Update | Our new FACS device

January 2026

Finding CTCs in a tube of blood is like looking for a needle in a house-sized haystack. Using FETCH, a procedure developed at Tzu, we can filter the blood and select for CTCs with a magnet. This will decrease the size of our metaphorical haystack to around the size of a big household bucket. At Tzu, we are interested in the number of proteins on the cell surface of the CTCs. As cancer is a heterogeneous disease, this number can vary between different CTCs. Therefore, we need to study these cells on a single-cell level. With FETCH, we already went from a house-sized haystack to a bucket-sized one, but what we really need is the individual needles, the cells. This is why we have acquired a FACS device: a Fluorescence Activated Cell Sorter.

The FACS device is a piece of laboratory equipment which uses lasers to analyse individual cells, which are present in a stream of droplets. We can first use antibodies with fluorescent labels to stain cells. Each antibody is directed to a specific protein in- or on the cell, such as PSMA, the prostate specific surface antigen, which is a drug target for prostate cancer. With the FACS, we can measure the amount of bound fluorescent antibodies, which is directly related to the expression of the protein. This can then already be used for therapy screening, and this is therefore a highly valuable tool for clinicians and pharmacists.

But the FACS is a Cell Sorter and can therefore do more than just measuring cells. We can select cells based on specific characteristics and sort these into tubes.

These characteristics are called gates, and a possible CTC gate could for example be: positive for epithelial markers, negative for white blood cell markers, has a nucleus. The sorted cells can then be transferred to the molecular biology lab for further analysis. At Tzu, we will there focus on RNA, protein and DNA analysis.

Normal FACS devices can only sort based on fluorescence and light scattering. The FACS we acquired, the BD FACSDiscover S8, however, has a function called CellView. This allows us to make images of every single cell analysed by the device. This is a significant improvement compared to ordinary FACS devices, as this allows you to study the cell morphology, integrity, size, and distribution of the measured proteins over the cell surface. This feature can especially be important when we are selecting for so-called CTC clusters, which are groups of CTCs present in the patient samples. As these are known to be more prevalent in patients with poor prognosis, they can potentially be of importance in therapy selection.

We have already processed our first samples with the FACS device and are eager to do many more! With FETCH and the FACS, we at Tzu are now fully equipped to process blood tubes and leukapheresis products from patient samples to single CTCs.