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T L Horton

Publications and source records attributed to T L Horton.

At least 19 recordsLinked to original sources
PubMed2000

Mitochondrial RNAs of myxomycetes terminate with non-encoded 3' poly(U) tails.

We examined the 3' ends of edited RNAs from the myxomycetes Stemonitis flavogenita and Physarum polycephalum using a modified anchor PCR approach. Surprisingly, we found that poly(A) tails are missing from the cytochrome c oxidase subunit 1 mRNA (coI) from both species and the cytochrome c oxidase subunit 3 mRNA (cox3) from P. polycephalum. Instead, non-encoded poly(U) tails of varying length were discovered at the 3' ends of these transcripts. These are the first described examples of 3' poly(U) tails on mature mRNAs in any system.

3' Untranslated Regions↗
PubMed2000

Xenopus NK cells identified by novel monoclonal antibodies.

Early-thymectomized (Tx) Xenopus frogs, which are permanently deficient in T cells, are used as a model sytem for the characterization of novel monoclonal antibodies (mAb) which identify candidate NK cells at the amphibian level of evolution. Hybridomas, generated from mice immunized with splenocytes from Tx Xenopus following B cell and thrombocyte depletion, were screened by flow cytometry. Three mAb (1F8, 4D4 and 1G5) were identified that stained increased proportions of splenocytes from Tx compared with control frogs. These mAb identified lymphoid populations from Xenopus spleen, liver and gut which, after 48 h culture in growth factor-rich medium, exhibited spontanous killing of MHC-deficient allotumor targets. mAb-defined splenocytes also rapidly induced apoptosis of such tumor targets. Dual color analysis confirmed that NK cells are neither T nor B cells. Cytospins of splenocytes isolated with anti-NK mAb revealed large lymphoid cells with distinct pseudopodia. Immunohistology indicated each anti-NK mAb routinely labeled cells within the gut epithelium but NK cells were difficult to visualize in spleen sections. Western blotting of spleen, liver and intestinal lysates subjected to sodium dodecyl sulfate polyacrylamide gel electrophoresis showed that 1G5 reacted strongly with protein bands of approximately 70 - 85 kDa, whereas mAb 1F8 and 4D4 stained less intensely, but identified similar protein bands.

Animals↗
PubMed2000

Evolution of four types of RNA editing in myxomycetes.

The myxomycete Physarum polycephalum requires extensive RNA editing to create functional mitochondrial transcripts. The cytochrome c oxidase subunit 1 (col) transcript exhibits a combination of editing forms not found together in any other eukaryotic RNA: 66 insertions of ribonucleotides (59 Cs, a single U, and three mixed dinucleotides) as well as base conversion of four Cs to Us (Gott et al., J Biol Chem, 1993, 268:25483-25486). Through a phylogenetic survey of col DNA genes and RNA transcripts in representative myxomycetes, we have decoupled the four types of editing in this lineage. Some myxomycetes share insertional editing with P. polycephalum, yet lack C--> U conversion, consistent with previous reports of separation of insertional and base conversion editing in P. polycephalum extracts (Visomirski-Robic & Gott, RNA, 1995, 3:821-837). Most remarkably, we detect unique evolutionary histories of the three different types of insertional editing, though these have been indistinguishable in vitro. For example, Clastoderma debaryanum exhibits insertions of Us, but not Cs or dinucleotides.

Amino Acid Sequence↗
PubMed1998

T-cell and natural killer cell development in thymectomized Xenopus.

The Xenopus early-thymectomy model system is used to investigate the extent to which the thymus controls T-cell development and to probe the evolution of natural killer (NK) cells. Loss of T-cell function following thymectomy, together with the paucity of cells expressing monoclonal antibody-defined T-cell surface markers, and greatly reduced expression of T-cell receptor beta transcripts in spleen, liver and intestine, indicate that T-cell development in minimal in the absence of the thymus. Our findings therefore mitigate against the idea that a substantial extrathymic pathway of T-cell development exists in early vertebrate evolution. Rather, they suggest that in this amphibian representative T cells are predominately thymus dependent. In vitro studies with control and thymectomized Xenopus splenocytes reveal that a non-T/non-B population and also two T-cell subsets all display natural cytotoxicity towards allogeneic thymus lymphoid tumour cells (which are deficient in MHC antigen expression). Since Xenopus thymectomized early in larval development are permanently deficient in T cells, they may provide a useful phylogenetic model for the study of NK cells.

