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Biomedical subjects

H K Webster

Publications and source records attributed to H K Webster.

At least 109 records · Page 6Linked to original sources
PubMed1989

Prolonged elevations of soluble interleukin-2 receptors in tuberculosis.

In order to determine whether tuberculosis is associated with elevated levels of soluble interleukin-2 receptors (SIL-2R), patients with both pulmonary (n = 12) and extrapulmonary (n = 8) disease were studied. SIL-2R were measured in sera using an enzyme-linked immunosorbent assay (ELISA). Prior to treatment, all 20 patients had levels of SIL-2R significantly greater than those of the 14 control subjects. Longitudinal study of patients with pulmonary disease revealed that levels of SIL-2R remained elevated in 11 of 12 patients after 2 months, and in four of six patients after 3 months of treatment. These findings suggest that tuberculosis is characterized by prolonged activation of the immune system despite optimal chemotherapy and that SIL-2R may distinguish active immunity from immunologic memory to this infection.

Adult↗
PubMed1989

ELISA method for detecting Plasmodium falciparum circumsporozoite antibody.

An enzyme-linked immunosorbent assay (ELISA) for circulating IgG mouse antibody to Plasmodium falciparum circumsporozoite (CS) protein was modified for use with human sera collected in an area of northern Zambia that was endemic for malaria and from individuals never exposed to malaria. Optimum sensitivity was achieved using Immulon 2 microtitration plates, boiled casein-Tween 20 blocking buffer, and by adding a solution of boiled casein (4 micrograms/ml) to the capture antigen diluent. The results for the detection of anti-CS IgG correlated well with those of sporozoite immunofluorescence antibody assays. Modification of the ELISA method permitted the simultaneous detection of anti-CS IgG and IgM antibody on a single serum sample in the same well of the microtitration plate and the detection of anti-CS IgG antibody in Kenyan dried whole-blood samples collected on filter-paper. The assay has been used to monitor human antibody levels in a phase-I malaria vaccine trial and in longitudinal studies of malaria transmission in Thailand and Kenya.

Adolescent↗
PubMed1988

Defective production of and response to IL-2 in acute human falciparum malaria.

Patients with acute Plasmodium falciparum malaria have defective cell-mediated immune responses to malaria-specific Ag (MA). This immunologic defect may partially explain the difficulty with which natural immunity to falciparum malaria develops and may have important implications for the efficacy of potential malaria vaccines in endemic areas. To investigate the basis of this immune defect, we have examined the capacity of PBMC from patients with acute falciparum malaria to produce IL-2 and to express I1-2R in response to Ag stimulation. The effect of exogenous IL-1 and IL-2 on lymphocyte proliferation was studied. Soluble IL-2R levels were measured in acute and convalescent sera. Our results showed that no detectable IL-2 was produced and no IL-2R were expressed by PBMC in response to MA during the acute infection. IL-2 production and IL-2R expression were also depressed when PBMC were exposed to streptococcal Ag. The specific immune defect was not reconstituted by the addition of graded doses of purified human IL-1 or IL-2 and could not be attributed to suppressor adherent cells. In contrast to the absence of IL-2 and cell-bound IL-2R, circulating soluble IL-2R was elevated in acute sera. These findings suggest that the lack of IL-2, through either a defect in its production or inhibition of its activity, may be the basis of the Ag-specific immune unresponsiveness in acute P. falciparum malaria.

Acute Disease↗
PubMed1988

IgM antibody responses to the circumsporozoite protein in naturally acquired falciparum malaria.

The circumsporozoite (CS) protein on the surface of sporozoites is the major target for antibody response(s) to the infective stage of the malaria parasite. Sera from malaria endemic areas contain both IgM and IgG antibodies that react with a dominant epitope in the tetrapeptide repeat region of the CS protein. In order to characterize the IgM CS antibody response in Plasmodium falciparum (PF) malaria, a prospective study was conducted in Thai Rangers. Variable IgM responses against the CS protein were detected in 81% of 47 PF-infected subjects. Similar to IgG response kinetics, IgM levels rose to a peak 6-10 days after detection of parasitemia and showed an apparent serum half-life of less than 25 days. The classic difference in isotype ratio (IgG/IgM) between primary and secondary antibody responses was observed to blood-stage, but not CS, antigens.

Adolescent↗
PubMed1988

Characterization of antibodies to sporozoites in Plasmodium falciparum malaria and correlation with protection.

