We have shown that human dermal fibroblasts, exposed to interferon-gamma (IFN-gamma) to induce surface class II major histocompatibility complex (MHC) antigens, were capable of presenting tetanus toxoid (TT) antigen to human TT-specific T cell clones. Antigen presentation by fibroblasts was antigen dependent, required HLA-DR expression by fibroblasts, and was MHC restricted. In contrast, we now report that IFN-gamma-treated fibroblasts are unable to present TT antigen to purified resting T cells obtained from the peripheral blood of TT-immune donors. In addition, although IFN-gamma-treated fibroblasts were able to stimulate alloreactive T cell clones, they were unable by themselves to stimulate primary allogeneic responses in resting T cells. The failure of fibroblasts to stimulate resting T cells was not due to suppressor effects by fibroblasts, because induction of TT and alloantigen responses in resting T cells by monocytes was not inhibited by the presence of fibroblasts. On the contrary, IFN-treated fibroblasts were synergistic with small numbers of monocytes in activating resting T cells. In addition, the failure of antigen presentation by fibroblasts to resting T cells was reversed by the addition of recombinant human interleukin 2 (rIL 2) to cultures, but not of purified human interleukin 1 (IL 1). These results emphasize that the requirements for activation of resting T cells differ from those of T cell clones. Although fibroblasts can efficiently present antigen to T cell clones, antigen presentation by fibroblasts to resting T cells requires the addition of exogenous IL 2. It is postulated that fibroblasts differ from classical antigen-presenting cells in that fibroblasts are incapable of stimulating the production of IL 2 in resting T cells.
Human peripheral blood T cells were purified by a four-step procedure which included depletion of plastic-adherent cells, rosetting with sheep red blood cells, nylon wool passage, and treatment with mouse monoclonal antibodies to human Ia antigens plus complement. The purified T cells completely failed to proliferate to phytohemagglutinin (PHA). Bacterially derived recombinant human interleukin 2 (IL 2) reconstituted the proliferative response of resting T cells to PHA. The optimal concentration of IL 2 required was 100 to 200 U/ml. IL 2 alone caused no T cell proliferation. Both PHA and IL 2 needed to be present together for the proliferation of T cells to occur. Incubation of T cells with either PHA or IL 2 alone for up to 18 hr, followed by washing, then by the addition of the reciprocal reagent, resulted in no T cell proliferation. Expression of IL 2 receptors and of Ia antigens, as assessed by indirect immunofluorescent staining, revealed that both PHA and IL 2 needed to be present for Tac and Ia antigen expression by T cells. T cells incubated with PHA and IL 2 for 18 to 42 hr acquired responsiveness to IL 2. These T cells remained absolutely dependent on IL 2 for proliferation to occur. In contrast to T cells stimulated with PHA in the presence of monocytes, T cells stimulated with PHA and IL 2 released no detectable IL 2. The failure of IL 2 secretion was not caused by down-regulation of IL 2 production by IL 2 itself, because the addition of IL 2 to cultures of T cells stimulated with PHA in the presence of monocytes did not interfere with IL 2 production. These results indicate that IL 2 is a sufficient signal to induce the expression of its receptor in PHA-stimulated T cells and subsequent proliferation but is not sufficient to cause endogenous IL 2 release.
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