Animals↗
PubMed1996

NK-like activity against allogeneic tumour cells demonstrated in the spleen of control and thymectomized Xenopus.

This paper addresses the issue of natural killer (NK) cell evolution by searching for NK-like activity in an amphibian representative, the immunologically well-characterized clawed frog, Xenopus laevis. Using in vitro 6 h 51chromium release assays, we have shown that splenocyte effectors from early thymectomized (Tx) year-old frogs, but not from control siblings, are able to spontaneously lyse allogeneic thymus tumour cell lines that lack MHC antigen expression. Such lytic capacity can be readily induced in control Xenopus and elevated in Tx frogs by a single injection of tumour cells, with maximal splenocyte cytotoxicity occurring 3 days postinjection, the amount of 51Cr-release correlating directly with effector: target ratios. Splenocytes, even those from tumour-injected frogs, are unable to lyse allogeneic splenic lymphoblasts or erythrocyte targets, even when the latter are coated with IgY (the Xenopus IgG equivalent); moreover, we were unable to demonstrate any splenocyte-induced lysis of the human NK cell target K562. Lymphokine-activated killing (LAK) in Xenopus is suggested, since Tx splenocytes cultured in cytokine-rich medium (from concanavalin A-stimulated control splenocytes) display significantly elevated killing of allogeneic tumour targets. Flow cytometric analysis highlights the loss of T cell markers from the spleen of Tx frogs and reveals a variable staining pattern of both control and Tx splenocytes when treated with a mAb that binds to both fish non-specific cytotoxic cells and human NK cells. Prospects for identifying the cellular basis of NK-like activity in Xenopus are discussed in the light of these experiments.

Animals↗
PubMed1992

Skin xenograft rejection in Xenopus--immunohistology and effect of thymectomy.

Immunohistology, using the T-lineage-specific monoclonal antibody XT-1 and an anti-IgM mAb, illustrates differences in the cellular basis of skin allograft and xenograft destruction displayed by control froglets (X. laevis). Thus T cells predominate within allografts, whereas B-lineage cells accumulate under xenografts (from X. tropicalis). The possibility that T cells do not play a central role in mediating xenograft rejection is consistent with the finding that early thymectomy (at 7 days) has minimal effect on rejection end points of X. tropicalis transplants. However, rejection of skin from a "phylogenetically less distant" xenogeneic species (X. borealis) is shown here to be impaired in early thymectomized X. laevis. Differences in the extent to which the thymus has been shown to influence skin xenograft rejection in Xenopus are discussed.

Animals↗
PubMed1991

Enterohepatic recirculation and renal metabolism of morphine in the rat.

Morphine (2.5 mg/kg) was administered iv to intact (I), bile duct-cannulated (BC), and bile duct-cannulated--renal-ligated (BC-RL) rats (n = 4 per group) to investigate the extent of enterohepatic recirculation and renal metabolism of the drug. A decrease in the serum area under the concentration-time curve (AUC) was observed for the BC in comparison with I rats. From these AUC values, it was determined that approximately 16% of the administered dose was subject to enterohepatic recirculation. In addition, a statistically significant (p less than 0.05) decrease in the systemic clearance of morphine was observed in the BC-RL rats compared with the BC animals (55.2 +/- 17.2 versus 31.4 +/- 8.5 mL/min/kg). This decrement in systemic clearance appeared to be the result of a significant decrease in the formation clearance of morphine glucuronide after ligation of the renal pedicles (23.2 +/- 4.8 versus 10.9 +/- 5.0 mL/min/kg). Renal metabolic clearance was calculated as 15.7 mL/min/kg, accounting for 28.5% of the systemic clearance of morphine. Hepatic clearance (31.4 +/- 8.5 mL/min/kg) accounted for 56.8% of total systemic clearance.

Animals↗
PubMed1991

Splenocyte response to T cell-derived cytokines in thymectomized Xenopus.