The antibody response to sporozoites of Plasmodium falciparum and the role of these antibodies in protection against malaria have not been systematically investigated. An understanding of antisporozoite antibodies in natural infection is, however, important to the development of a human malaria vaccine. In a prospective study in Thailand, an antibody response to sporozoites was observed only in individuals who developed parasitemia. Antibodies were detected against an epitope in the repeat region of the circumsporozoite (CS) protein. Current candidate sporozoite vaccines are based on CS repeat antigens. The CS antibody response was of low magnitude, peaked after detection of parasitemia, and had a serum half-life of less than 1 month. CS antibody boosting occurred in only 6% of reinfected individuals. These observations suggest that antisporozoite antibody is poorly developed under natural conditions and appears not to protect against development of malaria.

Adult↗
PubMed1988

Lymphocyte responsiveness to a candidate malaria sporozoite vaccine (R32tet32) of individuals with naturally acquired Plasmodium falciparum malaria.

Lymphocyte proliferative responses to the candidate malaria sporozoite vaccine antigen R32tet32 were evaluated in 29 patients with acute Plasmodium falciparum malaria, 20 convalescent patients, 11 nonimmune individuals, and 22 healthy residents of two endemic malarious areas in Thailand. The results indicate that 14 of 20 (70%) convalescent patients and 14 of 22 (64%) residents of endemic areas responded to the R32tet32 antigen. However, only 8 of 29 (28%) patients with acute P. falciparum malaria responded. When 4 of the convalescent patients who remained in a malaria-free area were restudied 5-10 months after the acute infection, they were either not responsive or their responses had greatly diminished. These findings show that sensitization to R32tet32 occurs following a natural P. falciparum infection, but the cellular immune response to sporozoite antigens may be short-lived and may be suppressed during acute P. falciparum malaria.

Acute Disease↗
PubMed1988

A comparative field study of radiolabeled and enzyme-conjugated synthetic DNA probes for the diagnosis of falciparum malaria.

Synthetic, 21-base, DNA probes to the genome of Plasmodium falciparum were either 32P-labeled or enzyme-conjugated for comparative field studies. The sensitivity of both probes was compared with microscopy in the examination of blood samples from 97 Thai villagers, 47 Thai Rangers, and 19 malaria-free Bangkok residents. The probes were also used to monitor the therapeutic response of 18 of the Rangers during 7 days of treatment. The probes proved highly specific. Both probes had lower limits of detection of about 100 parasites per microliter blood. Thus, the low parasite densities in partially immune villagers from an endemic area were often missed, while higher parasite densities in the nonimmune Rangers were usually detected. As monitors of response to treatment, the probes paralleled microscopy in identifying reversion from positive to negative parasitemia. The enzymelabeled DNA probe as shown to perform similarly to the radiolabeled probe in populations with different malarial immune status and during curative treatment.

Alkaline Phosphatase↗
PubMed1988

Lymphocyte subpopulations during acute and convalescence phases of malaria.

Lymphocytes of normal healthy persons were separated from blood by Ficoll-Hypaque gradient centrifugation and iron-magnet application. peripheral blood lymphocytes (PBL) were stained by various dye-labeled monoclonal antibodies. Cells positive for specific surface markers were enumerated by a fluorescence activated cell sorter (FACS) and fluorescence microscope (FM). The results revealed that the percentages of cells positive with one monoclonal antibody counted by these two techniques were similar while the percentages of cells with double staining were higher when counted by FACS than by FM. Lymphocyte subpopulations of 18 patients infected with Plasmodium falciparum during acute and convalescence period were studied. Lymphocytopenia occurred during the acute infection while total white blood cell counts were normal. PBL of the patients were stained with OKT3, OKT4, OKT8, Leu-11 and a combination of Leu-7, Leu-1 monoclonal antibodies. The absolute numbers of all lymphocyte subpopulations were decreased during the acute infection while T8 positive cells were decreased in both percentage and absolute number. Thus T4:T8 ratio (1.7:1) became higher than normal (1.3:1) at this period. During convalescence phase, absolute numbers and percentages of Leu-7+, Leu-1+ and perhaps Leu-7+, Leu-11- cells which had low NK cell activity were significantly higher than during acute illness. The finding might explain why the NK cell activity was low during the convalescence period.

Adolescent↗
PubMed1988

Circumsporozoite antibodies and falciparum malaria incidence in children living in a malaria endemic area.

