Using the monocyte activation test as a stand-alone release test for medical devices.

Monocyte activation tests (MAT) are widely available but rarely used in place of animal-based pyrogen tests for safety assessment of medical devices. To address this issue, the National Toxicology Program Interagency Center for the Evaluation of Alternative Toxicological Methods and the PETA International Science Consortium Ltd. convened a workshop at the National Institutes of Health on September 18-19, 2018. Participants included representatives from MAT testing laboratories, medical device manufacturers, the U.S. Food and Drug Administration's Center for Devices and Radiologic Health (CDRH), the U.S. Pharmacopeia, the International Organization for Standardization, and experts in the development of MAT protocols. Discussions covered industry experiences with the MAT, remaining challenges, and how CDRH's Medical Device Development Tools (MDDT) Program, which qualifies tools for use in evaluating medical devices to streamline device development and regulatory evaluation, could be a pathway to qualify the use of MAT in place of the rabbit pyrogen test and the limulus amebocyte lysate test for medical device testing. Workshop outcomes and follow-up activities are discussed.

The human plasma proteome: a nonredundant list developed by combination of four separate sources.

We have merged four different views of the human plasma proteome, based on different methodologies, into a single nonredundant list of 1175 distinct gene products. The methodologies used were 1) literature search for proteins reported to occur in plasma or serum; 2) multidimensional chromatography of proteins followed by two-dimensional electrophoresis and mass spectroscopy (MS) identification of resolved proteins; 3) tryptic digestion and multidimensional chromatography of peptides followed by MS identification; and 4) tryptic digestion and multidimensional chromatography of peptides from low-molecular-mass plasma components followed by MS identification. Of 1,175 nonredundant gene products, 195 were included in more than one of the four input datasets. Only 46 appeared in all four. Predictions of signal sequence and transmembrane domain occurrence, as well as Genome Ontology annotation assignments, allowed characterization of the nonredundant list and comparison of the data sources. The "nonproteomic" literature (468 input proteins) is strongly biased toward signal sequence-containing extracellular proteins, while the three proteomics methods showed a much higher representation of cellular proteins, including nuclear, cytoplasmic, and kinesin complex proteins. Cytokines and protein hormones were almost completely absent from the proteomics data (presumably due to low abundance), while categories like DNA-binding proteins were almost entirely absent from the literature data (perhaps unexpected and therefore not sought). Most major categories of proteins in the human proteome are represented in plasma, with the distribution at successively deeper layers shifting from mostly extracellular to a distribution more like the whole (primarily cellular) proteome. The resulting nonredundant list confirms the presence of a number of interesting candidate marker proteins in plasma and serum.

Characterization of the low molecular weight human serum proteome.

Serum potentially carries an archive of important histological information whose determination could serve to improve early disease detection. The analysis of serum, however, is analytically challenging due to the high dynamic concentration range of constituent protein/peptide species, necessitating extensive fractionation prior to mass spectrometric analyses. The low molecular weight (LMW) serum proteome is that protein/peptide fraction from which high molecular weight proteins, such as albumin, immunoglobulins, transferrin, and lipoproteins, have been removed. This LMW fraction is made up of several classes of physiologically important proteins such as cytokines, chemokines, peptide hormones, as well as proteolytic fragments of larger proteins. Centrifugal ultrafiltration of serum was used to remove the large constituent proteins resulting in the enrichment of the LMW proteins/peptides. Because albumin is known to bind and transport small molecules and peptides within the circulatory system, the centrifugal ultrafiltration was conducted under solvent conditions effecting the disruption of protein-protein interactions. The LMW serum proteome sample was digested with trypsin, fractionated by strong cation exchange chromatography, and analyzed by microcapillary reversed-phase liquid chromatography coupled on-line with electrospray ionization tandem mass spectrometry. Analysis of the tandem mass spectra resulted in the identification of over 340 human serum proteins; however, not a single peptide from serum albumin was observed. The large number of proteins identified demonstrates the efficacy of this method for the removal of large abundant proteins and the enrichment of the LMW serum proteome.

Differential affinity and cooperativity functions of the amino-terminal 70 residues of lambda integrase.

The site-specific recombinase (Int) of bacteriophage lambda is a heterobivalent DNA-binding protein that binds two different classes of DNA-binding sites within its recombination target sites. The several functions of Int are apportioned between a large carboxy-terminal domain that cleaves and ligates DNA at each of its four "core-type" DNA-binding sites and a small amino-terminal domain, whose primary function is binding to each of its five "arm-type" DNA sites, which are distant from the core region. Int bridges between the two classes of binding sites are facilitated by accessory DNA-bending proteins that along with Int comprise higher-order recombinogenic complexes. We show here that although the 64 amino-terminal residues of Int bind efficiently to a single arm site, this protein cannot form doubly bound complexes on adjacent arm sites. However, 1-70 Int does show the same cooperative binding to adjacent arm sites as the full length protein. We also found that 1-70 Int specifies cooperative interactions with the accessory protein Xis when the two are bound to their adjacent cognate sites P2 and X1, respectively. To complement the finding that these two amino-terminal domain functions (along with arm DNA binding) are all specified by residues 1-70, we determined that Thr75 is the first residue of the minimal carboxy-terminal domain, thereby identifying a specific interdomain linker region. We have measured the affinity constants for Int binding to each of the five arm sites and the cooperativity factors for Int binding to the two pairs of adjacent arm sites, and we have identified several DNA structural features that contribute to the observed patterns of Int binding to arm sites. Taken together, the results highlight several interesting features of arm DNA binding that invite speculation about additional levels of complexity in the regulation of lambda site-specific recombination.

Site-specific photo-cross-linking between lambda integrase and its DNA recombination target.

The site-specific recombinase (Int) of bacteriophage lambda is a heterobivalent DNA-binding protein and is composed of three domains as follows: an amino-terminal domain that binds with high affinity to "arm-type" sequences within the recombination target DNA (att sites), a carboxyl-terminal domain that contains all of the catalytic functions, and a central domain that contributes significantly to DNA binding at the "core-type" sequences where DNA cleavage and ligation are executed. We constructed a family of core-type DNA oligonucleotides, each of which contained the photoreactive analog 4-thiodeoxythymidine (4-thioT) at a different position. When tested for their respective abilities to promote covalent cross-links with Int after irradiation with UV light at 366 nm, one oligonucleotide stood out dramatically. The 4-thioT substitution on the DNA strand opposite the site of Int cleavage led to photo-induced cross-linking efficiencies of approximately 20%. The efficiency and specificity of Int binding and cleavage at this 4-thioT-substituted core site was shown to be largely uncompromised, and its ability to participate in a full site-specific recombination reaction was reduced only slightly. Identification of the photo-cross-linked residue as Lys-141 in the central domain provides, along with other results, several insights about the nature of core-type DNA recognition by the bivalent recombinases of the lambda Int family.