Cyclosporin A can switch the immune response induced by antigen from a humoral to a cell‐mediated mode

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Abstract
Cyclosporin A (CsA) is known as a nontoxic inhibitor of the immune response that is primarily employed in humans to prevent the rejection of allografts. We report here on a novel activity for this drug as an immunomodulator. CsA can either inhibit or facilitate the induction of delayed‐type hypersensitivity (DTH) depending on the class of immune response that would be induced in the absence of the drug. The drug inhibits the in vivo and in vitro induction of DTH when antigen is given under conditions that normally result in the induction of this response. The same concentration or dose of CsA inhibits the in vitro and in vivo induction of an antibody response and the antigen induces DTH instead. An explanation for these different effects of CsA on the induction of DTH is proposed. This novel activity of CsA in modulating the immune response from an antibody to a cell‐mediated mode may have clinical applications.