DA Class Research Articles

The ENU-3 protein family members function in the Wnt pathway parallel to UNC-6/Netrin to promote motor neuron axon outgrowth in C. elegans

The axons of the DA and DB classes of motor neurons fail to reach the dorsal cord in the absence of the guidance cue UNC-6/Netrin or its receptor UNC-5 in C. elegans. However, the axonal processes usually exit their cell bodies in the ventral cord in the absence of both molecules. Strains lacking functional versions of UNC-6 or UNC-5 have a low level of DA and DB motor neuron axon outgrowth defects. We found that mutations in the genes for all six of the ENU-3 proteins function to enhance the outgrowth defects of the DA and DB axons in strains lacking either UNC-6 or UNC-5. A mutation in the gene for the MIG-14/Wntless protein also enhances defects in a strain lacking either UNC-5 or UNC-6, suggesting that the ENU-3 and Wnt pathways function parallel to the Netrin pathway in directing motor neuron axon outgrowth. Our evidence suggests that the ENU-3 proteins are novel members of the Wnt pathway in nematodes. Five of the six members of the ENU-3 family are predicted to be single-pass trans-membrane proteins. The expression pattern of ENU-3.1 was consistent with plasma membrane localization. One family member, ENU-3.6, lacks the predicted signal peptide and the membrane-spanning domain. In HeLa cells ENU-3.6 had a cytoplasmic localization and caused actin dependent processes to appear. We conclude that the ENU-3 family proteins function in a pathway parallel to the UNC-6/Netrin pathway for motor neuron axon outgrowth, most likely in the Wnt pathway.

ENU-3 is a new player in UNC-6/Netrin directed axon outgrowth and guidance in C. elegans

UNC-6/Netrin is an axon guidance cue that works through UNC-40/DCC/Frazzled to attract cells and neurons and through UNC-5 and UNC-40 to repulse cells and neurons during nervous system development. The gene encoding the novel protein ENU-3 was identified in a mutant that enhanced the axon outgrowth defects of the DA and DB classes of motor neurons in a strain lacking functional UNC-5. Mutant ENU-3 also enhanced the axon outgrowth and guidance defects of the ventrally directed processes of the AVM and PVM touch receptor neurons in a strain lacking functional UNC-40. ENU-3 also plays a role in adhesion of axons to the substratum as does UNC-40. The protein is a putative single pass transmembrane protein expressed throughout the nervous system whose possible biochemical roles are currently unknown.

A population synthesis study of the luminosity function of hot white dwarfs

We present a coherent and detailed Monte Carlo simulation of the population of hot white dwarfs. We assess the statistical significance of the hot end of the white dwarf luminosity function and the role played by the bolometric corrections of hydrogen-rich white dwarfs at high effective temperatures. We use the most up-to-date stellar evolutionary models and implement a full description of the observational selection biases to obtain realistic simulations of the observed white dwarf population. Our theoretical results are compared with the luminosity function of hot white dwarfs obtained from the Sloan Digital Sky Survey (SDSS), for both DA and non-DA white dwarfs. We find that the theoretical results are in excellent agreement with the observational data for the population of white dwarfs with hydrogen deficient atmospheres (non-DA white dwarfs). For the population of white dwarfs with hydrogen-rich atmospheres (white dwarfs of the DA class), our simulations show some discrepancies with the observations for the brightest luminosity bins. These discrepancies can be attributed to the way in which the masses of the white dwarfs contributing to this luminosity bin have been computed, as most of them have masses smaller than the theoretical lower limit for carbon-oxygen white dwarfs. We conclude that the way in which the observational luminosity function of hot white dwarfs is obtained is very sensitive to the particular implementation of the method used to derive the masses of the sample. We also provide a revised luminosity function for hot white dwarfs with hydrogen-rich atmospheres.

