In my previous blog, I talked about the different routes used to deliver stem cells focusing on systemic delivery. Regardless of the delivery route chosen, labeling and tracking stem cells in vivo (within the organism) helps in determining their survival rate, where they migrated to and how many cells were retained in the area of interest. This provides us with insights into the therapeutic benefits of regenerative treatments. In this blog, I will discuss the current techniques being used to label stem cells.

Labeling a stem cell means introducing a detectable marker that differentiates stem cells from their surroundings. Once the stem cells are labeled, they are introduced into the body and tracked. There are two ways in which cells can be labeled (i) direct labeling and (ii) indirect labeling. Direct labeling is simpler. For example, cells are incubated with a radioactive probe which is taken up by the cells. Indirect labeling is more complex. Here, the cells are genetically modified i.e., a reporter gene that expresses a marker is introduced into the cell and an exogenously added probe or substrate interacts with this marker resulting in a signal. After direct or indirect labeling, cells are delivered into the body and imaging techniques are utilized to “see” the marker and in turn the cell population of interest in vivo.

When cells are radiolabeled, (a direct labeling method), the cell numbers increase after every cell division, but the number of probes remain the same. This “dilution” of the probe must be taken into consideration while visualizing and counting cells. Furthermore, a probe may be present and detectable even after cell death. Consequently, direct labeling methods, might not provide an accurate measurement of viable cells.

In contrast, in indirect labeling, the cell is genetically modified such that it expresses a marker, which is transferred to its daughter cell. Therefore, no “dilution” of the marker occurs. Also, there is no signal once the cell dies, therefore this type of labeling should provide accurate counts of a viable stem cell population. However, one disadvantage of indirect labeling is that there are safety concerns regarding genetic cell manipulation and due to this reason, most studies utilize direct labeling methods. Both methods have their strengths and weaknesses. The onus is on the scientific community to ascertain that the labeling technique used is safe, measures viable cells and doesn’t interfere with cellular function.

Several animal studies have been conducted where cultured mesenchymal stem cells (MSCs) have been labeled and tracked. When tracking stem cells in humans, we would have to manipulate (culture and or label) the cells before delivery. This is not preferred due to safety concerns and increased regulatory control by the FDA. Instead cell migration or localization may be studied in appropriate animal models. We can then extrapolate from the animal data and predict what might occur in humans. This approach seems reasonable, especially if the objective is to provide a minimally manipulated regenerative therapy.