Glutaraldehyde fixation of target cells to plastic for ELISA assays of monoclonal anti-HLA antibodies produces artefacts

Severe Acute Respiratory Syndrome Coronavirus 2 Pandemic

CONTEXT.—

The novel coronavirus disease 2019 (COVID-19) pandemic is significantly changing methodologic approaches in all branches of the health system. From a forensic point of view, this event is partly changing the manner in which forensic pathologists and all those who work in autopsy services operate, but above all, it is changing the patterns established for years by which cadavers are analyzed postmortem.

OBJECTIVE.—

To present a review of the literature and a proposal for COVID-19 autopsy protocols. To contain the infection risk, a revision of all the protocols that until now have been applied to the examination of bodies that require autopsy services is required.

DATA SOURCES.—

Currently, the diagnosis and postmortem analysis of positive or suspected COVID-19 cases plays a crucial role in scientific research. A review of the main recommendations proposed by international scientific societies regarding the risk of infection during autopsy was carried out. Scientific papers currently available via the PubMed NCBI search engine on COVID-19 postmortem diagnosis were also examined.

CONCLUSIONS.—

Throughout the history of medicine, autopsy has been fundamental to the understanding of multiple pathogenic processes that are investigated postmortem. The purpose of the study is to propose an operating protocol that can be useful for all clinical and forensic autopsies, with particular reference to the correct methods to be applied to the examination of positive or suspected COVID-19 cases, regarding both the autopsy procedure and the collection and analysis of biological samples.

Immunoenzymatic quantitative analysis of antigens expressed on the cell surface (cell-ELISA)

Cell-enzyme-linked immunosorbent assay (cell-ELISA) is an useful technique for the quantitative analysis of cell surface antigen expression that was developed on the basis of enzyme immunohistochemistry (EIH) and ELISA. Since its development, which was made possible by the establishment of monoclonal antibody technology, a wide range of cell types and surface molecules were analyzed by cell-ELISA. Here we show four variants of this method and provide a brief comparison of cell-ELISA with flow cytometry (FACS) and radioimmunobinding assay (RIA), which are other methods for the quantitative detection of cell-surface molecules. We describe step-by-step procedures for both direct and indirect cell-ELISA using either adherent or nonadherent live cells.

Wegener's granulomatosis is associated with organ-specific antiendothelial cell antibodies

BACKGROUND

Antiendothelial cell antibodies (AECA), usually detected using human umbilical vein endothelial cells (HUVEC), are frequently observed in systemic vasculitis, but their pathogenic role is unclear. Heterogeneity of endothelial cells necessitates use of clinically relevant endothelial cells for elucidation of the role of AECA in systemic vasculitis involving small blood vessels of specific organs.

METHODS

Human endothelial cells were isolated from normal tissue specimens from the nose, kidney, lung, liver, and umbilical vein. Using flow cytometry, AECA were detected against both unstimulated and cytokine-stimulated [tumor necrosis factor-alpha (TNF-alpha) and interferon-gamma (IFN-gamma)] endothelial cells. Functional capacity of AECA was determined by complement fixation assay. Sera from patients with Wegener's granulomatosis (16), limited Wegener's granulomatosis (8), renal limited disease (4), microscopic polyangiitis (MPA) (5), rheumatoid arthritis (10), and systemic lupus erythematosus (SLE) (9), and from healthy controls (20) were analyzed.

RESULTS

Compared with controls (1) Wegener's granulomatosis is significantly associated with noncytotoxic AECA that selectively bind surface antigens on unstimulated nasal, kidney, and lung endothelial cells; (2) binding of Wegener's granulomatosis AECA to kidney and nasal endothelial cells in particular was lost upon treatment with IFN-gamma and TNF-alpha; (3) the two cytokines per se were cytotoxic (30%) to nasal and lung endothelial cells and lysis was further increased (60%) by addition of systemic vasculitis serum; and (4) Wegener's granulomatosis serum caused agglutination of cytokine-stimulated nasal endothelial cells.

