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 gammaR leads to enhanced Ag-specific responses in vitro and in vivo. However, the ability to overcome immunologic nonresponsiveness by targeting poorly immunogenic Ags to Fc gammaR has not been investigated. To address this question in a simple model, we immunized transgenic mire expressing human CD64 (Fc 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 molecule and the Ag, We found that only CD64-expressing mice developed anti-Id titers to m22, Furthermore, chemically linked multimers of m22 Fab', which 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 react with other murine IgG1 Fab' fragments. Chemical addition of a second murine 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 immunological nonresponsiveness to a weak Immunogen, Therefore, targeting to CD64 may be an effective method to enhance the activity of nonimmunogenic tumor vaccines.