Status of activation of circulating vaccine-elicited CD8(+) T cells

Selective blunting of the status of activation of circulating tumor-specifi c T cells was invoked to explain their paradoxical coexistence with unhampe red tumor growth. By analogy, lack of tumor regression in the face of obser vable melanoma vaccine-induced T cell responses might be attributed to thei r status of activation. We enumerated with HLA-A*0201/peptide tetramers (tH LA) vaccine-elicited T cell precursor frequency directly in PBMC of patient s with melanoma undergoing vaccination with the HLA-A*0201-associated gp100 :209-217(210 M) epitope (g209-2 M), Furthermore, we tested by intracellular (IC)-FACS analysis and quantitative real-time PCR (qRT-PCR) the ability of postvaccination PBMC to produce cytokine in response to challenge with vac cine-related epitopes or vaccine-matched (HLA-A*0201) melanoma cells. Vacci ne-induced enhancement of T cell precursor frequency could be detected with tHLA in PBMC from six of eight patients studied at frequencies ranging bet ween 0.3 and 2.3% of the total CD8(+) population, Stimulation with vaccine- related epitopes induced IFN-gamma expression detectable by IC-FACS or qRT- PCR, respectively, in five and six of these patients. Furthermore, down-reg ulation of tHLA staining was noted upon cognate stimulation that could be u tilized as an additional marker of T cell responsiveness. Finally, we obser ved in six patients an enhancement of reactivity against vaccine-matched tu mor targets that was partly independent of documented vaccine-specific immu ne responses. A strong correlation was noted between tHLA staining of postv accination PBMC and IFN-gamma expression by the same samples upon vaccine-r elevant stimulation and assessed either by IC-FACS or qRT-PCR, Thus, blunti ng of the status of T cell activation on itself cannot easily explain the l ack of clinical responses observed with vaccination.