A mutant cell with a novel defect in MHC class I quality control

COS7 (African Green Monkey kidney) cells stably transfected with the mouse MHC class I allele H-2K(b) were mutagenized, selected for low surface expression of endogenous MHC class I products, and subcloned. A mutant cell line, 4S8.12, expressing very low surface MHC class I (approximately 5% of parental levels) was identified. This cell line synthesized normal levels of the MHC class I H chain and beta(2)-microglobulin, as well as normal levels of TAP, tapasin, GRP78, calnexin, calreticulin, ERp57, and protein disulfide isomerase. Full-length OVA was processed to generate presented H-2K(b)-SIINFEKL complexes with equal efficiency in wild-type and mutant cells, demonstrating that proteasomes, as well as TAP and tapasin, functioned normally. Therefore, all the known components of the MHC class I Ag presentation pathway were intact. Nevertheless, primate (human and monkey) MHC class I H chain and beta(2)-microglobulin failed to associate to form the normal peptide-receptive complex. In contrast, mouse H chains associated with beta(2)-microglobulin normally and bound peptide at least as well as in wild-type cells. The 4S8.12 cells provide strong genetic evidence for a novel component in the MHC class I pathway. This as-yet unidentified gene is important in early assembly of primate, but not mouse, MHC class I complexes.

Anti-peptide antibody blocks peptide binding to MHC class I molecules in the endoplasmic reticulum

The finding that MHC class I molecules are physically associated with the TAP transporter has suggested that peptides may be directly transported into the binding groove of the class I molecules rather than into the lumen of the endoplasmic reticulum (ER) where they subsequently would encounter class I molecules by diffusion. Such a mechanism would protect peptides from peptidases in the ER and/or escaping back into the cytoplasm. However, we find that an anti-peptide Ab that is cotranslationally transported into the ER prevents TAP-transported peptides from being presented on class I molecules. The Ab only blocks the binding of its cognate peptide (SIINFEKL) but not other peptides (KVVRFKDL, ASNENMETM, and FAPGNYPAL). Therefore, most TAP-transported peptides must diffuse through the lumen of the ER before binding stably to MHC class I molecules.

Expression of bcl-X(L) restores cell survival, but not proliferation off effector differentiation, in CD28-deficient T lymphocytes

Lymphocytes deficient in the T cell costimulatory molecule CD28 exhibit defects in cell survival, clonal expansion, and differentiation into effector cells. It is known that CD28-mediated signaling results in the upregulation of the Bcl family member Bcl-X(L). To investigate the role that Bcl-X(L) plays in the various functions of CD28, we expressed Bcl-X(L) in CD28-deficient primary T lymphocytes using retrovirus-mediated gene transfer. T cells were activated in vitro and infected with Bcl-X(L) or control retroviruses; this method allows gene expression in activated, cycling cells. Expression of Bcl-X(L) in naive T cells was achieved by reconstitution of the immune system of lethally irradiated recipient mice with retrovirus-infected purified bone marrow stem cells from CD28(-/)- or wild-type donor mice. Our studies demonstrate that Bcl-X(L) prolongs the survival of CD28(-/)- T cells but does not restore normal proliferation or effector cell development. These results indicate that the various functions of CD28 can be dissociated, and provide an experimental approach for testing the roles of downstream signals in the functions of cellular receptors such as CD28.

A synthetic peptide derived from the tumor-associated protein mdm2 can stimulate autoreactive, high avidity cytotoxic T lymphocytes that recognize naturally processed protein

Studies in melanoma patients have shown that unaltered self proteins can function as targets for tumor-reactive CTL. Here, we have investigated in a murine model whether autoreactive CTL can be found against the widely expressed proteins cyclin D1, mdm2, and p53, which are frequently overexpressed in transformed cells. Sixteen MHC class I binding peptides were identified in these proteins, and seven of them consistently stimulated primary CTL in vitro. Avidity measurements revealed that the avidity of peptide-induced CTL differed by >1000-fold. The highest avidity CTL were induced by a peptide derived from mdm2. These CTL recognized target cells expressing mdm2 endogenously, while CTL generated against the remaining peptides were of lower avidity and did not recognize cells expressing relevant proteins endogenously. Generation of high avidity anti-mdm2 CTL required several cycles of peptide stimulation, suggesting that the CTL precursor frequency was low. The data show the normal T cell repertoire contains small numbers of potentially autoreactive CTL. Expansion of these CTL may lead to beneficial autoimmunity against tumors, but, equally, it may be the basis of detrimental autoimmune diseases.

