Advancing our understanding of transplant recognition by the immune system
by Adam Burrack, PhD
Today I have the privilege of describing research I performed. A fundamental problem facing scientists seeking to cure type 1 diabetes (T1D) through beta cell replacement is the immune response to “foreign” tissues following transplantation. Ironically, the genes with the highest diversity in the human genome (“polymorphic”) are related to the activation of T cells of the immune system. Long story short, T cells recognize these genetic differences between proteins very effectively, and these differences promote very strong immune responses. Unfortunately, T cell responses against these differences resist tolerance-promoted therapies and must be suppressed for the lifetime of transplant recipients. These lifelong immune suppressive therapies limit wide application of organ transplantation to cure kidney failure or hypoglycemia unawareness in people with T1D.
Here is where details matter, a lot. The molecule we think T cells are targeting is the Major Histocompatibility Complex – MHC – which shows or presents T cells peptides derived from viruses, bacteria, or other pathogens. MHC is the most polymorphic gene in the human genome, so any transplant from anyone other than your identical twin (if you have one) will be targeted by your immune system. But it gets worse! In theory, if we understood precisely what the immune system is targeting during these responses, scientists would have a chance to develop target-specific therapies to promote specific tolerance, leaving the rest of the immune system intact. We think, as immunology researchers, that MHC is the key target of the immune response following transplantation. But MHC only gets to the surface of cells if it’s loaded with a peptide. So the question for transplant response is this; what are the peptides loaded in donor MHC? Is there a common peptide that loaded in any donor MHC would promote an immune response in transplant recipients?
Enter into this context the laboratory of Dr. Marc Jenkins. For the past 25 years, Dr Jenkins’ laboratory has pioneered and popularized the use of a research tool called peptide-MHC tetramers to study CD4 T cell biology also called helper T cells, in a variety of biological settings including vaccination studies, infectious disease, autoimmunity, and now transplantation. Peptide MHC tetramers are a method to study very specific set of T cells which are specific for particular peptides. For example, a vaccine is intended to expand virus-specific T cells and peptide-MHC tetramers are a great method to track that expansion. Using a machine called a flow cytometer immunologists can quantify both the expansion of peptide-specific T cells (using peptide-MHC tetramers) as well as determine the behavior of the cells by analyzing cell surface proteins characteristic of various types of activation. These tools and techniques give researchers useful information to help determine (a) the presence, (b) the expansion, and (c) the effector type of T cells in various normal biology and pathologic conditions.
So, given this context, I was trying to determine ‘what CD4 T cells see’ which precipitates transplant rejection, in general. For this study we used a skin transplantation model to study organ rejection in general. We were not addressing autoimmunity against the transplant, which would also be in play in a T1D recipient of beta cells. The reagents we used in this study were peptide-MHC tetramers specific for the MHC of the transplant donor – which would be foreign to the recipient – loaded with peptides derived from cells called dendritic cells. Dendritic cells are key antigen-presenting cells which interact with T cells to influence immune responses. A long-standing hypothesis in basic immunology is that “passenger leukocytes” from the transplant – ie, dendritic cells – are a key target promoting recipient T cell responses. We tested this premise in our paper. Following transplantation of skin, donor dendritic cells travel to lymph nodes or the spleen of the recipient and interact with recipient T cells. This promotes a massive immune response leading to transplant rejection. We made four key discoveries. First, several peptides that are expressed on the surface of dendritic cells and loaded in donor MHC promote CD4 T cell responses in transplant recipients; no individual response dominated, it was a lot of separate T cell responses all happening in parallel. Second, both the MHC of the donor and the specific peptides are required as targets for a specific set of T cells. We showed this using MHC knockout or over-expression models, as well as peptide knockout and over-expression molecules. Third, these individual sets of transplant-reactive T cells are much smaller than sets of T cells elicited by immunization protocols; the very large overall immune response to transplants is a consequence of many of these responses happening in parallel. Fourth, transplant-reactive T cells appear to have a characterize effector type, or phenotype, called Th1. This effector type is also associated with autoimmunity and responses against viruses. The other possibility was an effector type called Th17 which are critical for responses against fungal infections. Our results clearly demonstrated transplant-reactive T cells were not of the Th17 type.
Understanding targets of the immune response which leads to transplant rejection gets us, as a field, one step closer to being able to rationally develop therapies to modulate these immune responses. Combined with our ever-increasing knowledge of autoantigens targeted in T1D onset, we now have two separate, parallel immune responses to modulate in future attempts to prevent rejection of replacement beta cells in individuals with long-standing T1D.