The study investigates the role of CD4+ T cells in cancer immunotherapy, focusing on the inhibitory effects of cytotoxic Tr1 CD4 T cells. Using vaccines containing MHC class I (MHC-I) neoantigens and different doses of tumor-derived MHC-II neoantigens, researchers found that low doses of MHC-II neoantigens (LDVax) promoted tumor rejection, while high doses (HDVax) inhibited it. HDVax-induced inhibitory cells were identified as type 1 regulatory T (Tr1) cells expressing IL-10, granzyme B, perforin, CCL5, and LILRB4. These Tr1 cells suppressed tumor rejection induced by anti-PD1, LDVax, or adoptively transferred tumor-specific effector T cells. Mechanistically, HDVax-induced Tr1 cells selectively killed MHC-II tumor antigen-presenting type I conventional dendritic cells (cDC1s), leading to reduced cDC1 numbers in tumors. The study also explored strategies to overcome this inhibition, including anti-LILRB4 blockade, a CD8-directed IL-2 mutein, and targeted loss of cDC2/monocytes. These findings highlight the inhibitory role of cytotoxic Tr1 cells in cancer immunotherapy and provide insights into overcoming their suppressive effects.The study investigates the role of CD4+ T cells in cancer immunotherapy, focusing on the inhibitory effects of cytotoxic Tr1 CD4 T cells. Using vaccines containing MHC class I (MHC-I) neoantigens and different doses of tumor-derived MHC-II neoantigens, researchers found that low doses of MHC-II neoantigens (LDVax) promoted tumor rejection, while high doses (HDVax) inhibited it. HDVax-induced inhibitory cells were identified as type 1 regulatory T (Tr1) cells expressing IL-10, granzyme B, perforin, CCL5, and LILRB4. These Tr1 cells suppressed tumor rejection induced by anti-PD1, LDVax, or adoptively transferred tumor-specific effector T cells. Mechanistically, HDVax-induced Tr1 cells selectively killed MHC-II tumor antigen-presenting type I conventional dendritic cells (cDC1s), leading to reduced cDC1 numbers in tumors. The study also explored strategies to overcome this inhibition, including anti-LILRB4 blockade, a CD8-directed IL-2 mutein, and targeted loss of cDC2/monocytes. These findings highlight the inhibitory role of cytotoxic Tr1 cells in cancer immunotherapy and provide insights into overcoming their suppressive effects.