ENU‐3 functions in an UNC‐6/netrin dependent pathway parallel to UNC‐40/DCC/frazzled for outgrowth and guidance of the touch receptor neurons inC. elegans

UNC-6 and SLT-1 guide the migrations of the ventrally directed processes of the AVM and PVM touch receptor neurons and UNC-6 guides the axons of the DA and DB classes of motor neurons in C. elegans. The UNC-6 receptors are UNC-5 and UNC-40. The axon outgrowth defects of a subset of the DB motor neurons in the absence of UNC-5 are enhanced by mutations in enu-3. An enu-3 mutation enhances defects in ventral guidance of the processes of the AVM and PVM touch receptor neurons, the dorsal guidance of the distal tip cell and causes additional architectural defects in axons in unc-40 mutant strains in an UNC-6 dependent manner. These observations suggest that ENU-3 and UNC-40 function in parallel pathways dependent on UNC-6. ENU-3 depends on the presence of UNC-40 for its full effect on motor neuron axon outgrowth. ENU-3 works in an UNC-6 dependent pathway parallel to UNC-40 in ventral guidance of AVM and PVM and in dorsal guidance of the distal tip cells. Motor neuron axon outgrowth defects are caused by the presence of UNC-40 and the absence of functional UNC-5 or UNC-6 and defects are enhanced by the absence of functional ENU-3.

THE 1.17 DAY ORBIT OF THE DOUBLE-DEGENERATE (DA+DQ) NLTT 16249

New spectroscopic observations show that the double degenerate system NLTT 16249 is in a close orbit (a = 5.6+/-0.3 R_sun) with a period of 1.17 d. The total mass of the system is estimated between 1.47 and 2.04 M_sun but it is not expected to merge within a Hubble time-scale (t_merge ~ 10^11 yr). Vennes & Kawka (2012, ApJ, 745, L12) originally identified the system because of the peculiar composite hydrogen (DA class) and molecular (C_2--DQ class--and CN) spectra and the new observations establish this system as the first DA plus DQ close double degenerate. Also, the DQ component was the first of its class to show nitrogen dredged-up from the core in its atmosphere. The star may be viewed as the first known DQ descendant of the born-again PG1159 stars. Alternatively, the presence of nitrogen may be the result of past interactions and truncated evolution in a close binary system.

HELIUM-CORE WHITE DWARFS IN THE GLOBULAR CLUSTER NGC 6397

We present results of a study of the central regions of NGC 6397 using Hubble Space Telescope's Advanced Camera for Surveys, focusing on a group of 24 faint blue stars that form a sequence parallel to, but brighter than, the more populated sequence of carbon-oxygen white dwarfs (CO WDs). Using F625W, F435W, and F658N filters with the Wide Field Channel we show that these stars, 18 of which are newly discovered, have magnitudes and colors consistent with those of helium-core white dwarfs (He WDs) with masses ~ 0.2-0.3 Msun. Their H-alpha--R625 colors indicate that they have strong H-alpha absorption lines, which distinguishes them from cataclysmic variables in the cluster. The radial distribution of the He WDs is significantly more concentrated to the cluster center than that of either the CO WDs or the turnoff stars and most closely resembles that of the cluster's blue stragglers. Binary companions are required to explain the implied dynamical masses. We show that the companions cannot be main-sequence stars and are most likely heavy CO WDs. The number and photometric masses of the observed He WDs can be understood if ~1-5% of the main-sequence stars within the half-mass radius of the cluster have white dwarf companions with orbital periods in the range ~1-20 days at the time they reach the turnoff. In contrast to the CO WD sequence, the He WD sequence comes to an end at R625 ~ 24.5, well above the magnitude limit of the observations. We explore the significance of this finding in the context of thick vs. thin hydrogen envelope models and compare our results to existing theoretical predictions. In addition, we find strong evidence that the vast majority of the CO WDs in NGC 6397 down to Teff ~ 10,000 K are of the DA class. Finally, we use the CO WD sequence to measure a distance to the cluster of 2.34 +- 0.13 kpc.

