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Propanil N-(3,4,-dichlorophenyl)propanamide is commonly used as a post-emergent herbicide in paddy rice cultivation in Sri Lanka. This study examined propanil concentrations in paddy soil and water, in water and substrate of adjacent wetlands receiving runoff from the paddy and in tissues of two wetland macrophytes common in those wetlands. Field sampling was carried out during the wet season of 1993–1994 and the 1994 dry season. Propanil was detected in paddy soil and in paddy and wetland water samples as late as 14 days after treatment (DAT). It was not detected in the wetland substrate, but relatively high concentrations were found in leaf and stem tissues of the edible plants Ipomoea aquatica and Limnocharis flava 52 DAT in the wet season, but it was not detectable by 132 DAT. Tissue concentrations during the dry season were less consistent. This accumulation and retention by macrophytes may be important factors in reducing propanil concentrations in the wetland water below levels shown to be acutely toxic to other wetland biota. But, propanil so retained may pose a danger to humans who gather and eat these wetland plants regularly and who may, over several months, accumulate sufficient propanil to be harmful to their health.

Infection with HIV-1 requires expression of CD4 and the chemokine receptors CXCR4 or CCR5 at the target cell surface. Engagement of these receptors by the HIV-1 envelope glycoprotein is essential for membrane fusion, but may additionally activate intracellular signaling pathways. In this study, we demonstrate that chemokines and HIV-1 envelope glycoproteins from both T-tropic and macrophage-tropic strains rapidly induce tyrosine phosphorylation of the protein tyrosine kinase Pyk2. The response requires CXCR4 and CCR5 to be accessible on the cell surface. The results presented here provide the first evidence for activation of an intracellular signaling event that can initiate multiple signaling pathways as a consequence of contact between HIV-1 and chemokine receptors.

The loss of dry mass, nitrogen and phosphorus from shoot and root litter of mudflat annuals was examined in a series of experimental marshes in the Delta Marsh, Manitoba, Canada. Litter bags containing shoot material of three mudflat annuals (Aster laurentianus Fern., Atriplex patula L., and Chenopodium rubrum L.) were placed on the sediment surface of the marshes under drawdown conditions. In addition, litter bags containing root material of these three species were shallowly buried. Approximately 70% and 50% of both shoot and root litter, respectively, was still present after one year in the field. During the second year when the marshes were flooded, shoot and root litter lost an additional 20% and 0% of their mass, respectively. Except for Chenopodium roots, which accumulated nitrogen and phosphorus during both years, shoot and root litter lost from 0 to 50% of their nitrogen and phosphorus early in the first year, with levels generally remaining constant through the remainder of the study period. Our results indicate that mudflat annual litter decomposed slowly and would provide abundant habitat for aquatic invertebrates when these marshes were reflooded. However, most nutrient loss took place in the first year when the litter was unflooded, with little loss occurring in the second year when flooded.

Thymocytes co-expressing the CD4 and CD8 co-receptors differentiate into mature T cells that express either CD4 or CD8 and have helper or cytotoxic functions, respectively. Recent studies indicate that commitment to the CD4+ or CD8+ lineages occurs stochastically, but retention of the appropriate co-receptor is required to complete development.

Paspalum distichum L. has been the dominant species in the monsoonal wetlands of the Keoladeo National Park in northcentral India since 1982 when grazing by water buffalo and domestic cattle was halted. Maximum water levels in these wetlands occur immediately after the end of the summer monsoon in late September of early October and then decline until the next summer monsoon the following June. After the normal 1985 monsoon, maximum water depths were around 140 cm. After the poor 1986 monsoon, maximum water depths were only around 60 cm. Paspalum distichum maximum aboveground biomass at four sites ranged from 850 g m-2 at the shallowest site to 3400 g m−2 at a deep water site. The maximum biomass of other vegetation types, which had dominated this wetland prior to 1982, ranged from 1400 g m-2 at a deep water site (Ipomoea aquatica Forsk.) to only 240 g m-2 to 400 g m-2 at a deep-water submersed site (Hydrilla verticillata (L. f.) Royle/Cyperus alopecuroides Rottb.) and at a shallow emergent site (Scirpus tuberosus Desf./Sporobolus helvolus (Trin.) Dur. et Schinz). For all vegetation types, biomass changed seasonally in response to changing water levels and temperatures. After the 1986 monsoon, above-ground biomass for all vegetation types was much lower than it had been after the 1985 monsoon. Mean below-ground biomass was very low in all vegetation types (1 to 47 g m-2). Paspalum distichum had a higher aboveground biomass at nearly all water depths in all seasons than that of the pre-1982 vegetation types. Paspalum distichum belowground biomass, however, is comparable to, or less than, that of the pre-1982 vegetation types. During years with an average monsoon, the overall primary production of these wetlands is estimated to have increased 2.5 to 3.5-fold since they were overgrown with Paspalum distichum.

