Pituitary Autoantibodies in Lymphocytic Hypophysitis Target Both γ- and a-Enolase – A Link with Pregnancy?

Abstract

The first target autoantigen to have been identified in lymphocytic hypophysitis is a 49 kDa protein, identified as alpha-enolase. Pituitary autoimmunity is strongly associated with pregnancy and we have shown that pituitary autoantibodies from patients with peripartum lymphocytic hypophysitis also recognise enolase in the placenta. Enolase exists in different forms as a number of isoenzymes, which are homo- or heterodimers of three subunits, α, β and γ. aa-enolase is ubiquitous, ββ-enolase is muscle-specific and γγ-enolase, which is restricted to neuronal tissue and neuroendocrine cells, is known as neuron-specific enolase (NSE). NSE is expressed in normal human pituitary and pituitary neoplasms. The current study investigated which isoforms of enolase in pituitary and placenta reacted with the sera of patients with lymphocytic hypophysitis. Immunoblotting of two-dimensional gels of human pituitary cytosolic proteins showed that autoantibodies in patient sera react with both an acidic form, and more neutral forms of enolase. Immunoblotting with a monoclonal antibody to NSE confirmed the identity of the acidic enolase isoform as the γγ-isoform in both pituitary and placental samples. Gamma-enolase, i.e. NSE, was detected by immunohistochemistry in term placenta in decidua, syncytiotrophoblasts, anchoring villi and terminal villi. Our study is the first to describe the cellular localisation of NSE in normal human placenta, thus establishing a direct link between pituitary and placental autoantigens. This link provides a theoretical basis for the strong prediliction of lymphocytic hypophysitis to occur during or after pregnancy.