Extracellular Sugar Modifications Provide Instructive and Cell-Specific Information for Axon-Guidance Choices

Heparan sulfates (HSs) are extraordinarily complex extracellular sugar molecules that are critical components of multiple signaling systems controlling neuronal development. The molecular complexity of HSs arises through a series of specific modifications, including sulfations of sugar residues and epimerizations of their glucuronic acid moieties. The modifications are introduced nonuniformly along protein-attached HS polysaccharide chains by specific enzymes. Genetic analysis has demonstrated the importance of specific HS-modification patterns for correct neuronal development. However, it remains unclear whether HS modifications provide a merely permissive substrate or whether they provide instructive patterning information during development. We show here with single-cell resolution that highly stereotyped motor axon projections in C. elegans depend on specific HS-modification patterns. By manipulating extracellular HS-modification patterns, we can cell specifically reroute axons, indicating that HS modifications are instructive. This axonal rerouting is dependent on the HS core protein lon-2/glypican and both the axon guidance cue slt-1/Slit and its receptor eva-1. These observations suggest that a changed sugar environment instructs slt-1/Slit-dependent signaling via eva-1 to redirect axons. Our experiments provide genetic in vivo evidence for the "HS code" hypothesis which posits that specific combinations of HS modifications provide specific and instructive information to mediate the specificity of ligand/receptor interactions.

Wnt Signaling Positions Neuromuscular Connectivity by Inhibiting Synapse Formation in C. elegans

Nervous system function is mediated by a precisely patterned network of synaptic connections. While several cell-adhesion and secreted molecules promote the assembly of synapses, the contribution of signals that negatively regulate synaptogenesis is not well understood. We examined synapse formation in the Caenorhabditis elegans motor neuron DA9, whose presynapses are restricted to a specific segment of its axon. We report that the Wnt lin-44 localizes the Wnt receptor lin-17/Frizzled (Fz) to a subdomain of the DA9 axon that is devoid of presynaptic specializations. When this signaling pathway, composed of the Wnts lin-44 and egl-20, lin-17/Frizzled and dsh-1/Dishevelled, is compromised, synapses develop ectopically in this subdomain. Conversely, overexpression of LIN-44 in cells adjacent to DA9 is sufficient to expand LIN-17 localization within the DA9 axon, thereby inhibiting presynaptic assembly. These results suggest that morphogenetic signals can spatially regulate the patterning of synaptic connections by subdividing an axon into discrete domains.

Different in vivo tolerogenicity of MHC class I peptides.

The efficacy of MHC class I-derived peptides to induce tolerance was tested in a cardiac transplantation model. Two 25-mer peptides from the polymorphic region of the DA class I molecule (RT1.Aa) were synthesized by F-moc chemistry and injected intrathymically or intraperitoneally into LEW (RT1.1) responder animals. Intrathymic treatment of the recipient animals with peptide 1 (residues 56-80) accompanied by intraperitoneal treatment with peptide 4 (residues 96-120) led to indefinite survival of allogeneic DA cardiac allografts (n = 7; > 100 days). The tolerogenicity of both peptides differed according to the site of inoculation, as donor-specific tolerance was only observed after administration of peptide 1 into the thymus and injection of peptide 2 into the abdominal cavity of LEW recipients, but not vice versa. Donor-specific tolerance was confirmed in vivo by grafting of full-thickness skin and in vitro by appropriate proliferation and cytotoxicity assays using donor and third-party rats. Donor-specific tolerance was associated with up-regulation of interleukin-4, transforming growth factor beta, and interleukin-10 gene expression within cardiac allografts, thus suggesting intrathymic clonal deletion and external suppression with expansion of T-helper 2-type lymphocytes as the underlying mechanisms of tolerance induction.

Indirect T-cell allorecognition and the mechanisms of immunosuppression by allogeneic blood transfusions.

