(3,4,5)P3), at the plasma membrane. The PtdIns(3,4,5)P3 signaling pathway is negatively regulated by lipid phosphatases and inositol phosphates, which consequently play a critical role in controlling neutrophil function and would be expected to act as ideal therapeutic targets for enhancing or suppressing innate immune responses. Here, we comprehensively review current understanding about the action of lipid phosphatases and inositol phosphates in the control of neutrophil function in infection and inflammation.

The effects of the novel SHIP1 activator AQX-1125 on allergen-induced responses in mild-to-moderate asthma

BACKGROUND

SH2-containing inositol-5'-phosphatase 1 (SHIP1) is an endogenous inhibitor of the phosphoinositide-3-kinase pathway that is involved in the activation and chemotaxis of inflammatory cells. AQX-1125 is a first-in-class, oral SHIP1 activator with a novel anti-inflammatory mode of action.

OBJECTIVE

To evaluate the effects of AQX-1125 on airway responses to allergen challenge in mild-to-moderate asthmatic patients.

METHODS

A randomized, double-blind, placebo-controlled, two-way crossover study was performed in 22 steroid-naïve mild-to-moderate asthmatics with a documented late-phase response to inhaled allergen (LAR). AQX-1125 (450 mg daily) or placebo was administered orally for 7 days. Allergen challenge was performed on day 6 (2 h postdose), followed by methacholine challenge (day 7), and induced sputum collection and fractional exhaled nitric oxide (FeNO).

RESULTS

AQX-1125 significantly attenuated the late-phase response compared with placebo (FEV1 4-10 h: mean difference 150 mL, 20%; P = 0.027) and significantly increased the minimum FEV1 during LAR (mean difference 180 mL; P = 0.014). AQX-1125 had no effect on the early-phase response. AQX-1125 showed a trend in reduction of sputum eosinophils, neutrophils and macrophages although this did not achieve significance as there were only 11 paired samples for analysis. There was no effect on methacholine responsiveness or FeNO. Pharmacokinetic data showed AQX-1125 was rapidly absorbed with geometric mean Cmax and AUC0-24 h values of 1417 ng/mL and 16 727 h ng/mL, respectively. AQX-1125 was well tolerated, but mild GI side-effects (dyspepsia, nausea and abdominal pain) were described in 4/22 subjects on active treatment. These side-effects were mild self-limiting, required no further treatment and did not lead to discontinuation of therapy.

CONCLUSION AND CLINICAL RELEVANCE

AQX-1125, a novel oral SHIP1 activator, significantly reduces the late response to allergen challenge, with a trend to reduce airway inflammation. AQX-1125 was safe and well tolerated and merits further investigation in inflammatory disorders.

Downregulation of inhibitory SRC homology 2 domain-containing phosphatase-1 (SHP-1) leads to recovery of T cell responses in elderly

BACKGROUND

Immune responses are generally impaired in aged mammals. T cells have been extensively studied in this context due to the initial discovery of their reduced proliferative capacity with aging. The decreased responses involve altered signaling events associated with the early steps of T cell activation. The underlying causes of these changes are not fully understood but point to alterations in assembly of the machinery for T cell activation. Here, we have tested the hypothesis that the T cell pool in elderly subjects displayed reduced functional capacities due to altered negative feedback mechanisms that participate in the regulation of the early steps of T cell activation. Such conditions tip the immune balance in favor of altered T cell activation and a related decreased response in aging.

RESULTS

We present evidence that the tyrosine phosphatase SHP-1, a key regulator of T cell signal transduction machinery is, at least in part, responsible for the impaired T cell activation in aging. We used tyrosine-specific mAbs and Western blot analysis to show that a deregulation of the Csk/PAG loop in activated T cells from elderly individuals favored the inactive form of tyrosine-phosphorylated Lck (Y505). Confocal microscopy analysis revealed that the dynamic movements of these regulatory proteins in lipid raft microdomains was altered in T cells of aged individuals. Enzymic assays showed that SHP-1 activity was upregulated in T cells of aged donors, in contrast to young subjects. Pharmacological inhibition of SHP-1 resulted in recovery of TCR/CD28-dependent lymphocyte proliferation and IL-2 production of aged individuals to levels approaching those of young donors. Significant differences in the active (Y394) and inactive (Y505) phosphorylation sites of Lck in response to T cell activation were observed in elderly donors as compared to young subjects, independently of CD45 isoform expression.

