Antigen-specific IFN-γ responses against pre-erythrocytic antigen Pf CSP. PBMC from Pf CSP or CSLAM immunized monkeys (both PBS and CRL1005 formulations) were assayed against recombinant P fCSP protein by IFN-γ ELIspot. Results represent the mean net spot-forming cells (SFC) per million PBMC from all responders (n = 18) (solid circle) or non-responders (n = 2) (open circle) at the defined time points. DNA_1 = 1st DNA immunization; DNA_2 = 2nd DNA immunization; DNA_3 = 3rd DNA immunization; Virus = ALVAC-Pf 7 boost. Net SFC was calculated by correcting the SFC responses with antigen for background SFC without antigen. IFN-γ ELIspot responses were classified as positive if (1) the net SFC (mean SFC in experiment antigen wells – mean SFC in medium wells) was > 25 SFC per million PBMC, and (2) the stimulation index (ratio of mean SFC in experimental peptide wells to mean SFC in medium wells) was > 2. (*p < 0.05 and **p < 0.01).