Once all tissues from a single donor have been isolated and processed, they are recombined to form the cellular allograft product. Because cellular allograft products must be stored frozen at low temperatures (< -70°C), a cryopreservative solution is added to maintain cell viability.
At this point, additional batch testing is performed. This involves testing each batch for cell number, usually by hemocytometry, and viability, usually by trypan blue exclusion. In some products, specific antigens are stained and counted to ensure that the cells are MSCs. This ensures that the product sold has an acceptable number of nucleated cells and that the cells are viable. However, the methods used to determine cell numbers vary widely between manufacturers and are rarely reported. Specifications for cell viability and cell number at the time of packaging are reported, nominally at greater than 70% and approximately one million cells per milliliter (ml or cc). Although manufacturers provide cell concentration data, these numbers can be misleading as there is no consistent counting method used between manufacturers and different cell counts may be included.
After processing, testing and lot release of a cellular allograft, the product is frozen and distributed to hospitals. After the products arrive at the hospitalshould be stored on site in a low temperature freezer until use. Once it is time for use in the operating room (OR), the following process is typically performed:
- The cellular allotransplant unit is removed from the freezer and transported to the operating room. The unit is removed from the packaging and warmed to room temperature.
- Once the tissue is thawed, the cryopreservative is quickly removed and replaced with saline.
- Because it contains living cells, the thawed specimen must be implanted within 2 to 4 hours, otherwise it must be discarded.
Safety of cellular allotransplantation products
As previously mentioned, cellular allograft products cannot be terminally sterilized without killing the cells, so each step must be carefully processed using aseptic technique. This includes processing the tissue using sterilized equipment and supplies and clean room processing. Although the final product is tested for sterility, this testing is only done on a small number of samples, and units released for distribution are not tested.
Furthermore, since the tissue is processed gently to preserve viable cells, other biological contaminants from the donor may also survive (e.g., live bacteria and viruses). Due to this risk, the AATB requires a comprehensive review of the donor’s medical and social history to exclude high-risk patients. This includes a comprehensive list of common and uncommon bacteria, fungi, viruses, and prions. Furthermore, the AATB has always prohibited the combination of tissues from different donors. This ensures that each tissue product is traceable to a specific patient-recipient and a specific donor. Therefore, the living cells processed into allogeneic tissues are recombined with bone from the exact same donor.
Due to the pre-screening process of donors and the requirement of one donor per lot, the incidence of disease transmission is rare. However, it is virtually impossible to test for every conceivable pathogen and unfortunate incidents do occasionally occur. In 2021, one lot of allogeneic stem cells was processed from a donor who had a tuberculosis infection.1 The infection was present in the bone tissue and resulted in the implantation of contaminated cellular allograft product in over 100 patients, leading to multiple deaths. The FDA and AATB subsequently added testing for mycobacteria to their list of required microorganisms.
The Future of Cellular Allografts in Bone Transplantation
Cellular allograft bone graft products have been a popular choice for transplants for many years. Surgeons are attracted to the living cellular component of the graft. Although stem cells can also be obtained from the patient’s own bone marrow aspirate, some surgeons feel that stem cell allograft products do not require this secondary procedure. Despite their popularity, however, cellular allografts have come under increasing scrutiny from the FDA, heightened by recent disease outbreaks. Coupled with difficult hospital storage conditions and relatively high prices, some hospitals are now eliminating the use of cellular allografts in their facilities.
As such, harvesting BMA from the patient is seeing a resurgence in use. Using patient-derived stem cells solves the problems associated with cellular allotransplantation. Advanced bone graft systems such as Biogennix’s DirectCell® The system provides the surgeon with all the necessary tools to safely create biologically active bone grafts using living patient-derived stem cells without the risk of disease transmission.
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