RAPID INTERNALIZATION AND DEGRADATION OF SURFACE-BOUND ANTIBODIES BY TRITRICHOMONAS-FETUS

Tritichomonas foetus is a protozoan parasite of cattle that can be cul tured axenically. Three monoclonal antibodies specific for surface ant igens of T. foetus were found to be rapidly internalized and degraded by these cells after binding. Degradation was not due to secreted or a rtificially liberated proteases but depended on targeting to internal degradative compartments. Radiolabeled catabolites of the antibodies w ere subsequently incorporated into the parasite's own proteins. Antibo dy degradation could be inhibited by certain protease inhibitors or lo wered temperatures; a sharp reduction in degradation between 20 C and 15 C was similar to a well documented block in endocytic transport to degradative compartments of mammalian cells. Growth and proliferation of T. foetus in the continuous presence of the antibodies appeared unh indered, but there was a general shift toward expression of both more and less of each epitope among cells within each population. Subclones of these populations always exhibited striking variability in epitope expression levels, with patterns similar to the parent cultures. Thes e findings may lead to a better understanding of how T. foetus resists host immune responses.