Solid-phase immune electron microscopy (SPIEM) for rapid viral diagnosis

Investigations into shaking mink syndrome: an encephalomyelitis of unknown cause in farmed mink (Mustela vison) kits in Scandinavia

An apparently novel neurological disease clinically characterized by shaking, tremors, seizures, staggering gait, and ataxia was first observed in farmed mink kits in Denmark in 2000 and subsequently in Sweden, Denmark, and Finland in 2001, and again in Denmark in 2002. Lymphoplasmacytic encephalomyelitis was found in the affected kits. The lesions were most severe in the brainstem and cerebellum and consisted of neuronal degeneration and necrosis, neuronophagia, focal and diffuse gliosis, perivascular cuffs formed by lymphocytes, plasma cells and macrophages, and segmental loss of Purkinje cells. Testing was conducted to determine the cause of the disease, including general virological investigations (virus culture, negative-staining electron microscopy, immunoelectron microscopy, polymerase chain reaction for herpesviruses, adenoviruses, pestiviruses, and coronaviruses), tests for specific viral diseases (canine distemper, Borna disease, Louping ill, West Nile virus infection, tick-borne encephalitis, Aleutian disease), tests for protozoa (Toxoplasma gondii, Neospora caninum, Encephalitozoon cuniculi), bacteria (general culture, listeria, Clamydophila psittaci), and intracerebral inoculation of neonatal mice. The results of all these investigations were negative. One group of 3 mink kits inoculated intracerebrally with brain homogenate of affected mink developed clinical signs and histological lesions similar to those observed in naturally infected mink. Based on the histopathological features, it is postulated that the disease is caused by a yet unidentified virus.

Diagnostic electron microscopy is still a timely and rewarding method

BACKGROUND

Parallel to its technical development starting in the 1930s, electron microscopy (EM) became an important tool in basic and clinical virology. First utilized in the rapid diagnosis of smallpox, it developed to a diagnostic routine in the early 1960s using the negative staining technique. EM was applied to infected cell-cultures and also to 'dirty' specimens including urine, feces, vesicle fluid, liquor. With the implementation of molecular biological and genetic techniques, the use of diagnostic EM decreased.

OBJECTIVES

(1) To give a perspective on future indications and possible uses by discussing the past and the present of diagnostic EM, (2) To describe the system of External Quality Assessment on EM virus diagnosis (EQA-EMV) established in 1994 by our laboratory and its achievements.

STUDY DESIGN

EQA-EMV is run to evaluate, to confirm and to improve the quality of diagnostic EM. Two different types of specimen are sent out: (1) prepared grids to assess and train the diagnostic skills of the participants, (2) stabilized virus particle suspensions to assess preparation efficiency.

RESULTS

Diagnostic EM differs from other diagnostic tests in its rapidity and its undirected 'open view'. To emphasize these advantages, the indications for diagnostic EM are discussed, fundamental for a continuing future adaptation. Besides appropriate techniques, quality control measures are required to achieve and keep high diagnostic standards. The results from 6 years of EQA-EMV are presented.

CONCLUSIONS

In the history of diagnostic EM in virology, a change in use has been seen. Starting in the 1990s and coincident with the broad introduction of 'modern' diagnostic techniques, the number of EM diagnostic labs has decreased considerably--in spite of the obvious advantages of this technique. To guarantee the continuing performance of diagnostic EM in the future. EQA runs have to be performed as with other techniques in the diagnostic armament. The growing number of participants and participating countries indicates an interest in as well as a need for this program.

Detection of HIV-p24 antigen in body fluids by immunotrapping on Staphylococcus aureus (Cowan 1) bacteria, gold immunolabelling and backscattered electron analysis in a scanning electron microscope

An immunosorbent electron microscopical (ISEM) method, the Protein A-coated bacteria technique/gold, (PA-CBT/G), was developed for the detection of non-particulate soluble antigens. The method is based on immunotrapping of antigens on antibody coated, glutaraldehyde cross-linked, Protein A-rich, Staphylococcus aureus bacteria. The 'trapped' antigen is then identified by colloidal-gold immunolabelling. Gold particles are observed in a scanning/transmission electron microscope by analysis of backscattered electrons. With this method it was possible to detect the presence of p24 HIV antigens in blood, semen, saliva, crevicular and cerebrospinal fluids from HIV seropositive cases. Although the PA-CBT/G identified correctly the p24 antigen in only 80% of the ELISA HIV-antigen positive sera, it detected, more frequently than ELISA, HIV-antigen in seminal and oral fluids. The PA-CBT/G method could thus be useful to fully characterise individual HIV excretion patterns in body fluids other than sera even from patients negative for HIV-antigen by ELISA.

