Serum and organ-associated anti-hemoglobin humoral autoreactivity: Association with anti-Sm responses and inflammation

The release of hemoglobin (Hb) occurs in some infectious and autoimmune diseases characterized by inflammation. As levels of haptoglobin (Hp) fall, free Hb can cause pathology. Humoral autoreactivity to human Hb was demonstrated in the sera of systemic lupus erythematosus (SLE), leishmania and malaria patients. Serum anti-murine Hb antibody levels in lupus-prone mice also exhibited an age-dependent increase, with progressive organ sequestration; significant isotypic correlation was observed with anti-dsDNA antibodies. A suggestive link between anti-Hb and anti-Sm responses was observed: Human lupus sera expressing anti-Sm antibody reactivity preferentially contained heightened levels of anti-Hb autoantibodies, and immunization of lupus-prone mice with Sm led to enhanced anti-murine Hb reactivity. Human and murine anti-Hb monoclonal antibodies were generated, some of which were preferentially reactive toward disease-associated methemoglobin. Epitope-mapping studies revealed evidence of intra-molecular cross-reactivity. One such autoantibody synergized with Hb to enhance the secretion of pro-inflammatory cytokines while eliciting the increased production of monocyte migratory signals from endothelial cells. Preferential usage of specific variable region gene segments was not observed, although somatic mutations were documented. These studies reveal that, while the etiology, specificity and sequences of anti-Hb autoreactive antibodies can vary, they occur quite frequently and can have inflammatory consequences.