Xenoantibody directed against molecular components of the HLA system. Quantitation of antibody to serologically...

A quantitative absorption-blocking assay was used to measure the levels of antibody in immune rabbit sera that react with the serologically defined HLA molecular complex on the cell surface. Quantitation was determined by comparing the number of xenoantibody-treated cells with the number of untreated control cells needed to absorb a defined amount of cytolytic alloantibody. Complement-dependent cytolytic titers were used to standardize the total antibody content reactive with cell surface antigens of xenoantisera used to treat absorbing cells. The ability of xenoantibody to "block" or inhibit the reaction of alloantibody with the absorbing cell was observed to be the result of concomitant removal of both xeno- and alloantigenic determinants from the cell surface. Immunofluorescence data with xenoantibody suggested that removal was accomplished by temperature-dependent selective redistribution and endocytosis of the serologically defined HLA molecule complex. By using these procedures, xenoantisera have been characterized that contain different amounts of antibody to the two-component HLA complex relative to their total cytolytic antibody content. Variation ranges from a xenoantiserum with little or no blocking capacity to several highly effective serologically defined HLA blocking xenoantisera.