Tumor-specific CD8+ Tc9 cells activate host CD4+ T cells to control antigen-lost tumors

PROJECT SUMMARY Adoptive cell transfer using tumor-specific T cells is a promising option for cancer treatment and the most powerful tumor-killing effector T cells are CD8+ type-1 cytotoxic T cells (Tc1). We have shown that IL-9- secreting CD8+ Tc9 cells mediate stronger and long-lasting antitumor effects in vivo compared to the classical

IFN-γ-secreting Tc1. However, the underlying mechanisms remain unclear. Recently, we discovered that, when we rechallenged Pmel-1 Tc9- or Tc1-treated mice on the contralateral flank with gp100-knockout (KO) B16 tumor cells, these cells rapidly grew and established tumors in Tc1 but not Tc9-treated mice. Similarly,

relapsed gp100– B16 cells recovered from Pmel-1 Tc1-treated mice rapidly grew and established tumors after re-implanting to Pmel-1 Tc1- but not Tc9-treated mice, suggesting that Tc9- but not Tc1-treated mice develop a host immunity against other (than the cognate) antigens expressed by relapsed tumors. Importantly,

significantly larger numbers of CD4+ but not CD8+ T cells were detected in late-stage tumors, rechallenged tumors and tumor-draining lymph nodes (TDLNs) of Tc9-treated mice compared to Tc1-treated mice. These CD4+ T cells expressed high levels of IFNγ and granzyme-B and effectively killed wild-type B16, gp100–

relapsed and gp100-KO B16 tumor cells. Hence, these novel findings strongly indicate that adoptively transferred Tc9 but not Tc1 cells effectively induce a host CD4+ T cell response against relapsed tumors. To determine the mechanism underlying Tc9 cell-induced host CD4+ T cell response, we performed preliminary

studies to examine immune cells in tumor microenvironment (TME). We observed that dendritic cells (DCs) were enriched in Tc9-treated primary tumors, rechallenged tumors, and TDLNs. More importantly, Tc9 cells secreted a high level of IL-24, and Tc9-treated tumor supernatants contained abundant IL-24. As our

preliminary studies showed that IL-24 attracted DC migration in vitro, we speculate that Tc9 cells induce a host CD4+ T cell response through IL-24-DC circuit. We hypothesize that tumor-specific Tc9 cells may be a superb T-cell subset for cancer immunotherapy due to their capacity to elicit a host CD4+ T cell response to suppress

the growth of relapsed tumors. To test our hypothesis, Aim 1 will determine the role and importance of Tc9 cell- induced host CD4+ T cell responses in recognizing and eliminating relapsed tumor cells, and Aim 2 will determine the role and mechanism of IL-24-DC circuit in TME and TDLNs in eliciting tumor-specific CD4+ T cell

responses and preventing the recurrence of tumors. Completing this project will uncover a novel mechanism of Tc9 cells in mediating long-lasting antitumor activity in TME by inducing host CD4+ T cells to recognize tumor- associated antigens or neoantigens via IL-24-DC circuit and suppressing or preventing tumor recurrence in

Tc9-treated mice.