CONCLUSIONS

Our data suggest that the role of SHP-1 in T cell activation extends to its increased effect in negative feedback in aging. Modulation of SHP-1 activity could be a target to restore altered T cell functions in aging. These observations could have far reaching consequences for improvement of immunosenescence and its clinical consequences such as infections, altered response to vaccination.

Characterization of AQX-1125, a small-molecule SHIP1 activator: Part 1. Effects on inflammatory cell activation and chemotaxis in vitro and pharmacokinetic characterization in vivo

BACKGROUND

The SH2-containing inositol-5'-phosphatase 1 (SHIP1) metabolizes PI(3,4,5)P3 to PI(3,4)P2. SHIP1-deficient mice exhibit progressive inflammation. Pharmacological activation of SHIP1 is emerging as a potential therapy for pulmonary inflammatory diseases. Here we characterize the efficacy of AQX-1125, a small-molecule SHIP1 activator currently in clinical development.

EXPERIMENTAL APPROACH

The effects of AQX-1125 were tested in several in vitro assays: on enzyme catalytic activity utilizing recombinant human SHIP1, on Akt phosphorylation in SHIP1-proficient and SHIP1-deficient cell lines, on cytokine release in murine splenocytes, on human leukocyte chemotaxis using modified Boyden chambers and on β-hexosaminidase release from murine mast cells. In addition, pharmacokinetic and drug distribution studies were performed in rats and dogs.

RESULTS

AQX-1125 increased the catalytic activity of human recombinant SHIP1, an effect, which was absent after deletion of the C2 region. AQX-1125 inhibited Akt phosphorylation in SHIP1-proficient but not in SHIP1-deficient cells, reduced cytokine production in splenocytes, inhibited the activation of mast cells and inhibited human leukocyte chemotaxis. In vivo, AQX-1125 exhibited >80% oral bioavailability and >5 h terminal half-life.

CONCLUSIONS

Consistent with the role of SHIP1 in cell activation and chemotaxis, the SHIP1 activator AQX-1125 inhibits Akt phosphorylation, inflammatory mediator production and leukocyte chemotaxis in vitro. The in vitro effects and the pharmacokinetic properties of the compound make it a suitable candidate for in vivo testing in various models of inflammation.

Phosphoinositide lipid phosphatase SHIP1 and PTEN coordinate to regulate cell migration and adhesion

SHIP1 regulates PtdIns(3,4,5)P3 production in response to cell adhesion. Loss of SHIP1 leads to elevated PtdIns(3,4,5)P3 and Akt activation upon adhesion. SHIP1^−/− neutrophils lose polarity upon cell adhesion. They are extremely adherent, which impairs chemotaxis. Chemotaxis in SHIP1^−/− neutrophils can be rescued by reducing cell adhesion.

The second messenger phosphatidylinositol(3,4,5)P₃ (PtdIns(3,4,5)P₃) is formed by stimulation of various receptors, including G protein–coupled receptors and integrins. The lipid phosphatases PTEN and SHIP1 are critical in regulating the level of PtdIns(3,4,5)P₃ during chemotaxis. Observations that loss of PTEN had minor and loss of SHIP1 resulted in a severe chemotaxis defect in neutrophils led to the belief that SHIP1 rather than PTEN acts as a predominant phospholipid phosphatase in establishing a PtdIns(3,4,5)P₃ compass. In this study, we show that SHIP1 regulates PtdIns(3,4,5)P₃ production in response to cell adhesion and plays a limited role when cells are in suspension. SHIP1^−/− neutrophils lose their polarity upon cell adhesion and are extremely adherent, which impairs chemotaxis. However, chemo­taxis can be restored by reducing adhesion. Loss of SHIP1 elevates Akt activation following cell adhesion due to increased PtdIns(3,4,5)P₃ production. From our observations, we conclude that SHIP1 prevents formation of top-down PtdIns(3,4,5)P₃ polarity to facilitate proper cell attachment and detachment during chemotaxis.

