The passive films formed on the surface of normalFe19normalCr9normalNi and normalFe19normalCr9normalNi2.5normalMo alloys in 0.1M normalHCl have been characterized by AES and variable angle XPS. It is found that additions of Mo to normalFe19normalCr9normalNi results in the formation of a passive film with a more highly developed interfacial barrier film composed mainly of Cr2O3 , containing CrO3 in solid solution which forms a glassy phase. In addition, the passive films were found to contain more Fe. Molybdenum appears to be present as Mo4+ and Mo6+ identified as hydrated MoO2 and MoO42− , apparently as FeMoO4 . It is proposed that MoO42− anions are formed in the solid state along with CrO42− , which together are responsible for producing, in 0.1M normalHCl , a bipolar film consisting of a cation selective outer layer containing CrO42− and MoO42− and an intrinsically anion selective inner layer. The ion selective property of this duplex film is considered to be largely responsible for the development of the XCr2O3·YCrO3 barrier layer and resisting Cl− and OH− ingress. Both of these properties provide greater resistance to break down of passivity in Cl− ion media.

Although essential for T cell function, the identity of the T cell receptor "inside-out" pathway for lymphocyte function-associated antigen 1 (LFA-1) adhesion has proved elusive. Here, we define the "inside-out" pathway mediated by N-terminal SKAP1 (SKAP-55) domain binding to the C-terminal SARAH domain of RapL. TcR induced Rap1-RapL complex formation and LFA-1 binding failed to occur in Skap1(-/-) primary T cells. SKAP1 generated a SKAP1-RapL-Rap1 complex that bound to LFA-1, whereas a RapL mutation (L224A) that abrogated SKAP1 binding without affecting MST1 disrupted component colocalization in vesicles as well as T cell-dendritic cell (DC) conjugation. RapL expression also "slowed" T cell motility in D011.10 transgenic T cells in lymph nodes (LNs), an effect reversed by the L224A mutation with reduced dwell times between T cells and DCs. Overall, our findings define a TCR "inside-out" pathway via N-SKAP1-C-RapL that regulates T cell adhesion, motility, and arrest times with DCs in LNs.

Splenic marginal zone B (MZB) cells, positioned at the interface between circulating blood and lymphoid tissue, detect and respond to blood-borne antigens. Here we show that MZB cells in mice activate a homeostatic program in response to a high-cholesterol diet (HCD) and regulate both the differentiation and accumulation of T follicular helper (T) cells. Feeding mice an HCD resulted in upregulated MZB cell surface expression of the immunoregulatory ligand PDL1 in an ATF3-dependent manner and increased the interaction between MZB cells and pre-T cells, leading to PDL1-mediated suppression of T cell motility, alteration of T cell differentiation, reduced T abundance and suppression of the proatherogenic T response. Our findings reveal a previously unsuspected role for MZB cells in controlling the T-germinal center response to a cholesterol-rich diet and uncover a PDL1-dependent mechanism through which MZB cells use their innate immune properties to limit an exaggerated adaptive immune response.

T-cell receptors (TCRs) undergo microclustering and supramolecular activation cluster formation at the immunological synapse (IS) during conjugation between T cells and antigen-presenting cells (APCs) (5,21,35). Microclustering can in turn activate GTP-binding proteins, protein kinases, phosphatases, and the phosphorylation of adaptor proteins. CD4-and CD8-p56lck activation leads to immunoreceptor tyrosinebased activation motif phosphorylation on the CD3 and TCR chains, the recruitment of ZAP-70 (zeta-associated proteintyrosine kinase of 70 kDa), and the activation of TEC kinases ITK/RLK (interleukin-2-inducible/resting lymphocyte kinase) (1,(31)(32)(33)41). Adaptor proteins possess binding sites and domains needed for complex-complex formation (31,32,41). Immune cell-specific adaptors include LAT (linker for activation of T cells), GADS (Grb-2-like adaptor downstream of Shc), SLP-76 (SH2 domain-containing leukocyte protein of 76 kDa), ADAP (adhesion-and degranulation-promoting adaptor protein; previously known as FYN T-binding protein/SLP-76-associated protein [FYB/SLAP]), and SKAP-55 (Src kinase-associated phosphoprotein of 55 kDa; also known as SCAP1) (3,23,24,26,31,32,(40)(41)(42). Phosphorylation of LAT recruits phospholipase C␥1, Grb-2, and GADS-SLP-76 and induces Ca 2ϩ mobilization and cytokine transcription (32,33,41,44).Binding of leukocyte function-associated antigen 1 (LFA-1; also known as CD11/CD18 or ␣ L ␤ 2 ) to intercellular adhesion molecules 1 and 2 (ICAM-1 and -2) on APCs mediates T-cell-APC conjugation (5,8,11,21,37). Following initial adhesion, TCR/CD3 ligation induces signals (i.e., "inside-out signaling") that further activate integrin adhesion (2,5,8,11,37). Conversion of LFA-1 to intermediate-or higher-affinity forms involves changes in conformation and receptor clustering (11,37). Multiple signaling proteins mediate this process. They include the GTP-binding protein Rap-1, its ligand RapL (regulator of cell adhesion and polarization enriched in lymphoid tissues), RIAM (Rap1-GTP-interacting adaptor molecule), the guanine nucleotide exchange factor Vav-1, and the adaptors ADAP,10,13,15,16,17,18,20,22,27,28,34). The protein-tyrosine kinase ZAP-70 phosphorylates YESP sites in SLP-76, which allows binding to the Src homology 2 (SH2) domain of Vav-1 (29), while the SH2 domain of SLP-76 binds to two YDDV sites in ADAP (30, 39). T-cell lines lacking SLP-76 show impaired superantigen-induced conjugation (44). ADAP is an immune cell-specific adaptor with a unique N-terminal region, a proline-rich region, a canonical and a noncanonical SH3 domain, one Ena/VASP homology 1 (EVH1) binding domain, and two putative nuclear localization motifs (3,4,19,26,31). ADAP is preferentially phosphorylated by the Src kinase p59fynT (4,25) and can cooperate with p59 fynT and SLP-76 in amplifying TCR-induced interleukin-2 (IL-2) transcription (30). The adaptor can up-regulate integrin-mediated adhesion in certain basophilic cell lines (9), while ADAP Ϫ/Ϫ T cells show profound defects in ␤1 and ␤2 integrin clusterin...

