Reverse genetics in zebrafish.

The zebrafish has become a popular model system for the study of vertebrate developmental biology because of its numerous strengths as a molecular genetic and embryological system. To determine the requirement for specific genes during embryogenesis, it is necessary to generate organisms carrying loss-of-function mutations. This can be accomplished in zebrafish through a reverse genetic approach. This review discusses the current techniques for generating mutations in known genes in zebrafish. These techniques include the generation of chromosomal deletions and the subsequent identification of complementation groups within deletions through noncomplementation assays. In addition, this review will discuss methods currently being evaluated that may improve the methods for finding mutations in a known sequence, including screening for randomly induced small deletions within genes and screening for randomly induced point mutations within specific genes.

Positive selection of thymocytes induced by gene transfer: MHC class II-mediated selection of CD8 lineage cells.

Recombinant adenovirus vectors are powerful tools for inducing de novo gene expression in vivo. Here we have exploited them to study the specificity of CD4/CD8 lineage commitment during thymocyte positive selection, transferring MHC class II genes directly into thymi of mice deficient in both class I and II molecules. Expression of class II molecules was induced on cortical stroma, provoking the selection of a large population of mature CD4(+)CD8(-) cells, as expected, but also of a significant number of CD4(-)CD8(+) cells. The latter constituted a diverse population, containing both immature precursors and, though less frequent, cells that were mature according to several criteria. CD4(-)CD8(+) cells appeared with the same kinetics as their CD4(+)CD8(-) counterparts, but tended to be more prevalent at early times or when thymocyte reconstitution was only modest. These observations, derived from a dynamic selection system, indicate that CD4/CD8 lineage commitment is not irredeemably linked to the class of MHC molecule driving positive selection, a conclusion most compatible with selective models of commitment.

Targeted complementation of MHC class II deficiency by intrathymic delivery of recombinant adenoviruses.

De novo differentiation of CD4+ T cells was provoked in mice lacking major histocompatibility complex (MHC) class II molecules by intrathymic injection of adenovirus vectors carrying class II genes. This permits a new approach to questions concerning the dynamics of CD4+ T cell compartments in the thymus and peripheral lymphoid organs. Here two issues are explored. First, we show that mature CD4+ CD8- cells reside in the thymus for a protracted period before emigrating to the periphery, highlighting the potential importance of, and our ignorance of, the postselection maturation period. Second, we demonstrate that the survival of CD4+ cells in peripheral lymphoid organs is markedly curtailed when class II molecules are absent and is not further reduced in the absence of both class II and class I molecules, raising the possibility that MHC-mediated selection may continue in the periphery.

Positive selection of T cells induced by viral delivery of neopeptides to the thymus.

The relation between an antigenic peptide that can stimulate a mature T cell and the natural peptide that promoted selection of this cell in the thymus is still unknown. An experimental system was devised to address this issue in vivo-mice expressing neopeptides in thymic stromal cells after adenovirus-mediated delivery of invariant chain-peptide fusion proteins. In this system, selection of T cells capable of responding to a given antigenic peptide could be promoted by the peptide itself, by closely related analogs lacking agonist and antagonist activity, or by ostensibly unrelated peptides. However, the precise repertoire of T cells selected was dictated by the particular neopeptide expressed.

Albumin as a sealant for a polyester vascular prosthesis: its impact on the healing sequence in humans.

OBJECTIVE: Although the healing characteristics of albumin impregnated vascular prostheses have been extensively studied in animal models, they have never been studied in humans. We therefore examined the healing sequence and the albumin degradation rate of this type of prosthesis harvested from humans. We also addressed the possible relationship between the implantation of cross-linked albumin and a specific inflammatory reaction. METHODS: Thirty albumin-impregnated polyester vascular prostheses were collected in our institution from January 1991 to February 1993. The mean duration of implantation of the prostheses was 8.4+/-9.7 (SD) months (range: 1 hour to 26 months). Twenty two prostheses were patent at the time of explantation and 4 had been thrombosed for less than 24 hours. In 18 cases, the prostheses were surgically removed because of a complication or a reoperation, and during an autopsy in 12 cases. Each harvested specimen was submitted to histological and immunohistochemical studies in order to demonstrate the presence of human albumin sealant, and to determine the inflammatory cell constituents. RESULTS: The albumin-impregnated prostheses were poorly infiltrated by healing tissues after 2 years of implantation. An external capsule was constantly observed after 2 months of implantation with a nonspecific chronic inflammatory reaction localized between the capsule and the polyester yarns. We observed large amounts of albumin sealant after 2 months, a gradual degradation with time, and traces after 2 years of implantation in humans. The luminal surface of the explant was mainly covered with organized fibrin. No histological signs of a specific inflammatory reaction were observed. CONCLUSIONS: The healing of the albumin impregnated prosthesis was poor and the degradation rate of the albumin sealant was significantly delayed, when compared to animal models. This difference in degradation rate could be related to interspecies differences of phagocytic cells enzymatic machinery. Finally, implantation of glutaraldehyde cross-linked albumin in humans is safe, since we observed an aspecific chronic foreign body inflammatory reaction.