A miniaturized, "hanging-drop" bioassay reveals that splenocytes from early-thymectomized (Tx) Xenopus can respond (by enhanced thymidine incorporation) to thymic-dependent "cytokines" generated in PHA- or alloantigen-stimulated cultures. Preliminary evidence, using fluorescence activated cell sorting, indicates that surface IgM- splenocytes, rather than sIgM+ cells, from Tx toads are sensitive to the crude, splenocyte-derived, active supernatants. Although these responsive cells display residual, but low, reactivity to PHA, their thymus independence is suggested by flow cytometric observations using the anti-T cell monoclonal antibody XT-1. The development of "T-like" cells in Tx Xenopus is discussed.

Animals↗
PubMed1989

In vitro cytotoxicity in adult Xenopus generated against larval targets and minor histocompatibility antigens.

Our experiments reveal that application of several minor H antigen-disparate skin grafts to adult Xenopus over an 18-month period can lead to in vitro generation of CML reactivity toward these minor antigens. Furthermore, we demonstrate that, following MHC-disparate skin graft rejection, adult effectors can efficiently kill both adult and larval donor-strain targets; this killing is MHC-specific and requires MLC restimulation with cells syngeneic to the skin graft donor. The ability to kill larval lymphoblasts, which have been shown elsewhere to be MHC class I-negative but class II-positive, suggests the probable importance of class II-restricted killing in this species.

Animals↗
PubMed1989

Attempts to break perimetamorphically induced skin graft tolerance by treatment of Xenopus with cyclophosphamide and interleukin-2.

The maintenance of skin allotolerance induced by perimetamorphic application of MHC-disparate skin to isogeneic Xenopus is investigated. Removal of the perimetamorphically applied first-set graft after 4 weeks did not, in general, completely break allotolerance; however, many second-set semi-allogeneic grafts, applied up to 14 weeks after first-set removal, were no longer maintained in perfect condition. Skin allografts tolerated for up to 42 weeks continued to express donor histocompatibility antigens, as indicated by their survival times when transplanted back to the original donor or recipient strain. Treatment with human recombinant IL-2 (rIL-2), shown elsewhere to be an effective immunoregulatory lymphokine for Xenopus in vivo, failed to cause long-term-tolerated 1st-set allografts, or newly-applied 2nd-set grafts, to be rejected. In contrast, cyclophosphamide (CyP) treatment led to acute (less than 4 weeks) destruction of both 1st- and 2nd-set allografts; breaking of tolerance was regularly seen when donor and host differed by two MHC haplotypes, but occurred infrequently in semiallogeneic combinations. The experiments suggest that skin-induced allotolerance is maintained by an immunosuppressive mechanism, that is CyP-sensitive but resistant to rIL-2 treatment.

Animals↗
PubMed1987

Thymocyte/stromal cell chimaerism in allothymus-grafted Xenopus: developmental studies using the X. borealis fluorescence marker.

These experiments employ the X. borealis (quinacrine-fluorescence) cell marker to illustrate that froglet (normal or in vivo-irradiated) thymuses, alloimplanted to 4- to 6-week-old, 7-day-thymectomized hosts, become filled with host lymphoid cells, while a range of thymic stromal cell types (e.g. epithelial derivatives and reticuloendothelial cells) remain donor derived. A time-course study of 4 micron historesin-embedded sections reveals that for normal thymus implants, host cells begin to immigrate in good number only after metamorphosis. In contrast, 3000 rad-irradiated thymus implants begin to be repopulated with host lymphocytes within 2 weeks postimplantation, when hosts are still at a late larval stage of development. Despite rapid colonization by host lymphoid cells, irradiated thymuses remain small and often disappear in early adult life. Donor-derived lymphocytes frequent the blood and both the red pulp and perifollicular regions of the spleen following normal thymus implantation, whereas such thymic emigrants were not seen in the periphery of thymectomized hosts grafted with irradiated thymus glands.

Animals↗
PubMed1977

Critical role of the thymus in establishing humoral immunity in amphibians: studies on Xenopus thymectomized in larval and adult life.

Proliferative studies on spleen lymphocytes suggest the absnece of induced cellular activity to sheep erythrocytes in Xenopus thymectomized at 7 days of age. Immunological studies (measurement of splenic rosette-forming and plaque-forming cells and serum haemolysins) show that throughout larval life the thymus plays a critical role in establishing reactivity to red cells, since thymectomy as late as 40 days (stage 57) impairs the antibody response. In contrast, thymectomy during metamorphosis or in early adult life has no apparent debilitating effect.

