The proposed role of T-cell exhaustion in predicting clinical response to DLI. Our data support the idea that (A) response to DLI is associated with a preexisting reservoir of antitumor CD8⁺ T cells residing at the tumor site, to which CD4⁺ DLI provides immunologic help, not only to expand this reservoir but also to reverse T-cell exhaustion. The presence of T-cell exhaustion may signal that this reservoir exists. In contrast, in the absence of such a reservoir (B), a lack of DLI response is associated with both insufficient quantities of infiltrating T cells and the absence of phenotypic evidence of past T-cell activation.