Thymocytes that coexpress the CD4 and CD8 glycoproteins differentiate into mature CD4+ helper or CD8+ cytotoxic cells depending on whether their antigen receptors are specific for MHC class II or class I molecules, respectively. The mechanism of this decision process was investigated in mice whose T cell development was biased toward the class II-specific lineage. We found that constitutive expression of CD4 allows a developmentally arrested population of thymocytes that have mismatched class II-specific TCRs and the CD8 coreceptor to be rescued and to acquire a cytotoxic phenotype. This result is consistent with a two-step process of thymocyte maturation, in which there is stochastic down-regulation of either CD4 or CD8 and subsequent selection based on the ability of the TCR and remaining coreceptor to engage the same MHC molecule.

After the removal of domestic cattle and water buffalo in 1982, the monsoonal wetlands of the Keoladeo National Park, India were rapidly overgrown by the grassPaspalum distichum L. We studied the decomposition of above-ground litter ofPaspalum distichum and that of ten submersed, floating-leaved, free-floating and emergent species, including the dominants in these wetlands prior to 1982. The only species that had decomposition rates as low as those ofPaspalum were the emergents,Pseudoraphis spinescens (R. Br.) Vickery andTypha angustata Bory & Chaub, which had very restricted distributions both before and after 1982. All the previous dominants had much higher decomposition rates. The turnover time forPaspalum litter was estimated to be greater than one year, while litter of the previous dominants all had turnover times of less than one year. Thus, a permanent litter layer could develop in these wetlands that had not been present previously.

Wetlands in the Keoladeo National Park, Bharatpur, India, have been dominated by the grass Paspalum distichum L. since the removal of cattle and water buffalo from the park in the early 1980s. The mean water depth was lowest just before the onset of monsoon, highest immediately after it, and then declined slowly over the winter and spring. The mean cover of P. distichum was also lowest just prior to the onset of the monsoon (e.g. 19% in June 1984), increased after the monsoon (e.g. 35% in October 1985) and did not reach its maximum until late winter (e.g. 49% in March 1985). There was relatively little change in the species composition of the vegetation over an annual cycle. Because of the poor monsoon in 1986, water depths were much lower during the post-monsoonal period than normal. The cover of P. distichum was also reduced during this period and fell to only 2% by March 1987. The seed banks of areas dominated by Paspalum and four other emergent species contained a total of 55 species and between 1100 and 3100 seeds m−2. Seeds of P. distichum were found in all vegetation types. Paspalum-dominated areas grazed by geese had a seed bank that contained 44 species and a mean of about 1000 seeds m−2. Only goose droppings collected in November contained viable seeds, of which 90% were Paspalum seeds. Paspalum litter (6 cm or more) reduced recruitment from the seed bank by about 80% and the number of species recruited by about 50%. Only 12 species (total seedling density

Many cancers elicit an anti-tumor immune response, which is nevertheless unable to protect the patient. One approach to boost anti-tumor immunity is to target immunostimulatory cytokines to the tumor. Such targeting can be achieved by generating chimeric proteins (immunocytokines) in which the cytokine in question is fused to the C-terminus of a tumor-specific antibody. Immunocytokines containing interleukin-2 (IL-2) have been efficacious in mouse tumor models and have entered clinical trials. Numerous enhancements of immunocytokines are possible, including use of additional stimulatory cytokines, alternate modes of tumor targeting, structural modifications to improve pharmacokinetics, and removal of potentially immunogenic sequences from the fusion protein. In addition, immunocytokines are likely to be efficacious in combination with other therapies, including some forms of chemotherapy and cancer vaccines.