In a case-control study we examined the association of Plasmodium falciparum circumsporozoite antibodies (anti-R32tet32) with subsequent P. falciparum infections. A study population of 140 children living in an endemic area was followed longitudinally for 25 weeks with weekly blood smears for malaria parasites and, once every two weeks, serum samples for circumsporozoite antibody determinations. From the malaria cases, antibody measurements occurring between two and six weeks prior to the onset of parasitaemia were utilized. For each case, two controls were selected. The results from 17 cases and 34 controls failed to show a statistically significant difference in antibody levels prior to the infection (P=0.07, one-tailed Student's t-test). However, 8 of the 17 cases had antibody present, indicating a level that was not protective against patent infection.

Animals↗
PubMed1988

Malaria: treatment efficacy of halofantrine (WR 171,669) in initial field trials in Thailand.

Halofantrine (WR 171,669) hydrochloride was administered orally to 82 patients infected with Plasmodium falciparum malaria on the Thai-Kampuchean border between June 1982 and December 1983 in a randomized double-blind treatment trial which compared the efficacy of halofantrine with that of mefloquine. Halofantrine was curative with oral treatment on a single day in 65% of patients (13/20) who received 1000 mg followed 6 hours later by an additional 500 mg, and in 88% of patients (53/60) who received 500 mg every 6 hours for 3 doses. Mefloquine was curative in 88% of patients (22/25) given a single oral dose of 1000 mg and in 97% of patients (38/39) given a single oral dose of 1500 mg. The difference in cure rates between the 3-dose halofantrine regimen and either of the mefloquine regimens was not significant. The mean parasite clearance time for all regimens ranged from 75 to 84 hours. The mean fever clearance time for all four treatment groups was in the range 50-60 hours, with no significant differences between groups. Post-dosing side-effects in patients treated with halofantrine consisted of nausea, vomiting, abdominal pain and diarrhoea and were not significantly different from those treated with mefloquine. Halofantrine therefore appeared to be of comparable efficacy to mefloquine in the treatment of multidrug-resistant P. falciparum malaria.

Adult↗
PubMed1987

High-performance liquid chromatographic assay for pteroylpolyglutamate hydrolase.

A highly sensitive assay for pteroylpolyglutamate hydrolase is described employing high-performance liquid chromatography (HPLC) with ultraviolet detection at 280 nm. The method is based on the separation of pteroylpolyglutamates containing various glutamyl residues on a C18 muBondapak reversed-phase column. Individual pteroylpolyglutamates are eluted by a gradient of 2.5-8.5% acetonitrile in 0.1 M potassium phosphate buffer (pH 6.0) within 20 min. The polyglutamates with higher glutamyl residues were less well retained in the reversed-phase column. The relationship between the peak area and the amount of pteroylpolyglutamate was observed to be linear over the range 10 pmol to 2.5 nmol. Human serum pteroylpolyglutamate hydrolase was studied using pteroylpentaglutamate as substrate in 0.1 M sodium acetate buffer (pH 4.5). The enzyme appeared to function as an exopeptidase based on the detection of intermediates, pteroyltetra-, tri-, and -diglutamate, and the product, pteroylmonoglutamate. Using the HPLC assay, extracts of Plasmodium falciparum were found not to contain detectable enzyme activity.

Animals↗
PubMed1987

Polymorphism of proteins in malaria parasites following mefloquine treatment.

Proteins in malaria parasites (Plasmodium falciparum) isolated from a patient in Thailand before treatment, and after recrudescence of infection subsequent to mefloquine treatment, were compared by two dimensional polyacrylamide gel electrophoresis (2D-PAGE) analysis. Nine 'pre-treatment' and six 'recrudescent' clones were studied. Variants of the enzyme glucose phosphate isomerase were also noted and mefloquine susceptibility of each clone was measured by in vitro tests. The 'pre-treatment' isolate was found to contain at least four genetically distinct clones, all sensitive to mefloquine, while the 'recrudescent' isolate contained at least two other types of clone, both showing increased tolerance to mefloquine. These two more tolerant types of clone differed from all the sensitive ones studied in regard to several different protein variants as shown by 2D-PAGE analysis. It is concluded that at least two (and probably more) genetically distinct clones of parasites with increased tolerance to mefloquine were present in the parasite population before mefloquine treatment was given, and were selected under mefloquine pressure.

Animals↗
PubMed1987

Development of immunity in natural Plasmodium falciparum malaria: antibodies to the falciparum sporozoite vaccine 1 antigen (R32tet32).

