A mutant allele of the SUP45 (SAL4) gene of Saccharomyces cerevisiae shows temperature-dependent allosuppressor and omnipotent suppressor phenotypes

Using a plasmid-based termination-read-through assay, the sal4-2 conditional-lethal (temperature-sensitive) allele of the SUP45 (SAL4) gene was shown to enhance the efficiency of the weak ochre suppressor tRNA SUQ5 some 10-fold at 30°C. Additionally, this allele increased the suppressor efficiency of SRM2-2, a weak tRNA^Gln ochre suppressor, indicating that the allosuppressor phenotype is not SUQ5-specific. A sup ⁺ sal4-2 strain also showed a temperature-dependent omnipotent suppressor phenotype, enhancing readthrough of all three termination codons. Combining the sal4-2 allele with an efficient tRNA nonsense suppressor (SUP4) increased the temperature-sensitivity of that strain, indicating that enhanced nonsense suppressor levels contribute to the conditional-lethality conferred by the sal4-2 allele. However, UGA suppression levels in a sup ⁺ sal4-2 strain following a shift to the non-permissive temperature reached a maximum significantly below that exhibited by a non-temperature sensitive SUP4 suppressor strain. Enhanced nonsense suppression may not therefore be the primary cause of the conditional-lethality of this allele. These data indicate a role for Sup45p in translation termination, and possibly in an additional, as yet unidentified, cellular process.     

CD44 stimulation down-regulates Fas expression and Fas-mediated apoptosis of lung cancer cells

Cytotoxic T lymphocytes (CTL) play a major role in the rejection of tumor cells, but tumor rejection does not always occur in vivo, indicating that defects in anti-tumor immune responses may be common. We here document a novel function for CD44--using lung cancer cells, we showed that stimulation of CD44 reduced Fas expression and Fas-mediated apoptosis: (i) lung cancer cells expressed high levels of CD44; (ii) engagement of CD44 on the cells by a specific antibody or fragmented hyaluronan reduced Fas expression; (iii) CD44 cross-linking reduced Fas-mediated apoptosis; (iv) stimulation of CD44 on lung cancer cells decreased IFN-gamma production by autologous CTL; and (v) CD44 stimulation prevented killing of lung cancer cells by autologous CTL. Based on these findings, we postulate a new concept--that interaction of CD44 on lung cancer cells with fragments of extracellular hyaluronan present in the surrounding extracellular matrix reduces Fas expression as well as Fas-mediated apoptosis of cancer cells. This leads to reduced susceptibility of the cells to CTL-mediated cytotoxicity through the Fas-Fas ligand pathway.     

Absence of Epstein-Barr virus-specific, HLA class II-restricted CD4+ cytotoxic T lymphocytes in infectious mononucleosis.

Cytotoxic T lymphocytes (CTL) with the CD4+ phenotype that recognize major histocompatibility complex (MHC) class II antigens are detectable very frequently in cultures of human alloreactive or virus-specific T cells. The significance of these CD4+ CTL for an immune reaction in vivo is not clear. Since Epstein-Barr virus (EBV) transformed B cells express HLA-class I and class II antigens equally well both CD8+ and CD4+ CTL should be stimulated during an acute EBV infection. We analysed the MHC specificity and the phenotype of EBV-specific CTL from patients with infectious mononucleosis (IM). When tested directly without any previous culture, T cells from patients in the acute phase of IM showed specific MHC-restricted cytotoxicity against the autologous B cell line. Addition of a HLA class I specific monoclonal antibody (MoAb) but not of a HLA class II specific MoAb resulted in a complete blocking of the lytic activity. Cell sorting revealed that the entire cytotoxic activity was present in the CD8+ fraction whereas no specific CTL were detectable in the CD4+ fraction. The absence of cytotoxicity in CD4+ cells was not due to a lack of activation of these cells since both CD8+ and CD4+ cells were activated in situ, showing spontaneous growth in interleukin-2 (IL-2) and expressing the activation marker TP103. Frequency estimation revealed that 1/300-1/600 CD8+ but only 1/2000-1/4000 CD4+ T cells gave rise to a specific CTL colony after 10 days. If CD4+ colonies were tested repeatedly for cytotoxicity we found that CD4+ CTL acquired their cytotoxicity during in vitro culture. In addition, we isolated EBV-specific CD4+ T cell clones able to lyse their stimulator cells in the presence but not in the absence of lectin, even after a long period of culture. Taken together our results show that cytotoxicity mediated by CD4+ T cells does not play a role in an anti-viral immune response.     