LEW rats given twice weekly intravenous transfusions of DA blood for 10 weeks showed a strong antibody response to intact DA class I antigens at day 7 that declined to undetectable levels by week 6. The response remained undetectable for the remainder of the course, in spite of the repeated transfusions of DA blood. At week 6 during the blood transfusion course, the LEW rats were immunised with a DA class I peptide known to be recognised by LEW CD4+T cells in a LEW APC-dependent manner. This resulted in the prompt reappearance of a strong antibody response to intact DA class I antigen. However, in vitro T-cell proliferation responses to peptide 1 appeared to be partially suppressed by the blood transfusions. Immunisation of LEW rats with this peptide 4 weeks before commencement of the course of DA blood transfusions prevented the decline in antibody levels normally seen during the blood transfusion course. These data indicate that the multiple blood transfusions are able to induce, in non-sensitised recipients, a reversible suppression of the indirect T-helper response specific for allogeneic peptides in the blood transfusion. Although our protocol of twice weekly transfusions does not correspond to the clinical pattern of blood transfusions, our results raise the possibility that antigenic cross-reactivity at the level of small polymorphic MHC peptides between blood and organ donors might represent the immunological basis for the beneficial effects of random blood transfusions.

Indirect T-cell allorecognition and the mechanisms of immunosuppression by allogeneic blood transfusions

Abstract. LEW rats given twice weekly intravenous transfusions of DA blood for 10 weeks showed a strong antibody response to intact DA class I antigens at day 7 that declined to undetectable levels by week 6. The response remained undetectable for the remainder of the course, in spite of the repeated transfusions of DA blood. At week 6 during the blood transfusion course, the LEW rats were immunised with a DA class I peptide known to be recognised by LEW CD4 + T cells in a LEW APC-dependent manner. This resulted in the prompt reappearance of a strong antibody response to intact DA class I antigen. However, in vitro T-cell proliferation responses to peptide 1 appeared to be partially suppressed by the blood transfusions. Immunisation of LEW rats with this peptide 4 weeks before commencement of the course of DA blood transfusions prevented the decline in antibody levels normally seen during the blood transfusion course. These data indicate that the multiple blood transfusions are able to induce, in non-sensitised recipients, a reversible suppression of the indirect T-helper response specific for allogeneic peptides in the blood transfusion. Although our protocol of twice weekly transfusions does not correspond to the clinical pattern of blood transfusions, our results raise the possibility that antigenic cross-reactivity at the level of small polymorphic MHC peptides between blood and organ donors might represent the immunological basis for the beneficial effects of random blood transfusions.

The prevention of graft-versus-host disease by the serum of liver retransplanted rats

The effect of serum from orthotopic liver retransplanted plants (re-OLT serum) on graft-versus-host disease (GVHD) was studied in rats. In the re-transplantation model of rat liver, orthotopic liver transplantation (OLT) was carried out in the DA (RT1 a) into PVG (RT1 c) combination; two days later the DA liver was removed and a new PVG liver implanted into the same recipient (re-OLT). In the in vivo GVHD model, male PVG rats were sublethally irradiated and injected intravenously with 3 × 10 8 DA or BN (RT1 n) spleen cells through the penial vein. Within 1 h of the inoculation, rats of the experimenta;l group were injected with 1 ml of re-OLT serum taken at postoperative day (POD) 7. Rats in the control group received 1 ml of normal PVG serum or syngeneic re-OLT inoculation of DA or BN spleen lymphocytes. However, when the animals were treated with re-OLT serum, 100% ( 6 6 ) of the rats survived more than 60 days, following inoculation with DA lymphocytes but now with BN lymphocytes. The POD 7 re-OLT serum showed a strong inhibition against DA anti-PVG mixed lymphocyte reaction (MLR), although re-OLT serum did not contain soluble DA class I antigens, anti-DA class I or II antibody. The potential GVHD inhibitory factors in re-OLT serum may be two unique immunosuppressive proteins, which have been detected by SDS PAGE and reported previously. We conclude that re-OLT serum has immunisuppressive factors, which, at least in part, prevented the induction of GVHD in rats.

T and B cell responsiveness to donor class I MHC molecules and peptides in long survivors with kidney allografts.

