Lipopolysaccharides in the cell walls of Gram-negative bacteria elicit toxic as well as potentially beneficial inflammatory responses in animals. It is now reported that tissue toxicity caused by lipopolysaccharides is preferentially reduced by an enzymatic activity in human neutrophils. Acyloxyacyl hydrolysis removes fatty acyl chains that are linked to the hydroxyl groups of 3-hydroxytetradecanoyl residues in the bioactive lipid A moiety of the lipopolysaccharides. Maximal acyloxyacyl hydrolysis reduced lipopolysaccharide tissue toxicity, as measured in the dermal Shwartzman reaction, by a factor of 100 or more. In contrast, the ability of the deacylated lipopolysaccharides to stimulate B lymphocytes to divide was decreased only by a factor of 12. It is suggested that during tissue invasion by Gram-negative bacteria acyloxyacyl hydrolysis may be a defense mechanism that reduces the toxicity of lipopolysaccharides while preserving some of their potentially beneficial inflammatory and immune stimuli.

The problem of controlling the attitude of spacecraft within a formation is investigated. A class of decentralized coordinated attitude control laws using behavior-based control is developed. The decentralized coordinated attitude control laws that comprise the class differ by the coordination architecture used by the spacecraft formation. The choice of behavior weights defines the coordination architecture. A corollary of Barbalat's Lemma is used to prove that the class of control laws globally asymptotically stabilizes the spacecraft formation. Convergence of the system is shown to be a consequence of the closed-loop equations of motion. Numeric simulation is used to reinforce the analytic results, and to briefly investigate the effect of coordination architecture on performance.

A B S T R A C T Although phenol-extracted gram-negative bacterial lipopolysaccharides (LPS) have been used to study the properties of endotoxins for many years, nothing is known about the behavior of native (unextracted) LPS in vivo. Accordingly, we have compared extracted and native forms of LPS with regard to their biological activity, their ability to bind to plasma high density lipoproteins (HDL), and their fate after intravenous injection into rats. The LPS of Salmonella typhimurium G-30 were labeled with [3H]galactose, and whole bacteria, bacterial outer membranes, outer membrane fragments (harvested from the bacterial culture supernatant), and phenol extracts of the bacteria were prepared. After defining the LPS, phospholipid, and protein composition of these preparations, we compared the activity of the LPS in phenol extracts and membrane fragments in two assays. In both the Limulus lysate assay and the rabbit pyrogen test, the LPS in phenol extracts were slightly more potent than the LPS in membrane fragments. We next studied the ability of the LPS in each preparation to bind to rat lipoproteins in vitro, and each preparation was then injected intravenously into rats for measurements of LPS-HDL binding and tissue uptake in vivo. Two patterns of lipoprotein binding were observed. Less than 25% of the LPS in both outer membranes and whole bacteria bound to HDL in vitro. When the outer membranes and whole bacteria were injected into rats, their LPS again bound poorly to HDL and they were rapidly removed from plasma into the liver and spleen. In contrast, >50% of the LPS in both culture supernatant membrane fragments and

Although previous research in ESL composition suggests a link between writing in a first and second language, few studies have investigated this relationship in the context of the revising process. This article examines revision in controlled L1 and L2 writing tasks. Four advanced ESL writers with differing first language backgrounds wrote two argumentative essays in their native languages and two in English. Revisions were then analyzed for specific discourse and linguistic features. The results, for the most part, indicate striking similarities across languages. However, some differences are noted, suggesting that while proficient writers are capable of transferring their revision processes across languages, they are also capable of adapting some of those processes to new problems imposed by a second language.

An overview of air-bearing spacecraft simulators is provided. Air bearings have been used for satellite attitude determination and control hardware veri cation and software development for nearly 45 years. It is interesting to consider the history of this technology: how early systems were rst devised and what diverse capabilities current systems provide. First a survey is given of planar systems that give a payload freedom to translate and spin. Then several classes of rotational air bearings are discussed: those which simulate three-axis satellite attitude dynamics. The subsequent section discusses perhaps the most interesting facilities: those that provide both translational and three-dimensional rotational freedom. The diverse capabilities each style of air-bearing testbed provides, the many settings they can be found in, and ways to improve facility performance are described.