Ha et al. studied the potential complementary or opposing outcomes of anti-CTLA-4 antibody (mAb) treatment on CTLA-4+ suppressive CD4+ Tregs and newly antigen-activated CD8+ T cells using ADCC/ADCP-active, FcR affinity-enhanced or -silent mAbs in vitro with human PBMCs and in a mouse model. Concurrent FcR-enhanced mAb and tumor vaccine treatment effectively reduced CD45RA-FoxP3+ Tregs but also reduced activated CD8+ T cells, while delayed antibody treatment spared CD8+ T cell generation, translating to improved tumor control in mice. Timing of interventions may also be relevant to other Treg-depleting approaches.
Anti-CTLA-4 mAb is efficacious in enhancing tumor immunity in humans. CTLA-4 is expressed by conventional T cells upon activation and by naturally occurring FOXP3(+)CD4(+) Treg cells constitutively, raising a question of how anti-CTLA-4 mAb can differentially control these functionally opposing T cell populations in tumor immunity. Here we show that FOXP3(high) potently suppressive effector Treg cells were abundant in melanoma tissues, expressing CTLA-4 at higher levels than tumor-infiltrating CD8(+) T cells. Upon in vitro tumor-antigen stimulation of peripheral blood mononuclear cells from healthy individuals or melanoma patients, Fc-region-modified anti-CTLA-4 mAb with high antibody-dependent cell-mediated cytotoxicity (ADCC) and cellular phagocytosis (ADCP) activity selectively depleted CTLA-4(+)FOXP3(+) Treg cells and consequently expanded tumor-antigen-specific CD8(+)T cells. Importantly, the expansion occurred only when antigen stimulation was delayed several days from the antibody treatment to spare CTLA-4(+) activated effector CD8(+)T cells from mAb-mediated killing. Similarly, in tumor-bearing mice, high-ADCC/ADCP anti-CTLA-4 mAb treatment with delayed tumor-antigen vaccination significantly prolonged their survival and markedly elevated cytokine production by tumor-infiltrating CD8(+) T cells, whereas antibody treatment concurrent with vaccination did not. Anti-CTLA-4 mAb modified to exhibit a lesser or no Fc-binding activity failed to show such timing-dependent in vitro and in vivo immune enhancement. Thus, high ADCC anti-CTLA-4 mAb is able to selectively deplete effector Treg cells and evoke tumor immunity depending on the CTLA-4-expressing status of effector CD8(+) T cells. These findings are instrumental in designing cancer immunotherapy with mAbs targeting the molecules commonly expressed by FOXP3(+) Treg cells and tumor-reactive effector T cells.