Generation of Discriminative Human Monoclonal Antibodies from Rare Antigen-specific B Cells Circulating in Blood

The overall goal of this procedure is to produce fully human monoclonal antibodies against a desired antigen starting from human B lymphocytes circulating in the blood. This protocol allows the generation of monoclonal antibodies that can discriminate between highly related antigens. For some applications such as immunotherapy, precise targeting of an antigen can be indispensable.

The advantages of this technique are that this can be performed starting from the blood of healthy or immunized donors and allows the isolation of even rare antigen-binding B cells. While this method is described starting from human samples, it can also be applied to animals that can be pre-immunized. Visual demonstration of this method is critical as the procedure really is under quite complex multiple analytic gating strategy of discriminated B lymphocytes.

The procedure will be performed by Melinda. She is an assistant engineer from our laboratory. First pipette 35 milliliters of diluted cell suspension on 15 milliliters of density gradient in a 50 milliliter conical tube.

Then place the conical tube in a swing bucket rotor with brakes off and centrifuge at 1290 times gravity for 25 minutes at room temperature. Next, at the interface of the density gradient medium and plasma layer, carefully aspirate the mononuclear cell layer and transfer it to a new 50 milliliter conical tube. Add the cell suspension to a 15 milliliter conical tube and centrifuge at 460 times gravity for five minutes.

Then, resuspend the pellet in 200 microliters of phosphate buffered saline with 2%fetal bovine serum. Add conjugated tetramers to a final concentration of 10 micrograms per milliliter. Mix well and then incubate at room temperature for 30 minutes in the Laminar flow hood, protected from the light.

Then add 10 milliliters of ice cold cell separation buffer to the cell suspension. Centrifuge the cell suspension at 460 times gravity for five minutes. To the cell pellet obtained, add 500 microliters of ice cold cell separation buffer and 50 microliters of two variants of magnetic microbeads and incubate the cell suspension for 20 minutes.

Next, resuspend the cell pellet in ice cold cell separation buffer. Carefully transfer the cell suspension on the top of an equilibrated column and let it drain completely. Then rinse the tube with three milliliters of cell separation buffer and transfer the buffer to the column.

After rinsing the column three times, remove it from the magnet and place on a 25 milliliter conical collecting tube. After adding five milliliters of ice cold separation buffer on top of the column, quickly flush the cells from the column with the help of a plunger. Resuspend the cell pellet with a cocktail of anti-human antibodies and 7AAD in a total volume of 100 microliters of phosphate buffered saline with 2%fetal bovine serum.

Then, incubate the cells at four degrees Celsius for 30 minutes. Next, use a cell sorter cytometer to sort specific B cells at the single cell level not exceeding beyond on 1, 000 events per second. Then, collect the single sorted B cells into 8-strip PCR tubes previously filled with 10 microliters of 1X phosphate buffered saline and 10 units of RNAse inhibitor and place on a rack for 96 microtubes.

Quickly freeze the tubes at negative 80 degrees Celsius. Heat the frozen samples in a dry bath maintained at 70 degrees Celsius for five minutes to lyse the cells. Then place the strip on ice and start constituting the reverse transcription master-mix.

Once the reverse transcription master-mix is prepared, program instruct thermocycler. At the end of the reverse transcription, perform nested PCRs to amplify immunoglobulin genes. Identify the positive clones by visualizing on a 1.5%agarose gel the migration of a 500 base pair PCR product for variable heavy chain and coding segments and 350 base pair PCR product for variable light chain encoding segments.

Add one microliter of the cloning product to 45 microliters of electrocompetent TOP10 E.coli cells.Electroporate. Then quickly add 500 microliters of 2X YT medium to the cells and incubate in a water bath at 37 degrees Celsius for 30 minutes. Spread the transformed bacteria on 2X YT ampicillin plate.