A synthetic peptide derived from the tumor-associated protein mdm2 can stimulate autoreactive, high avidity cytotoxic T lymphocytes that recognize naturally processed protein

Studies in melanoma patients have shown that unaltered self proteins can function as targets for tumor-reactive CTL. Here, we have investigated in a murine model whether autoreactive CTL can be found against the widely expressed proteins cyclin D1, mdm2, and p53, which are frequently overexpressed in transformed cells. Sixteen MHC class I binding peptides were identified in these proteins, and seven of them consistently stimulated primary CTL in vitro. Avidity measurements revealed that the avidity of peptide-induced CTL differed by >1000-fold. The highest avidity CTL were induced by a peptide derived from mdm2. These CTL recognized target cells expressing mdm2 endogenously, while CTL generated against the remaining peptides were of lower avidity and did not recognize cells expressing relevant proteins endogenously. Generation of high avidity anti-mdm2 CTL required several cycles of peptide stimulation, suggesting that the CTL precursor frequency was low. The data show the normal T cell repertoire contains small numbers of potentially autoreactive CTL. Expansion of these CTL may lead to beneficial autoimmunity against tumors, but, equally, it may be the basis of detrimental autoimmune diseases.

Purification of egg-white allergens

Purification procedures for the four egg-white proteins ovomucoid, ovotransferrin, ovalbumin, and lysozyme are presented with reference to mechanistic studies at epitope levels of allergic reactions to these proteins. The applied procedures resulted in four preparations containing less than 0.1% contaminating proteins each. The purified protein preparations were characterized by SDS-PAGE and by crossed immunoelectrophoresis with polyclonal antibodies raised against an egg-white extract or the purified proteins. The necessity of these well-characterized proteins in studies on allergic reactions was shown by testing human sera in immunoblots of lysozyme, and by immunoblots of ovomucoid probing with antibodies against the proteins.

The Saccharomyces cerevisiae FKS1 (ETG1) gene encodes an integral membrane protein which is a subunit of 1,3-beta-D-glucan synthase

In Saccharomyces cerevisiae, mutations in FKS1 confer hypersensitivity to the immunosuppressants FK506 and cyclosporin A, while mutations in ETG1 confer resistance to the cell-wall-active echinocandins (inhibitors of 1,3-beta-D-glucan synthase) and, in some cases, concomitant hypersensitivity to the chitin synthase inhibitor nikkomycin Z. The FKS1 and ETG1 genes were cloned by complementation of these phenotypes and were found to be identical. Disruption of the gene results in (i) a pronounced slow-growth phenotype, (ii) hypersensitivity to FK506 and cyclosporin A, (iii) a slight increase in sensitivity to echinocandin, and (iv) a significant reduction in 1,3-beta-D-glucan synthase activity in vitro. The nucleotide sequence encodes a 215-kDa polypeptide predicted to be an integral membrane protein with 16 transmembrane helices, consistent with previous observations that the etg1-1 mutation results in echinocandin-resistant glucan synthase activity associated with the nonextractable membrane fraction of the enzyme. These results suggest that FKS1 encodes a subunit of 1,3-beta-D-glucan synthase. The residual activity present in the disruption mutant, the nonessential nature of the gene, and results of Southern blot hybridization analysis point to the existence of a glucan synthase isozyme.

Calcineurin-dependent growth of an FK506- and CsA-hypersensitive mutant of Saccharomyces cerevisiae

