Cellular Immune Suppression

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Although the phenomenon of immune suppression is clearly reproducible in certain experimental systems, the existence of “suppressor cells” as a separate functional T-cell subpopulation is not generally accepted. Suppression may be the result of the production of suppressive cytokines or of the negative interaction between two cytokines, in which case the “suppressor” cells are the T cells and/or macrophages producing the factor with inhibitory effects. In other experimental designs, suppression may be associated with the killing of antigen-presenting cells or with so-called Veto cells, i.e., cells that kill upon contact with any other cell which specifically recognizes antigen on the Veto cells. 1 In the mammalian systems studied, suppressor cells were originally thought to belong to the CD8+ T-cell subpopulation and to exhibit a phenotype distinct from cytotoxic CD8+ T cells. 2 - 9 In many cases they appeared to require induction by suppressor-inducer CD4+ T cells, which also exhibited phenotypic markers different from CD4+ helper T cells. 8, 10 - 18 However, it has since been shown that some immune suppression effects appear mediated directly by CD4+ T cells 19 and/or activated macrophages.