Accessory cell requirements for saponin adjuvant-induced class I MHC antigen-restricted cytotoxic T-lymphocytes.

The in vivo and in vitro accessory cell requirements of class I major histocompatability complex (MHC) antigen-restricted cytotoxic T-lymphocyte (CTL) responses were determined using cell-depletion experiments coupled with active immunizations using ovalbumin (OVA) as the immunogen and saponin adjuvant (QS-21). To paralyze macrophage activity in vivo, C57BL/6 mice were treated with particulate silica or carrageenan. In vivo depletion of helper T-lymphocytes was accomplished by treatment with GK1.5 rat monoclonal antibody, which is specific for the murine CD4 antigen, and by genetic depletion of class II MHC antigens. Following treatments, the mice were immunized with formulations containing OVA alone or mixed with QS-21 saponin adjuvant, which induces MHC class I antigen-restricted CTL responses. In vivo treatment to paralyze macrophages abrogated these CTL responses but not antigen-specific antibody or lymphocyte proliferative responses. Depletion of CD4+ T-lymphocytes had no effect on CTL responses but significantly reduced proliferation and antibody responses. In vitro depletion and reconstitution experiments were done to compare the contributions of different antigen-presenting cells (APC), specifically dendritic cells (DC) and macrophages. Again, the requirement for macrophages was absolute but there was no indication that DC were involved. These data suggest that antigen processing and presentation functions are critical to the induction of CTL and that they are a function of macrophages but that CD4+ helper T-lymphocyte functions are not required.