Antigen-specific therapy constitutes an attractive approach to re-establish a tolerant state in autoimmune disorders like type 1 diabetes, but has not been successful in clinical settings. Persistence of antigen-experienced memory cells likely contributes to lack of success in preventing established disease. Thus, antigen-specific therapies may need to be combined with additional immunomodulatory treatments in individuals with ongoing autoimmunity.

We generated non-obese diabetic (NOD) mice with tetracycline-regulated expression of islet-specific glucose-6-phosphatase catalytic subunit-related protein (IGRP) in antigen presenting cells (TII mice) and tracked IGRP-specific CD8⁺ T cells using tetramer enrichment. IGRP-specific T cells were absent in TII mice expressing IGRP from birth on, indicating tolerance of naïve cells to this antigen. Interestingly, antigen-experienced IGRP-specific T cells were readily detectable in TII mice expressing IGRP from 10 weeks of age, suggesting that they were refractory to cell depletion. However, several markers of exhaustion were up-regulated in response to antigen exposure, e.g. PD-1 (MFI of 14,987 ± 3609 vs 50,187 ± 13,191), Tim-3 (MFI of 473 ± 5 vs 1,156 ± 399) and TIGIT (MFI of 361 ± 115 vs 959 ± 273 in NOD vs TII mice, respectively). Nevertheless, these phenotypically exhausted IGRP-specific T cells were still able to kill IGRP peptide-loaded splenocytes and infiltrate transplanted islets in vivo.

We hypothesized that combining short-term anti-CD3 mAb treatment to delete pre-existing memory T cells followed by IGRP expression may induce effective tolerance in TII mice. Anti-CD3 mAb initially depleted antigen-experienced IGRP-specific cells. However, 5 weeks post treatment we detected similar numbers of IGRP-specific cells in NOD and TII mice, treated with either control or anti-CD3 mAb. These re-expanded cells derived from pre-existing memory cells and showed further up-regulation of exhaustion markers in TII mice treated with anti-CD3. Their function as well as diabetes incidence studies after combination treatment are currently under investigation.