Characterization and affinity isolation of xenoreactive human natural antibodies

Natural Abs, which are thought to provide an initial defense against invasive microorganisms, include isohemagglutinins, anti-phosphatidylcholine Abs, and anti-alpha-galactose Abs. We have evaluated the physiologic properties of the fraction of human natural Abs that bind to porcine endothelial cells and that would, as a result, initiate the rejection of a porcine organ transplanted into a human. The concentration of xenoreactive IgM in the serum varied widely in the population (5 to 105 micrograms/ml), but was highly dependent on the concentration of IgM in the serum (r = 0.85). Despite this variation and the potential diversity of epitopes recognized, human xenoreactive natural Abs exhibited surprisingly homogeneous binding characteristics, both in one individual and in the population. The apparent avidity determined by using a direct ELISA yielded a functional dissociation constant of 10(-8) M to 10(-10) M, depending on the temperature used. This high functional Kd apparently results from polyvalent interactions between the IgM and the porcine cell surface. Although the xenoreactive IgMs were absorbed by structurally diverse molecules such as ssDNA and thyroglobulin, about 80% of the xenoreactive Abs were specific for the terminal alpha-galactose determinant. A method was developed for affinity isolation of xenoreactive natural Abs by using a thermal extraction procedure. The method quantitatively accounts for all xenoreactive IgM, yielding functional IgM as evidenced by Ag binding and complement activation. Given the overlapping specificity of xenoreactive Abs in the population and the homogeneity of the functional Kd, the natural humoral immunologic barrier to xenotransplantation may be far less formidable than previously thought.