Autoimmune mechanisms in paraneoplastic neurological diseases: the paradigm of opsoclonus myoclonus ataxia syndrome]

Paraneoplastic Neurologic Diseases (PND) are remote complications of certain neoplastic diseases characterized by distinct neurological deficits in the central as well as the peripheral nervous system. The neurological disorders are usually clinically evident before the cancer is identified. PND are rare syndromes believed to have an immunological link where antibodies or activated T cells are acting against self antigens shared by the tumor cells and neurons. Most of these onconeural antigens are located in the cytoplasm-nuclear compartment of the cell, whereas others are located at the membrane and act as receptors or ion channels. This short review discusses the autoimmune mechanisms involved in PND in general and in opsoclonus-myoclonus-ataxia in particular. The article attempts to clarify why in most PND the immune effector mechanisms against onconeural antigens detected in sera of patients may not be the direct cause of the neurological deficit. We elaborate on the difference between the immune response against cytoplasmic-nuclear antigens as opposed to membranal antigens with regard to the fact that: a) the nervous system is considered immune privileged; b) neither the tumors nor the neurons have an optimal expression of MHC class I antigens and onconeural epitopes on the membrane. The availability of onconeural antigens to the immune effectors mechanisms is probably the main reason why there are few animal models for PND.