Aging↗
PubMed1977

Splenic involvement in amphibian transplantation immunity.

The spleen has been identified as a centre of alloimmune reactivity in control Xenopus. Levels of tritiated thymidine labelling and pyroninophilic cells are elevated in spleens of skin-allografted toadlets when compared with autografted and nongrafted animals. Second-set alloimmune reactivity can be transferred by implanting a spleen from a donor that has rejected one or two grafts into a nonsensitized host. Spleen donor and host in these experiments were mutually tolerant, following reciprocal transfer of embryonic tissue grafts. In contrast, studies on the uptake of tritiated thymidine and levels of pyroninophilia in animals thymectomized at 7 or 8 days of age suggest lack of splenic involvement in the chronic first-set allograft rejection that can still occur in the absence of the thymus. The lymphoid organ origin of "thymic-independent" alloimmunity still awaits clarification.

Animals↗
PubMed1976

The effect of thymectomy at different stages of larval development on the immune response of the clawed toad to sheep erythrocytes.

The role of the thymus during the first three weeks of larval life in the maturation of humoral immunity in the amphibian, Xenopus laevis, is examined. Thymectomy throughout this period dramatically affects the heterologous red cell response of young adults. Sheep erythrocyte (SRBC) administration elicited haemolytic antibody production in the blood and in the spleen (measured by the appearance of plaque forming cells) of control animals, but failed to do so in all thymectomized toadlets. Moreover, use of the immunocyto-adherence assay, which proved to be a sensitive test for SRBC reactivity in control Xenopus, indicated a complete absence of induced responsiveness to this antigen in the spleens of thymectomized toadlets, even in those animals thymectomized as late as 22 days of age. In contrast to allograft immunity in Xenopus, Which is suppressed only by thymic ablation during the first two weeks of larval life, the development of reactivity to heterologous erythrocytes therefore requires an additional period of thymic influence during ontogeny. These experiments support a concept of thymus-dependent cell heterogeneity in amphibians.

Animals↗
PubMedDate not supplied

Incomplete tolerance induced in Xenopus by larval tissue allografting: evidence from immunohistology and mixed leucocyte culture.

Application of adult skin allografts to Xenopus larvae has been a favoured protocol for probing the development of self-tolerance. A more physiologic approach is presented here that examines the immunologic outcome of grafting semi- or fully allogeneic larval skin or spleen to age-matched, larval Xenopus (X. laevis/X. gilli clonal hybrids). Following such grafting at 2 or 4 weeks-of-age, young froglets (4-5-months-old) are generally unable to reject second-set skin transplants, but destroy third-party skin vigorously, the MHC class II-rich spleen proving especially effective at inducing this tolerance. In contrast, following larval grafting of semiallogeneic tissues, mixed leucocyte culture performed at the end of metamorphosis (6 weeks) and again at 6 months reveals splenocyte reactivity toward donor-strain stimulators. Immunohistological findings extend this observation of anti-donor reactivity (suggesting incomplete tolerance) to the graft site. Thus despite excellent health when viewed externally, apparently tolerated second-set skin transplants display localised infiltration (especially into the epidermis) by CD8+ T cells and increased numbers of MHC class I and II-expressing cells by 3 weeks post-grafting. The immunologic implications of these findings are discussed.

Animals↗
PubMedDate not supplied

Ontogeny and thymus-dependence of T cell surface antigens in Xenopus: flow cytometric studies on monoclonal antibody-stained thymus and spleen.

Recently generated anti-Xenopus T cell monoclonal antibodies (mAbs) to the 120 kDa XTLA-1 determinant and against the putative CD5 and CD8 homologues, together with anti-IgM and anti-MHC class II mAbs, are used in dual colour flow cytometric experiments to characterize cell surface antigenic expression on lymphocytes in thymus and spleen of Xenopus laevis during larval and early adult life and also in metamorphosis-inhibited animals. Histological confirmation of T cell emergence early in larval ontogeny is supplied by cryostat sections stained for CD8. Five-day thymectomy, i.e. prior to T-lineage cell differentiation in the thymus, abolishes T cell marker expression in the spleen for up to 1 year. Moreover, late larval (20 days) or early adult (3 months) thymectomy (i.e. removal after peripheralization of T cells has occurred) also leads to severe depletion of mAb-defined T cells in the spleen.

Animals↗