Co-transplantation of mesenchymal stem cells improves spermatogonial stem cell transplantation efficiency in mice

Background: Spermatogonial stem cell transplantation (SSCT) could become a fertility restoration tool for childhood cancer survivors. However, since in mice, the colonization efficiency of transplanted spermatogonial stem cells (SSCs) is only 12%, the efficiency of the procedure needs to be improved before clinical implementation is possible. Co-transplantation of mesenchymal stem cells (MSCs) might increase colonization efficiency of SSCs by restoring the SSC niche after gonadotoxic treatment. Methods: A mouse model for long-term infertility was developed and used to transplant SSCs (SSCT, n=10), MSCs (MSCT, n=10), a combination of SSCs and MSCs (MS-SSCT, n=10), or a combination of SSCs and TGF beta 1-treated MSCs (MSi-SSCT, n=10). Results: The best model for transplantation was obtained after intraperitoneal injection of busulfan (40mg/kg body weight) at 4weeks followed by CdCl2 (2mg/kg body weight) at 8weeks of age and transplantation at 11weeks of age. Three months after transplantation, spermatogenesis resumed with a significantly better tubular fertility index (TFI) in all transplanted groups compared to non-transplanted controls (P<0.001). TFI after MSi-SSCT (83.3 +/- 19.5%) was significantly higher compared to MS-SSCT (71.5 +/- 21.7%, P = 0.036) but did not differ statistically compared to SSCT (78.2 +/- 12.5%). In contrast, TFI after MSCT (50.2 +/- 22.5%) was significantly lower compared to SSCT (P<0.001). Interestingly, donor-derived TFI was found to be significantly improved after MSi-SSCT (18.8 +/- 8.0%) compared to SSCT (1.9 +/- 1.1%; P < 0.001), MSCT (0.0 +/- 0.0%; P<0.001), and MS-SSCT (3.4 +/- 1.9%; P<0.001). While analyses showed that both native and TGF beta 1-treated MSCs maintained characteristics of MSCs, the latter showed less migratory characteristics and was not detected in other organs. Conclusion: Co-transplanting SSCs and TGF beta 1-treated MSCs significantly improves the recovery of endogenous SSCs and increases the homing efficiency of transplanted SSCs. This procedure could become an efficient method to treat infertility in a clinical setup, once the safety of the technique has been proven.