LEW rats with long-surviving (> 100 days) (DA x LEW)F1 kidney allografts were generated by treating the recipients with cyclosporine for 14 days after grafting. All rats were monitored after transplantation for the development of antibodies to intact donor class I MHC molecules. Cyclosporine completely suppressed the early antibody response to intact DA class I MHC molecules in all 19 LEW rats. However, 17 of the 19 rats developed antibodies between four and six weeks after grafting-i.e., between two and four weeks after the cessation of cyclosporine therapy, and maintained high levels of antibody to the donor class I molecules in spite of the long-term presence of the allograft. The 2 rats that did not produce antibodies to donor class I MHC molecules, along with one of the 17 that did produce antibodies, were immunized with a synthetic peptide corresponding to a region of the DA class I MHC molecule known to be recognized by LEW CD4+ T cells via the indirect recognition pathway. All 3 long survivors developed self APC-dependent CD4+ T cell proliferation to the immunizing donor peptides, and strong antibody responses to these peptides. However, none of these long survivors suffered rejection episodes as a consequence of the peptide immunization. In one of the two long-surviving rats without antibodies to intact donor class I MHC molecules at the time of peptide priming, the peptide priming resulted in the prompt development of strong antibodies to intact donor class I molecules. However, the other of these 2 rats did not produce such antibodies after peptide priming. Thus in this model of kidney allograft tolerance, with long-term exposure of the recipient's immune system to donor antigens without evidence of rejection, none of the animals develops tolerance for the indirect T cell recognition of donor class I MHC antigens. In occasional animals, B cells potentially reactive to intact donor class I molecules are present and are adequately exposed to antigen but are quiescent because of the absence of T cell help, perhaps as a consequence of reversible T cell suppression or anergy. In other occasional animals, B cell nonreactivity (anergy or tolerance) to intact donor class I molecules appears to develop.

A three-cell cluster hypothesis for noncognate T-B collaboration via direct T cell recognition of allogeneic dendritic cells.

In this article, we propose that T cell help for B cells can occur via an unusual three-cell cluster, with recipient CD4+ T helper cells interacting via direct allorecognition with donor dendritic cell class II MHC antigens, recipient B cells interacting with MHC class I (or any other) antigen on the donor dendritic cell surface, and noncognate (i.e., antigen nonspecific) T-B collaboration. In this noncognate pathway, antigen processing by B cells is not required and T cell help is potent because of the high precursor T cell frequency for direct recognition of allogeneic class II MHC molecules. The data supporting this hypothesis are: 1. LEW rat strain recipients of interstitial dendritic cell-free (DAxLEW)F1 kidney allografts were shown to have no detectable antibody to donor class I MHC antigens at day 7 after grafting. By contrast, LEW recipients of normal (DAxLEW)F1 kidneys had strong antibody responses. 2. Consistent wih important role for donor dendritic cells in the early antibody response to donor class I MHC antigens was the finding that it was dependent on donor class II MHC antigens. PVG recipients, previously immunized with pure DA RT1.B class II MHC antigens, had virtually no antibody response to the class I MHC antigens of DA kidney allografts. 3. We confirmed the low and high responder status of PVG and LEW rats, respectively, to DA class I antigens by studying antibody responses to pure DA class I antigens. However, PVG and LEW recipients of DA kidney allografts did not differ in their antibody response to the donor DA class I MHC antigens. This is consistent with this response not requiring the processing and presentation of DA class I antigen by PVG recipients. 4. LEW recipients of interstitial dendritic cell-free (DAxLEW)F1 kidney allografts did eventually develop a strong antibody response to DA class I antigens, but this was delayed by several weeks. That this delayed antibody response was probably mediated by conventional T-B collaboration and that T help was rate limiting in this situation, was demonstrated by immunizing LEW recipients with a DA class I peptide. This markedly accelerated the kinetics of the antibody response to the dendritic cell-free (DAxLEW)F1 kidneys.

