Durbaka V. R. Prasad

B7S1, a Novel B7 Family Member that Negatively Regulates T Cell Activation

T cell activation by antigen-presenting cells (APC) is regulated by positive and negative costimulatory molecules in the B7 family. Here we describe a novel addition in this family, designated as B7S1, which is uniquely anchored to the cell membrane via a GPI linkage. B7S1 is expressed on professional APC and widely distributed in nonlymphoid tissues. A soluble B7S1-Ig fusion protein binds to activated but not naive T cells. B7S1-Ig inhibits T cell activation and IL-2 production. A monoclonal antibody that blocks binding of B7S1 to its receptor enhances T cell proliferation in vitro and exacerbates experimental autoimmune encephalomyelitis in vivo. This study identifies a novel negative regulator of T cell activation and further reveals complex costimulatory regulation of immune responses.

Characterization of Mouse and Human B7-H3 Genes

T cell activation and immune function are regulated by costimulatory molecules of the B7 superfamily. Human B7-H3 is a recent addition to this family and has been shown to mediate T cell proliferation and IFN-γ production. In this work we describe the identification of the mouse B7-H3 homolog, which is ubiquitously expressed in a variety of tissues. Activated CD4 and CD8 T cells express a putative receptor that can be recognized by soluble mouse B7-H3-Ig molecules. While the mouse B7-H3 gene was found to contain a single copy, we discovered a novel isoform of human B7-H3 (named as B7-H3b hereafter) with four Ig-like domains that results from gene duplication and differential splicing. B7-H3b is the major isoform expressed in several tissues. This structural information suggests a genetic variation of the B7-H3 gene in mammalian species.

Plexin-A1 and its interaction with DAP12 in immune responses and bone homeostasis

Semaphorins and their receptors have diverse functions in axon guidance, organogenesis, vascularization and/or angiogenesis, oncogenesis and regulation of immune responses. The primary receptors for semaphorins are members of the plexin family. In particular, plexin-A1, together with ligand-binding neuropilins, transduces repulsive axon guidance signals for soluble class III semaphorins, whereas plexin-A1 has multiple functions in chick cardiogenesis as a receptor for the transmembrane semaphorin, Sema6D, independent of neuropilins. Additionally, plexin-A1 has been implicated in dendritic cell function in the immune system. However, the role of plexin-A1 in vivo, and the mechanisms underlying its pleiotropic functions, remain unclear. Here, we generated plexin-A1-deficient (plexin-A1−/−) mice and identified its important roles, not only in immune responses, but also in bone homeostasis. Furthermore, we show that plexin-A1 associates with the triggering receptor expressed on myeloid cells-2 (Trem-2), linking semaphorin-signalling to the immuno-receptor tyrosine-based activation motif (ITAM)-bearing adaptor protein, DAP12. These findings reveal an unexpected role for plexin-A1 and present a novel signalling mechanism for exerting the pleiotropic functions of semaphorins.

Murine B7-H3 Is a Negative Regulator of T Cells

T cell activation is regulated by the innate immune system through positive and negative costimulatory molecules. B7-H3 is a novel B7-like molecule with a putative receptor on activated T cells. Human B7-H3 was first described as a positive costimulator, most potently inducing IFN-γ production and cellular immunity. In this study we examined the expression and function of mouse B7-H3. B7-H3 is mostly expressed on professional APCs; its expression on dendritic cells appears to be up-regulated by LPS. In contrast to human B7-H3, we found that mouse B7-H3 protein inhibited T cell activation and effector cytokine production. An antagonistic mAb to B7-H3 enhanced T cell proliferation in vitro and led to exacerbated experimental autoimmune encephalomyelitis in vivo. Therefore, mouse B7-H3 serves as a negative regulator of T cell activation and function.

B Cells Activated by Lipopolysaccharide, But Not By Anti-Ig and Anti-CD40 Antibody, Induce Anergy in CD8+ T Cells: Role of TGF-β1

B cells recognize Ag through their surface IgRs and present it in the context of MHC class II molecules to CD4+ T cells. Recent evidence indicates that B cells also present exogenous Ags in the context of MHC class I to CD8+ T cells and thus may play an important role in the modulation of CTL responses. However, in this regard, conflicting reports are available. One group of studies suggests that the interaction between B cells and CD8+ T cells leads to the activation of the T cells, whereas other studies propose that it induces T cell tolerance. For discerning this dichotomy, we used B cells that were activated with either LPS or anti-Ig plus anti-CD40 Ab, which mimic the T-independent and T-dependent modes of B cell activation, respectively, to provide accessory signals to resting CD8+ T cells. Our results show that, in comparison with anti-Ig plus anti-CD40 Ab-activated B cells, the LPS-activated B cells (LPS-B) failed to induce significant levels of proliferation, cytokine secretion, and cytotoxic ability of CD8+ T cells. This hyporesponsiveness of CD8+ T cells activated with LPS-B was significantly rescued by anti-TGF-β1 Ab. Moreover, it was found that such hyporesponsive CD8+ T cells activated with LPS-B had entered a state of anergy. Furthermore, LPS-B expresses a significantly higher level of TGF-β1 on the surface, which caused the observed hyporesponsiveness of CD8+ T cells. Therefore, this study, for the first time, provides a novel mechanism of B cell surface TGF-β1-mediated hyporesponsiveness leading to anergy of CD8+ T cells.