CONCLUSION

Based on these findings we suggest that AECA may be one factor involved in the initiation of Wegener's granulomatosis. Antigen identification and elucidation of the pathogenic roles of AECA and inflammatory cytokines in systemic vasculitis using these cells will be particularly important.

HLA-DP epitope typing using monoclonal antibodies

We have made a panel of murine anti-DP monoclonal antibodies for serological typing of HLA-DP polymorphisms; they can be used in microcytotoxicity (for 7 epitopes) and binding assays (for 8 epitopes). The antibodies detect polymorphic differences in both alpha and beta chains. As immunogens we sometimes used B-lymphoblastoid lines or purified DP molecules but mostly used mouse fibroblast transfectants expressing DP molecules. The DP beta genes were made from a cloned DPB1*0201 gene by replacing its major area of polymorphism with matching stretches of DNA amplified from other alleles; cloned DPA1*01 and DPA1*02 genes were used for transfection along with the beta chain genes. The monoclonal antibodies showed reaction patterns that correlated with the presence of particular amino-acid sequence motifs; thus none of the antibodies is allele-specific. They bind instead to epitopes which are found on a number of different HLA-DP types. We have constructed frequency tables so that the epitope (motif) data can be interpreted as the most likely genotype in each case. The basic assumption to justify this work is that HLA-DP matching or mismatching will likely influence transplant outcome, particularly in bone marrow transplantation. The present challenge is to define permissive and nonpermissive combinations of HLA-DP; it may be that matching for epitopes, rather than for full alleles, will help to resolve this issue.

Detection of antiendometrial antibodies in patients with endometriosis by cell ELISA

PROBLEM

To determine whether infertile patients with endometriosis have serum antiendometrial antibodies.

METHODS

Sera from 40 infertile patients with or without endometriosis were tested by cell enzyme-linked immunosorbent assay (ELISA), in which endometrial cancer cells were used as endometrial antigens, and uterine cervix cancer cells as control antigens. As a negative control, eight healthy adult males were included. The level greater than the mean +/- 2 standard deviations (SD) of the male control group was judged positive.

RESULTS

The mean value of antiendometrial antibody level was significantly higher in patients with endometriosis than in those without endometriosis (ANOVA, P < 0.01). The frequency of antiendometrial antibody-positive patients was also higher in the former than in the latter (chi 2 test, P < 0.05). However, when uterine cervix cancer cells were used as antigens, no difference was observed in the mean antibody levels or in the positive rates between the two groups.

CONCLUSIONS

Endometriosis seems to be associated with autoantibody production against the endometrium-related antigen(s).

Identification of woodchuck class I MHC antigens using monoclonal antibodies

Two class I major histocompatibility complex (MHC) proteins with molecular masses of 43- and 39-kDa were identified in the cell surface membranes of normal woodchucks using a newly developed antiwoodchuck class I monoclonal antibody (mAb) B1b.B9 and immunoblotting. B1b.B9 was generated by immunizing mice with viable woodchuck peripheral blood mononuclear cells and was selected for anti-class I MHC reactivity using a cellular enzyme-linked immunoassay, indirect immunofluorescence on tissue sections and flow cytofluorimetry. The distribution pattern of class I MHC antigen on woodchuck lymphoid cells was found to be similar to that reported in other species. Also, the antigen expression on normal woodchuck hepatocytes was comparable to that observed on normal human liver parenchymal cells; thus, the antigen was not detected on hepatocytes by staining of liver tissue sections, but was found by indirect immunofluorescence staining of isolated liver cells. Western blot analysis of the plasma membranes from normal woodchuck hepatocytes revealed the presence of a single species of class I MHC heavy chain protein with a molecular mass of 43-kDa, whereas splenocyte plasma membranes showed intense expression of a 43-kDa species, as well as the presence of a 39-kDa protein. The 39- and 43-kDa proteins were extracted with Triton X-114 to the hydrophobic protein phase, suggesting that they both contain a hydrophobic transmembrane domain. The data obtained indicate that the B1b.B9 identifies a nonpolymorphic epitope of woodchuck class I MHC heavy chains, providing an important reagent for the study of the pathogenesis of hepatitis B virus infection in a woodchuck model.