INDUCTION OF NATURAL CHIMERISM AFTER RETRANSPLANTATION OF THE LIVER IN RATS

Immunological aspects after orthotopic rat liver retransplantation (re-OLT) were examined in association with cell migration and mixed chimerism. At day 2 after the first orthotopic liver transplantation (day 0) in the combination of DA (MHC haplotype, RT1a) donor into PVG (RT1c) recipient, the grafted DA liver was removed and a new PVG liver was implanted into the same PVG recipient (re-OLT). In the PVG recipient at various times after the re-OLT, DA-derived antigen and cells were detected using a DA-specific anti-class I mAb R3/13 in conjunction with ELISA, immunoblotting, and immunohistochemistry. The level of soluble class I antigen, which had risen to 270 ng/ml after the first OLT, substantially decreased within 24 hr after re-OLT. Using immunoblotting, DA class I antigen was detected in the PVG recipient's lymphoid organs at day 3 after DA liver grafting and persisted for up to 21 days after the DA liver was replaced by a new PVG liver. Immunohistochemistry on sections of spleen from re-OLT rats showed that the level of migratory cells expressing DA class I correlated with the findings obtained by immunoblotting. While the DA-derived antigen and cells were detected in the re-OLT recipient, the DA-specific inhibition of mixed lymphocyte reaction was observed in re-OLT serum. Our results suggest that the implanted DA liver graft was the source of DA soluble class I antigen, but DA-derived antigen and cells detected in the re-OLT recipient organs could persist for a relatively long time under immunosuppression after the implanted DA liver was removed by re-OLT.

Indirect T cell allorecognition: a cyclosporin A resistant pathway for T cell help for antibody production to donor MHC antigens

LEW (RT1 1) rats were primed for indirect allorecognition of DA (RT1 avl) MHC molecules by immunizing either with synthetic peptides corresponding to the polymorphic regions of the RT1.A avl classical class I MHC molecule, or with the isolated, denatured chains of the RT1.A class I, RT1.Bα class II and RT1.Bβ class II MHC molecules of the DA strain. These primed LEW rats received DA kidney allografts and were treated after grafting with cyclosporin A. Unprimed LEW controls mount a vigorous rejection response to DA kidney allografts and produce a strong antibody response to DA class I MHC antigens. Both the rejection and the antibody responses are virtually completely suppressed by cyclosporin A treatment in these controls. Priming to indirect allorecognition of donor MHC antigens did not diminish the effectiveness of cyclosporinA in suppressing the acute rejection of DA kidney grafts, but cyclosporin A could not suppress the early antibody response to the grafts in the primed rats. This findinf could be of importance in clinical transplantation, where antibody-mediated graft damage might play an important role in both acute vascular rejection and chronic rejection.

Immunohistochemical detection of strain-specific major histocompatibility complex class I antigens in paraffin-embedded rat osteochondral tissue

An immunohistochemical method using formalin-fixed, paraffin wax-embedded sections is described for detecting strain-specific major histocompatibility complex class I antigens in knee-joint tissue from DA and Lewis strains of rat. The fixed osteochondral tissues were additionally decalcified in formic acid before processing for paraffin wax embedding. For immunohistochemistry, two monoclonal antibodies, one specific for DA class I allele RT1Aa and the other for Lewis class I allele RT1A1, were used together with the avidin-biotin immunoperoxidase procedure. It was necessary to use strain-specific normal rat serum as a diluent for the antibodies to suppress cross-strain recognition. DA-specific antibody stained positively only on DA rat sections, not on Lewis rat sections, and Lewis-specific antibody stained positively only on Lewis rat sections, and not on DA. Positive staining was localized in the bone marrow, osteochondral cells and endothelium. We propose that the use of a decalcification medium may have enhanced the immunoreactivity of the tissue. The method described can be used on sections of allografts from the two strains of rat to assess morphologically the extent of cellular replacement of the graft by the host's cells.

