Cellular events in tolerance. I. Failure to demonstrate activation of lymphocytes, blocking factors, or suppressor cells during the induction of tolerance to a soluble protein

Lymph node cultures from Lewis rats rendered unresponsive to ultracentrifuged sheep immunoglobulins (soluble ShIg or ShIgSo) were examined at various times after exposure to tolerogen in vivo to determine if activation of macromolecular synthesis occurred during tolerance induction. Cells from ShIgSo treated animals could not be stimulated by antigen to incorporate precursors of DNA, RNA, or protein. 'Tolerant' cells could not be stimulated over a 10,000 fold dose range in vitro and did not show early or delayed DNA synthesis in vitro. In contrast, cells from rats injected with ShIg in adjuvant showed dose dependent stimulation at all times examined. In addition, thoracic duct lymphocytes from tolerant rats were not 'activated' in terms of their ability to support the replication of vesicular stomatitis virus. These results suggest that tolerance induction to this soluble protein does not proceed through the active steps in the immune response which are readily detected after the injection of an immunogen. Furthermore, the in vitro proliferation of ShIg immune lymph node cells, stimulated with ShIg, occurred whether these cells were cultured with normal rat serum or serum from rats tolerant to ShIg. In addition, co cultivation of lymph node cells from tolerant animals with immune lymphocytes did not lead to suppression of the responsiveness of the latter cells in vitro. These results suggest that the induction and maintenance of tolerance to ShIg occur through mechanisms involving neither blocking factors nor suppressor cells. Multiple pathways of tolerance to different types of antigens are proposed.