Loss of a single allele of SHIP exacerbates the immunopathology of Pten heterozygous mice

Monophosphorylation of CD79a and CD79b ITAM motifs initiates a SHIP-1 phosphatase-mediated inhibitory signaling cascade required for B cell anergy

Anergic B cells are characterized by impaired signaling and activation after aggregation of their antigen receptors (BCR). The molecular basis of this impairment is not understood. In studies reported here, Src homology-2 (SH2)-containing inositol 5-phosphatase SHIP-1 and its adaptor Dok-1 were found to be constitutively phosphorylated in anergic B cells, and activation of this inhibitory circuit was dependent on Src-family kinase activity and consequent to biased BCR immunoreceptor tyrosine-based activation motif (ITAM) monophosphorylation. B cell-targeted deletion of SHIP-1 caused severe lupus-like disease. Moreover, absence of SHIP-1 in B cells led to loss of anergy as indicated by restoration of BCR signaling, loss of anergic surface phenotype, and production of autoantibodies. Thus, chronic BCR signals maintain anergy in part via ITAM monophosphorylation-directed activation of an inhibitory signaling circuit involving SHIP-1 and Dok-1.

Genetic segregation of inflammatory lung disease and autoimmune disease severity in SHIP-1-/- mice

Alternatively activated M2 macrophages are implicated as both regulators and agents of lung disease, but their control is poorly understood. SHIP-1 is a 5' inositol phosphatase that negatively regulates the PI3K signaling pathway implicated in inflammation. SHIP-1-deficient mice have defects in hematopoiesis and B cell development, and die prematurely due to consolidation of lungs with M2-skewed macrophages. SHIP-1 is thought to restrain M2 macrophage polarization, with deregulated M2 skewing coinciding with severe lung disease in SHIP-1-deficient mice. To determine the influence of genetic background on the lung phenotype in SHIP-1(-/-) mice, we backcrossed the SHIP-1 null mutation onto C57BL/6 (Th2-resistant) and BALB/c (Th2-prone) backgrounds. Remarkably, we found that inflammatory lung disease was severe in C57.SHIP-1(-/-) mice, but absent in BALB.SHIP-1(-/-) mice. C57.SHIP-1(-/-), but not BALB.SHIP-1(-/-) mice had greatly increased myeloid progenitors, myeloid hyperplasia, markedly enhanced numbers of activated alveolar macrophages, and elevated amounts of Th2 and proinflammatory cytokines in bronchoalveolar lavage fluid and serum, suggesting that deregulated cytokine production induced disease. C57.SHIP-1(-/-) mice also developed severe B cell-dependent autoimmune disease, which was markedly attenuated on the BALB/c background. These data demonstrate that, contrary to current concepts, loss of SHIP-1 alone is not sufficient to cause lung inflammation, with disease only manifest on a permissive genetic background. This finding questions the nature of the lung disease in SHIP-1(-/-) mice, suggesting that its M2 classification is not strictly correct. Future identification of disease-promoting loci might reveal determinants of comorbid lung disease and autoimmunity and uncover potentially useful therapeutic targets.

IL-6 increases B-cell IgG production in a feed-forward proinflammatory mechanism to skew hematopoiesis and elevate myeloid production

Src homology 2 domain-containing inositol 5-phosphatase (SHIP(-/-)) animals display an age-related increase in interleukin-6 (IL-6), a decrease in B lymphopoiesis, and an elevation in myelopoiesis. We investigated the origin of the IL-6 production and show that it is largely produced by peritoneal and splenic macrophages. IL-6 production by these macrophages is not a direct result of the loss of SHIP: IL-6 production is not spontaneous, is absent from bone marrow-derived macrophages, declines with prolonged culture of macrophages, and requires a stimulus present in vivo. The IL-6-rich peritoneal cavity of SHIP(-/-) mice shows more than 700-fold more immunoglobulin G (IgG) than wild-type, approximately 20% of which is aggregated or in an immune complex and contains B220(+) cells that secrete IgG. The SHIP-deficient peritoneal macrophages show evidence of IgG receptor stimulation. Animals lacking both the signal-transducing gamma-chain of IgG receptors and SHIP or Ig and SHIP produce less IL-6. The data indicate a feed-forward process in which peripheral macrophages, responding through IgG receptors to secreted IgG, produce IL-6, to support further B-cell production of IgG. Because of the proinflammatory phenotype of SHIP(-/-) animals, these findings emphasize the importance of IL-6-neutralizing strategies in autoimmune and proinflammatory diseases.

