Biomedical subjects

Y X Fu

Publications and source records attributed to Y X Fu.

At least 37 records · Page 2Linked to original sources

PubMed2000

Enhanced growth of primary tumors in cancer-prone mice after immunization against the mutant region of an inherited oncoprotein.

One major objective of tumor immunologists is to prevent cancer development in individuals at high risk. (TG.AC x C57BL/6)F1 mice serve as a model for testing the feasibility of this objective. The mice carry in the germline a mutant ras oncogene that has an arginine at codon 12 instead of glycine present in the wild-type, and after physical (wounding) or chemical promotion, these mice have a high probability for developing papillomas that progress to cancer. Furthermore, F1 mice immunized with Arg(12) mutant ras peptide in complete Freund's adjuvant (CFA) develop T cells within 10 d that proliferate in vitro on stimulation with the Arg(12) mutant ras peptide. Within 14 d, these mice have delayed-type hypersensitivity to the peptide. Immunization with CFA alone or with a different Arg(12) mutant ras peptide in CFA induced neither response. To determine the effect of immunization on development of tumors, mice immunized 3 wk earlier were painted on the back with phorbol 12-myristate 13-acetate every 3 d for 8 wk. The time of appearance and the number of papillomas were about the same in immunized and control mice, but the tumors grew faster and became much larger in the mice immunized with the Arg(12) mutant ras peptide. Thus, the immunization failed to protect against growth of papillomas. The peptide-induced CD4(+) T cells preferentially recognized the peptide but not the native mutant ras protein. On the other hand, mice immunized with Arg(12) mutant ras peptide and bearing papillomas had serum antibodies that did bind native mutant ras protein. Together, these studies indicate that active immunization of cancer-prone individuals may result in immune responses that fail to eradicate mutant oncogene-expressing tumor cells, but rather induce a remarkable enhancement of tumor growth.

Animals↗

PubMed2000

Lymphotoxin-alpha-dependent spleen microenvironment supports the generation of memory B cells and is required for their subsequent antigen-induced activation.

Lymphotoxin alpha-deficient (LTalpha-/-) mice show dramatically reduced IgG responses after either primary or secondary immunizations with sheep red blood cells (SRBC). When splenocytes from SRBC-primed wild-type donor mice were infused into irradiated naive wild-type recipient mice, they generated a robust memory IgG response, but not when infused into LTalpha-/- recipients, indicating that the microenvironment that develops in LTalpha-/- mice is incompetent to support the activation of this memory response. When irradiated wild-type mice were reconstituted with splenocytes from primed LTalpha-/- donors and then challenged with the same immunizing Ag, no memory response was observed, indicating further that memory cells could not be generated in the LTalpha-/- environment. To address which lymphocyte subsets were impaired in the LTalpha-/- mice, we performed reconstitution experiments using a hapten/carrier system and T cells and B cells from different primed donors. There was no detectable defect in either the generation or expression of memory T cells from LTalpha-/- donors. In contrast, B cells were not primed for memory in the microenvironment of LTalpha-/- mice. Additionally, primed wild-type memory B cells could not express a memory IgG response in the LTalpha-/- microenvironment. Thus, splenic white pulp structure, which depends on the expression of LTalpha for its development and maintenance, is needed to support the generation of memory B cells and to permit existing memory B cells to express an isotype switched memory Ig response following antigenic challenge.

Animals↗

PubMed2000

Antigen persistence is required for somatic mutation and affinity maturation of immunoglobulin.

