Rapid Internalization and Degradation of Surface-Bound Antibodies by Tritrichomonas foetus

Abstract

Tritrichomonas foetus is a protozoan parasite of cattle that can be cultured axenically. Three monoclonal antibodies specific for surface antigens of T. foetus were found to be rapidly internalized and degraded by these cells after binding. Degradation was not due to secreted or artificially liberated proteases but depended on targeting to internal degradative compartments. Radiolabeled catabolites of the antibodies were subsequently incorporated into the parasite's own proteins. Antibody degradation could be inhibited by certain protease inhibitors or lowered 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 unhindered, 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. These findings may lead to a better understanding of how T. foetus resists host immune responses.

Transient invagination of flagella by Tritrichomonas foetus

“…The size of internalized particles or structures does not necessarily determine which mechanism predominates, although in mammalian macrophages the contribution of phagocytosis generally increases with increasing particle size (Pratten and Lloyd 1986;Koval et al 1998). A very active endocytic machinery exists at the T. foetus cell surface (Benchimol et al 1981(Benchimol et al , 1986(Benchimol et al , 1990(Benchimol et al , 1992Queiroz et al 1991;Aonso et al 1994Aonso et al , 1997Granger and Warwood 1996;Tachezy et al 1996Tachezy et al , 1998, raising the possibility that the¯agella might be laterally internalized by a process akin to receptor-mediated endocytosis. Hypertonic medium inhibits clathrin-mediated endocytosis in mammalian cells by preventing clathrin from interacting with adaptor proteins (Daukas and Zigmond 1985;Hansen et al 1993); these conditions also inhibited¯a-gellar internalization by T. foetus.…”

“…Hypertonic medium inhibits clathrin-mediated endocytosis in mammalian cells by preventing clathrin from interacting with adaptor proteins (Daukas and Zigmond 1985;Hansen et al 1993); these conditions also inhibited¯a-gellar internalization by T. foetus. However, additional evidence argues against an endocytic process: endocytosis from the plasma membrane is inhibited by cooling of human granulocytes to 15°C (Jesaitis et al 1984); cooling to the 12±20°C range also inhibits early stages in the endocytic pathways of mammalian cells (Helenius et al 1983) and trypanosomes (Hager et al 1994;Brickman et al 1995) as well as T. foetus (Granger and Warwood 1996;Aonso et al 1997). This is the same temperature range within which¯agellar internalization is triggered in T. foetus (present paper); internalization can occur at temperatures lower than this, but subsequent externalization is inhibited.…”

“…Prolonged existence as a pseudocyst may, however, result in agellar degradation, as has been observed in some fecal pseudocysts (Mattern et al 1973;Stachan et al 1984). The¯agella might then be a source of nutrients for the cell (Lehker and Alderete 1992;Granger and Warwood 1996) and prolong its viability. It should be pointed out that pseudocysts can also be seen in cultures maintained under standard culture conditions.…”

“…For quantitation of¯agellar display, Tritrichomonas foetus isolate MT330.1 (Burgess 1988) was grown as described elsewhere (Granger and Warwood 1996) at 37°C in Dulbecco's modi®ed Eagle's medium (DMEM; Sigma Chemical Co., St. Louis, Mo., USA) containing penicillin, streptomycin, and 6±10% fetal bovine serum (FBS). Growth was aerobic in polystyrene petri plates under an atmosphere containing 5% CO 2 in a humidi®ed incubator.…”

Experimentally induced intravaginal Tritrichomonas foetus infection in a mouse model

“…There rose interest of many researchers on the pathogenic capacity of T. foetus strains (Hook et al 1995, Soto et al 1997, the enzymatic activity responsible for tissue damage in the reproductive tract (Lockwood et al 1984, Thomford et al 1996, the molecules of the protozoa involved in the colonization process, the persistence of infection, the protective immunity (Felleisen 1999), and the evasion mechanisms of the host immune response , Talbot et al 1991, Granger & Warwood 1996.…”