Antigen delivery by alpha(2)-macroglobulin enhances the cytotoxic T lymphocyte response

alpha(2)M* targets antigens to APCs for rapid internalization, processing, and presentation. When used as an antigen-delivery vehicle, alpha(2)M* amplifies MHC class II presentation, as demonstrated by increased antibody titers. Recent evidence, however, suggests that alpha(2)M* encapsulation may also enhance antigen-specific CTL immunity. In this study, we demonstrate that alpha(2)M*-delivered antigen (OVA) enhances the production of specific in vitro and in vivo CTL responses. Murine splenocytes expressing a transgenic TCR specific for CTL peptide OVA(257-264) (SIINFEKL) demonstrated up to 25-fold greater IFN-gamma and IL-2 secretion when treated in vitro with alpha(2)M*-OVA compared with soluble OVA. The frequency of IFN-gamma-producing cells was increased approximately 15-fold, as measured by ELISPOT. Expansion of the OVA-specific CD8+ T cell population, as assayed by tetramer binding and [3H]thymidine incorporation, and OVA-specific cell-mediated cytotoxicity, as determined by a flow cytometric assay, were also enhanced significantly by alpha(2)M*-OVA. Furthermore, significant CTL responses were observed at antigen doses tenfold lower than those required with OVA alone. Finally, we also observed enhanced humoral and CTL responses by naïve mice following intradermal immunization with alpha(2)M*-OVA. These alpha(2)M*-OVA-immunized mice demonstrated increased protection against a s.c.-implanted, OVA-expressing tumor, as demonstrated by delayed tumor growth and prolonged animal survival. The observation that alpha(2)M*-mediated antigen delivery elicits specific CTL responses suggests the cross-presentation of antigen onto MHC class I. These results support alpha(2)M* as an effective antigen-delivery system that may be particularly useful for vaccines based on weakly immunogenic subunits or requiring dose sparing.