Anti-gp41 antibodies induced by immunization. (A) Immunization protocol. Rabbits were immunized twice with DNA encoding for trimer-stabilized constructs and twice with homologous VLPs at indicated time points. (B) Three groups receiving gp41CTM and trimer-stabilized constructs were assigned. One group receiving gp145 DNA and VLPs served as control group. (C,D) Terminal bleeds of rabbits were analyzed by IgG ELISA. Peptides spanning the MPER (C) and CHR (D) served as coating peptides. Anti-rabbit-HRP coupled antibody was used as secondary antibody. Titers are calculated as reciprocal dilutions of specific signals which are at least two-fold above pre-bleed signals. Asterisks above single groups mark a mean titer significantly above background level (p < 0.05).