Levels of natural IgM antibodies against phosphorylcholine in healthy individuals and in patients undergoing isolated limb perfusion.

Natural IgM antibodies against phosphorylcholine (anti-Pc IgM) resemble C-reactive protein (CRP) regarding specificity and have gained increasing attention because of their supposed role in clearance of damaged cells and in cardiovascular disease. In order to quantify these antibodies in human plasma, we have developed an ELISA system, in which p-aminophenylphosphorylcholine (PCH) coupled to human serum albumin (HSA) was coated on microtiters plates. Human plasma or serum samples were incubated in the plates, after which bound anti-Pc IgM was detected with mouse anti-human IgM-HRP. Pre-incubation of plasma with competitors such as phosphorylcholine, phosphorylethanolamine, phosphorylserine or glycine-HSA, confirmed that the ELISA was specific for anti PC IgM. Levels of anti Pc IgM in a cohort of healthy donors differed by more than 100-fold, whereas the fluctuation of anti-Pc IgM levels in individuals over time was small (coefficient of variation between 6% to 25%). Furthermore, there was no correlation between CRP and anti-Pc IgM in this cohort. Levels of anti-Pc IgM in the normal donors correlated significantly with IgM binding to apoptotic cells. To test the hypothesis that anti-Pc IgM can bind to neo-antigens expressed on necrotic or apoptotic cells, anti-Pc IgM was also quantified in patients with tumors undergoing isolated limb perfusion with tumor necrosis factor-alpha (TNF-alpha). Following this procedure, a significant decrease of circulating anti-Pc IgM relative to total IgM was found in all five patients tested. In conclusion, we have developed a specific and reproducible ELISA for anti Pc IgM quantification. Fluctuation of levels of these natural antibodies over time in healthy individuals was limited, although the variation among individuals was large. Significant decreases of levels of anti-Pc IgM were found to occur during tissue damage.