Adjustment for time-invariant and time-varying confounders in 'unexplained residuals' models for longitudinal data within a causal framework and associated challenges.

'Unexplained residuals' models have been used within lifecourse epidemiology to model an exposure measured longitudinally at several time points in relation to a distal outcome. It has been claimed that these models have several advantages, including: the ability to estimate multiple total causal effects in a single model, and additional insight into the effect on the outcome of greater-than-expected increases in the exposure compared to traditional regression methods. We evaluate these properties and prove mathematically how adjustment for confounding variables must be made within this modelling framework. Importantly, we explicitly place unexplained residual models in a causal framework using directed acyclic graphs. This allows for theoretical justification of appropriate confounder adjustment and provides a framework for extending our results to more complex scenarios than those examined in this paper. We also discuss several interpretational issues relating to unexplained residual models within a causal framework. We argue that unexplained residual models offer no additional insights compared to traditional regression methods, and, in fact, are more challenging to implement; moreover, they artificially reduce estimated standard errors. Consequently, we conclude that unexplained residual models, if used, must be implemented with great care.

Improved methods for immunoassay of mycothiol.

Improved enzyme-linked immunosorbent assay (ELISA) methods have been developed for the determination of femtomole amounts of mycothiol (MSH), the main low-molecular-weight thiol in mycobacteria. The immunoassays utilize an affinity-purified rabbit polyclonal antibody that is highly specific for the pseudodisaccharide moiety of MSH. MSH was first biotinylated by the thiol-specific reagent 3-(N-maleimidopropionyl)biocytin. The MSH-biotin adduct was then captured with immobilized avidin and detected with anti-MSH antibody (biotin-capture ELISA) or was captured with immobilized anti-MSH antibody and detected with alkaline phosphatase-labelled avidin (MSH-capture ELISA). The MSH-capture ELISA was the most sensitive method, measuring as little as 0.3 fmol of MSH. Methods for biotinylating MSH directly from Mycobacterium spp. are described. The MSH-capture ELISA was tested for the detection of M. avium seeded in human urine or cerebrospinal fluid samples and for screening mutant M. smegmatis strains to detect MSH production.

Neutralization of meningococcal endotoxin by antibody to core glycolipid.

Antibodies to Escherichia coli J5, a uridine 5'-diphosphate-galactose epimerase-less mutant of E. coli 0111, neutralized meningococcal endotoxemia from all three major capsular serogroups. We chose the dermal necrosis of the local Shwartzman phenomenon and the renal cortical necrosis of the general Shwartzman phenomenon as assays because these are the hallmarks of meningococcemia, and because meningococcal lipopolysaccharide (LPS) is a uniquely potent cause of dermal purpura and necrosis. Meningococcal antisera raised against LPS from MGC A, B, and C also provided good protection against endotoxemia from the homologous capsular groups, but it was inconsistent against the heterologous serogroups. The superiority of J5 antibodies (purified IgG as well as antiserum) is probably due to the fact that J5 LPS contains only the endotoxin core. Consequently, immunization with this mutant stimulates production of antibodies to core LPS without interference by the "0" antigenic determinants of the side chains. These observations indicate that the endotoxin core is the toxic moiety of meningococcal LPS, that the core LPS of meningococcus (MGC) is immunologically similar to enteric LPS, and that the antigenically variable "0" side chains of MGC LPS interfere with antibody production against the common core. They also suggest that antibodies prepared against this E. coli mutant could interrupt the devastating course of meningococcal endotoxemia in man, regardless of the capsular serogroup of the infecting strain.