Regulation of in vitro and in vivo immune functions by the cytosolic adaptor protein SKAP-HOM.

SKAP-HOM is a cytosolic adaptor protein representing a specific substrate for the Src family protein tyrosine kinase Fyn. Previously, several groups have provided experimental evidence that SKAP-HOM (most likely in cooperation with the cytosolic adaptor protein ADAP) is involved in regulating leukocyte adhesion. To further assess the physiological role of SKAP-HOM, we investigated the immune system of SKAP-HOM-deficient mice. Our data show that T-cell responses towards a variety of stimuli are unaffected in the absence of SKAP-HOM. Similarly, B-cell receptor (BCR)-mediated total tyrosine phosphorylation and phosphorylation of Erk, p38, and JNK, as well as immunoreceptor-mediated Ca(2+) responses, are normal in SKAP-HOM(-/-) animals. However, despite apparently normal membrane-proximal signaling events, BCR-mediated proliferation is strongly attenuated in the absence of SKAP-HOM(-/-). In addition, adhesion of activated B cells to fibronectin (a ligand for beta1 integrins) as well as to ICAM-1 (a ligand for beta2 integrins) is strongly reduced. In vivo, the loss of SKAP-HOM results in a less severe clinical course of experimental autoimmune encephalomyelitis following immunization of mice with the encephalitogenic peptide of MOG (myelin oligodendrocyte glycoprotein). This is accompanied by strongly reduced serum levels of MOG-specific antibodies and lower MOG-specific T-cell responses. In summary, our data suggest that SKAP-HOM is required for proper activation of the immune system, likely by regulating the cross-talk between immunoreceptors and integrins.

Signaling by human herpesvirus 8 kaposin A through direct membrane recruitment of cytohesin-1.

The induction of a transformed cellular phenotype by viruses requires the modulation of signaling pathways through viral proteins. We show here that the phenotypic changes induced by the kaposin A protein of human herpesvirus 8 are mediated through its direct interaction with cytohesin-1, a guanine nucleotide exchange factor for ARF GTPases and regulator of integrin-mediated cell adhesion. Focus formation, stress fiber dissolution, and activation of the ERK-1/2 MAP kinase signal cascade were reverted by the cytohesin-1 E157K mutant, which is deficient in catalyzing guanine nucleotide exchange. Furthermore, liposome-embedded kaposin A specifically stimulates cytohesin-1 dependent GTP binding of myristoylated ARF1 in vitro. These results suggest a previously unknown involvement of ARF GTPases in the control of cellular functions by herpesviruses.

VEGF receptor signaling and endothelial function.

Angiogenesis, the formation of new blood vessels from preexisting ones, is a central process during normal development and during pathological repair. Vascular endothelial growth factor-A (VEGF-A) can stimulate both physiological and pathological angiogensis. VEGF-A is a ligand for the two receptor tyrosine kinases VEGFR-1 (Flt-1) and VEGFR-2 (KDR/Flk-1). Most biological functions of VEGF-A are mediated via VEGFR-2, whereas the role of VEGFR-1 is largely unknown. Activation of mitogen-activated kinase, stress-activated kinase, protein kinase C, and the Akt pathway are implicated in VEGF-A-dependent endothelial function, including cell survival, proliferation, generation of nitric oxide, and the induction of angiogenesis. Induction of metalloproteinases, activation of focal adhesion kinase and of PI3-kinase are implicated in VEGF-A-induced endothelial cell migration. The important role of nitric oxide as a mediator of endothelial function in vivo links the receptor signaling network to other biological effects.

Persistent infection of Epstein-Barr virus-positive B lymphocytes by human herpesvirus 8.

In patients with Kaposi's sarcoma (KS), human herpesvirus 8 (HHV-8) can invariably be detected in KS tumor tissue and, at a lower frequency, in prostate tissue and peripheral blood B lymphocytes. Whereas the majority of KS spindle cells are latently infected by HHV-8, linear HHV-8 genomes characteristic for lytic infection are found predominantly in the peripheral blood cells of KS patients. In this study, we show that HHV-8 can stably infect B lymphocytes in vitro in the presence of Epstein-Barr virus (EBV). We were able to generate immortalized HHV-8(+)/EBV+ lymphoblastoid cell lines (LCLs) derived from peripheral blood mononuclear cells (PBMC) of EBV- and EBV+ donors. In HHV-8(+)/EBV+ LCLs, which have the phenotype of activated B lymphocytes (CD19(+), surface immunoglobulin M, CD23(+), CD30(+), CD80(+)), HHV-8 was still present after more than 25 passages (more than 9 months of culture). Latent viral transcripts and proteins were present in nonstimulated HHV-8(+)/EBV+ LCLs. After induction by phorbol ester and n-butyrate, HHV-8(+)/EBV+ LCLs expressed lytic HHV-8 transcripts and proteins. Moreover, HHV-8 could be serially passaged from HHV-8(+)/EBV+ LCLs to fresh PBMC.

Characterization of the Borna disease virus phosphoprotein, p23.

Borna disease virus infection is diagnosed by the presence of serum antibodies reactive with the major viral proteins, p40 and p23. Although p40 and p23 are unrelated in amino acid sequence structure, cross-reactive antibodies are described. Protein fragments and synthetic peptides were analyzed to characterize the specificities of antibodies to p23. Epitope mapping revealed eight continuous epitopes accessible on the surface of a predicted structural model for the monomeric and the disulfide-linked dimeric forms of p23. None of these epitopes was reactive with antibodies to p40. Cross-reactivity with monospecific sera and monoclonal antibodies to p40 was found for one discontinuous epitope located at the amino terminus of p23.

Neutralizing antibodies in Borna disease virus-infected rats.

Borna disease is a neurologic syndrome caused by infection with a nonsegmented, negative-strand RNA virus, Borna disease virus. Infected animals have antibodies to two soluble viral proteins, p40 and p23, and a membrane-associated viral glycoprotein, gp18. We examined the time course for the development of neutralization activity and the expression of antibodies to individual viral proteins in sera of infected rats. The appearance of neutralizing activity correlated with the development of immunoreactivity to gp18, but not p40 or p23. Monospecific and monoclonal antibodies to native gp18 and recombinant nonglycosylated gp18 were also found to have neutralizing activity and to immunoprecipitate viral particles or subparticles. These findings suggest that gp18 is likely to be present on the surface of the viral particles and is likely to contain epitopes important for virus neutralization.

Enzyme-linked immunosorbent assay for detecting antibodies to Borna disease virus-specific proteins.

Borna disease virus is a unique neurotropic RNA virus that causes neurologic disease in a wide variety of animal hosts. We established an enzyme-linked immunosorbent assay for the detection of antibodies to Borna disease virus on the basis of the use of three recombinant viral proteins (recp40, recp23, and recp18). This assay system is more sensitive and rapid than the methods currently used for the serologic diagnosis of infection such as Western blotting (immunoblotting), indirect immunofluorescence test, or immunoprecipitation.

Characterization of a Borna disease virus glycoprotein, gp18.

Borna disease virus is a nonsegmented negative-strand RNA virus that causes neurologic disease in a wide variety of animal hosts. Here we describe identification and characterization of the first glycoprotein in this viral system. The 18-kDa glycoprotein, gp18, has been purified from infected rat brain. Isolation and microsequencing of this protein allowed identification of a 16.2-kDa open reading frame in the viral antigenome. Lectin binding and endoglycosidase sensitivity assays indicate that gp18 is an unusual N-linked glycoprotein.