Immune regulation by novel costimulatory molecules

CD4 helper T (Th)-cells and the cytokines that they produce play essential regulatory roles in immune and autoimmune responses. Th activation and differentiation is regulated by costimulatory receptors. CD28 and CTLA-4 are important in maintaining the threshold of T-cell activation. ICOS and PD-1 are novel costimulatory receptors expressed on activated T-cells. B7-H3 recognizes a putative costimulatory receptor on activated T-cells. Here we summarize the latest developments in the novel costimulatory molecules and their roles in regulating Th activation, differentiation, and function.

Recognition of bacterial infection by innate immune sensors

Microbial challenges to the host initiate an array of defense processes through the activation of innate and adaptive immunity. Innate immunity consists of sensors or pattern-recognition receptors (PRRs) that are expressed on immune and non-immune cells and sense conserved pathogen-derived molecules or pathogen-associated molecular patterns (PAMPs) in various compartments of the host cells. Recognition of the PAMPs by PRRs triggers antimicrobial effector responses via the induction of proinflammatory cytokines and type I IFNs. Several families of PRRs, such as Toll-like receptors (TLRs), NOD-like receptors (NLRs), RIG-I-like receptors (RLRs), and DNA sensors and their respective PAMPs have been well studied in innate immunity and host defense. Here, we review the recent findings on bacterial recognition by TLRs and NLRs and the signaling pathways activated by these sensors.

Evidence That Glycoprotein 96 (B2), a Stress Protein, Functions as a Th2-Specific Costimulatory Molecule

After the engagement of Ag receptor, most of the Th cells for their optimal activation require a second (costimulatory) signal provided by the APCs. We demonstrate the isolation and characterization of a 99- to 105-kDa protein (B2), from LPS-activated B cell surface, and its function as a Th2-specific costimulatory molecule. Appearance of B2 as a single entity on two-dimensional gel electrophoresis and as a distinct peak in reverse-phase HPLC ascertains the fact that B2 is homogeneous in preparation. Electron microscopy as well as competitive binding studies reveal that 125I-labeled B2 specifically binds anti-CD3-activated T cell surface and also competes with its unlabeled form. Internal amino acid sequences of B2 are found to be identical with stress protein gp96. The identity of B2 as gp96 is also revealed by immunological characterization and by confocal microscopic colocalization studies of B2 and gp96 on LPS-activated B cells. Confocal imaging studies also demonstrate that gp96 can be induced on B cell surface without association of MHC molecules. Furthermore, the novel role of gp96 in Th cell proliferation skewing its differentiation toward Th2 phenotype has also been established. Ab-mediated blocking of gp96-induced signaling not only abrogates in vitro proliferation of CD4+ T cells, but also diminishes the secretion of Th2-specific cytokines. Notably, the expression of CD91 (receptor of gp96/B2) is up-regulated on anti-CD3-activated Th cells and also found to be present on Th1 and Th2 subsets.

T Cells from Programmed Death-1 Deficient Mice Respond Poorly to Mycobacterium tuberculosis Infection

Background Programmed Death-1 (PD-1; CD279) receptor molecule is widely believed to be a negative regulator predominantly expressed by exhausted/activated mouse T cells. Upon interaction with its ligands, PD-L1 and PD-L2, PD-1 inhibits activation of T cells and cytokine production, which has been documented in various viral and fungal infections as well as in vitro studies. Therefore, inhibition of T cell responses by PD-1 resulted in disease resistance in a variety of mouse infection models studied heretofore. Methodology/Principal Findings Here, we report that PD-1 deficient (PD-1−/−) mice infected with Mycobacterium tuberculosis (M. tb) H37Rv by the aerosol route have increased susceptibility as compared with their wild type littermates. Surprisingly, M. tb antigen-specific T cell proliferation was dramatically reduced in PD-1 deficient animals compared with wild-type littermates, and this was due to increased numbers of regulatory T cells (Tregs) and recruitment of mesenchymal stem cells. Furthermore, PD-1−/− mice exhibited decreases in the autophagy-induced LC3-B marker protein in macrophages. Conclusions/Significance Our findings suggest that PD-1 does not play an inhibitory role during M. tb infection and instead promotes mycobacterial clearance in mice.

Immunobiology of CD28 expression on human neutrophils. I. CD28 regulates neutrophil migration by modulating CXCR-1 expression.

CD28, described as a T cell costimulatory molecule so far, is expressed on human peripheral blood neutrophils, as shown by cell surface staining and immunoprecipitation with anti‐CD28 monoclonal antibody, and by reverse transcription PCR. The phorbol 12‐myristate 13‐acetate‐augmented expression of CD28 on these cells can be blocked by actinomycin D, an RNA transcription inhibitor, and staurosporin, a protein kinase inhibitor. Cross‐linking of CD28 results in an early increase in IL‐8 receptor A (IL‐8RA or CXCR‐1) expression and a concurrent increase in IL‐8‐induced chemotaxis. The expression of CXCR‐1 is down‐regulated by receptor internalization 3 h after CD28 cross‐linking with concurrent decrease in IL‐8‐induced chemotactic migration. Thus, our results demonstrate for the first time that CD28 is expressed on human peripheral blood neutrophils and that CD28 may play an important role in the regulation of IL‐8RA expression and migration of neutrophils in response to IL‐8.