Whether germinal centers (GC) with follicular dendritic cell (FDC) clusters are the essential sites for affinity maturation of immunoglobulin is still controversial. To re-evaluate the role of GC / FDC in affinity maturation and somatic mutation in a defined antigen system, lymphotoxin-alpha(- / -) and TNF receptor I(- / -) mice, lacking GC / FDC, were immunized with (4-hydroxy-3-nitrophenyl) acetyl-sheep RBC (NP-SRBC). In contrast to soluble hapten-carrier systems, NP-SRBC allows us to compare affinity maturation in the presence or absence of adjuvant. These mice showed a dramatically impaired ability to generate high-affinity IgG to NP, but retained the ability to produce low-affinity anti-NP IgG when NP-SRBC was used in the absence of adjuvant. In contrast to wild-type mice, somatic mutation of the expressed IgG heavy chain gene was rarely detected in these GC / FDC-deficient mice. This suggests that GC / FDC are essential for affinity maturation. Trapping antigen-specific B cells inside the T cell zone of TNFRI(- / -) mice may prolong the interaction between T and B cells, which allows class switching but no further affinity maturation of IgG. Interestingly, GC / FDC-deficient mice could be induced to generate high-affinity, somatically mutated IgG antibodies by immunization with the same amount of NP-SRBC antigen emulsified in incomplete Freund's adjuvant or repeated immunization with the antigen alone. Thus, these data support a model in which prolonged availability of antigen is required for somatic mutation and affinity maturation, and FDC or adjuvants facilitate such processes by slowly releasing antigens.

Animals↗

PubMed2000

Modulation of T-cell-mediated immunity in tumor and graft-versus-host disease models through the LIGHT co-stimulatory pathway.

LIGHT was recently described as a member of the tumor necrosis factor (TNF) 'superfamily'. We have isolated a mouse homolog of human LIGHT and investigated its immunoregulatory functions in vitro and in vivo. LIGHT has potent, CD28-independent co-stimulatory activity leading to T-cell growth and secretion of gamma interferon and granulocyte-macrophage colony-stimulating factor. Gene transfer of LIGHT induced an antigen-specific cytolytic T-cell response and therapeutic immunity against established mouse P815 tumor. In contrast, blockade of LIGHT by administration of soluble receptor or antibody led to decreased cell-mediated immunity and ameliorated graft-versus-host disease. Our studies identify a previously unknown T-cell co-stimulatory pathway as a potential therapeutic target.

Amino Acid Sequence↗

PubMed1999

The requirement of membrane lymphotoxin for the presence of dendritic cells in lymphoid tissues.

Although several cytokines, including tumor necrosis factor (TNF), can promote the growth of dendritic cells (DCs) in vitro, the cytokines that naturally regulate DC development and function in vivo have not been well defined. Here, we report that membrane lymphotoxin (LT), instead of TNF, regulates the migration of DCs in the spleen. LTalpha(-/-) mice, lacking membrane LTalpha/beta and LTalpha(3), show markedly reduced numbers of DCs in the spleen. Unlike wild-type mice and TNF(-/-) mice that have densely clustered DCs in the T cell zone and around the marginal zone, splenic DCs in LTalpha(-/-) mice are randomly distributed. The reduced number of DCs in lymphoid tissues of LTalpha(-/-) mice is associated with an increased number of DCs in nonlymphoid tissues. The number of splenic DCs in LTalpha(-/-) mice is restored when additional LT-expressing cells are provided. Blocking membrane LTalpha/beta in wild-type mice markedly diminishes the accumulation of DCs in lymphoid tissues. These data suggest that membrane LT is an essential ligand for the presence of DCs in the spleen. Mice deficient in TNF receptor, which is the receptor for both soluble LTalpha(3) and TNF-alpha(3) trimers, have normal numbers of DCs. However, LTbetaR(-/-) mice show reduced numbers of DCs, similar to the mice lacking membrane LT alpha/beta. Taken together, these results support the notion that the signaling via LTbetaR by membrane LTalpha/beta is required for the presence of DCs in lymphoid tissues.

Animals↗

PubMed1999

Requirement for membrane lymphotoxin in natural killer cell development.

