Novel insights into exosome-induced, tumor-associated inflammation and immunomodulation.

The immune system of cancer patients is often suppressed. Accumulating evidence suggests that exosomes released from tumor cells may play an essential role in this process but the mechanisms are not fully understood. Here we review recent papers showing that exosomes trigger the release of cytokines/chemokines from immune cells. We suggest that this process will either result in the stimulation of anti-tumor immune reactions or in a systemic immunosuppression. The direction appears to be largely dependent on the duration of interactions between immune cells and exosomes leading to the accumulation of inflammatory factors, i.e. on the length of the exposure to these factors. We propose that a long-term interaction of the immune system with elevated levels of tumor exosomes contributes to the development of immunosuppression in cancer patients.