Primary liver tumors and liver metastasis currently represent the leading cause of cancer-related death. Commensal bacteria are important regulators of antitumor immunity, and although the liver is exposed to gut bacteria, their role in antitumor surveillance of liver tumors is poorly understood. We found that altering commensal gut bacteria in mice induced a liver-selective antitumor effect, with an increase of hepatic CXCR6 natural killer T (NKT) cells and heightened interferon-γ production upon antigen stimulation. In vivo functional studies showed that NKT cells mediated liver-selective tumor inhibition. NKT cell accumulation was regulated by CXCL16 expression of liver sinusoidal endothelial cells, which was controlled by gut microbiome-mediated primary-to-secondary bile acid conversion. Our study suggests a link between gut bacteria-controlled bile acid metabolism and liver antitumor immunosurveillance.

Bumblebees of any one species in Maine forage for pollen and/or nectar from a large variety of morphologically diverse flowers, but individuals have limited foraging repertoires at any one time. Unspecialized individuals were sometimes unsuccessful in extracting nectar and/or pollen from highly rewarding flowers.In any one area with a variety of concurrently blooming plants, the bumblebees had apparent species preferences. Superimposed on the species preferences were individual preferences. Individuals had primary foraging specialties (their majors) and secondary specialities (their minors). Minors were often bridges to new majors. Queens in Maine necessarily have several successive majors during their lifetime since the blooming time of the plants they utilize are brief relative to their life-span as foragers. However, the blooming time of most plants available to Bombus fervidus workers are long relative to their lifetime. Switching was rarely observed in these bees, even in some individuals observed daily for up to 1 mo at the same foraging area containing other plant species in bloom that were highly attractive to other individuals of the same bumblebee species.On a per flower basis, those flowers producing the most food rewards generally had the largest number of bees majoring from them, and the food rewards available were roughly comparable between different kinds of flowers, regardless of their differences in rates of nectar production.Specializing was usually preceded by sampling a variety of rewarding as well as nonrewarding flowers. When the flowers from which bees were majoring in an area were experimentally removed many of the bees sampled flowers of other concurrently blooming plants, but they generally did not switch to flowers from which the food rewards were being depleted by specialists, unless these were experimentally fortified with syrup. Upon finding superior food rewards in enriched blossoms, they switched immediately.Flower-specificity is related to site-specificity. Many bees shared the same foraging area, but different individual bees at the same site utilized the flowers of different plant species. When the foraging area contained landmarks, the bees visited clumps of flowers in a sequence (foraging path) that was generally repeated several times on the same foraging trip when the foraging site was small.The foraging behavior of bumblebees is discussed from a comparative standpoint with other bees and in relation to food distribution and availability in the environment.

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On their first 2 foraging trips out of the hive, young Bombus vagans workers visited, on the average, 4 or 5 different kinds of rewarding as well as unrewarding flowers, and few of each kind in succession. But, after 3—7 foraging trips, most of the bees specialized on jewelweed, which was the most numerous flower available with high nectar reward. When jewelweed specialists became numerous, and the food rewards in jewelweed declined, the bees resampled the reward spectrum. They again visited, and continued to visit, at least 3—4 different kinds of flowers on successive foraging trips in an enclosure where patch size was limited. Flowers in open inflorescences (aster, goldenrod) were handled appropriately from the start, but handling accuracy at zygomorphic flowers (jewelweed, turtlehead) was initially 40—50% at the first 10 flowers encountered, and increased to >90% in 60—100 flower visits. It is concluded that the most important problem faced by foraging bees attempting to enhance food intake is that of assessing the resources, which often change rapidly. Individual bees specialize on flowers yielding rewards that are "perceived" to exceed some minimum. However, the difference between perceived and actual rewards is, in part, determined by handling skills that are affected by foraging experience. Thus, optimal foraging in the bees must be explored from the perspective of long— rather than short—term energy balance.

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Differences in the foraging behavior of B. terricola workers on white clover, Trifolium repens, were examined on previously unvisited (filled) and depleted flowers, and as a function of flower-head density. 1. The number of florets visited per unit time was independent of flower head density from 20 to at least 290 heads/m, in part because the bees utilized more florets per head at low flower head densities, and also because they approached but rejected more flower heads at high rather than at low flower head densities. 2. Previously visited clover-heads were approached but often rejected, while unvisited heads were not rejected. 3. The bees behaved markedly different while foraging in patches of flowers which were available to all foragers, than in those which had been screened and contained on the average 3.9 times more sugar; they tendent to move through depleted areas and to concentrate in rich areas. On successive flower-head visits in depleted areas they moved more forward than backward (82% vs. 18%), while in rich areas they tended to move almost as much backward as forward 47% vs. 53%). 4. The distances of inter-head moves were approximately twice as long in depleted as in rich areas. 5. The bees visited almost as many florets per unit time in the rich as in the depleted areas (32 vs. 35 per min). But in the rich patches they probed on the average into 11.6 florets per head in contrast to only 2.3 florets per depleted head. 6. In an experiment with B. vagans workers foraging from Aconitum napellus inflorescences, the bees did not reject previously visited flowers, and they moved upward in successive flower visits on inverted as well as on unaltered inflorescences. On horizontal inflorescences they moved both right and left. The movements are not a direct response to nectar differences, nor to differences in average nectar distributions. The systematic foraging behavior on vertical inflorescences may thus be a mechanism of reducing the revisiting of just-emptied flowers.

Four common species of bumblebees in central Maine each utilized the nectar from a great variety of both native and exotic flowers. The number of individuals that utilized ~ nect.ar reso~,rce was directly re~ated to supplies available. Seasonally, = 92% of the st~~dmg crop of food energy available per day was utilized in 1972. No interference com-petitiOn was .observed, and competition was primarily exploitative. Small wild bees generally foraged at h~gh temperatures from the small food rewards left after Bombus exploitation.Overlap m resource utilization in the guild of bumblebees was avoided by differences in tongue length. Open (short-corolla) flowers were visited primarily by short-and secondarily by long-tongued bumblebees. Deep-corolla flowers were visited primarily by long-tongued bumblebees. Pollen flowers were utilized more by short-tongued than by long-tongued bumbleb~es. However, the bumblebee forager spectra on given flower species varied from one lo~ahty to ~he ne_xt as well as seas

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