Development of natural killer (NK) cells is thought to depend on interactions between NK progenitors and the bone marrow (BM) microenvironment; however, little is known about the molecular signals involved. Here we show that lymphotoxin (LT) provides an important signal for the development of both NK cells and NK/T cells. LTalpha-/- mice show marked reduction in splenic and BM NK and NK/T cell numbers and dramatically impaired NK and NK/T cell function. Mice deficient in either tumor necrosis factor receptor (TNFR)-I or TNFR-II have normal numbers of NK and NK/T cells, implying that neither of the TNFRs nor soluble LTalpha3 is required for development of these cell types. Reciprocal BM transfers between LTalpha-/- and wild-type mice suggest that close interactions between membrane LT-expressing NK cell precursors and LT-responsive radioresistant stromal cells are necessary for NK cell development. When LT-deficient BM cells are incubated with IL-15, NK cells are formed. In addition, LT-deficient BM cells produce IL-15 after activation. Thus, membrane LT appears to deliver a signal for NK cell development that is either independent of IL-15 or upstream in the IL-15 pathway. These results reveal a novel function for membrane LT in NK and NK/T cell development. They also support a cellular and molecular mechanism by which NK cell precursors themselves deliver essential signals, through the membrane ligand, that induce the microenvironment to promote further NK cell and NK/T cell development.

Animals↗

PubMed1999

Coalescing into the 21st century: An overview and prospects of coalescent theory.

Coalescent theory represents the most significant progress in theoretical population genetics in the past two decades of this century. It is now widely recognized as a cornerstone for rigorous statistical analyses of molecular data from populations. In the future, challenges from the rapidly expanding body of molecular data will continue to inject fresh blood into the development of coalescent theory. As the 21st century looms, we review recent progress in coalescent theory and discuss some prospects for future developments, particularly with regard to human population genetics.

Algorithms↗

PubMed1999

Receptive field characteristics of neurons in the nucleus of the basal optic root in pigeons.

Optokinetic nystagmus is a reflex to stabilize an object image on the retina by compensatory eye movements. In lower vertebrates, the nucleus of the basal optic root participates in generating this reflex. Visual responses of 135 neurons were extracellularly recorded from the nucleus in pigeons and their receptive field properties were analysed on-line with a workstation. These cells could be categorized into slow (84%), intermediate (3%) and fast (13%) cells, preferring motion velocities of 0.25-8, 16 and 32-64 deg./s, respectively. Using whole-field gratings as stimuli revealed that 97% of the cells were selective for direction of motion and 3% were not. The directional cells preferred motion in the dorsoventral (35%), nasotemporal (34%), ventrodorsal (23%), or temporonasal (8%) directions. The omni-directional neurons were equally excited or inhibited by motion in all directions. The receptive field of basal optic neurons usually consisted of an excitatory receptive field and an inhibitory receptive field, both of which possessed opposite (heterodirectional) or identical (homodirectional) directionalities. In the case of homodirectional co-existence of both fields, whether whole-field gratings could produce visual responses from the cells would depend on the interaction between excitation and inhibition evoked in their excitatory and inhibitory receptive fields, respectively. Therefore, in some cases a single object was more effective than whole-field gratings in eliciting visual responses from basal optic neurons in pigeons. All of these receptive field properties revealed by on-line computer analysis may underlie the detection of optic flow and the induction of optokinetic responses.

Animals↗

PubMed1999

Rapid selection of complement-inhibiting protein variants in group A Streptococcus epidemic waves.

Serotype M1 group A Streptococcus strains cause epidemic waves of human infections long thought to be mono- or pauciclonal. The gene encoding an extracellular group A Streptococcus protein (streptococcal inhibitor of complement) that inhibits human complement was sequenced in 1,132 M1 strains recovered from population-based surveillance of infections in Canada, Finland and the United States. Epidemic waves are composed of strains expressing a remarkably heterogeneous array of variants of streptococcal inhibitor of complement that arise very rapidly by natural selection on mucosal surfaces. Thus, our results enhance the understanding of pathogen population dynamics in epidemic waves and infectious disease reemergence.

Animals↗

PubMed1999

Selenium-dependent cellular glutathione peroxidase protects mice against a pro-oxidant-induced oxidation of NADPH, NADH, lipids, and protein.