Coordinate defects in human histocompatibility leukocyte antigen class II expression and antigen presentation in bare lymphocyte syndrome

The human immunodeficiency, type II bare lymphocyte syndrome (BLS), has been attributed to a defect in the transcription of class II histocompatibility genes. Immunocompetence, as assessed by functional exogenous antigen presentation, was not restored in immortalized B cells, derived from a BLS patient, after transfection with HLA-DR class II structural genes. Incubation of protein antigens, as well as infectious virus, with DR-transfected BLS cells failed to induce activation of antigen-specific helper T lymphocytes. Peptide antigens were presented by class II molecules displayed on BLS cells, although the conformation of these class II proteins was altered as indicated by epitope mapping. This defect in antigen presentation was independent of the specific class II DR allele transfected into BLS cells. Genetic complementation analysis has been used with BLS cells to demonstrate that the defect in class II gene transcription is linked to the absence of a trans-acting factor. Similarly, functional class II dimers were restored after in vitro fusion of cells derived from two distinct BLS complementation groups, implying that specific transcriptional control elements are shared by a gene critical for antigen presentation and genes encoding HLA class II antigens. Thus, two important functionally linked pathways of class II molecules, structural gene expression and antigen presentation, share a common regulatory pathway defective in BLS.

Amino acids in the peptide-binding groove influence an antibody-defined, disease-associated HLA-DR epitope

A shared amino-acid sequence on the alpha helix of certain DR beta 1 chains is predicted to generate a 'shared epitope' that is implicated in susceptibility to the development of rheumatoid arthritis (RA). Different relative risks (RR) for disease susceptibility and severity conferred by these DR beta 1 chains suggest that their 'shared epitopes' are not equivalent. A set of monoclonal antibodies (MoAb) that map to the critical region, and for which optimal binding depends on DR context and cell lineage, was used to test this idea. Mapping experiments using mutated DR beta 1* molecules showed that the antibody-binding epitopes are overlapping; residue 70Q is pivotal for each, but neighbouring residues on the alpha helix and on the floor of the groove are also involved. Importantly, these epitopes are profoundly modified by peptide loading of DR beta 1*0401 molecules. These data suggest that 'shared epitopes' on DR molecules that are associated with RA are influenced by their context; such structural modifications may be the basis for the varying susceptibilities conferred by these DR molecules for the development of RA.

A cell surface ELISA for the screening of monoclonal antibodies to antigens on viable cells in suspension

To simplify the screening of monoclonal antibodies to different human T cell surface molecules a live cell enzyme-linked immunosorbent assay (cell ELISA) has been established and optimized. The assay was performed in 96-well plates. By using living human T lymphocytes in suspension surface modification by fixation or insolubilization of the cells was avoided. Several parameters influencing sensitivity and specificity were studied. About 150 ng/ml of mouse monoclonal antibodies to cell surface antigens could be detected when using 5 x 10(4) cells per well and a 1/1000 dilution of the anti-mouse IgG-alkaline phosphatase conjugate. This sensitivity permitted the primary screening of cell specific antibodies from hybridoma supernatants. The same detection limit was obtained in flow cytometric analysis. If required, the sensitivity of the cell ELISA could be increased using higher cell numbers and conjugate concentration. When analysing different cell lines with selected antibodies the cell ELISA was found to be as sensitive and specific as the fluorescence assay. The assay was applied to the screening of supernatants from hybridomas developed against human T helper cell clones and the detection of V beta specificities of T cell clones.

