Testicular germ cells, which appear after the establishment of central tolerance, express novel cell surface and intracellular proteins that can be recognized as ‘foreign antigens’ by the host’s immune system. However, normally these germ cells do not evoke an auto-reactive immune response. The focus of this manuscript is to review the evidence that the Blood-Testis-Barrier (BTB)/Sertoli cell (SC) barrier along with the SCs ability to modulate the immune response is vital for protecting auto-antigenic germ cells. In normal testis, the BTB/SC barrier protects the majority of the auto-antigenic germ cells by limiting access by the immune system and sequestering these ‘new antigens’. SCs also modulate testis immune cells (induce regulatory immune cells) by expressing several immunoregulatory factors, thereby creating a local tolerogenic environment optimal for survival of nonsequesetred auto-antigenic germ cells. Collectively, the fortress created by the BTB/SC barrier along with modulation of the immune response is pivotal for completion of spermatogenesis and species survival.

The acute cell-mediated immune response presents a significant barrier to xenotransplantation. Immune-privileged Sertoli cells (SC) can prolong the survival of co-transplanted cells including xenogeneic islets, hepatocytes, and neurons by protecting them from immune rejection. Additionally, SC survive as allo- and xenografts without the use of any immunosuppressive drugs suggesting elucidating the survival mechanism(s) of SC could be used to improve survival of xenografts. In this study, the survival and immune response generated toward neonatal pig SC (NPSC) or neonatal pig islets (NPI), nonimmune-privileged controls, was compared after xenotransplantation into naïve Lewis rats without immune suppression. The NPSC survived throughout the study, while NPI were rejected within 9 days. Analysis of the grafts revealed that macrophages and T cells were the main immune cells infiltrating the NPSC and NPI grafts. Further characterization of the T cells within the grafts indicated that the NPSC grafts contained significantly more cluster of differentiation 4 (CD4) and cluster of differentiation 8 (CD8) regulatory T cells (Tregs) at early time points than the NPI grafts. Additionally, the presence of increased amounts of interleukin 10 (IL-10) and transforming growth factor (TGF) β and decreased levels of tumor necrosis factor (TNF) α and apoptosis in the NPSC grafts compared to NPI grafts suggests the presence of regulatory immune cells in the NPSC grafts. The NPSC expressed several immunoregulatory factors such as TGFβ, thrombospondin-1 (THBS1), indoleamine-pyrrole 2,3-dioxygenase, and galectin-1, which could promote the recruitment of these regulatory immune cells to the NPSC grafts. In contrast, NPI grafts had fewer Tregs and increased apoptosis and inflammation (increased TNFα, decreased IL-10 and TGFβ) suggestive of cytotoxic immune cells that contribute to their early rejection. Collectively, our data suggest that a regulatory graft environment with regulatory immune cells including CD4 and CD8 Tregs in NPSC grafts could be attributed to the prolonged survival of the NPSC xenografts.

Immune-privileged Sertoli cells (SCs) exhibit long-term survival after allotransplantation or xenotransplantation, suggesting they can be used as a vehicle for cell-based gene therapy. Previously, we demonstrated that SCs engineered to secrete insulin by using an adenoviral vector normalized blood glucose levels in diabetic mice. However, the expression of insulin was transient, and the use of immunocompromised mice did not address the question of whether SCs can stably express insulin in immunocompetent animals. Thus, the objective of the current study was to use a lentiviral vector to achieve stable expression of insulin in SCs and test the ability of these cells to survive after allotransplantation. A mouse SC line transduced with a recombinant lentiviral vector containing furin-modified human proinsulin cDNA (MSC-EhI-Zs) maintained stable insulin expression in vitro. Allotransplantation of MSC-EhI-Zs cells into diabetic BALB/c mice demonstrated 88% and 75% graft survival rates at 20 and 50 days post-transplantation, respectively. Transplanted MSC-EhI-Zs cells continued to produce insulin mRNA throughout the study (i.e., 50 days); however, insulin protein was detected only in patches of cells within the grafts. Consistent with low insulin protein detection, there was no significant change in blood glucose levels in the transplant recipients. Nevertheless, MSC-EhI-Zs cells isolated from the grafts continued to express insulin protein in culture. Collectively, this demonstrates that MSC-EhI-Zs cells stably expressed insulin and survived allotransplantation without immunosuppression. This further strengthens the use of SCs as targets for cell-based gene therapy for the treatment of numerous chronic diseases, especially those that require basal protein expression.

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