It has been shown previously that a single intravenous injection of mouse F, LNC into either parent results in a rapid reduction in the ability of the recipient to generate CTL reactive against donor antigens in an in vitro MLR. The underlying mechanism appears to be the in-activation of host CTL precursors that can recognize donor lymphocytes that have entered the recirculating pool. The donor lymphocytes may be acting as functionally deleting APC, or veto cells. Here, we have injected C57BL/6 (B6) mice with (C57BL/6XDBA/2) F, (FO LNC. The CTL response against donor LNC was maximally reduced by 2 days and stayed reduced for at least 6 weeks, but ultimately recovered to normal levels. The response reduction mechanism remained operative during the period when the response was reduced: Fresh FITC-labeled B6 LNC introduced into B6 recipients of an earlier injection of Fi LNC were as effectively deleted of F^reactive CTLp as the original host B6 LNC population, the fresh FITC-labelled B6 LNC being separated from host B6 LNC by cell sorting before testing. When B6 mice injected with Fi LNC ultimately recovered their response against Fi donor antigens, reinjection ofF¡ LNC did not induce a new response reduction. Instead of entering the recirculating pool, the injected F, cells were rapidly removed by a specific immune process.
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