The proapoptotic and antimitogenic protein p66SHC acts as a negative regulator of lymphocyte activation and autoimmunity

The ShcA locus encodes 3 protein isoforms that differ in tissue specificity, subcellular localization, and function. Among these, p66Shc inhibits TCR coupling to the Ras/MAPK pathway and primes T cells to undergo apoptotic death. We have investigated the outcome of p66Shc deficiency on lymphocyte development and homeostasis. We show that p66Shc−/− mice develop an age-related lupus-like autoimmune disease characterized by spontaneous peripheral T- and B-cell activation and proliferation, autoantibody production, and immune complex deposition in kidney and skin, resulting in autoimmune glomerulonephritis and alopecia. p66Shc−/− lymphocytes display enhanced proliferation in response to antigen receptor engagement in vitro and more robust immune responses both to vaccination and to allergen sensitization in vivo. The data identify p66Shc as a negative regulator of lymphocyte activation and show that loss of this protein results in breaking of immunologic tolerance and development of systemic autoimmunity.

PIP3 pathway in regulatory T cells and autoimmunity

Regulatory T cells (Tregs) play an important role in preventing both autoimmune and inflammatory diseases. Many recent studies have focused on defining the signal transduction pathways essential for the development and the function of Tregs. Increasing evidence suggest that T-cell receptor (TCR), interleukin-2 (IL-2) receptor (IL-2R), and co-stimulatory receptor signaling are important in the early development, peripheral homeostasis, and function of Tregs. The phosphoinositide-3 kinase (PI3K)-regulated pathway (PIP3 pathway) is one of the major signaling pathways activated upon TCR, IL-2R, and CD28 stimulation, leading to T-cell activation, proliferation, and cell survival. Activation of the PIP3 pathway is also negatively regulated by two phosphatidylinositol phosphatases SHIP and PTEN. Several mouse models deficient for the molecules involved in PIP3 pathway suggest that impairment of PIP3 signaling leads to dysregulation of immune responses and, in some cases, autoimmunity. This review will summarize the current understanding of the importance of the PIP3 pathway in T-cell signaling and the possible roles this pathway performs in the development and the function of Tregs.

Role of phosphoinositide 3-kinase signaling in autoimmunity

Activation of the phosphoinositide 3-kinase (PI3K) pathway promotes proliferation and survival in many different cell types of the immune system. PI3K acts downstream of receptors that mediate proliferation and survival in T cells, and required roles for individual class I PI3K catalytic isoforms have been established. Interestingly, mice with either augmented or diminished PI3K activity in T cells develop lymphoproliferation and signs of autoimmunity. Here, we summarize our current knowledge of mouse strains with hyperactive or reduced PI3K, different isoforms of class I PI3K in T cell-mediated immunity and autoimmunity, and the therapeutic implications for modulating this pathway for treatment of various autoimmune diseases.

PI3K/AKT signaling and systemic autoimmunity

The phosphoinositide 3-kinase (PI3K)/Akt signaling axis plays an important role in cellular proliferation and growth signaling. With respect to the immune system, a growing body of data is helping to elucidate the role of this pathway in lymphocyte development, as well as to show how perturbations that lead to unregulated activation in this pathway may produce systemic autoimmunity or malignancy. Various knockout and transgenic murine models have been described for key mediators of this signaling pathway. Many of these models resulting in the activation of this pathway demonstrate features of systemic autoimmunity, linking this pathway to autoimmune disease. Here, we review recently described murine models that exhibit activated PI3K/Akt signaling and the potential role this pathway in autoimmune disease, and also discuss the therapeutic implications of these findings.

Compound heterozygosity for Pten and SHIP augments T-dependent humoral immune responses and cytokine production by CD(4+) T cells

Tight regulation of the phosphatidylinositiol 3-kinase (PI3K) pathway is essential not only for normal immune system development and responsiveness, but also in the prevention of immunopathology. Indeed, unchecked activation of the PI3K pathway in T cells induces lymphoproliferation and systemic autoimmunity. Evaluating the importance of threshold levels of two key PI3K pathway phosphoinositol phosphatases, we previously reported that mice heterozygous for both Pten and SHIP develop a more rapid progression of a lymphoproliferative autoimmune syndrome than do Pten(+\-) mice. Investigating the basis for this difference, we now describe a quantitative and qualitative difference in the antibody responses of C57BL\6 Pten(+\-) SHIP(+\-) mice upon challenge with a T-dependent antigen. Suspecting that this phenotypic difference might be the result, at least in part, of a T-helper cell defect, an in vitro analysis of anti-CD3/interleukin (IL)-2-expanded CD4(+) T cells was performed. After stimulation with anti-CD3, cells from mice heterozygous for both Pten and SHIP exhibited a striking increase in IL-4 secretion (> 10-fold), without a corresponding increase in T helper 2 (Th2) cell numbers being evident by intracellular staining for this cytokine. Modest increases were also seen for both IL-13 and IFN-gamma. Perhaps in keeping with this abnormal in vitro cytokine profile, IgG1 serum levels were significantly elevated in young C57BL\6 Pten(+\-) SHIP(+\-) mice. Thus, the relative levels of Pten and SHIP appear to be key variables in CD4(+) T-cell function, primarily via their ability to regulate IL-4 production.

