IntroductionNatural killer (NK) cells take part in the defense against virusinfected, neoplastic, and allogeneic cells. 1,2 NK-cell function is balanced by signals from inhibitory and activating receptors. If the balance is tipped toward activation, cytolysis and cytokine production are initiated. 3 Murine inhibitory receptors belong to the Ly49 or the NKG2/CD94 family. 4 Their ligands are MHC class I molecules, and MHC class I deficiency, or the presence of MHC class I alleles that fail to bind the inhibitory receptors, therefore results in "missing self" recognition. 5 NK cells from mice and humans without MHC class I expression are incapable of rejecting MHC class I-deficient cells, showing that they have developed hyporesponsiveness in the MHC-deficient environment. [6][7][8] Conversely, NK cells lacking inhibitory receptors for self-MHC class I are hyporesponsive despite normal MHC class I expression. [9][10][11] Studies with MHC class I transgenic mice provided evidence that individual MHC class I alleles deliver educational signals to NK cells, and this conveys novel reactivity to the NK-cell system. 12,13 Altogether, these data imply an education mechanism, in which host MHC class I molecules both secure self-tolerance and convey functional capacity to NK cells. 6,7,9,10 The question of how MHC class I alleles influence NK-cell functionality is important for the understanding of hematopoietic stem cell transplantation across killer cell Ig-like receptor (KIR)/ HLA donor-recipient mismatched barriers, in which NK cells elicit therapeutically beneficial missing self-rejections of recipient leukemic cells. 14 Epidemiologic correlations between certain KIR/HLA genotypes and disease susceptibility and outcome constitute another area in which NK-cell education is of potential importance. [15][16][17] Several models have been proposed to explain the educational effects of MHC class I molecules on NK-cell function.Kim and colleagues suggested that NK cells are initially hyporesponsive and become "licensed" when their Ly49 receptors engage self-MHC class I during maturation. 10,18 In another model, NK cells are responsive by default, and become hyporesponsive, or "disarmed," in the absence of inhibitory input. 9,19 A commonly held view in NK-cell education is that an NK cell is either educated or not, depending on whether its inhibitory receptors are engaged during NK-cell education. This view is based on the notions that Ly49 receptors discriminate sharply between MHC class I alleles 20,21 and that the functional capacity of NK-cell subsets to develop missing self-activity depend on the MHC setup. 22 Our recent studies showed, however, that individual MHC class I alleles are not equally efficient in educating NK cells for the capacity to reject MHC-deficient cells in vivo; some display a good "educating impact" on the NK-cell system, while some MHC class I alleles are less efficient. 23 Here, we provide data that explain the difference in educating impact between MHC class I alleles. We show that the number and ...

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The ability of murine NK cells to reject cells lacking self MHC class I expression results from an in vivo education process. To study the impact of individual MHC class I alleles on this process, we generated mice expressing single MHC class I alleles (Kb, Db, Dd, or Ld) or combinations of two or more alleles. All single MHC class I mice rejected MHC class I–deficient cells in an NK cell–dependent way. Expression of Kb or Dd conveyed strong rejection of MHC class I–deficient cells, whereas the expression of Db or Ld resulted in weaker responses. The educating impact of weak ligands (Db and Ld) was further attenuated by the introduction of additional MHC class I alleles, whereas strong ligands (Kb and Dd) maintained their educating impact under such conditions. An analysis of activating and inhibitory receptors in single MHC class I mice suggested that the educating impact of a given MHC class I molecule was controlled both by the number of NK cells affected and by the strength of each MHC class I–Ly49 receptor interaction, indicating that NK cell education may be regulated by a combination of qualitative and quantitative events.