肺腺癌细胞微环境及转移与黏附分子表达的关系

目的研究肺腺癌细胞生长环境及转移性与黏附分子CD44v6和CD29的表达关系。方法将起源相同、转移性不同的两个肺腺癌细胞系AGZY和Anip分别用简便肿瘤多细胞球体(MTS)培养法培养，并设常规单层贴壁细胞培养对照。通过倒置显微镜、扫描及透射电镜观察MTS形成情况，并用免疫组化法分别对MTS及贴壁细胞上CD44v6和CD29表达进行检测。结果MTS培养成功，贴壁细胞与MTS在细胞结构及细胞连接结构上相似，两种MTS在形态及结构上差异无显著性。免疫组化结果显示，CD29在高转移性的Anip细胞及其MTS上呈阳性表达；在低转移性的AGZY细胞及其MTS上阴性表达。CD44v6在Anip和AGZY细胞及MTS上均呈阳性表达，差异无显著性。贴壁细胞与MTS上两种黏附分子表达均无差异。结论成功建立了一种简易制备MTS的方法。细胞生长方式(单层贴壁与MTS)可能不影响CD44v6和CD29的表达。CD29表达可能与肺腺痛转移性相关；CD44v6表达可能与肺腺癌转移无关。     

TGF-β1 Null Mutation Leads to CD154 Upregulated Expression in Affected Tissues

The TGF-1(–/–) mouse is a murine model for systemic autoimmune disease. The aim of this study is to elucidate the immunological mechanism that leads to multifocal tissue inflammation and autoantibody production in TGF-1(–/–) mice. Heart, lung, liver, and salivary gland from TGF-1(–/–) were assessed for CD154 expression by RT-PCR and immunohistochemistry. Compared to wild-type littermates, CD154 expression was elevated in all tissues studied. Furthermore, IL-12 mRNA was expressed in the salivary gland and heart of TGF-1(–/–) mice and not in wild-type littermates. This suggests that the CD154 pathway is activated in these tissues. This shows that TGF-1 regulates CD154 expression leading to spontaneous IL-12 production and autoimmunity.     

Enhancement of cALL immunogenicity by co-culture with a CD154 expressing 293 cell line

Pre-B cell acute lymphoblastic leukaemia (cALL) commonly occurs in young patients and although successful conventional therapies are available (such as cytotoxic drugs and bone marrow transplantation) for a proportion of patients (approximately 30%) these are ultimately unsuccessful. Recurrence of disease is a result of the failure of the immune system to recognize these abnormal cells and down-regulation of crucial molecules required for cognate CD4(+) T cell recognition has been postulated as a means of immune escape. In this study we show that an embryonic kidney cell line (293 cells) transfected with CD154 (40 L.1) are capable of not only maintaining the viability of primary ALL cells in culture but can also up-regulate the expression of a number of crucial molecules involved in antigen recognition. We show that 40 L.1 cell stimulation of primary ALL cell cultures can not only enhance the allogeneic and autologous MLR response to such cells but will also induce CTL effectors which are capable of lysing wild-type autologous ALL cells. It is therefore conceivable that such an approach could be used to generate an active anti-tumour response in patients, following conventional therapy, reducing the incidence of recurrence.     

Activation and internalization of p56lck upon CD45 triggering of Jurkat cells

Relationships between CD45 and p56^Ick have been suggested by co-immunoprecipitation of both proteins and by dephosphorylation of the p56^lck regulatory site, Tyr 505, by CD45 in vitro. We investigated whether the kinase activity of p56^lck is modulated in T cells triggered via CD45. We showed that incubation of Jurkat cells with a combination of two anti-CD45 monoclonal antibodies (mAb) (MC5/2 + D3/9) induced an increase in p56^lck kinase activity, while a single mAb did not. Under these conditions, p56^lck underwent two consecutive waves of activation. This was accompanied by internalization of the kinase and by a time-dependent increased accessibility of CD45 phosphatase at the plasma membrane. Similarly, activation and internalization of p56^lck were observed using a combination of anti-CD45 (MC5/2) and anti-CD2(T11₂) mAb, suggesting that a functional complex consisting of CD45, CD2 and p56^lck was formed upon cell triggering. Taken together, these results suggests that: (i) CD45 participates in the regulation of p56^lck kinase activity in vivo and that (ii) CD45 could play a mediator role in the stimulation and endocytosis of p56^lck through the CD2 pathway.     

