FLUORESCENCE – ACTIVATED CELL SORTING

Ana Luisa Caetano

Unidade de Citometria de Fluxo Instituto de Medicina Molecular

1) How does a FACS sort cells?

2) Examples of cell sorting experiments.

3) The future of FACS and other cell sorting technologies.

4) How to do cell sorting at the IMM.

FLUORESCENCE – ACTIVATED CELL SORTING

How does a FACS sort cells

– Aligns the particles in a continuous laminar fluid flow.

– Detects the “physical” properties and fluorescence of

the particles as they flow past a light source (laser).

– Records and displays the measured parameters.

– Applies regions and gates to define sub-populations.

and then…

SORTERS have the ability to physically isolate these sub-populations.

Flow Cytometry Sorting Schematic

Laser interrogation and signal processing followed by sort decision: sort right, sort left, or no sort.

Electronic delay until cell reaches breakoff point. Then the stream is charged : + or -.

Charged droplets deflected by electrostatic field from plates held at high voltage (+/-3000 volts).

Besides tubes can sort onto slides or multi-well plates.

The nozzle/flow cell is vibrated by a transducer (converts electrical energy into mechanical energy) so it produces a stream breaking into droplets.

BASIC THEORY OF SORTING

Sort Setup:1. Droplets are created by vibrating

the Nozzle at a very precise amplitude and frequency.

2. The “drop delay” is calculated which defines the time duration for a particle to travel from the interrogation point at the laser to the “last attached drop”.

3. Sort decisions are defined by the user in the Analysis Software.

The Sort:4. A decision is made at the

interrogation point whether to sort a cell or abort a cell.

5. If a cell is to be sorted, the electronics waits for the “Drop Delay” time when the cell will be contained in the “Last Attached Drop”.

6. A charge is sent down the sheath/sample stream.

7. The “Last Attached Drop” breaks off carrying the charge which can be positive or negative depending on the direction to be sorted.

8. As the charged droplet falls between the electrical field created by the charged Sort Deflection Plates, it is deflected into the proper sorting tube for collection.

4-WAY SORTING

It is possible to sort up to 4 populations at once by applying 4 different charges to the droplet (+1,+0.5,-0.5,-1) for each of the different populations.

SORT ABORTS

It may happen that within the drop envelope charge that has your wanted cell there may also be an unwanted cell.

The sorter knows to stop sort if unwanted cell is so close to wanted cell that it would be included in sort window.

This is termed a Sort Abort.

Can operate sorter in different ways to deal with aborts.

SORT ABORTS

Enrich modeAll positive events sorted.

Purify modeAll negative events aborted.

Single modeAll negative events aborted and must only have one positive cell.

2 drop sort mode

Positive event

Negative event

SORT MODES

Mode Characteristic General ApplicationEnrich Tries to capture all positive

events regardless of thepresence of negative events

High Recovery

Purify Sorts positives only in theabsence of a negative event

High Purity

Single Same as Purify mode exceptwill only accept one positiveevent per sort decision

Single Cell Deposition

Mixed Mode Sorts Purify Mode left andall the aborts to the right

High Purity one directionwhile recovering all abortedpositive events the otherdirection

WHAT EFFECTS THE SORT ABORT RATE?

At a cell analysis rate of 10 000/sec average of 1 cell every 9 drops.

Increasing the analysis frequency (while drop drive frequency remains same) will increase the chance of aborts.

Sort abort rate also affected by what proportion of the population you are sorting: the lower the proportion the more likely that it will be an unwanted cell close to your wanted cell.

To decrease aborts need to trade off between analysis rate and how long you want to sort for.

Yield: Proportion of cells wanted in sample actually sorted.Purity: Proportion of wanted cells in the sorted population.

PREPARING SAMPLES FOR SORTING

1. Need to have a clean, single cell suspension.

2. Need to know how many sorted cells you require.

3. Need to ensure you have adequate number of cells to get the number of sorted cells you want. To cover for losses during sample preparation and sorting best to have at least double the amount from basic calculation.

4. Need to know what you want to sort the cells into (tubes, plates, slides, type of media).

SPLEEN DENDRITIC CELL SORT

BEFORE

AFTER

SORTING RULES

Always filter the sample as last step of cell preparation to prevent clogs.

Adjust cell concentration so you can run at correct analysis rate.

Use color gating to ensure the populations are gated correctly.

Use Doublet Discrimination for optimum results. Always double-check the Sort Logic and Mode before

starting a sort.

Before performing a re-analysis of a sorted population, backflush and check for the absence of any carry over.

OTHER CELL SORTING TECHNOLOGIES

Magnetic beads (MACS):Coated with antibody, run past magnet to remove

unwanted cells or enrich wanted cells.

Panning:Coat tissue culture plate with antibody and then

incubate cells on it, can remove unwanted cells or enrich wanted cells.

Mechanical catch (FACScalibur or Partec):Wanted cells physically isolated by a movable catch

(slow).

Combine beads and flow; do pre-enrichment by MACS or panning.

FLUORESCENCE-ACTIVATED CELL SORTING

At the IMM we have 3-laser FacsAria. It can do 4-way sorting and sort into plates as well as tubes.

For booking please contact myself or Ana Luisa Caetano on ext 47222 /47224 (IMM 21 799 9530).

At moment booked up to 2 months in advance so need to book early!