Presence and activation status of insulin-specific T cells is related to insulin autoantibodies
by Adam Burrack, PhD
In today͛s edition, I will describe a project that I was involved with, in a minor role. It has been an on-going
debate in the field whether immune responses against the insulin molecule itself are the driving force behind beta cell destruction. George Eisenbarth͛s laboratory put this question to the test in a 2005 report which
demonstrated that one key mutation to the insulin protein abrogated autoimmunity in NOD mice. Whether
this paradigm applies to human T1D has remained an open question. Addressing this question in human
samples has been a major challenge, because while determining individual risk of developing T1D is possible,
predicting when an individual will develop T1D is not straight-forward.
In addition, CD4 T cells are relatively rare in the peripheral blood. To address this question in human subjects therefore requires relatively large volumes. As a caveat, obtaining pancreas or pancreatic lymph node biopsies is not feasible in most cases from human patients, limiting our analyses to peripheral blood. These logistic
challenges restrict the number of clinical institutions capable of executing these experiments very short. Such institutions include the Barbara Davis Center in Denver, the Benaroya Institute in Seattle, the Joslin Center in
Boston, and the University of Florida. Perhaps a handful of others.
Lastly, the human HLA DQ8 molecule, which is very unstable chemically, is the HLA molecule most closely
related to the mouse MHC molecule I-Ag7. These MHC alleles are most closely associated with T1D onset –
presumably through their ability to load unique peptide sequences and activate CD4 T cell responses. Since
insulin is the hypothesized key target of CD4 T cells during diabetes pathogenesis, making a peptide-MHC
tetramer reagent for insulin loaded in DQ8 is a key road block. Enter into this conversation the laboratory of Dr. Brian Fife at the University of Minnesota and his post-
doctoral trainee Dr Justin Spanier. Dr Fife has been interested in the peripheral regulation of autoreactive CD4 T cells since his post-doctoral work with Dr Jeff Bluestone at
UCSF and has been on the faculty of the University of Minnesota since 2008. Dr Spanier is a trained biochemist and learned to make peptide-MHC tetramers for this study, including the DQ8-insulin reagent described
above. These authors quantified and characterized autoreactive insulin-specific CD4 T cells in the peripheral
blood of people with T1D or control subjects. To summarize, Dr Fife and Spanier found that shorter disease
duration correlated with the presence of more insulin-specific CD4 T cells in the peripheral blood, which suggests that insulin-specific CD4 T cell number ͞peaks͟ around the time of diabetes onset, and then decreases as beta cells are destroyed and progressively fewer target cells remain. Dr Spanier also found that
insulin-specific CD4 T cells tended to have an ͚effector memory͛ phenotype in these individuals, suggesting
recent antigen experience, or that these CD4 T cells had recently fulfilled their function in promoting beta cell destruction.
Dr͛s Fife and Spanier then collaborated with Dr Aaron Michels at the Barbara Davis Center to determine
insulin-specific autoantibody production in these individuals. Reminder that insulin-specific autoantibody is a marker of disease but does not appear to be directly pathogenic to beta cells. In contrast, the current
understanding in the field is that T cells are the perpetrator of beta cell death (see figure 1 of this recent
review about beta cell destruction from the Fife lab. Regardless, this quantification is key,
because self-tolerance to insulin is lost among both T cells and B cells during the development of T1D. The
question of which response occurs first and ͚helps the other along͛ is key to the future design of therapies to
perturb this disease process and prevent T1D onset. Work with Dr Michels demonstrated a direct correlation between the number of insulin-specific CD4 T cells in the blood and insulin autoantibody titers. This means the more insulin-specific CD4 T cells detected, the higher than anti-insulin antibody levels. This suggests a
functional relationship between autoreactive T cells and B cells. Since CD4 T cells ͞help͟ B cells to mature,
class-switch, and produce antibodies within germinal centers of lymph nodes, these results suggest CD4 T cell responses to insulin are
critical for promoting B cell responses and subsequent beta cell destruction. An important note is that CD4 T
cells are also key for activating CD8 T cell (or ͞killer͟ T cell) responses. As such, due to this critical role for CD4
T cells in activating both cell-mediated and humoral immunity, an interpretation of these results is that
depletion or inhibition of CD4 T cells specific for insulin might delay or prevent T1D in at-risk individuals. In
other words, without insulin-specific CD4 T cell help it is unlikely that human T1D would develop. This report
demonstrates the ability to detect insulin-specific CD4 T cells in the blood of recent-onset T1D patients and
that the number of these cells correlates with insulin autoantibody levels.
Perturbing the development and function of insulin-reactive CD4 T cells in at-risk individuals is the next
frontier of this type of immune-profiling research. There are a number of on-going clinical trials which have
the potential to affect these cells at clinicaltrials.gov. If you or a loved one live with T1D, please contact your
Congressional representatives and express your strong support for the Special Diabetes Program. To continue to move clinical
care forward, as a society we need to make evidence-based decisions, decisions based on scientific research.