Expression of a 32-kDa ligand for the CD40 antigen on activated human T lymphocytes

To identify the ligand(s) of the human CD40 antigen, a cDNA encoding the extracellular domain of the CD40 antigen was fused to a cDNA encoding the constant region (Fc) of human IgGl. The CD40-Fc fusion protein was able to specifically bind to CD4⁺ and various CD8⁺ T cell clones activated with immobilized anti-CD3. The ¹²⁵I-labeled CD40-Fc fusion protein bound anti-CD3 activated CD4⁺ T cell clone (MT9) with an equilibrium dissociation constant (Ka) of 10-20 nM. The human CD40-binding protein expressed on the cell surface of activated T lymphocytes is a monomeric protein of ≈ 32 kDa. Minor components of 29 kDa and 17 kDa were also detected. A small proportion of CD4+ and CD8⁺ blood mononuclear T cells activated by anti-CD3 expressed the CD40 ligand but its detection was best observed following depletion of B cells. Addition of B cells to purified T cells abolished the binding of CD40-Fc obtained after anti-CD3 activation.     

Subpopulations of CD4+ cells in lpr/lpr mice: differences in expression of T cell receptor/CD3 complex and proliferative responses.

CD4+ cells from autoimmune-prone C57BL/6 lpr/lpr mice contain two subpopulations, B220-CD4+ and B220+CD4+ cells. Highly purified B220-CD4+ cells from C57BL/6 +/+ and lpr/lpr mice were examined by comparing functional characteristics and expression of cell surface antigens and T cell receptor (TcR)/CD3 complex. Both lpr B220+CD4+ and B220+CD4-CD8- cells, most of which were PgP-1 positive, expressed TcR/CD3 complex on the cell surface at lower level as compared with B220-CD4+ cells of age-matched normal mice. In addition, the B2200-CD4+ cells were heterogeneous on the basis of surface expression of PgP-1 and CD3 antigens. Normal levels of TcR C alpha-, C beta- and V beta 8-specific mRNA were found in the B220-CD4+ cells and B220+CD4+ cells as compared with normal B220-CD4+ cells, while V beta 8-specific mRNA was preferentially expressed only by B220+CD4-CD8- cells. Either B220+CD4+ cells and B220+CD4-CD8- cells failed to respond to anti-CD3 monoclonal antibody (MoAb) as assessed by proliferative responses and production of interleukin-2 (IL-2). However, appreciable levels of reactivity to anti-CD3 MoAb were detected in the B220-CD4+ cells, although the responsiveness of this subset to such stimuli were reduced, compared with those of normal control. These results indicate that the B220-CD4+ cells in lpr mice are phenotypically and functionally distinct from normal B220-CD4+ cells.     

CD40-mediated lymphotoxin α expression in human B cells is tyrosine kinase dependent

The cytokine lymphotoxin (LT)α is known to play a role in B cell activation. As the engagement of the B cell antigen CD40 is known to lead to B cell proliferation and differentiation, we studied LTα expression in human B cells after CD40 ligation. We demonstrate that anti-CD40 monoclonal antibody (mAb) induces strong LTα mRNA and surface expression in human tonsil B cells. Induction of LTα mRNA and surface expression by CD40 ligation is inhibited by the protein tyrosine kinase (PTK) inhibitors herbimycin and genistein in a dose-dependent manner. The protein kinase C (PKC)-specific inhibitors sphingosine and bisindolylmaleimide caused negligible inhibition of anti-CD40-induced LTα mRNA and surface expression. No inhibition is observed with the protein kinase (PKA) inhibitors H89 and HA1004. Cross-linking of the transmembrane phosphatase CD45 to CD40 by using goat-anti-mouse F(ab')₂ fragments strongly inhibits CD40-mediated LTα expression in human B cells, confirming the role of PTK activation in CD40-mediated induction of LTα expression. Inhibitors of the serine/threonine protein phosphatases PP1 and PP2A, okadaic acid and calyculin induce LTα mRNA expression. In contrast, cyclosporin A, an inhibitor of the serine/threonine phosphatase calcineurin has no effect on anti-CD40-induced LTα expression. These results suggest that induction of LTα expression in B cells following engagement of CD40 involves activation of protein tyrosine kinases.