Immune responses in vitro. VII. Loss of susceptibility of functional theta-bearing cells to cytotoxic action of anti-theta serum and complement in vitro

θ-Bearing lymphocytes become progressively less susceptible to cytolytic effects of specific anti-θ serum and complement with incubation in tissue culture. Thymus-derived cell function, in terms of “helper cell” capacity for antibody responses and DNA synthetic responses to phytomitogens, however, remains intact. The data indicate that the loss of susceptibility to cytoloytic effects of anti-θ serum and complement is due to a decrease in the amount of θ antigen on these cells. This phenomenon hampers attempts to determine the length of time that collaborative or “helper” influences of thymusderived cells are required for development of optimal antibody responses in vitro.