Then, incubate the plates overnight at 37 degrees Celsius. Next, setup a reaction premix and add 25 microliters of it in per 8-strip PCR tube kept on ice. Then, use a sterile toothpick to pick colonies and streak the colonies on the 2X YT ampicillin plate.

Then, dip the toothpicks in the PCR reaction tube. Identify the positive clones by visualizing on a 1.5%agarose gel the migration of an 850 base pair PCR product for the heavy chain vector and a 600 base pair PCR product for the light chain vector. In two milliliters of LB medium, inoculate four positive colonies.

Incubate the inoculate at 37 degrees Celsius overnight at 200 revolutions per minute shaking. Seed 15, 000 human embryonic kidney cells in 200 microliters of Dulbecco's Modified Eagle Medium supplemented with 10%fetal bovine serum in each well of a flat bottom 96 well plate a day before the transfection. Then incubate the plate at 37 degrees Celsius overnight at 5%carbon dioxide.

Cotransfect the cells using a linear polyethylenimine derivative transfection reagent. Dilute 0.5 microliters of DNA transfection reagent with 10 microliters of 150 millimolar of sodium chloride. Next, dilute 0.125 microgram of each variable heavy and light chain expressing vectors in 10 microliters of 150 millimolar sodium chloride.

Vortex all the dilutions for 10 seconds each. Then, mix the previously diluted DNA transfection reagent with 10 microliters of DNA solution. Quickly vortex the DNA transfection mixture for 15 seconds and incubate for 15 minutes at room temperature.

Add the DNA transfection mixture drop-wise on the cells and then gently swirl the plate. Replace the medium 16 hours after transfection and culture the cells for five days in serum-free medium at 37 degrees Celsius. Next, use a multichannel pipette, to aspirate the supernatant, and transfer it into the v-bottom 96 well plate.

Coat each well in a 96 well ELISA plate with relevant antigen at four degrees Celsius for overnight in 100 microliters of reconstituted ELISA coating buffer at a final concentration of two micrograms per milliliter. Next add 50 microliters of a transfected cell supernatant to the wells of a 96 well ELISA plate and incubate the plate for two hours at room temperature. Then add 100 microliters of antihuman immunoglobulin G antibody conjugated to horseradish peroxidase enzyme at one microgram per milliliter concentration and incubate the well at room temperature for one hour.

Finally, add 50 microliters of chromogenic substrate to the well and incubate for another 20 minutes. At the end of incubation, read the optical density at 450 nanometers on a spectrophotometer. To isolate the specific B cells a gating strategy was used that discriminated the B cells expressing receptors against the HLA-A2/Pp65 complex.

Initially, the cells were CD19 positive, CD3 negative gated. Finally, cells were stained with both HLA-A2/Pp65PE and HLA-A2/Pp65APC, but not HLA-A2/MelABV421 tetramers, were gated to obtain the highly specific B cells. Using this gating strategy, selective B cells can be isolated, which can differentiate between HLA-A2/Pp65 and other related HLA-A2 peptide complex.

Next, to study the presence of PC1.02 antibody against HLA-A2/Pp65 tetramer an ELISA assay was performed. In the bar plot obtained, the y-axis corresponds to the optimal density readings and the x-axis correspond to the HLA-A2 peptides. From the assay, it shows that a monoclonal antibody binds with HLA-A2/Pp65 tetramer in a peptide and major histocompatibility complex dependent manner.

Next, surface plasmin residence was used to study the binding affinity of the PC1.02 antibody. Then, a tabular data was obtained to analyze the HLA-A2/Pp65 specific B cells. The impact of the exclusion strategy of unspecific B cells, the yields of immunoglobulin gene amplification, and monoclonal antibody production from HLA-A2/Pp65 specific B cells have been shown.

Once mastered, the technique ranging from isolation of antigens specific B cells from human blood to monoclonal antibody production can be done in as little as one month. When performing this procedure, remember go straight from sorting of B cells to RT-PCR without pausing.