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Hematopoiesis is sustained by the proliferation and development of an extremely low number of hematopoietic stem cells resident in the bone marrow. These stem cells can migrate from their bone marrow microenvironment and can be found at low levels in the peripheral blood. The factors that regulate egress or ingress of the stem cells from the marrow include cytokines and chemokines. This process of stem cell trafficking is fundamental to both stem cell biology and stem cell transplantation. We show that primitive hematopoietic cells with cobblestone area-forming cell activity express receptors for and display enhanced motility in response to a new class of stem cell agonists, namely lysophospholipids. These agents synergistically promote chemokinestimulated cell chemotaxis, a process that is crucial in stem cell homing. The response to lysophospholipids is mediated by Rac, Rho, and Cdc42 G proteins and the hematopoi-

CTLA-4 inhibits T-cell activation and protects against the development of autoimmunity. We and others previously showed that the coreceptor can induce T-cell motility and shorten dwell times with dendritic cells (DCs). However, it has been unclear whether this property of CTLA-4 affects both conventional T cells (Tconvs) and regulatory T cells (Tregs). Here, we report that CTLA-4 had significantly more potent effects on the motility and contact times of Tconvs than Tregs. This was shown firstly by anti-CTLA-4 reversal of the anti-CD3 stop-signal on FoxP3-negative cells at concentrations that had no effect on FoxP3-positive Tregs. Secondly, the presence of CTLA-4 reduced the contact times of DO11.10 x CD4 ؉ CD25 ؊ Tconvs, but not DO11. IntroductionT-cell responses require an antigen-specific signal generated by the TCR/CD3 and CD4-CD8-p56 lck complexes 1,2 as well as by coreceptors, such as CD28, inducible T-cell costimulator (ICOS), CTLA-4, and programmed death-1 (PD-1). 3,4 CTLA-4 is inhibitory as demonstrated by the phenotype of Ctla4 Ϫ/Ϫ mice that develop a lymphoproliferative disorder characterized by massive tissue infiltration and early lethality. 5,6 CTLA-4 raises the threshold that is needed for T-cell activation by cell intrinsic and extrinsic pathways. 7 Cell intrinsic mechanisms include inhibition of ZAP-70 microcluster formation, 8 an altered immunologic synapse (IS), 9 modulation of TCR signaling by phosphatases SHP-2 and PP2A, 10,11 and interference with the expression or composition of lipid rafts on the surface of T cells. [12][13][14] Cell extrinsic functions include the ecto-domain competition for CD28 binding to CD80/CD86, 15 occupancy or removal of CD80/86 16 or the release of indoleamine 2,3-dioxygenase (IDO) 17 and transforming growth factor ␤ (TGF-␤). 18 CTLA-4 expression is also needed for optimal suppression by regulatory T cells (Tregs). 19 The external domain of CTLA-4 has been reported to mediate anergy induction, whereas its effect in reducing TCR signaling required the internal domain. 20 In this context, we have shown that CTLA-4 induces T-cell motility (ie, motility activator) and limits contact times with DCs as well as affecting the migration of T cells in lymph nodes. 8,21,22 Reduced contact times by CTLA-4 would be expected to limit TCR ligation events and raise the threshold for T-cell activation. [21][22][23][24] The motility inducing effects of CTLA-4 have been confirmed by several laboratories in various in vivo models, [25][26][27] and involves CTLA-4 activation of the GTP binding protein Rap1 and LFA-1 adhesion. 24,28 Immune responses are regulated by a balance of inflammatory responses by conventional or responder T cells (Tconvs) and suppression by Tregs that maintain tolerance to self-antigen. 29,30 Treg suppression of the in vitro proliferation of Tconvs is cellcontact-dependent. 31 Further, Tregs constitutively express CTLA-4 which can be released to the cell surface and is needed for optimal suppressor function. 19,32,33 In this context, it has been unclear whethe...

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