The immunosuppressants FK506 and cyclosporin A (CsA) bound to their receptors, FKBP12 or cyclophilin, inhibit the Ca2+/calmodulin-dependent protein phosphatase, calcineurin, preventing T cell activation or, in yeast, recovery from alpha-mating factor arrest. Vegetative growth of yeast does not require calcineurin, and in strains sensitive to FK506 or CsA, growth is inhibited by concentrations of drug much higher than those required to inhibit T cell activation or recovery from mating factor arrest. We now describe the isolation of a mutant of Saccharomyces cerevisiae which is 100-1000-fold more sensitive to the growth inhibitory properties of these drugs. The mutation (fks1) also confers a slow growth phenotype which is partially suppressed by exogenously added Ca2+ and exacerbated by EGTA. Simultaneous disruption of the two genes (CNA1 and CNA2) encoding the alternative forms of the catalytic A subunit of calcineurin, or of the gene (CNB1) encoding the regulatory B subunit, is lethal in an fks1 mutant. Disruption of the gene encoding FKBP12 (FKB1) or the major, cytosolic cyclophilin (CPH1) in fks1 cells results in the loss of hypersensitivity to the relevant drug. Overexpression of CNA1 or CNA2, in conjunction with CNB1, results in a significant decrease in hypersensitivity to FK506 and CsA. The results show that the hypersensitivity of the fks1 mutant is due to the inhibition of calcineurin phosphatase activity by the receptor-drug complexes. The growth dependence of the mutant on the Ca2+/calcineurin signal pathway provides an important tool for studying in yeast certain aspects of immune suppression by these drugs.

Calcineurin mediates inhibition by FK506 and cyclosporin of recovery from alpha-factor arrest in yeast

The structurally unrelated immunosuppressants FK506 and cyclosporin A (CsA) act similarly, inhibiting a Ca(2+)-dependent signal required for interleukin-2 transcription and T-cell activation. Each drug binds to its cytosolic receptor, FKBP-12 and cyclophilin, respectively, and the drug-receptor complexes inhibit the Ca2+/calmodulin-dependent protein phosphatase, calcineurin. In yeast, calcineurin has been implicated in recovery from alpha-mating factor arrest. Here we show that FK506 bound to yeast FKBP-12 appears to form a complex with yeast calcineurin. Moreover, recovery from mating factor arrest is highly sensitive to FK506 or CsA, and this sensitivity requires the presence of FKBP-12 or cyclophilin, respectively. These results define a key physiological target of an FK506- and CsA-sensitive signal pathway in yeast, suggest a high degree of mechanistic conservation with mammalian cells, and indicate that further examination of the yeast system should provide insight into the same process in T cells.

Yeast FKBP-13 is a membrane-associated FK506-binding protein encoded by the nonessential gene FKB2

The immunosuppressants FK506 and rapamycin prevent T-cell activation and also inhibit the growth of certain strains of the yeast Saccharomyces cerevisiae. It has previously been shown that yeast contains a 12-kDa cytosolic FK506-binding protein (yFKBP-12), which also possesses peptidylprolyl cis-trans isomerase activity, and that fkb1 strains lacking yFKBP-12 are resistant to rapamycin and sensitive to FK506. The absence of yFKBP-12 permitted the detection and isolation of a second FK506- and rapamycin-binding protein, which is about 13 kDa in size (yFKBP-13) and membrane-associated. Purified yFKBP-13 binds FK506 with 15-fold lower affinity than yFKBP-12 and has peptidylprolyl cis-trans isomerase activity with a similar substrate profile. The sequence of the first 37 N-terminal amino acids was determined, and the yFKBP-13 gene (FKB2) was cloned and sequenced. A hydrophobic putative signal sequence precedes the N terminus of the mature protein. yFKBP-13 most closely resembles the membrane-associated human FKBP-13, which also possesses a signal peptide, whereas yFKBP-12 most closely resembles human FKBP-12. fkb2 and fkb1 fkb2 mutants are viable and unaltered in their sensitivity to FK506, suggesting that yeast possesses an additional target for this drug. Furthermore, fkb2 null mutations confer no change in rapamycin sensitivity. These findings show that yFKBP-13 and yFKBP-12 have distinct functions within the cell.

Body contact swimming rescues--what are the risks?

A survey of 482 lifeguards and Water Safety Instructors revealed that at least one-half had performed body contact swimming rescues (BCRs). Over two-thirds of these BCRs occurred when the lifesaver was relatively inexperienced in actual water rescues of any kind although virtually all rescuers had received formal lifesaving training. The victim tried to grasp the rescuer in 26 per cent of the BCRs which took place in swimming pools and 39 per cent of those occurring in lakes. Most of these attempts were made during the approach and were predominantly from the front, around the rescuer's head and neck or wrists and arms. Since the majority of victims were younger and smaller than their rescuers and got into difficulty less than 3 m from safety, only a small proportion of the grasps were perceived as endangering the lifesaver. Consequently, the victim was often permitted to retain the hold while being rescued. Grasps which were reported to be threatening tended to be applied by individuals of equal or larger physical size than the rescuer and usually occurred at distances exceeding 3 m from safety.