Induction of regulatory T cell-resistant helper CD4⁺ T cells by bacterial vector

Salmonella typhimurium engineered to deliver cancer/testis antigen NY-ESO-1 through type III secretion (S typhimurium-NY-ESO-1) was shown to be an efficient cancer vaccine construct in mice and to stimulate NY-ESO-1-specific CD8⁺/CD4⁺ T cells in vitro in patients with cancer with NY-ESO-1 spontaneous immunity. e also showed that individuals without spontaneous immunity to NY-ESO-1 had specific CD4⁺ T-cell precursors with high avidity to NY-ESO-1 under tight control by CD4⁺CD25 ⁺ regulatory T (Treg) cells. We now found that in healthy donors and patients with melanoma without NY-ESO-1 spontaneous immunity, S typhimurium-NY-ESO-1 elicits CD4⁺ T helper 1 (Th1) cells in vitro recognizing naturally processed antigen from these high-avidity NY-ESO-1-specific naive precursors. In contrast to peptide stimulation, induction of specific Th1 cells with S typhimurium-NY-ESO-1 did not require in vitro depletion of CD4⁺CD25⁺ Treg cells, and this prevailing effect was partially blocked by disruption of interleukin-6 or glucocorticoid-nduced TNF receptor (GITR) signals. Furthermore, S typhimurium-induced Th1 cells had higher GITR expression than peptide-nduced Th1 cells and were resistant to suppression by CD4⁺CD25⁺ Treg cells in a GITR-dependent fashion. We propose that S typhimurium-NY-ESO-1 induces antigenspecific T-cell responses that are resistant to suppression by CD4⁺CD25⁺ Treg cells.