Are we ready for placenta cells to enter the clinic?

Perinatal cells have been gaining impact in the field of regenerative medicine for over a decade¹. In addition to the well-known hematopoietic stem cells from cord blood, other cells with stem/progenitor properties can be isolated from the human term placenta.

More specifically, different stem cells can be isolated from four major regions of the placenta: amniotic epithelial, amniotic mesenchymal stromal, chorionic mesenchymal stromal, and chorionic trophoblastic tissues, as established by the First International Workshop on Placenta-Derived Stem Cells². Cells with characteristics of mesenchymal stromal/stem cells (MSC) can also be isolated from different placental regions, such as chorionic villi^3-6, from the umbilical cord^7-9, and also from the maternal component of placenta, known as the decidua¹⁰. Perinatal cells from the human amniotic fluid have also gained much attention in the field of regenerative medicine¹¹.

There are several reasons for the increased recognition of perinatal cells. First, the use of human term placenta lacks ethical concerns since it is generally regarded as biological waste, and there are also few costs associated to obtaining the placenta after birth. Second, it is easy to isolate cells from human term placenta. Moreover, of utmost importance is that placental MSC lack expression of human leukocyte antigens class II and other co-stimulatory molecules, and have intrinsic, potent immunomodulatory properties^12-14, making them very attractive for transplantation in allogeneic settings. In fact, the intrinsic immunomodulatory properties of perinatal cells seem higher than that compared to other cells used in regenerative medicine (e.g. bone marrow MSC)^15-17, thus substantiating their suitability for therapeutic approaches in regenerative medicine.

There is a lot of research to understand how perinatal cells contribute to the regeneration of damaged tissues. Initially it was thought that the cells differentiated into tissue-specific cell types and engrafted in order to replace damaged tissues. A more recent and more widely-documented mechanism is that perinatal cells act via paracrine signaling through the release of bioactive mediators^18,19. The paracrine signals induce regeneration by stimulating resident target cells to proliferate, and/or by inducing resident progenitor cells to differentiate. This mechanism has opened a new era in regenerative medicine.

Even though the regenerative properties of perinatal cells have been recognized in a variety of disorders, a critical aspect that remains under discussion is the regulatory criteria for the clinical use of cells obtained from the placenta.

To illustrate the issues that need further clarification, we focus on perinatal cells that have mesenchymal properties. The first important parameter to characterize these cells is the number of culture passages, or even better the population doubling time. Differences in the scale up of cell production between preclinical lab work versus human clinical trials could contribute to the failure of some therapies to transition from bench to bedside. Indications of population doubling time should be mandatory for defining the cell populations.

Another parameter to consider for placenta-derived cells, as for MSC derived from other sources, is cell potency. Regulatory authorities have recently mandated the development of potency assays as part of the release criteria for advanced-phase clinical trials²⁰. The well-established notions of sterility and cell viability, together with markers that identify MSC²¹, are the current accepted criteria for MSC release. However, the field of MSC therapy lacks functional-based assays for given disease indications, and defining robust and predictive markers and assays of potency remains a major issue in the cell-therapy field²².

The International Society for Cell Therapy (ISCT) has stressed the need to include immunophenotype and also immune-cell responder assays as an essential part of the release criteria for cells with immunomodulatory properties²⁰. We consider this recommendation to be highly relevant to perinatal cells.

Another important parameter when considering cells from human placenta is contamination with trace amounts of maternal cells that can only de detected after several passages in culture. The consensus from the First International Workshop on Placenta Derived Stem Cells² proposed the analysis of the fetal or maternal origin of cells. As mentioned previously, the placenta is a feto-maternal organ, and there are placental tissues where this could result in a mixed cell population. The cells should be characterized with highly sensitive techniques, such as quantitative PCR, not only immediately after cell isolation but also after cell culture.

To underline the need for caution, we reiterate our previous study which compared maternal cell contamination both after cell isolation and later after several culture passages²³. We tested freshly isolated cell populations from the mesenchymal fraction of both the amnion and chorion regions of the placenta. In both cases the initial genomic PCR amplification did not detect maternal cells in the freshly isolated cell populations. Several passages later, maternal cells were detected in the populations isolated from the chorionic membrane, while cell populations from the amniotic membrane did not show presence of maternal alleles²³.

As the community works to develop release criteria for placental MSC, the presence of maternal cell populations should be included among the tested parameters.

The corresponding author Ornella Parolini, PhD is Professor of Applied Biology at the Università Cattolica del Sacro Cuore (Rome, Italy) and Director of the Centro di Ricerca “E. Menni”, Fondazione Poliambulanza - Istituto Ospedaliero  (Brescia, Italy). Prof. Parolini has over 100 publications on placental stem cells and has been President of the International Placenta Stem Cell Society (IPLASS) for the past eight years.

Antonietta R. Silini is a Research Coordinator at the Centro di Ricerca “E. Menni”, Fondazione Poliambulanza - Istituto Ospedaliero (Brescia, Italy).  Her current recent research interests are focused on placental cells from the amniotic membrane, and their potential therapeutic effects in different diseases, including cancer.

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