Negative staining in the detection of viruses in clinical specimens

Viruses have unique morphology and are therefore good candidates for negative staining. Negative staining with phosphotungstic acid (PTA) or uranyl acetate has facilitated the detection of many viruses in clinical specimens. Enhancement procedures have included the use of centrifugation and agar diffusion for concentrating virus particles, the use of solid phase capture reagents to trap virus particles and the use of secondary antibodies and electron dense markers to help visualize them. Techniques currently in use and employing negative staining include direct EM, immune electron microscopy (IEM), solid phase immune electron microscopy (SPIEM), colloidal gold-labeled protein A (PAG), solid phase IEM employing a second decorator antibody (SPIEMDAT), and solid phase IEM using colloided gold-labeled secondary antibodies (SPEIMDAGT). IEM methods assist with the detection of small viruses or viruses present in low numbers while PAG offers increased sensitivity over direct EM and IEM. In our experience the serum-in-agar (SIA) method is the most sensitive of the PAG IEM techniques for detection of rotavirus particles in clinical specimens. SPIEMDAT enhances the detection of small viruses which are often missed by other techniques due to background staining in specimens. SPEIMDAGT employing colloidal gold-labeled secondary antibody has increased sensitivity and offers the advantage of detecting viral antigen when whole virus particles are not visible. IEM techniques have recently been used for typing viruses using either monospecific antisera or monoclonal antibodies and colloidal gold-labeled secondary antibody.

The grid-cell-culture technique: the direct examination of virus-infected cells and progeny viruses

We describe a method for the structural analysis and identification of viruses, without purification or concentration steps which could alter virus morphology. Virus-infected cells grown on carbon-Parlodion-coated electron microscope grids release large numbers of progeny viruses which adsorb to the surface of the grid and are revealed by negative staining. The technique is rapid, sensitive and can be used at three levels. Negative staining of whole cell preparations revealed both extracellular and intracellular viruses or nucleocapsids beneath the plasma membrane; non-ionic detergent extraction of cells infected with certain viruses reveals cytoskeleton-associated, virus-specific structures normally only observed after thin sectioning; cultures prepared by either procedure are suitable for colloidal gold immunological studies. Extracellular and cytoskeletal-associated viruses were heavily and specifically labelled with gold. The results indicate that the technique may be used to rapidly identify unknown viruses on the basis of size, topography, morphology and mode of maturation from the infected cell, as well as the presence of characteristic intracellular cytoskeletal-associated structures. The technique also has potential use in the sero-grouping and sero-typing of viruses with specific monoclonal antibodies.

Demonstration of calicivirus in human faeces by immunosorbent and immunogold-labelling electron microscopy methods

Immunosorbent electron microscopy (ISEM) and immuno-gold staining (IGS) electron microscopy methods have been applied to human faeces, shown by direct electron microscopy (EM) to contain calicivirus. Caliciviruses were successfully trapped on grids coated with positive rabbit or human antisera against calicivirus, but not with negative sera. Caliciviruses were specifically labelled with gold particles, when treated with positive rabbit or human antisera against calicivirus followed by protein A-gold and goat anti-rabbit or anti-human IgG gold conjugates in an indirect method in suspension. Goat anti-human IgM gold complexes did not react with the available antisera. Attempts to label caliciviruses trapped on the grids were unsuccessful. No cross-reactions were observed with Norwalk agent-like particles, hepatitis A virus or poliovirus type 3 by any of the methods. The results indicate that ISEM and IGS may be useful techniques for detection and identification of small viruses present in low concentrations in faeces.

