Nanoimmunotherapy: new outlook on allogeneic organ transplantation

One of the main challenges faced in allogeneic organ transplantation is the rejection of the donor tissue by the recipient’s immune system. Using a mixture of immunosuppressive drugs, the acceptance of organ transplantation is highly favorable for a short period of time (Gardiner et al. 2016). However, long term graft survival rates are low due to response from the immune system initiated by innate immune cells which triggers allograft rejection (Liu et al. 2012). While it is known that innate immune cells mediate allograft reject, the specific means by which this is achieve is not fully understood. Learning more about this pathway and the molecular mechanisms involved can lead to the development of specific immunotherapy that can promote long term allograft survival.

A recently published article by Braza et al. in Immunity explored the molecular pathways leading to graft rejection and explored therapeutic approaches that could potentially lead to long term allograft survival. To conduct their experiments, the researchers developed a nanoimmunotherapy strategy which allowed them to deliver proteins to highly targeted regions of the body, in this case, localized to the allograft. Using an experimental transplantation mouse model, they demonstrated that allograft rejection follows a macrophage activation pathway. Additionally, through downregulation of this pathway, by mTOR manipulation and inhibition of co-stimulatory signals, they were able to promote allograft survival indefinitely.

To elucidate the role of macrophage mediated allograft rejection, they performed heart transplant between two genetically different strains of mice. They focused on proteins that might be involved in promoting inflammation and found that vimentin and HMGB1, both agonists to dectin-1 and TLR4, were upregulated in the donor allograft following transplantation. Since macrophages express dectin-1 and TLR4, an increase in TNFα and IL-6 production indicated that vimentin and HMGB1 were able to train macrophages that infiltrated the graft. They then developed a nanoimmunotherapic approach that targets and inhibits the mTOR [referred to as mTORi-HDL in their research] signaling pathway. This pathway is linked to the production of cytokines through trained immunity (Netea et al. 2016). This targeted therapy demonstrated reduced cytokine production during the training period. Interestingly, one area where the mTORi-HDL accumulated was in the bone marrow, where it could potentially facilitates the development of prolonged therapeutic effects by association with myeloid cells and their progenitors.

mTORi-HDL localize to the liver, spleen, kidney, and bone marrow, and
is preferentially taken up by myeloid cells. Uptake of mTORi-HDL
by T and B cells is very limited.

In the allogeneic heart transplant mouse model, mTORi-HDL was preferentially taken up by macrophages compared to other myeloid cells; mTORi-HDL uptake in T cells were poor, suggesting a preference for myeloid cells. Treatment with mTORi-HDL following heart transplantation resulted in lower numbers of macrophages, neutrophils, and DC present in the allograft, blood, and spleen. Additionally, macrophages isolated from heart allografts treated with mTORi-HDL displayed significantly lower production of TNFα and IL-6. So far, Braza et al. have demonstrated that allograft rejection is triggered by the upregulation of proteins such as vimentin and HMGB1 which trains infiltrating macrophages. The attenuation of macrophages can be achieved by using nanoimmunotherapy, mTORi-HDL. This therapy targets myeloid cells, mainly macrophages, and lead to lower number of these cells present in the graft following transplantation. Lastly, to tie it all together, they investigated whether mTORi-HDL nanoimmunotherapy can promote organ transplant acceptance.

To assess the function of infiltrating macrophage in allografts they looked at two different subsets of macrophages. While the mTORi-HDL treatment decreased the number of reactive macrophages, it promoted the number of regulatory macrophages in the allografts. Grafts were rejected when regulatory macrophage population was depleted, suggesting that they play a key functional role in organ transplant acceptance. In fact, the application of mTORi-HDL therapy after heart transplant significantly increased the graft survival after five days. They extended their methods further to create another nanoimmunotherapy that inhibited CD40 – an essential costimulatory molecule required for T cell activation. Remarkably, when the two nanoimmunotherapy were applied together – mTOR inhibition and DC40 inhibition – they led to graft survival past 100 days post heart transplantation without showing any signs of toxicity or off-target side effects. In their model, allograft tolerance is therefore achieved by preventing reactive macrophage production of TNFα and IL-6, and promote regulatory macrophage which leads to CD4⁺ Treg expansion and CD8⁺ T cell inhibition.

Mice with both therapies showed a higher percentage of grafts surviving
past 100 days after transplantation compared to the mice that received one therapy.

This study is important because it demonstrate a significant breakthrough in allograft survival. Braza et al. showed that allogeneic heart transplant in a mouse model can survive, without immune rejection, by using targeted immunotherapies that inhibits mTOR and CD40 signaling. This form of therapy does not rely on global suppression of the immune system, which could lead to infections, cancer and metabolic toxicity (Naesens et al. 2009). Rather, nanoimmunotherapy targets specific cells in a specific region, leading to graft acceptance and survival. Nanoimmunotherapy seem to be a viable option that can be used to promote long term survival of allogeneic organ transplant. While this is promising, further research needs to be conducted to examine how these results translate to humans. Perhaps conducting the same experiments in primates could give us better insights on how this might work in humans. Further research also needs to be conducted on the long term survival of the graft. The research stopped at 100 days post transplantation, however, it would also be interesting to examine the graft at 200 or 500 days after the transplantation. Nonetheless, the results presented in this paper is promising and opens the door to the development of targeted immunotherapy that can be used as a treatment for allogeneic organ transplant.

References:
Braza, Mounia S., et al. "Inhibiting inflammation with Myeloid cell-specific nanobiologics promotes organ transplant acceptance." Immunity 49.5 (2018): 819-828.
Gardiner, Kyle M., et al. "Multinational evaluation of mycophenolic acid, tacrolimus, cyclosporin, and everolimus utilization." Annals of Transplantation 21 (2016): 1-11. 

Liu, Wentao, et al. "Innate NK cells and macrophages recognize and reject allogeneic nonself in vivo via different mechanisms." The Journal of Immunology (2012): 1102997.

Naesens, Maarten, Dirk RJ Kuypers, and Minnie Sarwal. "Calcineurin inhibitor nephrotoxicity." Clinical Journal of the American Society of Nephrology 4.2 (2009): 481-508.
Netea, Mihai G., et al. "Trained immunity: a program of innate immune memory in health and disease." Science 352.6284 (2016): aaf1098.