Since our prior work indicated that Se-dependent cellular glutathione peroxidase (GPX1) was necessary for protection against paraquat lethality, the present studies were to elucidate the biochemical mechanisms related to that protection. Four groups of mice [Se-deficient or -adequate GPX1 knockout and wild-type (WT)] were injected (i.p.) with 50 mg paraquat/kg body weight and tissues were collected 0, 0.5, 1, 2, 3, or 4 h after the injection. Whereas the ratios of NADPH/NADP and NADH/NAD in lung were reduced by 50-70% only 0.5 h after the injection in all groups, these two ratios in liver of the Se-adequate WT were significantly higher than those of the three GPX1 knockout or deficient groups 2-4 h after the injection. The paraquat-induced pulmonary lipid peroxidation and hepatic protein oxidation, measured as F(2)-isoprostanes and carbonyl contents, respectively, peaked at 1 h in these three groups. No such oxidative events were shown in any tissue of the Se-adequate WT throughout the time course. Whereas the F(2)-isoprostane formation was accelerated by both GPX1 knockout and Se deficiency in liver, it was not significantly elevated by the paraquat treatment in brain of any group. The paraquat injection also resulted in temporal changes in lung GPX activity and GPX1 protein in the Se-adequate WT, and significant reductions in lung total SOD activity in the GPX1 knockout or deficient groups. In conclusion, GPX1 plays a critical role in maintaining the redox status of mice under acute oxidative stress, and protects against paraquat-induced oxidative destruction of lipids and protein in vivo. These protections of GPX1 seem to be inducible and coordinated with those of other antioxidant enzymes.

Animals↗

PubMed1999

Development and maturation of secondary lymphoid tissues.

The secondary lymphoid tissues are located at strategic sites where foreign antigens can be efficiently brought together with immune system regulatory and effector cells. The organized structure of the secondary lymphoid tissues is thought to enhance the sensitivity of antigen recognition and to support proper regulation of the activation and maturation of the antigen-responsive lymphoid cells. Although a substantial amount is known about the cellular elements that compose the lymphoid and nonlymphoid components of the secondary lymphoid tissues, information concerning the signals that control the development of the tissues and that maintain the organized tissue microenvironment remain undefined. Studies over the past few years have identified lymphotoxin as a critical signaling molecule not only for the organogenesis of secondary lymphoid tissues but for the maintenance of aspects of their microarchitecture as well. Additional signaling molecules that contribute to the formation of normal lymphoid tissue structure are being identified at an accelerating pace. Analyses of mouse strains with congenital defects in different aspects of secondary lymphoid tissue development are beginning to clarify the role of these tissues in immune responses and host defense. This review focuses on studies defining recently identified crucial signals for the biogenesis of secondary lymphoid organs and for the maintenance of their proper microarchitecture. It also discusses new insights into how the structure of these tissues supports effective immune responses.

Animals↗

PubMed1998

Expression of complement receptors 1 and 2 on follicular dendritic cells is necessary for the generation of a strong antigen-specific IgG response.

Two mechanisms could account for the impaired humoral immune response found in Cr2-/- mice. The absence of complement receptors 1 and 2 (CR1, CR2) on B cells could affect their activation. Alternatively, impaired Ag trapping by follicular dendritic cells (FDC) could affect B cell maturation into Ig-secreting or memory B cells. To compare the roles of CR1 and CR2 on B cells vs FDC in this abnormal response, bone marrow (BM) chimeric mice were generated and immunized with specific T-dependent Ags. The primary and secondary Ab response was measured. Cr2+/+ animals reconstituted with a Cr2-/- BM generated a diminished but detectable humoral immune response compared with controls. When injected with preformed immune complexes (IC), these mice maintained follicular IC localization. Cr2-/- animals reconstituted with a Cr2+/+ BM had an initial rise in the Ab titer, but were unable to maintain it as shown by a pronounced decrease in the IgG titer. This defect persisted during the secondary immune response. Follicular IC trapping was also impaired. Despite the abnormal Ab response, germinal center formation was retained in all of the chimeric animals. These experiments are the first to demonstrate an absolute requirement for CR1 and CR2 expression on FDC in the generation of a normal humoral immune response.