Antibodies that reacted with a candidate sporozoite vaccine antigen (R32tet32) were found in 20 of 21 patients treated for acute infection with Plasmodium falciparum and monitored longitudinally over 67 days. R32tet32 contains 32 tandem copies of a tetrapeptide sequence that constitutes the immunodominant epitope of the circumsporozoite surface protein. The magnitude of the antibody response varied considerably among individuals and appeared to be independent of the number of previous clinical infections. Recrudescence of infection or infection with Plasmodium vivax had no demonstrable effect on antibody levels, although reinfection with P. falciparum produced a rapid rise in antibody titer. Antibody levels were observed to decline rapidly after treatment of clinical infection with mefloquine. The apparent antibody half-life was 27 days, which is comparable to the half-life of circulating immunoglobulin G in humans. The data suggest that antisporozoite antibody production ceased on about day 4 after treatment of acute infection. A similar pattern of response was observed for antibodies against the erythrocytic forms of the parasite. The cessation of antibody synthesis was interpreted as being due to immunosuppression induced by the presence of intraerythrocytic parasites and may explain in part why protective immunity is poorly developed in natural malaria infections.

Acute Disease↗
PubMed1987

In vitro mefloquine resistance of Plasmodium falciparum isolated from the Burmese border region of Thailand.

The in vitro susceptibility of twenty isolates of Plasmodium falciparum from Tha Song Yang, Tak province, Thailand were determined. The isolates were resistant to chloroquine (IC50 = 220 nM; MIC = 762 nM), quinine (IC50 = 252 nM; MIC = 1010 nM), and pyrimethamine (IC50 = 16400 nM; MIC = 43100 nM) but generally sensitive to mefloquine (IC50 = 6.90 nM; MIC = 20.9 nM) and halofantrine (IC50 = 8.73 nM; MIC = 2.71 nM). Two isolates were identified which appeared resistant to mefloquine (IC50 = 23.1 nM; MIC = 56.6 nM). These isolates may represent an extension of a population of parasites from eastern Thailand.

Animals↗
PubMed1986

Expression of human malaria parasite purine nucleoside phosphorylase in host enzyme-deficient erythrocyte culture. Enzyme characterization and identification of novel inhibitors.

The intraerythrocytic human malaria parasite, Plasmodium falciparum, requires a source of hypoxanthine for nucleic acid synthesis and energy metabolism. Adenosine has been implicated as a major source for intraerythrocytic hypoxanthine production via deamination and phosphorolysis, utilizing adenosine deaminase and purine nucleoside phosphorylase, respectively. To study the expression and characteristics of human malaria purine nucleoside phosphorylase, P. falciparum was successfully cultured in purine nucleoside phosphorylase-deficient human erythrocytes to an 8% parasitemia level. Purine nucleoside phosphorylase activity was undetectable in the uninfected enzyme-deficient host red cells but after parasite infection rose to 1.5% of normal erythrocyte levels. The parasite purine nucleoside phosphorylase was not cross-reactive with antibody against human enzyme, exhibited a calculated native molecular weight of 147,000, and showed a single major electrophoretic form of pI 5.4 and substrate specificity for inosine, guanosine and deoxyguanosine but not xanthosine or adenosine. The Km values for substrates, inosine and guanosine, were 4-fold lower than that for the human erythrocyte enzyme. In these studies we have identified two novel potent inhibitors of both human erythrocyte and parasite purine nucleoside phosphorylase, 8-amino-5'-deoxy-5'-chloroguanosine and 8-amino-9-benzylguanine. These enzyme inhibitors may have some antimalarial potential by limiting hypoxanthine production in the parasite-infected erythrocyte.

Animals↗
PubMed1986

Analysis of resistance to pyrimethamine/sulphadoxine of isolates of Plasmodium falciparum from Eastern Thailand.

The in vitro susceptibility of five field isolates of Plasmodium falciparum from the region of the Thai-Kampuchean border to pyrimethamine, sulphadoxine, and their combination, was determined using a microtitre test system and media deficient in p-aminobenzoic acid and folic acid. Two-fold serial dilutions of pyrimethamine ranging from concentrations of 8.0 to 0.125 microM and sulphadoxine ranging from 800 to 50 microM were evaluated for antimalarial activity. Viability was based on the maturation of the ring stages to normally-appearing schizonts. Tested singly the parasites had an average ID90 of 3.82 microM for pyrimethamine and greater than 800 microM for sulphadoxine. Analysis of the drugs interaction showed maximum potentiation at approximately 0.8 microM of pyrimethamine and 80 microM of sulphadoxine. These results suggest that resistance to Fansidar is due to the resistance to both components. Although there was a potentiating effect it was probably not sufficient enough for the drugs to be effective in vivo. This may, in part, explain the reduction in clinical cures with the sulphadoxine-pyrimethamine combination in eastern Thailand.

Drug Resistance, Microbial↗