Different mechanisms for the modulation of TL antigens on murine lymphoid cells.

An indirect radioimmunoassay has been used to study the antibody-induced changes in expression of TL antigens on mouse thymus and leukemia cells. The results obtained indicate that the incubation of TL+ cells in relatively high concentrations of anti-TL antisera results in a detectable, but not major, loss in the quantity of antigen detected in the plasma membranes. This decrease is not inhibited by azide. Although the antigen loss seems to coincide with the appearance of patching of membrane TL antigens on leukemia cells observed in indirect immunofluorescence microscopy, this is not the case when thymocytes are used as targets. Fab fragments prepared from anti-TL antibodies can also induce a quantitative decrease in antigen expression. The major mechanism involved in membrane antigen loss seems to be endocytosis, although some shedding may also occur. Resistance to immune cytolysis (antigenic modulation) occurs during incubation in anti-TL antibodies before either endocytosis or antigen redistribution has progressed enough to be directly responsible. Since antigenic modulation does not represent a depletion in TL antigens being expressed on the cell surfaces, another mechanism must account for the failure of guinea pig complement to achieve cell lysis.