Isolation of Human Peripheral Blood Mononuclear Cells (PBMCs) via Ficoll-Paque Density Gradient & Viability Assaying
๐ฌ Principle & Isopycnic Density Gradient Separation
Whole human peripheral blood contains distinct cellular lineages with characteristic buoyant densities:
- Platelets / Thrombocytes: Density $rho < 1.030,text{g/mL}$. Remain suspended in the plasma layer.
- Mononuclear Cells (PBMCs): Lymphocytes ($rho approx 1.060-1.070,text{g/mL}$) and Monocytes ($rho approx 1.065-1.072,text{g/mL}$).
- Polymorphonuclear Granulocytes (PMNs – Neutrophils, Eosinophils, Basophils): Density $rho > 1.080,text{g/mL}$.
- Erythrocytes (RBCs): Density $rho > 1.090,text{g/mL}$.
Ficoll-Paque is an aqueous solution consisting of Ficoll 400 (a synthetic, highly branched, neutral polymer of sucrose that aggregates erythrocytes into rouleaux, accelerating their sedimentation) and Sodium Diatrizoate (an iodinated radiopaque compound that adjusts the refractive index and osmotic density strictly to $1.077 pm 0.001,text{g/mL}$ at $20^circtext{C}$).
When diluted whole blood is layered gently over Ficoll-Paque and centrifuged at $400times g$ for 30 minutes with the centrifuge brake disengaged, cells migrate based on their buoyant densities:
Erythrocytes and granulocytes are denser than the Ficoll medium ($rho > 1.077,text{g/mL}$) and sediment rapidly to the bottom pellet. Conversely, mononuclear cells (lymphocytes and monocytes) have buoyant densities lower than Ficoll ($rho < 1.077,text{g/mL}$) and are retained as a dense, opaque white ring (the buffy coat ring) at the horizontal interphase between the plasma/saline layer and the Ficoll-Paque layer.
๐ Step-by-Step Isolation Protocol
- Blood Collection & Dilution: Aseptically collect $5,text{mL}$ of human venous blood into an EDTA or heparin vacutainer tube. Dilute the blood $1:1$ with sterile Phosphate-Buffered Saline (PBS, pH 7.4) at room temperature.
- Gradient Layering (Critical Checkpoint): Add $3.0,text{mL}$ of Ficoll-Paque ($1.077,text{g/mL}$) into a sterile $15,text{mL}$ conical polypropylene centrifuge tube. Hold the tube at a $45^circ$ angle. Using a sterile pipette, very slowly and gently layer $6.0,text{mL}$ of diluted blood over the Ficoll surface without disturbing the meniscus or mixing the two phases.
- Density Gradient Centrifugation: Centrifuge at $400times g$ ($~1600,text{rpm}$ in a standard swinging-bucket rotor) for 30 minutes at $20^circtext{C}$. Crucial: Ensure the centrifuge deceleration brake is set to OFF (zero brake) to prevent vibration-induced disruption of the cell interphase.
- Harvesting the PBMC Interphase: Following centrifugation, observe the four distinct layers: (1) Upper straw-yellow plasma/PBS, (2) Opaque white buffy coat interphase, (3) Clear Ficoll layer, (4) Dark red RBC/granulocyte pellet. Carefully aspirate and discard the top $2-3,text{mL}$ of plasma without touching the ring. Using a sterile Pasteur pipette, aspirate the cloudy PBMC buffy coat ring and transfer into a fresh $15,text{mL}$ centrifuge tube.
- Washing: Add $10,text{mL}$ of cold PBS to the harvested PBMCs. Centrifuge at $300times g$ for 10 minutes (washes away residual Ficoll and platelets). Discard supernatant, resuspend pellet in $5,text{mL}$ PBS, and centrifuge at $200times g$ for 10 minutes. Resuspend final cell pellet in $1.0,text{mL}$ complete RPMI-1640 medium.
- Trypan Blue Viability Counting: Mix $20,mutext{L}$ of cell suspension with $20,mutext{L}$ of $0.4%$ Trypan Blue solution. Load $10,mutext{L}$ onto an improved Neubauer hemocytometer. Under $10times$ objective, count viable cells (unstained, clear, glistening) and non-viable cells (dark blue, ruptured membranes) in the 4 large corner squares:
$$text{Viability (%)} = left( frac{text{Total Unstained Viable Cells}}{text{Total Cells (Viable + Blue Dead)}} right) times 100$$
A successful preparation yields $>95%$ cell viability and approximately $1-2 times 10^6,text{PBMCs/mL}$ of whole blood.
