Specialized antigen-presenting cells. Splenic dendritic cells and peritoneal-exudate cells induced by mycobacteria activate effector T cells that are resistant to suppression.

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Abstract
The ability of several types of trinitrophenyl (TNP)-labeled Ia+ [mouse] cells to induce contact hypersensitivity (CS) after i.v. injection was tested. Most labeled cell types (spleen cells, splenic macrophages, various types of peritoneal-exudate cells) not only fail to induce CS after this type of inoculation but, rather, activate T suppressor cells leading to specific immunological tolerance. Occasionally, some of these immunizing cells managed to bypass the T suppressor system and induced CS. In those cases the response was short-lived and could be blocked by concomitant injection of trinitrobenzene sulfonic acid (TNBS), a potent inducer of T suppressor cells. TNP-labeled splenic dendrite cells and TNP-labeled peritoneal-exudate cells induced by complete Freund''s adjuvant had several distinctive features. They were always able to sensitize when injected i.v. and the degree of sensitization was roughly equivalent to that achieved by cutaneous application of picryl chloride, the chemically reactive form of TNP. The response they elicited was long lived (i.e., lasted for > 3 wk). Their sensitizing capacity could not be blocked by the concomitant injection of TNBS. They elicited a response that could be adoptively transferred to untreated, normal recipients. The type of cell that first presents antigen to the immune system apparently plays an important, even essential, role in determining the strength and duration of the subsequent immune response. Some special antigen-presenting cells apparently can induce a response that is relatively resistant to host suppressor mechanisms. Evidence that they do so by activating contrasuppressor cells is discussed.

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