Animals↗

PubMed1998

B lymphocytes induce the formation of follicular dendritic cell clusters in a lymphotoxin alpha-dependent fashion.

Lymphotoxin (LT)alpha is expressed by activated T cells, especially CD4(+) T helper type 1 cells, and by activated B and natural killer cells, but the functions of this molecule in vivo are incompletely defined. We have previously shown that follicular dendritic cell (FDC) clusters and germinal centers (GCs) are absent from the peripheral lymphoid tissues of LTalpha-deficient (LTalpha-/-) mice. LTalpha-/- mice produce high levels of antigen-specific immunoglobulin (Ig)M, but very low levels of IgG after immunization with sheep red blood cells. We show here that LTalpha-expressing B cells are essential for the recovery of primary, secondary, and memory humoral immune responses in LTalpha-/- mice. It is not necessary for T cells to express LTalpha to support these immune functions. Importantly, LTalpha-expressing B cells alone are essential and sufficient for the formation of FDC clusters. Once these clusters are formed by LTalpha-expressing B cells, then LTalpha-deficient T cells can interact with B cells to generate GCs and productive class-switched antibody responses. Thus, B cells themselves provide an essential signal that induces and maintains the lymphoid microenvironment essential for GC formation and class-switched Ig responses.

Animals↗

PubMed1998

Hypervariability generated by natural selection in an extracellular complement-inhibiting protein of serotype M1 strains of group A Streptococcus.

In many countries, M1 strains of the human pathogenic bacterium group A Streptococcus are the most common serotype recovered from patients with invasive disease episodes. Strains of this serotype express an extracellular protein that inhibits complement [streptococcal inhibitor of complement (Sic)] and is therefore believed to be a virulence factor. Comparative sequence analysis of the 915-bp sic gene in 165 M1 organisms recovered from diverse localities and infection types identified 62 alleles. Inasmuch as multilocus enzyme electrophoresis and pulsed-field gel electrophoresis previously showed that most M1 organisms represent a distinct streptococcal clone, the extent of sic gene polymorphism was unexpected. The level of polymorphism greatly exceeds that recorded for all other genes examined in serotype M1 strains. All insertions and deletions are in frame, and virtually all nucleotide substitutions alter the amino acid sequence of the Sic protein. These molecular features indicate that structural change in Sic is mediated by natural selection. Study of 70 strains recovered from two temporally distinct epidemics of streptococcal infections in the former East Germany found little sharing of Sic variants among strains recovered in the different time periods. Taken together, the data indicate that sic is a uniquely variable gene and provide insight into a potential molecular mechanism contributing to fluctuations in streptococcal disease frequency and severity.

Antigens, Bacterial↗

PubMed1998

Probability of a segregating pattern in a sample of DNA sequences.

Mutations that result in segregating sites (polymorphic sites) in a sample of DNA sequences can be classified into different types. A pattern of segregating sites is an array of the numbers of various types of mutations. Using an urn model, the probability of a pattern of segregating sites can be expressed as a recurrence equation and its value can be computed sequentially. Among those that can be computed by this method are the probability of obtaining k external mutations (mutations that occur in external branches of the genealogy of a sample), the probability of obtaining k internal mutations (mutations that occur in internal branches), the probability of obtaining k singletons (segregating sites at which one of the two segregating nucleotides is present in only one sequence), and the probability of obtaining k non-singletons. Two applications of the method are discussed. One is a maximum likelihood estimation of straight theta and another is a Bayesian statistical test of the hypothesis of neutral mutations.

Bayes Theorem↗

PubMed1998

Receptive field properties of visual neurons in the avian nucleus lentiformis mesencephali.

