CHARACTERIZATION AND AFFINITY ISOLATION OF XENOREACTIVE HUMAN NATURALANTIBODIES

Natural Abs, which are thought to provide an initial defense against i nvasive microorganisms, include isohemagglutinins, anti-phosphatidylch oline Abs, and anti-alpha-galactose Abs. We have evaluated the physiol ogic properties of the fraction of human natural Abs that bind to porc ine endothelial cells and that would, as a result, initiate the reject ion of a porcine organ transplanted into a human. The concentration of xenoreactive IgM in the serum varied widely in the population (5 to 1 05 mu g/ml), but was highly dependent on the concentration of IgM in t he serum (r = 0.85). Despite this variation and the potential diversit y of epitopes recognized, human xenoreactive natural Abs exhibited sur prisingly homogeneous binding characteristics, both in one individual and in the population. The apparent avidity determined by using a dire ct ELISA yielded a functional dissociation constant of 10(-8) M to 10- (10) M, depending on the temperature used. This high functional K-d ap parently 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-galac tose determinant. A method was developed for affinity isolation of xen oreactive natural Abs by using a thermal extraction procedure. The met hod quantitatively accounts for all xenoreactive IgM, yielding functio nal IgM as evidenced by Ag binding and complement activation. Given th e overlapping specificity of xenoreactive Abs in the population and th e homogeneity of the functional K-d, the natural humoral immunologic b arrier to xenotransplantation may be far less formidable than previous ly thought.