anthracis by murine macrophages [20] and human NK cells [21] involve IFN-γ; although IFN-γ production by NK cells may be down-regulated somewhat by anthrax lethal toxin [21]. In mice, IFN-γ-inducible chemokines CXCL9, -10 and -11, contributed directly to in vitro anti-microbial effects against B. anthracis Sterne strain spores [22], and IFN-γ was produced by NK cells in response to B. anthracis spores [23]. Human peak TNA response occurs at different time points for different individuals [1], but typically between 28 and

35 days after CB-839 concentration the first dose in the series. The timeframe of peak circulation of T cells is not known. It is clear that sampling only at one time point (7 days after the second dose) provides an indication of the potential of two doses of AV7909 to induce T cell responses, but does not fully capture the differential kinetics of in vivo T cell activation, migration to lymphoid organs and recirculation in peripheral blood. Hence, because sampling the blood compartment only detects T cells in transit, these data are biased by sampling one time point. However, studies of T cell responses with Melan-A peptide vaccine adjuvanted with 0.5 mg of CpG demonstrated circulating levels of Melan-A specific T cells peaked at 7 days (4/7 subjects) and 10 days (3/7 subjects) after second immunization, and decline Bcl 2 inhibitor to near baseline by day 14 [24], suggesting that our PBMC samples were obtained within an

appropriate window for sampling. Nonetheless, the use of a CYTH4 single post-immunization sampling point may explain some inter-group variability in this small study population. Of note is the observation that of subjects that had positive ELISpot responses, half responded to both rPA and PAp, revealing an overlap in processed epitopes and predicted peptides (PAp). This overlap in responses of rPA compared to the pool of PAp suggests predicted peptides to be a suitable strategy for ELISpot testing in unknown HLA populations with

limited PBMC samples. In summary (1) immunization with two doses (14 days apart) of an anthrax vaccine candidate consisting of AVA plus CPG 7909 was sufficient to induce IFN-γ-positive T cell recall responses in ex vivo-stimulated PBMC collected 21 days following the first immunization; (2) in this pilot study, a dose (0.25 mg) of CPG 7909, lower than used in other vaccines in development, was adequate to increase innate and adaptive immune responses beyond that elicited by AVA (BioThrax) alone; (3) rPA and predicted peptides of PA may be adequate as recall antigens in assessing anthrax vaccine-induced T cell recall responses of frozen PBMC; finally, (4) the innate responses to CpG, such as decreased ALC and increased CRP, explain a contribution of roughly 60% to the later peak anti-PA antibody titer ( Fig. 3B); the remaining variability is attributed to Subject differences in response to PA antigen, perhaps HLA-related. This work was supported by BARDA/NIAID contract number HHSN272200800051C.