Targeting weak antigens to CD64 elicits potent humoral responses in human CD64 transgenic mice

Previous studies have documented that targeting foreign Ags to Ige Fc gamma R leads to enhanced Ag-specific responses in vitro and in vivo. However, th e ability to overcome immunologic nonresponsiveness by targeting poorly imm unogenic Ags to Fc gammaR has not been investigated. To address this questi on in a simple model, we immunized transgenic mire expressing human CD64 (F c gamma RI) and their nontransgenic littermates with Fab' derived from the murine anti-human CD64 mAb m22, The m22 Fab' served as both the targeting m olecule and the Ag, We found that only CD64-expressing mice developed anti- Id titers to m22, Furthermore, chemically linked multimers of m22 Fab', whi ch mediated efficient internalization of the human CD64, were significantly more potent than monomeric m22 F(ab')(2) at inducing anti-Id responses. In all cases, the humoral responses were specific for m22 Id and did not reac t with other murine IgG1 Fab' fragments. Chemical addition of a second muri ne Fab' (520C9 anti-human HER2/neu) to m22 Fab' multimers demonstrated that IgG1 and IgG2a anti-Id titers could be generated to 520C9 only in the CD64 -expressing mite. These results show that targeting to CD64 can overcome im munological nonresponsiveness to a weak Immunogen, Therefore, targeting to CD64 may be an effective method to enhance the activity of nonimmunogenic t umor vaccines.