Use of protein A in the serum-in-agar diffusion method in immune electron microscopy for detection of virus particles in cell culture

A modified technique using protein A in the serum-in-agar (SIA) method for immune electron microscopy (IEM) was presented. Grids coated with staphylococcal protein A were floated on samples mounted on agar containing 2% antiserum and incubated at 37 C, for 60 min. After washing and staining, the grids were observed in an electron microscope. The effects of protein A on virus detection were evaluated using poliovirus and bovine rotavirus infected cell culture fluids. The results showed that the technique using protein A (PA-SIA) had at least 10-fold higher sensitivity for virus detection than the original SIA. The optimal concentration of protein A was 1 to 10 micrograms/ml for coating the grids to trap virus particles. The PA-SIA method was also compared with immunosorbent electron microscopy (ISEM). The former showed higher or at least the same sensitivity and some advantages in detecting antigen-antibody reaction than the latter method. These results indicate that our PA-SIA method may be superior to other IEM techniques presented previously for the detection and identification of viruses.

Immunonegative stain techniques for electron microscopic detection of viruses in human faeces

Immune electron microscopy techniques have for some years been applied to detection of viruses in clinical specimens, especially faecal samples, as both sensitivity and specificity are improved by use of specific antibody. The following review describes in detail different preparation methods and illustrates some of the results that may be obtained.

A simplified microwell pseudoreplica for the detection of viruses by electron microscopy and immunoelectron microscopy

Simplified procedures for immunoelectron microscopy (IEM) and electron microscopy (EM) are described. The procedures employ the principle of agar filtration and pseudoreplication. The modification consisted of the use of microwells for storage of gels with or without antiserum (for IEM or EM, respectively) and an array of containers in which pseudoreplication and negative staining were performed. The containers were prepared from 5 ml syringes from which the needle holding parts were cut. This device enabled simultaneous and rapid handling of specimens. With Sindbis virus as a model, our microwell pseudoreplica IEM (MW-PR-IEM) was compared to six other IEM techniques and was found to be the most rapid and sensitive technique. With the MW-PR-IEM technique, the specific minimal detection limit (detection of clumps) was 1.5 x 10(7) virus particles per ml, and the non-specific detection limit (detection of single virions) was 1.8 x 10(6) virus particles per ml.

Visualization of herpes simplex virus type 1 attachment to target cells using Staphylococcus aureus as a morphologic tag

Staphylococcus aureus was used as a morphologic tag to allow light microscopic localization of herpes simplex virus type 1 (HSV-1) binding and attachment to HEp-2 target cells. The virus was bound to S. aureus through an anti-HSV-1 linkage. The complex was stable and the attached virus still infectious.

Evaluation of immunoadsorbent electron microscopic techniques for detection of Sindbis virus

Two immunosorbent electron microscopic techniques (ISEM), the protein A coated grid technique (PA-CGT) and the antibody coated grid technique (AB-CGT) were applied and evaluated for the detection of Sindbis virus from infected tissue culture fluids. At optimal conditions, the efficiency of trapping the virions was only about 1.5 higher with the PA-CGT as compared to the AB-CGT, but the PA-CGT was less dependent on the antiserum dilution used in the test. Both methods were suitable for quantitation experiments, since the number of virions trapped was proportional to the virus concentration. The influence of virus incubation time and temperatures, staining solutions, buffers and washing procedures on the trapping efficiency and specificity was further studied with the PA-CGT. Maximal trapping on coated grids was obtained after 3 h incubation of the virus. At room temperature, less debris was found on the grids, as compared to 37 degrees C, and the numbers of virions counted were only slightly lower. The optimal staining solution was alcohol uranyl acetate. The specificity of the PA-CGT was dependent on washing steps with phosphate buffered saline containing bovine serum albumin. With the standard procedure, at room temperature around 3 X 10(7) virions/ml (1 X 10(6) PFU/ml) were specifically detected in about 1.5 h.

Herpes simplex virus binding and entry modulate cell surface protein mobility

Fluorescence photobleaching recovery measurements showed that herpes simplex virus type 1 attachment to target cells rapidly induced an anchorage modulation of cell surface protein mobility, an activity mediated by the cytoskeleton and associated with the multivalent attachment of other ligands (e.g., cells, lectins, or anti-immunoglobulin) to cell surfaces. The restriction in cell surface protein mobility was released concurrently with virus penetration. The effects of attachment and penetration on cell surface protein mobility and cytoskeletal function are some of the earliest cellular changes induced by herpes simplex virus infection.

