Pere Santamaria, IGRP, autoreactive CD8+ T cells
by Adam Burrack, PhD
In previous blogs I’ve described several of the heavy hitters in the basic research of type 1 diabetes (T1D) research, including several investigators who focus on T cells of the immune system. T cells are amenable to study in the NOD mouse model of T1D because these cells can be tracked using reagents I’ve previously described called MHC tetramers, which allow researchers to follow T cells that target specific peptides. In T1D research, investigators can look for the presence of various types of autoreactive T cells in the blood of pre-diabetic patients. This information can be combined with clinical assays to measure the levels of auto antibodies specific for insulin and other diabetes-related targets which measure B cell responses. New assays for the clinical detection of these B cell responses will be the topic of a future blog in this series. A third clinical measure is a metabolic glucose tolerance test, which quantifies insulin production in response to a standard meal. A new diabetic patient or someone with fewer beta cells than normal – as would be the case before overt disease onset – will have less insulin production than normal (and less C-peptide present in their circulation) following this meal. Taken together, these three data points help clinicians develop a gauge of how imminent disease onset may be in a particular patient with known genetic risk factors for T1D.
I’ve previously profiled two prominent researchers who study T cells in type 1 diabetes, and today will describe a third key player. Katie Haskins at the University of Colorado has made a career of studying several different CD4-expressing T cells that target beta cells for destruction. Importantly, work done by post-doctoral researchers in Dr Haskins’ lab has shown that the peptides targeted by autoreactive CD4 T cells in diabetic NOD mice are also relevant targets of CD4 T cells in human T1D patients. In addition, Jeff Bluestone at UC-San Francisco studies the biology of regulatory T cells expressing CD4 and hopes to one day use these cells as a potential cell therapy to cure T1D. Down-regulatory CD4 T cells are powerful regulators of other T cell responses, and as such are an intriguing potential route for cell therapy and personalized medicine. Today I will profile Pere Santamaria at the University of Calgary.
Dr Santamaria first identified a set of CD8-expressing ‘killer’ T cells that target pancreatic beta cells and play a role in type 1 diabetes onset in the NOD mouse and appear to play a role in the human disease as well. Demonstrating that autoreactive CD8 T cells in both diabetic mice and humans respond to this peptide validates the utility of studying T cell responses against this target in the mouse model of disease. The autoreactive CD8 T cells that Dr Santamaria isolated and characterized from the NOD mouse do not recognize a peptide derived from the insulin molecule, these cells recognize a peptide derived from a beta cell-specific protein called islet-specific glucose 6-phosphatase catalytic subunit-related protein (say that three times fast!), which I will abbreviate IGRP. In the human study mentioned above, 65% of T1D patients tested had detectable CD8 T cell responses against IGRP, which is not as high a proportion as insulin-specific responses but comparable to other autoantigen targets (such as chromogranin A or Zinc transporter 8). The high percentage of patients responding to the IGRP peptide means that studying the behavior of the analogous T cells in diabetic mice and how these cells respond to various treatments is a clinically useful endeavor: tracking these T cells in NOD mice gives us a window on the disease process in humans.
One avenue Dr Santamaria and his collaborators have pursued is to study the importance of T cell responses against this target compared to the requirement for responses against the insulin molecule. In two papers published in 2006 by the Thomas/Kay group at St Vincent’s Insitute in Melbourne, Australia, and in 2008 by Dr Santamaria’s group, the authors concluded that T cell tolerance to the IGRP-derived protein did not prevent T1D in NOD mice, whereas tolerance to the insulin molecule does prevent disease. This results suggests – in agreement with the paper from George Eisenbarth’s lab which I have previously described – that insulin is a required auto antigen target for T1D development. This report suggests that a T cell response against IGRP is not required for development of T1D and that T cell tolerance to IGRP does not prevent other T cell responses leading to disease. From these data, it appears that T cell responses directed against the insulin molecule drive disease, whereas T cell responses against IGRP are not required for disease.
Moving from disease onset to curing T1D with pancreatic islet transplantation, a major barrier to long-term function of transplanted islets in T1D recipients (or NOD mice) is the presence of memory T cells (expressing CD4 or CD8) specific for islet-derived proteins (including IGRP). Unfortunately for recipients of transplanted islets, memory T cells specifically target insulin, IGRP, and other proteins expressed by the transplanted cells. These T cell responses lead to transplant destruction. Without therapies to stop these T cell responses, islet transplantation into subjects with T1D is an exercise in frustration. There are several broad strategic approaches to try to achieve transplant tolerance. One is to inhibit all immune responses, called global suppression, which is the current state-of-the-art in pancreas and islet transplantation. An alternative to this approach is to encapsulate insulin-producing cells within a capsule, which I will return to in future blogs. Another strategy is to try to re-establish tolerance to insulin-producing cells, called immune modulation – another strategy to which I will return later. A third option is to remove targets of autoreactive T cells from the transplant (or changing the insulin molecule to be unrecognizable) to prevent the activation of graft-reactive T cells. This last strategy would require genetic manipulation of the transplanted islets to remove or stop expression of the proteins which are attacked by the T cells.
Dr Santamaria has pursued studying whether auto antigen (IGRP) expression by transplanted beta cells is required for memory T cells (specific for IGRP) to infiltrate islet transplants in diabetic recipients. The rationale for this study was that IGRP is not critical for the physiologic function of beta cells – secreting insulin – so if islets lacking expression of IGRP (or in theory other auto antigen targets of T cells) were protected from destruction, this might represent a clinically viable strategy to prolong islet transplant function. In a 2012 manuscript in Diabetes, Dr Santamaria’s group demonstrated that islets which did not express IGRP were destroyed in diabetic NOD transplant recipient mice in the same time frame as islets which expressed IGRP. CD8-expressing T cells specific for IGRP did not infiltrate rejected transplants which did not express the IGRP protein, but were found within destroyed transplanted islets which did express IGRP. These experiments established that auto antigen expression is required for autoreactive T cells – which are specific to that auto antigen – to infiltrate transplants, but that the absence of one specific T cell specificity does not prevent rejection of the transplant and disease recurrence. Importantly, these data demonstrate that IGRP expression is not required for other autoreactive, islet-specific, T cells to mediate disease recurrence. In other words, in a recipient that is autoreactive against many islet-derived proteins, re-establishing tolerance to any one antigen will probably not be sufficient to prevent disease recurrence.
Overall, these data from the Santamaria lab demonstrate that while the CD8 T cell response to the IGRP peptide is an alternate to studying T cell responses to insulin, the T cell response against IGRP does not drive disease. On the bright side, the ability of tolerance-promoting therapies to render T cells tolerant to IGRP using pre-clinical candidate therapies is a potential use of autoreactive T cell lines and TCR transgenic mice. Positive results in this type of model system can help researchers develop therapies to – one day – prevent disease onset.