Analysis of monoclonal antibodies specific for unique and shared determinants on HLA-DR4 molecules

The specificities for seven mAbs to HLA-DR4 were determined initially using homozygous BCLs and L-cell transfectants expressing wild-type DR molecules. Three antibodies (NFLD.D1, NFLD.M1, and NFLD.D7) bound all DR4 molecules, but only one was specific for DR4. Four antibodies (NFLD.D2, NFLD.D3, NFLD.D8, and NFLD.D10) reacted with some but not all DR4 subtypes and had extra reactions, particularly with DR gene products associated with susceptibility to RA. To localize the antibody-binding epitopes on DR4 molecules, the antibodies were then analyzed on transfectants expressing hybrid genes, which were generated by exon shuffling of DRB1*0403 and DRB1*0701. Two of the pan-DR4 antibodies bound epitopes that require the beta 2 domain while the third mapped primarily to the HVR-I region. One antibody NFLD.D10 to subtypes of DR4 mapped to residues 40-97 on DR beta 1*0403 chains. Comparison of reaction patterns with amino acid sequences suggest that the antibodies against subtypes of DR4 are specific primarily for a region containing sequences postulated to determine susceptibility to RA.

Immunoenzymatic assay that measures the expression of murine histocompatibility antigens in macrophages and lymphocytes

A convenient--simple, sensitive, rapid and reproducible--enzyme immunoassay to measure H-2 particulated and solubilized cellular antigens is described. Cellular antigens were measured by ELISA through the binding of specific biotinylated antibodies and streptoavidin-peroxidase conjugate to cells in suspension. Endogenous peroxidase activity of activated cells was inhibited by addition of sodium azide and H2O2 in acid conditions. The assay proved capable of distinguishing between two cell lines (EL-4/H-2b and P815/H-2d) and even between the cells of three congenic mouse strains (BALB/B, H-2b, BALB/c, H-2d, and BALB/K, H-2k) and was sensitive to as few as 2.5 x 10(4) cells/well. Results were comparable to those obtained with FACS. An inhibition version of this assay was found to be very useful for the detection of H-2 antigens present in whole antigen cells extracts.

Cellular enzyme-linked immunocircle assay. A rapid assay of hybridomas produced against cell surface antigens

A novel method has been developed for the initial screening of hybridomas produced against cell surface antigens. Glutaraldehyde-fixed cells were immobilized as targets on the lid of a 96-well tissue culture plate which had been precoated with poly-L-lysine. Antibody binding was determined by an immunoenzymatic method in an arrangement permitting both macro- and microscopic examination. After optimization with mouse thymus cells using existing rat monoclonal antibodies, new rat-mouse hybridoma cell lines against mouse thymocytes and bone marrow cells were screened. The antibodies could be characterized immediately both by the localization of the immune reaction (surface or intracellular) or as estimated by the frequency of positive cells recognized by the antibody in the sample.

Improved fixation of frozen lympho-haemopoietic tissue sections with hexazotized pararosaniline

In this study, a simple single-step fixation method for frozen tissue sections is introduced using the hexazotized salt of Pararosaniline as preservative agent. Tissue preservation by this method was shown to be superior to the commonly-used fixation with acetone. Fixation with hexazotized Pararosaniline caused a minimal loss of antigenicity as demonstrated using twenty-three monoclonal antibodies directed against lympho-haemopoietic and stromal cells.

Transfection of HLA genes using genomic DNA

Mouse L cells expressing HLA genes are potentially useful for producing and analyzing monoclonal antibodies (mAb) to HLA molecules. This paper describes the preparation of transfectants using uncloned human DNA and three methods to isolate the HLA-expressing transfectants. Transfectant libraries were made by cotransfecting mouse thymidine kinase (tk)-deficient L cells with a calcium phosphate precipitate containing genomic DNA and tk plasmid DNA. Transfectants expressing HLA genes were isolated using these methods: immunomagnetism, replicate-plating combined with cellular enzyme-linked immunoassay, and sorting using a fluorescence-activated cell sorter. Two HLA-A2 transfectant were isolated using immunomagnetism, two HLA-A24 transfectants by replicate plating, and one HLA-Bw60 transfectant by FACS. However, no transfectants were isolated that stably expressed class II genes. The class I transfectants have been useful in characterizing several locally prepared mAbs which bind to monomorphic determinants on class I HLA molecules. Two of the transfectant lines, one expressing HLA-A2 (8001) and the other HLA-A24 (8008), have been included in the collection of lines distributed for use in the Eleventh International Histocompatibility Workshop.

