High immunosuppressive burden in cancer patients: a major hurdle for cancer immunotherapy.

A bottleneck for immunotherapy of cancer is the immunosuppressive microenvironment in which the tumor cells are located. Regardless of the fact that large numbers of tumor-specific T cells can be generated in patients by active immunization or adoptive transfer, these T cells do not readily translate to tumor cell killing in vivo. The immune regulatory mechanism that prevents autoimmunity may be harnessed by tumor cells for the evasion of immune destruction. Regulatory T cells, myeloid-derived suppressor cells, inhibitory cytokines and immune checkpoint receptors are the major components of the immune system acting in concert with causing the subversion of anti-tumor immunity in the tumor microenvironment. This redundant immunosuppressive network may pose an impediment to efficacious immunotherapy, thus facilitating tumor progression. Cancer progression clearly documents the failure of immune control over relentless growth of tumor cells. Detailed knowledge of each of these factors responsible for creating an immunosuppressive shield to protect tumor cells from immune destruction is essential for the development of novel immune-based therapeutic interventions of cancer. Multipronged targeted depletion of these suppressor cells may restore production of granzyme B by CD8(+) T cells and increase the number of IFN-γ-producing CD4(+) T cells.