CD4 assists in T cell recognition of peptides bound to MHC class II molecules by binding to a non-polymorphic region on class II and stabilizing TCR recognition of the peptide-class II complex. Overexpression of CD4 in transgenic mice expressing a class II-restricted TCR resulted in a dramatic loss of thymocytes that became evident soon after the TCR and CD4 were co-expressed. Both the thymus and lymph nodes of the double-transgenic mice had reduced numbers of CD4 lineage T cells. A large proportion of the remaining CD4 lineage T cells lost either the transgenic TCR alpha or beta chains. The double-transgenic mice continued to generate thymocytes and T cells that expressed the transgenic TCR, but these cells did not express endogenous CD4. Overexpression of CD4 thus severely disrupts the normal developmental pathway of these thymocytes, supporting a model in which avidity of the TCR complex for self class II molecules determines the outcome of thymocyte development.

The CD4 molecule has important roles in signaling during both T cell development and peripheral helper T cell activation1,2. The ectodomain of CD4 binds to a membrane-proximal domain of MHC class II molecules on antigen presenting cells3, while its cytoplasmic tail interacts with the cytoplasmic protein tyrosine kinase (PTK) p56 lck (lck), a member of the src family of protein tyrosine kinases (reviewed in reference 4). These properties of CD4 contribute to the simultaneous interaction of the T cell antigen receptor (TCR) and CD4 with the same MHC class II molecule and to the localization of lck to the TCR complex. It has been thought that the function of CD4 (and of the class I-specific coreceptor CD8) in signal transduction is to facilitate localization of lck within the TCR complex, thus initiating a PTK signaling cascade. However, recent studies suggest that the primary function of CD4 may be to physically stabilize the interaction of the TCR with MHC, and that a direct signaling role of the CD4-associated lck molecule may not be required4,5. In this chapter, we discuss a non-catalytic role of the CD4-associated lck molecule in facilitating T cell activation and developmental signaling.

The ectodomains of the T cell surface glycoproteins CD4 and CD8 bind to membrane-proximal domains of MHC class II and class I molecules, respectively, while both cytoplasmic domains interact with the protein tyrosine kinase (PTK) p56lck(lck) through a shared cysteine-containing motif. Function of CD4 and CD8 requires their binding to the same MHC molecule as that recognized by the T cell antigen receptor (TCR).In vitrostudies indicate that CD4-associated lck functions even in the absence of kinase activity.In vivoexperiments show that, whereas helper T cell development is impaired in CD4-deficient mice, high level expression of a transgenic CD4 that cannot bind lck rescues development of this T cell subset. These studies suggest that CD4 is an adhesion molecule whose localization is regulated through protein-protein interactions of the associated PTK and whose function is to increase the stability of the TCR signalling complex by binding to the relevant MHC. The function of CD4 in development has been further studied in the context of how double positive (CD4+ CD8+ ) thymocytes mature into either CD4+T cells with helper function and TCR specificity for class II or into CD8+T cells with cytotoxic function and specificity for class I. Studies using CD4- transgenic mice indicate that development of single positive T cells involves stochastic downregulation of either CD4 or CD8, coupled to activation of a cytotoxic or helper program, respectively, and subsequent selection based on the ability of the TCR and remaining coreceptor to engage the same MHC molecule.

The CD4 and CD8 coreceptor molecules on immature thymocytes participate in T cell repertoire selection. To examine more definitively the role of CD4 and CD8 in the negative and positive selection of immature thymocytes, we generated transgenic mice with elevated surface CD4 expression and mated them with mice expressing a transgenic T cell receptor. Augmented CD4 expression was found to markedly alter CD8-dependent negative and positive selection of T cells specific for the male (H-Y) antigen presented by H-2Db major histocompatibility complex class I molecules. Moreover, the cytoplasmic tail of CD4 was essential for effecting these alterations, since the overexpression of tailless CD4 molecules failed to influence the outcome of CD8-dependent selection. The inhibition of positive and negative selection in double-transgenic mice expressing the full-length CD4 molecule was associated with a decreased interaction between the protein tyrosine kinase p56lck and CD8. These results strongly implicate p56lck in T cell repertoire selection.