Comparative study of cell-immunoenzymatic methods for the estimation of IgG and IgM anti-Brucella antibodies in the diagnosis of human brucellosis

Two methods which employ whole cells are described and compared for the detection of human IgG and IgM anti-brucella antibodies. Dot ELISA and ELISA were shown to be suitable for a screening diagnosis of human brucellosis. Titres of antibodies obtained by dot ELISA showed 100% coincidence for IgG and 97% for IgM, compared with agglutination and complement fixation tests; when ELISA was used 11% positive sera were not detected. The comparison of these two methods with the conventional serological test kit indicated that both dot ELISA and ELISA were sensitive, reproducible and specific for the quantification of IgG and IgM antibrucella antibodies.

Optimal fixation of cells for use in solid-phase ELISA

Glutaraldehyde (GA) is commonly used to achieve the stable fixation of whole cells onto microtitre plates for use in solid-phase ELISA. Concentrations of GA greater than 0.10% (v/v) were found to result in increased levels of non-specific binding of antibodies in ELISA. However, the use of 0.05-0.025% (v/v) GA permitted the stable fixation of cells whilst minimizing the non-specific effects seen with higher GA concentrations. The binding of cells onto microtitre plates was also found to be highly variable depending on both the type of plates and the nature of the cells being used.

Enzyme-linked bio-immunoassay for IFN-gamma by HLA-DR induction

A novel enzyme-linked bio-immunoassay (bio-ELISA) has been developed to detect interferon-gamma (IFN-gamma) induced HLA-DR antigen on the surface of human tumor cells. Cells are cultured at 37 degrees C in 96-well microtiter plates in the presence of IFN-gamma for 2 days. After fixation with reagent alcohol the HLA-DR antigen is detected using a monoclonal antibody, followed by goat anti-mouse IgG-HRP conjugate. Four human cell lines were evaluated and all expressed HLA-DR in response to IFN-gamma in a dose-related fashion. Based on sensitivity, reproducibility and absence of antiproliferative effect by IFN-gamma, the COLO 205 cells (colon adenocarcinoma) were determined to be optimal. The bioassay is sensitive to 0.3 ng/ml IFN-gamma with a range to 10 ng/ml. The specificity of HLA-DR induction by IFN-gamma was demonstrated using an isotype specific monoclonal antibody as well as IFN-gamma neutralizing monoclonal and polyclonal antibodies. The effect of other cytokines on HLA-DR induction with COLO 205 cells was also investigated in this bioassay and only IFN-beta and interleukin-1 (IL-1) showed slight induction of HLA-DR. IFN-alpha had no effect at the concentration tested. Evaluation of assay parameters including reproducibility, sensitivity, simplicity, speed, cost and ability to standardize support the conclusion that this bioassay is a substantial improvement over the routinely used viral inhibition assay as a measure of IFN-gamma biological activity. The bio-ELISA technique also has potential applications for the quantitation of other cellular surface antigens induced by cytokines.