The in situ variation of dimethyl sulfide (CH3SCH3, DMS) at a fixed station in a coastal area of the Baltic Sea has been studied for a period of time (January 1987 to June 1988) covering the annual biological cycle. DMS in the surface waters of the brackish Baltic Sea showed a clear seasonal variation, ranging from 2 to 200 ng S L -1 . Lowest concentrations were in winter, peak values followed the spring bloom, and a pronounced maximum was found during the summer (July-August). Concentrations above low winter levels occurred only when the trophogenic layer was depleted of inorganic nitrogen. From our data it is clear that the seasonal variation in DMS concentration is related to biological activity. However, we did not find any correlations between DMS concentration and gross parameters such as chlorophyll a, total phytoplankton biomass, or primary production on an annual basis. Further, we were not able to relate high DMS concentrations to any particular phytoplankton species or species assemblages. It appears that DMS production is primarily associated with phytoplankton growth under nitrogen-limited conditions and not with certain species. We found a significant correlation of ambient DMS concentration with copepod and total zooplankton biomass, suggesting zooplankton grazing pressure as the major factor responsible for the liberation of /3-dimethylsulfoniopropionate (DMSP) from phytoplankton cells and thus for the DMS production. The turnover time of DMS in the water column was calculated to be of the order of 2 days, and the most effective sink process seems to be of microbiological and/or chemical origin. Previous to this study, no in situ data set has been available to test the relative importance of the various factors responsible for the DMS production in seawaters. We have demonstrated that variations in DMS concentration must be looked upon as the result of complex physiological as well as ecological interactions. INTRODUCTIONThe ocean surface layer plays an important role in the global biogeochemical sulfur cycle. Dimethyl sulfide (DMS) constitutes about 90% of the reduced volatile sulfur in surface seawaters [Wakeham et al., 1987; Leck and Bggander, 1988] and is far in excess of the concentration expected at equilibrium with atmospheric concentration [Lovelock et al., 1972; Maroulis and Bandy, 1977; Barnard et al., 1982; Cline and Bates, 1983; Dacey and Wakeham, 1984]. This gives rise to a flux of DMS from the ocean to the atmosphere estimated to be in the range 30-50 Tg S a -1 globally [Andreae, !986]. The total natural flux of gaseous sulfur to the atmosphere originating from oceans, continents, and volcanoes is estimated to be around 80 Tg S a -• . This figure is of the same order of magnitude as the anthropogenic sulfur flux from fossil fuel combustion [Cullis and Hirschler, 1980; Mbller, 1984]. In the boundary layer of the marine atmosphere, DMS is photochemically oxidized to non-sea-salt sulfate (NSS-SO42-) via intermediates such as sulfur dioxide (SO2) and methane sulfonic acid (CH3SO3 H) [And...

We used stable nitrogen isotopes to describe the pelagic food-web structure of three coastal Baltic Sea areas, each of which was sampled twice. Two of the areas were influenced by 15 N-rich nutrient discharges from a sewage treatment plant. Analyses were made of particulate organic matter (Ͻ35 m, mainly phytoplankton), zooplankton, mysids (Mysis mixta and M. relicta), sprat (Sprattus sprattus), smelt (Osmerus eperlanus), four size classes of herring (Clupea harengus), and pikeperch (Stizostedion lucioperca). Discharges from the sewage treatment plant significantly increased ␦ 15 N values in the whole food web, from phytoplankton to piscivorous fish. Based on nitrogen isotopic compositions, consistent trophic food-web structures were observed on both occasions and in all three areas. The results indicate that zooplankton and mysids may have more complex diets than assumed before. Apparent trophic fractionation, i.e., differences in ␦ 15 N between a consumer and its assumed food, averaged 2.4‰ with a standard error of Ϯ0.5‰. Differences between areas in fish ␦ 15 N show young-of-the-year herring, sprat, smelt, and pikeperch to be relatively non-migratory.

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We studied the seasonal dynamics of zooplanktivory by the major zooplanktivores (sprat Sprattus sprattus, herring Clupea harengus and the mysid shrimp Mysis mrxta) in 3 coastal areas of the northern Baltic Sea proper from the beginning of July through the end of October, 1985. The 3 areas are within 30 km of each other and dffer in nutrient loading and primary productivity. Consumption rates were obtained by combining abundance estimates of the planktivores (from Bongo nets, gill nets and acoustics) with diet analysis and bioenergetics models. Both the dominating planktivore groups and total planktivory rates changed over the study period. Sprat and yearling herring were the major zooplanktivores in July and August whereas young-of-year herring and M. mixta were more important in September and October. Planktivory rates increased from low levels at the beginning of July to a peak in August coinciding with a late summer decline in crustacean zooplankton biomass. Planktivory rates were lower than estimated zooplankton production rates in July and early August when zooplankton biomass was increasing and similar to or higher than production in the autumn when zooplankton biomass declined. Both clupeids and mysids consistently selected prey in the order cladocerans (Bosmina longispina maritima and Pleopis polyphemoides) > Eurytemora affinis hirundoides copepods > Acartia copepods. The selected species represented a smaller proportion of total zooplankton biomass and decreased earlier in the season in the least productive area, indicating a larger and earlier effect of planktivory in that area compared to the most productive area.

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