Anemia, thrombocytopenia, leukocytosis, extramedullary hematopoiesis, and impaired progenitor function in Pten+/-SHIP-/- mice: a novel model of myelodysplasia

The myeloproliferative disorder of mice lacking the Src homology 2 (SH2)-containing 5' phosphoinositol phosphatase, SHIP, underscores the need for closely regulating phosphatidylinositol 3-kinase (PI3K) pathway activity, and hence levels of phosphatidylinositol species during hematopoiesis. The role of the 3' phosphoinositol phosphatase Pten in this process is less clear, as its absence leads to embryonic lethality. Despite Pten heterozygosity being associated with a lymphoproliferative disorder, we found no evidence of a hematopoietic defect in Pten(+/-) mice. Since SHIP shares the same substrate (PIP(3)) with Pten, we hypothesized that the former might compensate for Pten haploinsufficiency in the marrow. Thus, we examined the effect of Pten heterozygosity in SHIP(-/-) mice, predicting that further dysregulation of PIP(3) metabolism would exacerbate the pheno-type of the latter. Indeed, compared with SHIP(-/-) mice, Pten(+/-)SHIP(-/-) animals developed a myelodysplastic phenotype characterized by increased hepatosplenomegaly, extramedullary hematopoiesis, anemia, and thrombocytopenia. Consistent with a marrow defect, clonogenic assays demonstrated reductions in committed myeloid and megakaryocytic progenitors in these animals. Providing further evidence of a Pten(+/-)SHIP(-/-) progenitor abnormality, reconstitution of irradiated mice with marrows from these mice led to a marked defect in short-term repopulation of peripheral blood by donor cells. These studies suggest that the regulation of the levels and/or ratios of PI3K-derived phosphoinositol species by these 2 phosphatases is critical to normal hematopoiesis.

SHIP, SHIP2, and PTEN activities are regulated in vivo by modulation of their protein levels: SHIP is up-regulated in macrophages and mast cells by lipopolysaccharide

The phosphatidylinositol-3 kinase (PI3K) pathway plays a central role in regulating numerous biologic processes, including survival, adhesion, migration, metabolic activity, proliferation, differentiation, and end cell activation through the generation of the potent second messenger PI-3,4,5-trisphosphate (PI-3,4,5-P(3)). To ensure that activation of this pathway is appropriately suppressed/terminated, the ubiquitously expressed 54-kDa tumor suppressor PTEN hydrolyzes PI-3,4,5-P(3) to PI-4,5-P(2), whereas the 145-kDa hematopoietic-restricted SH2-containing inositol 5'-phosphatase SHIP (also known as SHIP1), the 104-kDa stem cell-restricted SHIP sSHIP, and the more widely expressed 150-kDa SHIP2 break it down to PI-3,4-P(2). In this review, we focus on the properties of these phospholipid phosphatases and summarize recent data showing that the activities of these negative regulators often are modulated by simply altering their protein levels. We also highlight the critical role that SHIP plays in lipopolysaccharide-induced macrophage activation and in endotoxin tolerance.

Phosphoinositide signaling disorders in human diseases

Phosphoinositides (PIs) play an essential role in diverse cellular functions. Their intracellular level is strictly regulated by specific PI kinases, phosphatases and phospholipases. Recent discoveries indicate that dysfunctions in the control of their level often lead to pathologies. This review will focus on some human diseases whose etiologies involve PI-metabolizing enzymes. The role of PTEN (phosphatase and tensin homolog deleted on chromosome ten) in cancer, the impact of the Src homology 2-containing inositol-5-phosphatase phosphatases in acute myeloid leukemia or diabetes, the involvement of myotubularin family members in genetic diseases and the implication of OCRL1 in Lowe syndrome will be emphasized. We will also review how some bacterial pathogens have evolved strategies to specifically manipulate the host cell PI metabolism to efficiently infect them.