SPECIFIC SUPPRESSION OF ALLOGRAFT REJECTION BY SOLUBLE CLASS I ANTIGEN AND COMPLEXES WITH MONOCLONAL ANTIBODY

In this experiment, we investigated the effect of daily injection or continuous slow infusion of either DA (MHC haplotype, RT1a) serum or soluble DA class 1 MHC antigen or its complexes with monoclonal antibody on rejection of heterotopic heart allograft in the combination of PVG.RT1a (RT1a) donor into PVG (RT1c) recipient. DA serum delayed significantly both the early and late rejection of PVG.RT1a heart grafts in PVG recipients. Removal of soluble class I MHC antigen from DA serum by affinity chromatography on a monoclonal anti-class I antibody column completely abolished the immunosuppressive effect. Continuous infusion of purified soluble class I antigen from DA rat liver, even from day 4 after heart grafting, induced a significant prolongation of graft survival. This effectiveness was donor-specific and amplified by the mixture of monoclonal anti-class 1 (RT1a) antibody with DA serum--this being induced only by using continuous infusion but not by daily injection. The results indicate that soluble class I antigen can act as a specific immunosuppressive agent in allograft rejection and that its effect is amplified by monoclonal anti-class 1 antibody.

A detailed analysis of the potential of water-soluble classical class I MHC molecules for the suppression of kidney allograft rejection and in vitro cytotoxic T cell responses.

Water-soluble classical (RT1-A) class I MHC molecules were purified from aqueous extracts of DA strain liver. Following monoclonal antibody affinity, lentil lectin affinity, and gel filtration chromatography, 600 micrograms of soluble RT1-A class I molecules with antigen activity equivalent to 1.3 x 10(11) nucleated DA spleen cells (greater than 500 DA spleens) was obtained. Both PVG and LEW strain recipients of DA kidney allografts were pretreated with intravenous injections of the DA soluble class I molecules, in doses with antigen activity equivalent to 10(8) nucleated DA spleen cells. Three protocols of pretreatment were used: twice-weekly injections for 4-5 weeks, with grafting 3 or 4 days after the last injection; a single injection 7 days pregraft; or a single injection 1 day pregraft. The PVG and LEW rats received the soluble class I pretreatment either alone or in combination with suboptimal doses (2 mg/kg/day) of cyclosporine after grafting, making a total of 12 experimental groups treated with soluble class I antigen. In no case did treatment with soluble class I antigen elicit an antibody response in prospective graft recipients; influence kidney graft survival in any way; or enhance or suppress the antibody response to the kidney graft. The soluble DA class I MHC molecules were tested in vitro for their effect on the generation and effector function of allospecific PVG and LEW anti DA RT1-A class I cytotoxic T cells and TNP specific, self RT1-Aa restricted cytotoxic T cells. Concentrations up to 5 micrograms/ml (10(-7) M), equivalent to 10(9) nucleated DA spleen cells/ml, were without any effect. We conclude that monomeric forms of water-soluble classical class I molecules are poor immunogens--and, at doses conventionally used for active enhancement, do not influence cytotoxic T cell responses and have little potential for donor-specific immunosuppression.

The Effects of Time-Dependent Convection on White Dwarf Radial Pulsations

AbstractAfter the discovery of pulsations in white dwarfs, predictions were made that these DA and the hotter DB stars should be pulsating in radial modes with periods of a few seconds or less. The mechanisms are the normal kappa and gamma effects that periodically block the flow of radiative luminosity and the blocking effect of the frozen-in convection at the bottom of the convection zone. Blue edges of the instability strips are between 12,000K and 13,000K for the DA and between 32,000K and 33,000K for the DB variables. Extensive observations, however, have shown that these stars pulsate only in the few-hundred-second nonradial modes and not in any few-second radial modes. We have added the time dependent convection model of Cox, Brownlee, and Eilers (1966) to our pulsation analyses to further investigate the white dwarf radial modes. Since the time scale of the convection is usually short compared to the radial pulsation periods, convection is able to carry luminosity rapidly enough to nullify the kappa and gamma effects periodic radiation blocking. We find that most, and maybe all, radial pulsations for 0.6 solar mass carbon-oxygen white dwarfs with thin hydrogen or helium surface layers are stabilized for both these DA and DB classes, now finally in agreement with observations.