We next performed experiments to determine the quantitative aspects of the isotype-specific humoral immune responses that were generated in response to the anti-HIV immunization vectors. The sera were collected from mice for ELISA assays at 54 d.p.p and 14 d.p.b., as described previously. Purified MPER antigenic peptide was bound to the plate. The plates were then incubated with immunized mouse sera and the binding was detected with HRP conjugated secondary antibody. Ad5/ HVR2-MPER-L15DE1 and Ad5/HVR2-MPER-L15(Gag) vectors produced an isotype-specific anti-MPER humoral response in vaccinated mice; whereas there was no isotype-specific response seen in mice vaccinated with Ad5, AdCMVGag or AdCMVEnv. The presence of isotypes IgG2b and IgG1 corresponds to Th1 and Th2 activation, respectively (Figure 7) . T cell activation is important for host protection against foreign antigens.