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New characterization and safety evaluation of human limbal stem cells used in clinical application: fidelity of mitotic process and mitotic spindle morphologies

Marija Zekušić1 Marina Bujić Mihica1 Marija Skoko1 Kruno Vukušić2 Patrik Risteski2 Jelena Martinčić2 Iva M. Tolić2 Krešo Bendelja3 Snježana Ramić1 Tamara Dolenec1 Ivana Vrgoč Zimić1 Dominik Puljić1 Ivanka Petric Vicković1 Renata Iveković1 Ivanka Batarilo4 Uršula Prosenc Zmrzljak5 Alan Hoffmeister6 Tiha Vučemilo1

Background Limbal stem cells(LSCs) are crucial for the regeneration of the corneal epithelium in patients with limbal stem cell deficiency (LSCD). Thus, LSCs during cultivation in vitro should be in highly homogeneous amounts, while potency and expression of stemness without tumorigenesis would be desirable. Therefore, further characterization and safety evaluation of engineered limbal grafts is required to provide safe and high-quality therapeutic applications. Methods After in vitro expansion, LSCs undergo laboratory characterization in a single-cell suspension, cell culture, and in limbal grafts before transplantation. Through a clinically applicable protocol,the collected data on LSCs at passage 1 were summarized: identity (cell size, morphology); potency (yield, viability, population doubling time, colony-forming efficiency were analyzed by trypan blue staining); expression of putative stem cell markers by flow cytometry, immunofluorescence and immunohistochemistry. Then, mitotic chromosome stability and normal mitotic outcomes were explored by using live-cell imaging. Finally, impurities, bacterial endotoxins and sterility were determined. Results Expression of the stemness marker p63 in single-cell suspension and in cell culture showed high values by different methods. Limbal grafts showed p63-positive cells (78.7 ± 9.4%), Ki67 proliferation (41.7 ± 15.9%), while CK3 was negative. Impurity with 3T3 feeder cells and endotoxins was minimized. We presented mitotic spindles with a length of 11.40 ± 0.54 m and a spindle width of 8.05 ± 0.55 m as new characterization in LSC culture. Additionally, live-cell imaging of LSCs (n=873) was performed, and only a small fraction <2.5% of aberrant interphase cells was observed; 2.12 ± 2.10% of mitotic spindles exhibited a multipolar phenotype during metaphase, and 3.84 ± 3.77% of anaphase cells had a DNA signal present within the spindle midzone, indicating a chromosome bridge or lagging chromosome phenotype. Conclusion This manuscript provides, for the first time, detailed characterization of the parameters of fidelity of the mitotic process and mitotic spindle morphologies of LSCs used in a direct clinical application. Our data show that p63-positive CK3-negative LSCs grown in vitro for clinical purposes undergo mitotic processes with extremely high fidelity, suggesting high karyotype stability. This finding confirms LSCs as a high-quality and safe therapy for eye regeneration in humans.