Cell-free chromatin particles activate immune checkpoints in human T cells: Implications for cancer therapy

Immune checkpoint blockade is an exciting breakthrough in cancer therapy, but how immune checkpoints are activated is unknown. We have earlier reported that cell-free chromatin particles (cfChPs) that circulate in the blood, or those that are released locally from dying cells, are readily internalized by healthy cells with biological consequences. Here we show that treatment of human lymphocytes with cfChPs isolated from sera of cancer patients led to marked activation of immune checkpoints viz. PD-1, CTLA-4, LAG-3, NKG2A, and TIM-3. Concurrently activated were stress-related markers cJun, cFos, JunB, FosB, NF[Kcy]B, and EGR1. The above immune checkpoints were also activated when lymphocytes were treated with cfChPs released from dying HeLa cells; the latter could be abrogated by three cfChPs deactivating agents. These results suggest that immune checkpoints are activated by lymphocytes as stress response to cfChPs. Simultaneous downregulation of multiple immune checkpoints may herald a new approach to immunotherapy of cancer. Statement of SignificanceWe show that cell-free chromatin particles (cfChPs) that circulate in the blood of cancer patients, or those released from dying cancer cells, simultaneously activate five immune checkpoints as a stress response by human lymphocytes. Activation of checkpoints was abrogated by cfChPs deactivating agents suggesting a novel approach to cancer treatment.