Interference by Parvolex with chloride estimation by the Ag/AgCl method.

We demonstrate significant interference by Parvolex (acetylcysteine) with the Ag/AgCl method for chloride estimation. A study of four patients who had taken a paracetamol overdose and been treated with Parvolex implied this interference, and an in vitro study confirmed it. The interference was shown to decline upon storage of the serum at a similar rate in both the patient and in vitro studies. Suggested mechanism for these phenomena are given.

Zidovudine-resistant and -sensitive HIV-1 isolates from patients on drug therapy: in vitro studies evaluating level of replication-competent viruses and cytopathogenicity.

OBJECTIVE: To compare biological properties of zidovudine-resistant variants of HIV-1 isolated from subjects on long-term drug therapy with drug-sensitive parental isolates obtained from the same patients before initiation of treatment. METHODS: Clinical HIV-1 strains were isolated following co-incubation of patient peripheral blood mononuclear cells with mitogen-stimulated umbilical cord blood lymphocytes. Drug resistance was evaluated by infecting MT-4 cells pretreated with zidovudine and maintained under drug pressure. RESULTS: The drug-resistant phenotype remained stable, following many viral replication cycles in the absence of zidovudine. Drug-resistant variants contained fewer replication-competent viruses but were more cytopathogenic than their corresponding zidovudine-sensitive parental strains. CONCLUSIONS: These results suggest that drug-resistant strains possess biological properties that may differ from those of drug-sensitive variants of HIV-1.

Characterization of zidovudine resistant variants of HIV-1 isolated from patients on prolonged therapy.

As part of a clinical trial to assess zidovudine related toxic effects, the authors followed 48 initially asymptomatic individuals who received prolonged therapy with this drug at several tertiary care institutions. Blood samples had been obtained for viral isolation every three months and had yielded infectious HIV-1 progeny in over 94 percent of cases. After one year of therapy, over 30 percent of individuals had developed variants of HIV-1 that displayed in vitro resistance to zidovudine. Six of these zidovudine resistant variants of HIV-1 were compared with drug sensitive isolates obtained from the same subjects prior to initiation of treatment. The drug resistant variants were generally as infectious per mg viral protein for both susceptible T cell lines and peripheral blood mononuclear cells as the corresponding parental isolates from which they were derived. The drug resistance phenotype remained stable, in that zidovudine-insensitive species could still be identified, following many replication cycles in the absence of drug pressure. However, the percentage of zidovudine resistant viruses appeared to diminish in culture over time under such conditions. This was demonstrated by the fact that lower percentages of cells became infected in the presence of the drug, if the viruses used for infection had been propagated in the absence of the drug. In addition, higher concentrations of such viruses were required to initiate infection in the presence of the drug, and these viruses possessed lower IC50's for zidovudine. This suggests that zidovudine resistant variants of HIV-1 may be unlikely to possess any growth advantage in the absence of the drug.

Biological comparison of wild-type and zidovudine-resistant isolates of human immunodeficiency virus type 1 from the same subjects: susceptibility and resistance to other drugs.

We used a viral endpoint dilution assay to show changes in the proportion of zidovudine (azidothymidine; AZT)-resistant viruses within a heterogeneous mixture of human immunodeficiency virus type 1 (HIV-1) quasispecies isolated from patients on long-term AZT therapy. Several HIV-1 isolates, which could replicate in 10 microM AZT, were susceptible to both 2',3'-dideoxycytidine and a novel cytosine analog BCH-189, in which a sulfur atom replaces the 3' carbon of the pentose ring. In certain instances, cross-resistance was seen with 3'-didehydro-2',3'-dideoxythymidine. Although most strains of AZT-resistant HIV-1 displayed reduced susceptibility to 3'-azido-2',3'-dideoxyuridine, two strains were identified for which this was not the case.

Clinical significance and characterization of AZT-resistant strains of HIV-1.

A number of laboratories have now independently confirmed that zidovudine (AZT)-resistant strains of human immunodeficiency virus type 1 (HIV-1) may be isolated from patients undergoing prolonged therapy with this drug. In certain instances, such drug-resistant viral isolates have been obtained from patients with clinical acquired immune deficiency syndrome (aids), while in others, isolation of drug-resistant strains has been achieved in the case of HIV seropositive, asymptomatic subjects. Most of the evidence points to a series of mutations within the polymerase gene of HIV-1, which encodes viral reverse transcriptase, as being responsible for development of the drug-resistant phenotype. It further appears that over 50% of patients treated with AZT for periods longer than six months are likely to yield drug-resistant strains of HIV-1 in their circulation. Furthermore, the development of drug resistance soon after initiation of AZT therapy may potentially be correlated with the likelihood of AZT treatment failure. In several instances, cross resistance has been observed between AZT and other nucleosides being considered for potential therapy of HIV-1-associated disease.

