1746-P: Differential Depletion and Expansion of Autoreactive CD8+ T Cell Subsets after Alefacept Treatment in Type 1 Diabetes (T1D)

The T1DAL clinical trial, NCT00965458, demonstrated that alefacept (LFA-3-Ig) treatment was associated with preservation of c-peptide in subjects with new-onset T1D. Previous analysis of peripheral blood from these subjects showed reduction of CD2-expressing lymphocytes, including memory T cell populations. We now have interrogated the islet antigen-specific (autoreactive) T cell profiles from subjects in this trial using single-cell mass cytometry in combination with peptide-loaded MHC class I tetramer (Tmr) staining. We then applied a new analytical method, DISCOV-R, for phenotyping rare cell subsets using a multiparameter clustering algorithm. As expected among polyclonal CD8+ T cells, alefacept treatment preferentially reduced memory subsets, particularly those with higher expression of CD2 (Helios- transitional memory, exhausted effector memory, and Helios- effector memory CD45RA+). Prior to treatment, autoreactive Tmr+ CD8+ T cells were phenotypically heterogeneous, with three prominent phenotypes across T1D subjects (CXCR3+ naïve, Helios- and Helios+ transitional memory). Following alefacept treatment, the overall frequencies of autoreactive cells did not consistently change; however, their phenotypes were altered. CD2-high CD8+ T cells were diminished; naïve cells were expanded; and notably, a stem cell-like memory phenotype, indicative of early memory with high proliferative potential but low effector function, was also substantially expanded among autoreactive cells in most individuals. Similar phenotypic alterations occurred among polyclonal CD8+ T cells, indicating that the effects were not limited to the islet-specific T cell population. These findings suggest that alefacept treatment alters the composition of autoreactive CD8+ T cells that are not depleted, and that phenotype, not just frequency, of autoreactive cells is an important consideration for development of other immunomodulatory therapies.