Immunosorbent electron microscopy for detection of viruses

This chapter discusses the newer modifications of immunosorbent electron microscopy (ISEM) methods in both plant and animal virology. ISEM methods presented in the chapter include all the techniques where the “solid phase principle” is essential in a way similar to other solid phase immunoassays. These methods include (1) the antibody-coated grid technique (AB-CGT); (2) the protein A-coated grid technique (PA-CGT); (3) the protein A-coated bacteria technique (PA-CBT); and (4) the antigen-coated grid technique (AG-CGT). In all ISEM methods, one of the components of the system is adsorbed to a solid phase. In AG-CGT, PA-CGT, and AB-CGT, one of the reagents is adsorbed to an electron microscopic grid, while in PA-CBT protein A is naturally present on the surface of a bacterium that serves as a solid support. In ISEM methods, the viruses can be statistically evaluated and numerically expressed as number of virions per unit of area, and can, therefore, be statistically evaluated. Thus, these methods optimize the results of a test by quantifying the effects of the quality of the supporting grid, the time of adsorption, the pH, the presence of salts, and the type of staining. The ISEM also permits a detailed study of antigenic variations in the same genus of virus, and thus would visually pinpoint the type or strain differences.

Demonstration of virus particles within immune complexes by electron microscopy

A method has been developed to detect virus particles within immune complexes by electron microscopy (VICEM test). Radioiodinated or unlabeled adenovirus type 2 immune complexes (IC) in human serum were used to determine the optimal conditions. Best results were obtained when the immune complexes were precipitated with 5% polyethylene glycol 6000, adsorbed with 0.5% of protein A-containing Staphylococcus aureus (Cowan 1) and eluted, with brief sonication, in 2 M propionic acid. When the eluate was examined by the pseudoreplica method of electron microscopy, morphologically intact viruses, concentrated over five-fold and free from background serum proteins, were readily demonstrated. The method is applicable to virus IC with a wide-range of antigen-to-antibody ratios and requires a minimum of 10(4)--10(5) virus particles. This method should be useful for the characterization of IC in clinical specimens obtained from patients with viral infections and for the possible demonstration of viral agents in immune complex diseases of unknown etiology. A modification of the technique can also be used for the rapid serological identification of viruses by electron microscopy, as exemplified by studies using adeno- and herpes simplex viruses.

Comparison of two methods of virus detection by immunosorbent electron microscopy (ISEM) using protein A

The efficiency of two methods of immunosorbent electron microscopy has been compared. The first method consists in trapping virus particles by means of Staphylococcus aureus cells coated with a layer of viral antibodies; the second method consists in trapping virus particles o electron microscope grids coated with specific antibody. A suspension containing 107 antibody-coated bacteria trapped the total number of virions present in 1 ml of a 500 ng/ml virus preparation; the cells were then fully saturated with virions, and approximately 100 virions (of 30 nm diameter) were visible at the periphery of each cell. When 107 cells/ml were used the minimum virus concentration needed to see one virion at the cell periphery was 5 ng/ml. Antibody-coated grids allowed for the detection of approximately the same quantity of virus, but the data obtained with the method were more reproducible and suitable for quantitation.

Solid-phase immune electron microscopy-double-antibody technique for rapid detection of papovaviruses

The solid-phase immune electron microscopy-double-antibody technique, which takes less than 1 h to perform, was applied as a rapid, sensitive, and specific diagnostic tool in the demonstration of papovavirus particles. BK virus propagated in 82C human skin fibroblasts and a monospecific high-titer immune serum to BK virus were used to establish the test procedure. When Formvar-carbon-coated grids were treated with appropriately diluted antibody, a 28-fold increase of virus particles per square micrometer was observed. Viewing of the virus particles was facilitated by the addition of a second "decorator" antibody. BK virus preparations at concentrations of 10(2) to 10(3) PFU/ml could be detected by this technique. There was no cross-reaction with mouse polyomavirus.