Inhibition of HIV-1 transmission by interferon and 3'-azido-3'-deoxythymidine during de novo infection of promonocytic cells.

HIV-1 infection of promonocytic U937 cells was used to examined induction of IFN-alpha/beta gene expression and to determine the inhibitory effects of IFN-alpha 2 and/or AZT on de novo HIV-1 infection, initiated either by coculture of virus-shedding U9-IIIB cells with uninfected cells or by incubation of U937 cells with virus-containing supernatants. Usually 21-28 days were required to transmit virus to greater than 90% of the cell culture. HIV-1 infection did not stimulate constitutive production of endogenous IFN-alpha 1/alpha 2 or IFN-beta genes, although induction of IFN RNA was observed following coinfection with the paramyxovirus Sendai. Exogenous rIFN-alpha 2 treatment decreased the intracellular accumulation of viral RNA 5- to 20-fold as determined by Northern blot analysis and the extracellular levels of HIV-1 as measured by p24 ELISA antigen capture. The effect was most dramatic at a time coincident with the rapid increase in virus spread through the cell culture (Days 10-18). During the fourth week of infection, HIV-1 multiplication was able to overcome the IFN-induced block in virus spread. AZT was not effective in limiting virus spread in the cocultivation experiments. When U937 cells were infected with virus-containing supernatants from U9-IIIB cells, IFN and AZT acted in combination to limit the number of infected cells and to inhibit intracellular accumulation of viral RNA. These experiments suggest that subtle differences in the mode of virus transmission in monocytic cells may alter the efficacy of combination antiretroviral therapy.

AZT (zidovudine) may act postintegrationally to inhibit generation of HIV-1 progeny virus in chronically infected cells.

The reverse transcriptase enzyme of HIV-1 is known to be error-prone. We were interested in the possibility of isolating a variant HIV-1 strain that might be capable of replication in the presence of AZT, thought to act by antagonizing reverse transcriptase activity. Toward this end, chronically infected H-9 cells were exposed to various concentrations of AZT for at least 500 days. No mutant has yet arisen from such cultures, which continued to produce high levels of each of the viral proteins p24, p17, gp41, and gp51/66 in the presence of the drug. Notwithstanding such expression of viral antigens, culture fluids from these various AZT-treated cultures were not capable of infecting otherwise susceptible target cells. Electron microscopic observations of AZT-treated chronically infected H-9 cells indicated a lower production of viral structures, in comparison with control cultures. Furthermore, those particles seen at the plasma membrane of AZT-treated cells often appeared to be envelope-deficient. These data suggest that AZT may be able to interfere in some way with proper assembly and/or packaging of infectious progeny HIV-1 at the cell membrane, although other modes of action for a postintegrational effect of AZT cannot be excluded.

New CD4(+) cell line susceptible to infection by HIV-1.

Infection of a newly described human T lymphoid cell line, CEM-CL10, with three different variants of HIV-1 resulted in cytopathic effects followed by cell lysis. Following primary lytic infection, proviral DNA could not be detected by Southern blot analysis in the outgrowth of the surviving CEM-CL 10 cells 60 days after initial exposure to HIV-1. These surviving cells could be further grown as a separate line, derived from CEM-CL10, and were found to be resistant to subsequent infection by HIV-1. A marked decrease in CD4 antigen expression was observed in these latter cells but not of the CD3 and transferrin receptor antigens. This decline in cell surface CD4 expression was correlated with both an absence of specific CD4 mRNA and with changes in structure of the CD4 gene. Both the HIV-1-sensitive CEM-CL10 cell line and its CD4(-), HIV-1-resistant derivative line, will be made available to interested investigators.

Isolation of drug-resistant variants of HIV-1 from patients on long-term zidovudine therapy. Canadian Zidovudine Multi-Centre Study Group.

We determined whether drug-resistant variants of HIV-1 could be isolated from the peripheral blood mononuclear cells of 20 individuals with HIV infection (Centers for Disease Control groups II and III) on long-term zidovudine (AZT) therapy. Toward this end, zidovudine (10 microM) has been included in the tissue culture medium used to isolate HIV-1. Under these circumstances, virus with a zidovudine-resistant phenotype was successfully obtained in five out of 20 cases. This property of drug resistance appeared to be stable, and did not disappear upon extended replication of such virus in the absence of drug pressure. Drug-resistant virus could also be isolated from these subjects on subsequent occasions, but was not present in samples obtained prior to therapy. Replication of these zidovudine-resistant isolates in tissue culture was inhibited by each of four other nucleoside analogues. Thus, other drugs may be useful in controlling selective zidovudine-resistant variants of HIV-1.