Phosphoinositide phosphatases: just as important as the kinases

Phosphoinositide phosphatases comprise several large enzyme families with over 35 mammalian enzymes identified to date that degrade many phosphoinositide signals. Growth factor or insulin stimulation activates the phosphoinositide 3-kinase that phosphorylates phosphatidylinositol (4,5)-bisphosphate [PtdIns(4,5)P(2)] to form phosphatidylinositol (3,4,5)-trisphosphate [PtdIns(3,4,5)P(3)], which is rapidly dephosphorylated either by PTEN (phosphatase and tensin homologue deleted on chromosome 10) to PtdIns(4,5)P(2), or by the 5-phosphatases (inositol polyphosphate 5-phosphatases), generating PtdIns(3,4)P(2). 5-phosphatases also hydrolyze PtdIns(4,5)P(2) forming PtdIns(4)P. Ten mammalian 5-phosphatases have been identified, which regulate hematopoietic cell proliferation, synaptic vesicle recycling, insulin signaling, and embryonic development. Two 5-phosphatase genes, OCRL and INPP5E are mutated in Lowe and Joubert syndrome respectively. SHIP [SH2 (Src homology 2)-domain inositol phosphatase] 2, and SKIP (skeletal muscle- and kidney-enriched inositol phosphatase) negatively regulate insulin signaling and glucose homeostasis. SHIP2 polymorphisms are associated with a predisposition to insulin resistance. SHIP1 controls hematopoietic cell proliferation and is mutated in some leukemias. The inositol polyphosphate 4-phosphatases, INPP4A and INPP4B degrade PtdIns(3,4)P(2) to PtdIns(3)P and regulate neuroexcitatory cell death, or act as a tumor suppressor in breast cancer respectively. The Sac phosphatases degrade multiple phosphoinositides, such as PtdIns(3)P, PtdIns(4)P, PtdIns(5)P and PtdIns(3,5)P(2) to form PtdIns. Mutation in the Sac phosphatase gene, FIG4, leads to a degenerative neuropathy. Therefore the phosphatases, like the lipid kinases, play major roles in regulating cellular functions and their mutation or altered expression leads to many human diseases.

Role of SHIP in cancer

The SH2-containing inositol-5'-phosphatase, SHIP (or SHIP1), is a hematopoietic-restricted phosphatidylinositide phosphatase that translocates to the plasma membrane after extracellular stimulation and hydrolyzes the phosphatidylinositol-3-kinase-generated second messenger PI-3,4,5-P(3) to PI-3,4-P(2). As a result, SHIP dampens down PI-3,4,5-P(3)-mediated signaling and represses the proliferation, differentiation, survival, activation, and migration of hematopoietic cells. There are multiple lines of evidence suggesting that SHIP may act as a tumor suppressor during leukemogenesis and lymphomagenesis. Because of its ability to skew macrophage progenitors toward M1 macrophages and naïve T cells toward T helper 1 and T helper 17 cells, SHIP may play a critical role in activating the immune system to eradicate solid tumors. In this review, we will discuss the role of SHIP in hematopoietic cells and its therapeutic potential in terms of suppressing leukemias and lymphomas and manipulating the immune system to combat cancer.

Reversible phosphorylation in haematological malignancies: potential role for protein tyrosine phosphatases in treatment?

Most aspects of leukocyte physiology are under the control of reversible tyrosine phosphorylation. It is clear that excessive phosphorylation of signal transduction elements is a pivotal element of many different pathologies including haematological malignancies and accordingly, strategies that target such phosphorylation have clinically been proven highly successful for treatment of multiple types of leukemias and lymphomas. Cellular phosphorylation status is dependent on the resultant activity of kinases and phosphatases. The cell biology of the former is now well understood; for most cellular phosphoproteins we now know the kinases responsible for their phosphorylation and we understand the principles of their aberrant activity in disease. With respect to phosphatases, however, our knowledge is much patchier. Although the sequences of whole genomes allow us to identify phosphatases using in silico methodology, whereas transcription profiling allows us to understand how phosphatase expression is regulated during disease, most functional questions as to substrate specificity, dynamic regulation of phosphatase activity and potential for therapeutic intervention are still to a large degree open. Nevertheless, recent studies have allowed us to make meaningful statements on the role of tyrosine phosphatase activity in the three major signaling pathways that are commonly affected in leukemias, i.e. the Ras-Raf-ERK1/2, the Jak-STAT and the PI3K-PKB-mTOR pathways. Lessons learned from these pathways may well be applicable elsewhere in leukocyte biology as well.

PI3Ks in lymphocyte signaling and development

Lymphocyte development and function are regulated by tyrosine kinase and G-protein coupled receptors. Each of these classes of receptors activates phosphoinositide 3-kinase (PI3K). In this chapter, we summarize current understanding of how PI3K contributes to key aspects of the adaptive immune system.