Tumor-induced immune dysfunctions caused by myeloid suppressor cells

In the late 1970s, several findings suggested that accessory cells distinct from lymphocytes might suppress immune reactivity in tumor-bearing hosts. Studies in animal models and patients later confirmed that cells driven to act as dominant immune suppressors by growing cancers could subvert the imm une system. These cells have also been termed natural suppressors, a functi onal definition connoting their ability to hamper various T- and B-lymphocy te responses without prior activation and independently from antigen and MH C restriction. These properties were attributed to distinct cell population s. The phenotypic discrepancies, together with the lack of antigen specific ity, have generated serious restraints to research on tumor-induced suppres sion. Recent evidence indicates that suppressor cells are closely related t o immature myeloid precursors and can be found in several situations that c an exert adverse effects on the immunotherapy of cancer. The present review is an attempt to address the nature and properties of immature myeloid sup pressors and their relationship to dendritic cells and macrophages, with th e aim of clarifying the complex network of tumor-induced, negative regulato rs of the immune system.