Differential binding of natural monoclonal antibodies to the surface of fixed or living cells

A few hundred monoclonal antibodies derived from normal mice were tested for binding to cell surface antigens in one T-cell hybridoma and one I-Ek-transfected fibroblast cell line. The assay, which is suitable for large screenings, used glutaraldehyde-fixed cells followed by immunoenzymatic detection of immunoglobulin. Of the 331 antibodies tested, 75 showed significant binding, not only on these cells, but also on a macrophage, fibroblast, thymoma, and pre-B cell line, and on normal syngeneic and allogeneic thymocytes. If the assay was modified so as to use live cells in a simplified ELISA on living cells, only 10 of 253 antibodies were found to be positive with the T-cell hybridoma line and 7 with the transfected fibroblast cell line. In both sets of conditions, about 75% of the positive antibodies were found to be 'multireactive' after being tested on a panel of antigens. In contrast, conventional 'immune antibodies' to cell surface antigens could be tested by routine methods in either type of assay. We conclude that, while glutaraldehyde fixation does not affect the reactivity of conventional antibodies, this technique is inappropriate for testing the binding of natural antibodies to cell surface antigens.

Characterization of a method using viable human target cells as the solid phase in a cell concentration fluorescence immunoassay (CCFIA) for screening of monoclonal antibodies and hybridoma supernatants

A new method has been characterized for the use of viable target cells as the solid phase for screening of hybridoma supernatants in a cell concentration fluorescence immunoassay (CCFIA). Briefly, the specific target antigen on the cells is bound by the monoclonal antibodies and revealed by use of a fluoresceinated second antibody. Separation of free from bound antibody is accomplished by filtration in the 0.2 micron filter-bottom wells of specialized assay plates. Processing is automated in a Pandex screen machine, resulting in numerical fluorescence values for each well. This method is rapid (under 1 h per 96-well plate), highly sensitive (down to 0.2 ng/ml) and sparing of target cells (0.3-2.5 X 10(4) cells per assay well). It has been applied to 37 different varieties of human solid tumor cells, as well as to human peripheral blood mononuclear cells. The cells used as targets for the characterization of this method were still capable of attachment and growth when recovered post-assay. This method was compared with a viable cell enzyme-linked immunosorbent assay (ELISA) method, showing similar sensitivity and greatly shortened assay time. Comparison of the results from this method with those obtained from flow cytometric analysis performed on viable cells showed close correlation, whereas a lower correlation was seen with immunohistochemical methods using acetone-fixed cells. Development of this method made it possible to rapidly screen many thousands of hybridoma supernatants and successfully select those which were specific for surface antigens on viable cells.

An enzyme immunoassay for the measurement of HLA-DR antigen expression

A simple, sensitive, rapid and reproducible method is described for the measurement of HLA-DR expression on monocytes. The assay is based on an enzyme-linked immunostaining of the cells in suspension form followed by colour development. The non-specific binding of anti-HLA-DR to Fc receptors was taken into account by incubation of cells with an irrelevant monoclonal antibody (antirubella-G7H9). The binding of the anti-rubella monoclonal with the cells was subtracted from the binding of anti-HLA-DR to cells. Using three different cell lines (K562, CTLL and Daudi), the specificity of the assay proved to be acceptable and the assay remained sensitive even when only 10(4) cells/well were used. The assay has a number of potential applications.

A solid-phase immunoassay to screen anti-HLA monoclonal antibodies

A solid-phase assay to detect anti-HLA monoclonal antibodies was developed. In this assay microtiter plates are coated with antigens solubilized from cultured lymphoid cells by sonication and then incubated with anti-HLA monoclonal antibodies. The antigen-antibody interaction is indicated by the development of color following the addition of peroxidase-conjugated anti-mouse Ig xenoantibodies and its substrate. The assay is rapid since it does not require centrifugations during the washing steps. Furthermore the assay is simple, reproducible and suitable to screen large numbers of samples and to detect antibodies recognizing determinants not exposed on the membrane of viable cells. The sensitivity of the assay is influenced by the pH of the buffer used to coat plates with antigens, by the number of cells used to prepare soluble antigens, by the incubation time of antigen preparations with plates and by the incubation time of antibody preparations with antigen-coated plates. Titration of anti-HLA monoclonal antibodies with known specificity and screening of hybridomas generated with splenocytes from mice immunized with cultured human lymphoid cells indicate that the sensitivity of the solid-phase assay is similar to that of the ELISA with lymphoid cells.