A Simple Cell-based Immunofluorescence Assay to Detect Autoantibody Against the N-Methyl-D-Aspartate (NMDA) Receptor in Blood

The overall goal of this assay is to detect autoantibodies against the NMDA receptor in the blood of patients with suspected autoimmune encephalitis. The measure help answer the key questions in differential diagnosis of patients with acute neuro-psychiatric symptoms. The main advantage of this technique is that it is a simple and reliable screening measure.

Begin this procedure by preparing gelatin-coated culture plates. Aliquot 200 microliters of a 2%gelatin solution to each well of a 48-well culture plate and incubate the plate at 37 degrees Celsius for at least 30 minutes. After 30 minutes, aspirate the gelatin solution from the wells.

Seed in each well 5 times 10 to the 4th HEK293 cells in 200 microliters of cell culture medium, containing 10%FBS. Incubate the plate overnight at 37 degrees Celsius in a humidified incubator supplied with 5%carbon dioxide. On the following day, replace the spent culture medium in each well with 200 microliters of fresh culture medium, containing 10%FBS.

Scaling up according to the total number of wells in the experiment, Prepare solution A by mixing the NR1-GFP plasmid with culture medium. Prepare solution B by mixing the transfection reagent with culture medium. Prepare solution C by mixing solution A and solution B and incubating the mixture at room temperature for 20 to 30 minutes.

Next, aliquot 40 microliters of solution C to each well containing the HEK293 cells. Swirl the plate gently and incubate the plate at 37 degrees Celsius in a humidified incubator supplied with 5%carbon dioxide overnight. Check the expression of the NR1-GFP recombinant protein in the host cells under a fluorescent microscope with 40 times to 200 times magnification, before proceeding to the cell-based immunofluorescence assay.

To begin this assay, first aspirate the spent medium from the wells of the culture plate and then wash each well with 200 microliters of PBS three times. Add 200 microliters of 4%paraformaldehyde solution to each well and incubate the plate at room temperature for 15 minutes. Aspirate the paraformaldehyde solution from the wells and wash each well with 200 microliters of PBS three times.

Next, add 200 microliters of 10%skim milk in PBST solution to each well and incubate the plate at room temperature with gentle shaking for one hour. Aspirate the 10%skim milk in PBST solution from the wells and wash the wells with 200 microliters of PBST once. Incubate the wells with plasma diluted one to 100 in PBST as the primary antibody with gentle shaking at room temperature for one hour.

After one hour, aspirate the diluted plasma from the wells and wash each well with 200 microliters of PBST three times. To each well, add 200 microliters of goat anti-human IgG, conjugated with Alexa Fluor 594 and incubate the culture plate at room temperature with gentle shaking for one hour. Next, aspirate the goat anti-human IgG conjugated with Alexa Fluor 594 solution and wash each well with 200 microliters of PBST three times.

After the third PBST wash, add 100 microliters of glycerol solution, containing DAPI, to each well. Observe and image the cells under a fluorescent microscope using the correct filter for each dye and a 10 times eye piece with four times, 10 times and 20 times objectives. These representative images taken 24 to 30 hours after transfection, show HEK293 cells that express NR1-GFP.

Approximately 30%of the cells had the green signal under the fluorescent microscope. In the corresponding Alexa Fluor 594 images, the red signal in panel B indicates the binding of the anti-NMDA receptor autoantibodies to the NR1 expressed in the HEK293 cells. The weak red signal in panel E indicates the background noise of the Alexa Fluor 594 images.

The cells expressing NR1-GFP were used for screening the presence of anti-NMDA receptor autoantibodies in a human plasma sample. In these merged images, the yellow color indicates colocalization of the NR1-GFP and anti-NMDA receptor autoantibodies. The arrows indicate examples of cells with prominent colocalization.

A plasma sample that shows a greater than 30%overlap of green and red signals will be interpreted as positive in this case. In the merged image of the negative control, there is little overlap of green and red signals, which confirms the specific binding of the anti-NMDA receptor autoantibodies to the NR1-GFP in the assay. Once mastered, the technique can be done in four hours if it is performed properly.

While attempting this technique, it is important to remember it is just a screening measure. Following these procedures, other measures such as western blot analysis, and tissue-based analysis can be performed to answer additional questions like specificity of positive cases. After development, the technique pave the way for researchers in the field of psychiatry to explore differential diagnosis of patients with acute mental health emergency.

After watching this video, you should have a good understanding of how to perform the assay to screen for autoantibodies against NMDA receptor in the blood.