How to avoid rejection: T cells, timing, and TGFβ

Cytotoxic CD8+ T cells play a major role in graft rejection

Rejection remains a significant problem facing organ transplant recipients, despite the remarkable progress in the field. Antigen presenting cells (APCs) present in the graft can migrate to lymph nodes and activate host CD8+  T cells through direct presentation, leading to acute rejection. Simultaneously, APCs in the recipient also capture and process alloantigens from the graft, and through indirect presentation to alloreactive CD8+ T cells, contribute to graft rejection over both acute and chronic timeframes. Graft survival is predicated on a “dysfunctional” state of host CD8+ T cells, wherein they become hyporesponsive, non-proliferative, and unable to produce IL-2. The mechanisms driving this T cell anergy are poorly understood and are under active investigation across multiple human disease disciplines. For example, in cancer research, reversing the anergic state of tumor-associated T cells offers the possibility of a therapeutic breakthrough; whereas, in transplant biology and autoimmune diseases, inducing or maintaining CD8+ T cell anergy may be a therapeutic goal. A recent study from Dr. Sylvaine You’s laboratory at the French Institute of Health and Medical Research in Paris, suggests that one mechanism that underlies CD8+ T cell anergy involves TGFβ and the PD-1/PD-L1 pathway (Baas et al, 2016).

When given after T cell priming, CD3 antibody therapy prevents graft rejection

Novel work by Dr. You’s group showed previously that delayed CD3 antibody treatment could induce tolerance in a mouse pancreatic islet allogenic transplant model (You et al, 2012). Timing was critical to the grafts’ survival. Preventative treatment with CD3 antibody begun simultaneously with the islet transplant modestly prolonged graft survival, but ultimately the grafts were rejected.  Only when T cells had already been primed to graft antigen - that is, when treatment was begun a few days after transplantation - did the anti-CD3 antibody promote long-term graft acceptance. Antigen-specific tolerance was demonstrated in this study by performing a second transplant into the same, treated host using islets from a third-party allogenic donor.  This tissue was rapidly rejected.  Although activated effector T cells were preferentially depleted within the engrafted islets following CD3 antibody treatment, the mechanism responsible remained unclear.

TGFβ signaling regulates PD-1 and PD-L1 to maintain transplant tolerant CD8+ T cells

In the present study, Dr. You and her colleagues address the mechanism of CD3 antibody-induced CD8+ T cell tolerance using their allogenic islet transplantation model in mice as well as single-cell gene profiling and functional studies. Islets from facility-bred BALB/c mice were transplanted under the kidney capsule of diabetic (streptozotocin-induced) facility-bred C57BL/6 recipients.  The mice were treated for five days with anti-CD3 antibody starting on day 7 post-transplantation. Intragraft CD8+ T cells isolated on post-engraftment day 14 showed increased expression of PD-1 and PD-L1.  Next, the team repeated their study, but this time, CD3 antibody therapy was given with or without monoclonal antibodies to PD-1 or PD-L1. Grafts were rejected within ~30 days for both the PD-1 and PD-L1 groups, while the CD3 antibody-only treatment group tolerated the grafts. Further, treating the anti-CD3 tolerized recipients on day 100 with anti-PD-L1 antibody precipitated rapid graft rejection.

Intragraft CD8+ T cells isolated from CD3-antibody-treated mice on day 14 post-transplant also showed increased expression of Tgfb1 mRNA. To analyze whether TGFβ played an active role in anti-CD3-induced graft tolerance, the You team repeated their study again with or without co-administration of neutralizing anti-TGFβ antibody with the CD3 antibody treatment on day 7 post-transplant. Anti-TGFβ antibody significantly reduced CD3-induced graft tolerance. To determine if the effect was due to TGFβ signaling within T cells, B6.Cg-Tg(Cd4-TGFBR2)16Flv/J (stock # 005551) mice were used as graft recipients.  Also known as DnTGFβII mice, these mice express a dominant-negative form of the TGFβ receptor II regulated by the mouse CD4 antigen promoter.  The mutant TGFβ receptor blocks TGFβ signaling in both CD4+ and CD8+ T cells.  When DnTGFβII were engrafted with allogenic islets and were treated with anti-CD3, the grafts were rejected, demonstrating that blocking TGFβ signaling in T cells abrogated tolerance.

These data suggest a novel role for TGFβ signaling in maintaining T cell in an anergic state that permits long-term allograft survival by regulating the PD-1/PD-L1 checkpoint inhibitory pathway. A better understanding of these mechanisms supports the therapeutic potential of anti-CD3 antibody for preventing transplant rejection, as well as for mitigating autoimmune conditions involving a loss of self-tolerance by CD8+ T cells, such as in Type 1 diabetes and lupus.