Flow Techniques for Counting Cells
Michael Keeney from the London Health Sciences Centre is working to improve flow cytometry methods in cases where cell counts are particularly low. Keeney has collaborated with breast cancer researchers to detect circulating tumor cells in peripheral blood at 1 cell/mL. He has also worked on the detection of minimal residual disease in pediatric patients with acute lymphoblastic leukemia after induction therapy.
Keeney tells GEN that rare event detection of acute lymphoblastic leukemia is critical for re-stratifying a patient group based on their response to a particular therapy. These small populations of residual tumor cells, which may be on the order of one in 10,000 cells, establish the patient’s prognosis and will inform the clinician regarding whether or not to alter the treatment.
Leukemic patients may have up to 1012 leukemic cells, and in classical morphologic "remission" there can still be up to 1010 residual leukemic cells. In his study, Keeney examined antigen expression in the remission state—i.e., 29 days after therapy initiation—in order to quantify residual tumor cells. He noted that minimal manipulation of the sample is ideal, as certain preparation methods may in fact lead to a loss of the abnormal cell population in the sample. Samples should be prepared at a higher concentration than what is normally used for flow cytometry; if a sensitivity of 0.01% is target, at least 1 x 106 cells should be collected.
Keeney and his colleagues are working with a quality assurance group from the U.K. called NEQAS, which provides blood samples containing small numbers of tumor cells to participating labs, so that researchers can test their samples against these controls. Additionally, Keeney is collaborating with the FNIH on a recent initiative to investigate standardization methods for detecting these small numbers of residual tumor cells. As a form of quality control, he has also sent many flow cytometry reference images to new labs utilizing these procedures, so that other researchers can understand the differences between normal and abnormal cell populations.
Keeney indicates that rare event detection has its limitations—both statistical and bench work expertise are required to effectively identify these cell populations. However, flow cytometry still has many advantages, he notes. “Flow cytometry is well-suited for rare event analysis in different hematological diseases. Compared to PCR or genome sequencing, it remains the least expensive method for identifying minimal residual disease, and it is a speedy process—results are obtained in the same day as running the sample,” he says.
Rare event detection has mostly been used in acute lymphoblastic leukemias, but the methodology is now being applied to adults with acute myeloid leukemias. Similar to Prof. Porwit’s studies, Keeney has also utilized 10-color flow cytometry to study blood samples from these leukemia patients, as it is much harder to clearly identify abnormal from normal cells in these patients. The testing is also being rolled out to a broader patient set, such as a chronic lymphocytic leukemia group in Europe.
According to Keeney, “We will continue to make improvements to the standardization protocols over the next few years. In the meantime, any labs performing these methods should participate in the quality assurance program.”
Along with their colleagues at Abbott Laboratories and South Western Area Pathology Service Liverpool Hospital, Keeney and London Health Sciences Centre’s Benjamin Hedley, Ph.D., are investigating standardization and validation of flow cytometric methods for counting reticulated platelets. Reticulated blood cells and platelets are useful tools for differentiating causes of anemia, but fewer laboratories utilize a flow cytometric method for reticulated platelet enumeration.
Dr. Hedley explains that a major application for counting reticulated platelets is to monitor the efficiency of certain drugs that mimic the body’s natural ability to make platelets.
“We need to be able to identify early on which patients are responding to these drugs,” he says. “This requires determining if the patient is producing more platelets, is not producing more platelets, or is producing platelets that are somehow getting destroyed by another factor. By measuring the number of immature, reticulated platelets, this tells us if the drug is helping the patient produce platelets in the bone marrow, whether or not they may be destroyed later on.”
However, the problem remained that there was no standard flow cytometric method for enumerating immature platelets newly released from the bone marrow. To tackle this issue, Dr. Hedley and his team utilized a previously published flow cytometric method from 1990 for analyzing reticulated platelets using Thiazole Orange dye, and combined this with a CD41/CD61 (platelet glycoproteins-IX and IIIa) platelet enumeration method that was standardized by the International Society for Laboratory Hematology.
“The main advantage of combining the platelet enumeration method with Thiazole Orange is that the number of platelets and percentage of immature platelets is obtained at once from the same experimental tube,” Dr. Hedley reports. The researchers used normal donor samples to establish a normal range of reticulated platelets. The samples were analyzed using Kaluza flow cytometry software from Beckman Coulter.
Dr. Hedley explains that Thiazole Orange, which is a fluorochrome that binds nucleic acids, distinguishes reticulated platelets because mature platelets have lost their RNA content. He also notes that using an eight- or 10-color flow cytometer is not necessary for counting platelets; rather, one fluorochrome to identify platelets and another to identify if they are immature or mature platelets is sufficient. He and his collaborators will be comparing their methodology using different instruments, such as Beckman Coulter and BD flow cytometers.
Dr. Hedley and his team determined that a final concentration of 10% of the initial Thiazole Orange used was sufficient for staining reticulocytes for 30 minutes, could be quenched with formaldehyde, and was stable for one hour.
“Stability is key. Clinically, we cannot expect to run our samples within minutes of being prepped. The samples are prepared all at once but need to be stable until each has been run through the flow cytometer,” he says.
Hematology counters that can screen complete blood counts may not accurately measure low numbers of platelets, and immature platelets are also larger than mature platelets and may fall outside of the set size or volume dimensions recognized by hematology analyzers. Instead, flow cytometric methods rely on antibodies and thus can still accurately count larger, immature platelets in low quantities.