Arginase activity in alternatively activated macrophages protects PI3Kp110δ deficient mice from dextran sodium sulfate induced intestinal inflammation.

Alternatively activated or M2 macrophages have been reported to protect mice from intestinal inflammation, but the mechanism of protection has not been elucidated. In this study, we demonstrate that mice deficient in the p110δ catalytic subunit activity of class I phosphatidylinositol 3-kinase (PI3Kp110δ) have increased clinical disease activity and histological damage during dextran sodium sulfate (DSS) induced colitis. Increased disease severity in PI3Kp110δ-deficient mice is dependent on professional phagocytes and correlates with reduced numbers of arginase I+ M2 macrophages in the colon and increased production of inflammatory nitric oxide. We further demonstrate that PI3Kp110δ-deficient macrophages are defective in their ability to induce arginase I when skewed to an M2 phenotype with IL-4. Importantly, adoptive transfer of IL-4-treated macrophages derived from WT mice, but not those from PI3Kp110δ-deficient mice, protects mice during DSS-induced colitis. Moreover, M2 macrophages mediated protection is lost when mice are cotreated with inhibitors that block arginase activity or during adoptive transfer of arginase I deficient M2 macrophages. Taken together, our data demonstrate that arginase I activity is required for M2 macrophages mediated protection during DSS-induced colitis in PI3Kp110δ-deficient mice.

Temporal stability of the mouse gut microbiota in relation to innate and adaptive immunity.

Gut microbial community properties of mammals are thought to be partly shaped by a combination of host immunity and environmental factors, but their relative importance is not firmly established. To address this gap, we first characterized the faecal bacteria of mice with a functioning immune system (wild-type, WT), mice with defective immune responses (CD45), mice lacking an adaptive immune system (RAG), and mice with both immune dysfunctions (45RAG). Using fingerprinting of 16S rRNA genes, we observed significant differences in gut microbiota composition across all mouse strains (P < 0.001) and identified several mouse strain-specific genera via pyrosequencing, including Turicibacter sp. (in WT mice) and Allobaculum sp. (in CD45-deficient animals). To define the role of the host immune system in constraining gut microbiota stability after perturbation, we cohoused CD45-deficient and WT mice and monitored gut bacterial community dynamics during 8 weeks. Cohousing caused the WT bacterial communities to become indistinguishable from those of CD45 mice (P > 0.05). Time-series analysis indicated that the communities of cohoused mice changed directionally as opposed to the relatively stable communities of non-cohoused controls. When we considered only taxonomic membership, it was the communities of CD45 non-cohoused mice that experienced the highest rate of change. Rather than be governed by fluctuations in the relative abundance of taxa, we suggest that CD45-regulated immune responses either are stimulated by the presence of bacteria per se or promote temporal stability by selecting for the occurrence of specific taxa.

Transforming growth factor-beta-activated kinase 1 regulates natural killer cell-mediated cytotoxicity and cytokine production.

Carma1, a caspase recruitment domain-containing membrane-associated guanylate kinase, initiates a unique signaling cascade via Bcl10 and Malt1 in NK cells. Carma1 deficiency results in reduced phosphorylation of JNK1/2 and activation of NF-κB that lead to impaired NK cell-mediated cytotoxicity and cytokine production. However, the precise identities of the downstream signaling molecules that link Carma1 to these effector functions were not defined. Here we show that transforming growth factor-ß (TGF-ß)-activated kinase 1 (TAK1) is abundantly present in NK cells, and activation via NKG2D results in its phosphorylation. Lack of Carma1 considerably reduced TAK1 phosphorylation, demonstrating the dependence of TAK1 on Carma1 in NKG2D-mediated NK cell activations. Pharmacological inhibitor to TAK1 significantly reduced NK-mediated cytotoxicity and its potential to generate IFN-γ, GM-CSF, MIP-1α, MIP-1ß, and RANTES. Conditional in vivo knockdown of TAK1 in NK cells from Mx1Cre(+)TAK1(fx/fx) mice resulted in impaired NKG2D-mediated cytotoxicity and cytokine/chemokine production. Inhibition or conditional knockdown of TAK1 severely impaired the NKG2D-mediated phosphorylation of ERK1/2 and JNK1/2 and activation of NF-κB and AP1. Our results show that TAK1 links Carma1 to NK cell-mediated effector functions.

Rap1b facilitates NK cell functions via IQGAP1-mediated signalosomes.

Rap1 GTPases control immune synapse formation and signaling in lymphocytes. However, the precise molecular mechanism by which Rap1 regulates natural killer (NK) cell activation is not known. Using Rap1a or Rap1b knockout mice, we identify Rap1b as the major isoform in NK cells. Its absence significantly impaired LFA1 polarization, spreading, and microtubule organizing center (MTOC) formation in NK cells. Neither Rap1 isoform was essential for NK cytotoxicity. However, absence of Rap1b impaired NKG2D, Ly49D, and NCR1-mediated cytokine and chemokine production. Upon activation, Rap1b colocalized with the scaffolding protein IQGAP1. This interaction facilitated sequential phosphorylation of B-Raf, C-Raf, and ERK1/2 and helped IQGAP1 to form a large signalosome in the perinuclear region. These results reveal a previously unrecognized role for Rap1b in NK cell signaling and effector functions.

Murine NKG2D ligands: "double, double toil and trouble".

Unlike T and B cells, NK cells lack variable, clonotypic receptors that recognize foreign antigens. Instead, NK cells depend on conserved receptors such as NKG2D. NKG2D recognizes a variety of inducible self-proteins that belong to the non-classical MHC class I family. They include ULBP (1-3), MIC (A & B) in human and H60 (a, b & c), Rae-1 (alpha-epsilon) and Mult1 in mice. These self-proteins are expressed due to pathological stimuli, share limited amino acid homology and form the molecular basis for NKG2D-mediated activation. Recent studies have vastly improved our understanding of NKG2D receptor-mediated activation, signaling and function. However, a detailed knowledge on the immunobiology of its ligands is lacking. How many is too many? Is NKG2D the only receptor for these ligands? Where are these ligands expressed? What are the molecular mechanisms that regulate their expression? Do normal cells express these ligands? Does the communication between NKG2D receptor and its ligands travel through a two way road? If so, what do the 'target' cells get in turn, only death? How efficient are these ligands as molecular targets for NK cell-mediated tumor immunotherapy?

The p110 delta of PI3K plays a critical role in NK cell terminal maturation and cytokine/chemokine generation.

Phosphatidylinositol 3-kinases (PI3Ks) play a critical role in regulating B cell receptor- and T cell receptor-mediated signaling. However, their role in natural killer (NK) cell development and functions is not well understood. Using mice expressing p110 delta(D910A), a catalytically inactive p110 delta, we show that these mice had reduced NK cellularity, defective Ly49C and Ly49I NK subset maturation, and decreased CD27(High) NK numbers. p110 delta inactivation marginally impaired NK-mediated cytotoxicity against tumor cells in vitro and in vivo. However, NKG2D, Ly49D, and NK1.1 receptor-mediated cytokine and chemokine generation by NK cells was severely affected in these mice. Further, p110 delta(D910A/D910A) NK cell-mediated antiviral responses through natural cytotoxicity receptor 1 were reduced. Analysis of signaling events demonstrates that p110 delta(D910A/D910A) NK cells had a reduced c-Jun N-terminal kinase 1/2 phosphorylation in response to NKG2D-mediated activation. These results reveal a previously unrecognized role of PI3K-p110 delta in NK cell development and effector functions.