The induction of T cell unresponsiveness by rapidly modulating CD3.

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Abstract
The immunomodulatory effects of an IgM anti-CD3 mAb (38.1) were investigated. 38.1 was distinct from other anti-CD3 mAb, in that it was rapidly modulated from the cell surface in the absence of a secondary antibody. Although 38.1 induced an immediate increase in intracellular free calcium [Ca2+]i by highly purified T cells, it did not induce entry of the cells into the cell cycle in the absence of accessory cells (AC) or a protein kinase C-activating phorbol ester. Clearing of 38.1 from the surface of AC-depleted T cells, documented both by immunofluorescence and by functional activity, was rapid, with markedly reduced levels of initially bound mAb observed after a 1 to 2 h incubation at 37 degrees C and complete modulation noted after a 5-h incubation. Despite rapid modulation of 38.1, the T cells continued to express substantial amounts of surface CD3, suggesting there is a rapid rate of turnover of CD3 molecules on resting T cells. After modulation of 38.1 bound CD3, T cells were markedly inhibited in their capacity to respond to PHA. Inhibition could be overcome by culturing the cells with supplemental AC or IL-2. The inhibitory effects of 38.1 could be mimicked by briefly pulsing cells with the calcium ionophore, ionomycin, that had no effect on surface expression of CD3. 38.1- or ionomycin-pulsed cells were inhibited in their subsequent response to PHA even when exposures were carried out in the presence of EGTA to prevent increases in [Ca2+]i from extracellular sources. Inhibition could not be accounted for by an inability of the ionomycin-treated or 38.1-modulated T cells to increase [Ca2+]i in response to PHA. These studies demonstrate that a state of T cell nonresponsiveness can be induced by modulating CD3 with an anti-CD3 mAb in the absence of co-stimulatory signals. A brief increase in [Ca2+]i resulting from mobilization of internal calcium stores appears to be sufficient to induce this state of T cell nonresponsiveness.

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