Mature CD4+ and CD8+ T cells recognize and respond to processed antigens associated with major histocompatibility complex-encoded molecules (MHC). It is during thymic development that selection processes act on immature CD4+CD8+ thymocytes to ensure the fonnation of a repertoire of functional T cells1,2. Thus, immature T cells expressing an ∝β TCR with specificity for self-peptides plus MHC class I or class II molecules are positively selected, differentiating into CD8+ or CD4+ T cells, respectively3,4,5,6. Immature thymocytes lacking or expressing a non-selectable TCR undergo programmed cell death1. Additionally, those CD4+CD8+ T cells expressing an autospecific TCR may undergo programmed cell death 7,8 (negative selection). The CD4 and CD8 molecules are thought to actively participate in these T cell repertoire selection events through their coreceptor functions9. In a coreceptor model, the TCR and appropriate coreceptor molecule recognize and interact with the same MHC molecule, with CD4 or CD8 potentiating TCR-derived intracellular signals10,1l.12. Recent reports have demonstrated that mutations affecting CD8/class I MHC interactions disrupt both positive and negative selection in the thymus13,14.

DURING thymocyte development, progenitor cells bearing both CD4 and CD8 coreceptor molecules mature into functional T lymphocytes that express these proteins in a mutually exclusive way1. Although T-cell specificity is determined primarily by the structure of the T-cell antigen receptor (TCR) heterodimer2, a developmentally regulated process acts to ensure that cells bearing class II-restricted TCRs are CD4+ and those bearing class I-restricted TCRs express only CD8 (ref.3). To investigate this maturation process, we have engineered transgenic mice in which CD4 is expressed in all thymocyte subsets and in all peripheral T cells. Peripheral CD4+8+ T lymphocytes from these mice react with both class I and class II alloantigens. Moreover, expression of the CD4 transgene disrupts the positive selection of doubly transgenic thymocytes bearing a class I-restricted TCR specific for the male (H–Y) antigen. Hence the CD4 coreceptor participates directly in T-cell repertoire selection.

Biweekly elevation of available nitrogen and phosphorus concentrations in marsh surface waters did not alter the rate of Sparganium eurycarpum Engelm. or Typha glauca Godr. shoot decomposition. After 505 days, losses of shoot material totaled 39% for both species under fertilized and unfertilized conditions. However, Sparganium shoot litter with an initial nitrogen concentration of 1.41% lost 27% more dry weight over a 505-day period than did Sparganium with an initial concentration of 0.59%. Typha shoot litter with an initial concentration of 0.55% lost 2% more dry weight during 505 days than Typha tissues with an initial nitrogen concentration of 0.48%. During the 505-day sampling interval, quantities of nitrogen in shoot litter increased 31% for Sparganium and decreased 10% for Typha under unfertilized conditions. Unfertilized Sparganium shoot litter lost 2% and unfertilized Typha litter gained 25% of original phosphorus quantities. Under fertilized conditions, nitrogen quantities increased 35% for the 2 species. The quantity of phosphorus in Sparganium and Typha shoot litter increased 7 and 121%, respectively, with fertilization. Sparganium shoot litter with a high initial nitrogen concentration lost 50% of its original quantity of nitrogen over 505 days, but high-nitrogen Typha shoot litter gained an additional 19% nitrogen.

In 1979 and 1980, NH 4 NO 3 and (NH 4 ) 2 HPO 4 were applied monthly to the surface water of enclosed experimental plots dominated by either Sparganium eurycarpum Engelm. or Typha glauca Godr. in Eagle Lake, Iowa, a natural prairie marsh. The impact of nitrogen and phosphorous loading on concentrations of NH 4 N, NO 3 N, and PO 4 P in 15-cm deep interstitial water was unclear in 1979 because of muskrat damage to many interstitial water-sampling devices. However, in 1980, fertilizer application resulted in significant increases in NH 4 N (+189%) and PO 4 P (+60%) in interstitial waters. In 1980, significant increases of 57% and 19% in net annual above-ground production occurred for Sparganium and Typha , respectively, in the fertilized enclosures. Root—rhizome production was not altered by fertilization.