The receptive field (RF) properties of visual neurons extracellularly recorded from the nucleus lentiformis mesencephali (nLM) in pigeons (Columba livia) were quantitatively analyzed using a workstation computer. These cells were actively spontaneous, and direction-and velocity-selective. Using spatial gratings as visual stimuli, these cells could be divided into three groups: uni- (74%), bi- (17%), and omnidirectional (9%) cells in terms of their directionality. On the basis of their velocity selectivity, they could be named slow cells (84%), preferring low velocity (0.1-11 degrees/s), and fast cells (14%), preferring rapid motion (34-67 degrees/s), with one cell (2%) responding maximally to an intermediate velocity of 18 degrees/ s. These two properties were correlated in the way that all unidirectionals were slow cells, omnidirectionals were fast cells, and bidirectionals were either slow or fast cells including the intermediate cell. Using small targets as visual stimuli, it was found that the majority of cells examined had RFs that each consisted of an excitatory RF (ERF) and an inhibitory RF (IRF) that overlapped. The unidirectionals were mainly of this type of RF structure, whereas the omnidirectionals apparently had ERFs alone. The direction preference of ERF was opposite to that of IRF for unidirectional cells tested, whereas they were perpendicular to each other for one bidirectional cell. The overall responses of these cells resulted from interaction between excitation and inhibition induced by directionally different motion. Under certain conditions, visual responses of a particular cells to a small target moving through its ERF were equal in responsive strength to those to whole-field gratings swept over the screen. It was suggested that optokinetic nystagmus produced by wholefield gratings results from population activity of large group(s) of neurons in some optokinetic nuclei, at least one of which is nLM.

Animals↗

PubMed1998

On the three methods for estimating deleterious genomic mutation parameters.

Due to the tremendous cost of the traditional mutation-accumulation approach (the Bateman-Mukai technique), data are rare for deleterious mutation parameters such as genomic mutation rate, selection and dominance coefficients. Two alternative approaches have been developed (the Morton-Charlesworth and Deng-Lynch techniques). Except for the Deng-Lynch method, the statistical properties (bias and sampling variance) of these techniques are poorly understood; therefore we investigated them using computer simulation. With constant fitness effects of mutations, the Bateman-Mukai (assuming additive effects) and Deng-Lynch (assuming multiplicative effects) techniques are unbiased; the Morton-Charlesworth technique (assuming multiplicative effects) is very biased if fitness is used in the regression to estimate h, but slightly biased if the logarithm of fitness is used. With variable fitness effects, all techniques are biased. The Deng-Lynch technique is statistically better than the others except when fitness is used to estimate the average degree of dominance in selfing populations with the Morton-Charlesworth technique. If fitness effects are multiplicative but additivity is assumed, the Bateman-Mukai technique is biased under constant fitness effects, and less biased under variable fitness effects relative to when fitness effects are additive (as assumed by the technique). Our study not only quantifies the degree of bias under the biologically plausible situations investigated, thus forming a basis for correct inference of the true parameters by using these techniques, but also provides insights into the relative efficiencies of these techniques when the same number of genotypes are handled experimentally.

Computer Simulation↗

PubMed1998

Conditions for positive and negative correlations between fitness and heterozygosity in equilibrium populations.

The past decades have witnessed extensive efforts to correlate fitness traits with genomic heterozygosity. While positive correlations are revealed in most of the organisms studied, results of no/negative correlations are not uncommon. There has been little effort to reveal the genetic causes of these negative correlations. The positive correlations are regarded either as evidence for functional overdominance in large, randomly mating populations at equilibrium, or the results of populations at disequilibrium under dominance. More often, the positive correlations are viewed as a phenomenon of heterosis, so that it cannot possibly occur under within-locus additive allelic effects. Here we give exact genetic conditions that give rise to positive and negative correlations in populations at Hardy-Weinberg and linkage equilibria, thus offering a genetic explanation for the observed negative correlations. Our results demonstrate that the above interpretations concerning the positive correlations are not complete or even necessary. Such a positive correlation can result under dominance and potentially under additivity, even in populations where associated overdominance due to linked alleles at different loci is not significant. Additionally, negative correlations and heterosis can co-occur in a single population. Although our emphasis is on equilibrium populations and for biallelic genetic systems, the basic conclusions are generalized to non-equilibrium populations and for multi-allelic situations.

Adaptation, Biological↗