Rapid deletion and inactivation of CTLs upon recognition of a number of target cells over a critical threshold.

Initiation of CTL responses against foreign pathogens also primes anti-self CTLs. Mechanisms of CTL inactivation inhibit anti-self CTLs to prevent tissue damage. These mechanisms are exploited by pathogens and tumors to evade the immune response, and present a major hurdle to adoptive CTL therapies. It is unclear whether CTL inactivation is Ag specific and, if so, which APCs are involved. Potential candidates include the target cells themselves, dendritic cells, myeloid-derived suppressor cells, and macrophages. In this study, we show that lymphoma-specific CTLs are rapidly deleted in an Ag-specific manner after adoptive transfer into lymphoma-bearing mice, and the surviving CTLs are functionally impaired. The only APCs responsible were the target cells directly presenting Ag, notwithstanding the presence of myeloid-derived suppressor cells, and CD8(+) dendritic cells cross-presenting tumor Ag. The capacity to inactivate CTLs critically depended on the number of tumor/target cells; small numbers of targets were readily killed, but a large number caused quick deletion and functional inactivation of the CTLs. Application of mild, noninflammatory, and nonlymphoablative chemotherapy to specifically reduce tumor burden before CTL injection prevented CTL deletion and inactivation and allowed eradication of tumor. Our results advocate the use of adoptive CTL therapy soon after mild chemotherapy. They also suggest a simple mechanism for Ag-specific impairment of anti-self CTLs in the face of an active anti-foreign CTL response.