Molecular and Cellular Requirements for Enhanced Antigen Cross-Presentation to CD8 Cytotoxic T Lymphocytes
Journal of Immunology
MHC class I-mediated cross-priming of CD8 T cells by APCs is critical for CTL-based immunity to viral infections and tumors. We have shown previously that tumor-secreted heat shock protein gp96-chaperoned peptides cross prime CD8 CTL that are specific for genuine tumor Ags and for the surrogate Ag OVA. We now show that tumor-secreted heat shock protein gp96chaperoned peptides enhance the efficiency of Ag cross-priming of CD8 CTL by several million-fold over the cross-priming activity of
... activity of unchaperoned protein alone. Gp96 also acts as adjuvant for cross-priming by unchaperoned proteins, but in this capacity gp96 is 1000-fold less active than as a peptide chaperone. Mechanistically, the in situ secretion of gp96-Ig by transfected tumor cells recruits and activates dendritic cells and NK cells to the site of gp96 release and promotes CD8 CTL expansion locally. Gp96mediated cross-priming of CD8 T cells requires B7.1/2 costimulation but proceeds unimpeded in lymph node-deficient mice, in the absence of NKT and CD4 cells and without CD40L. Gp96-driven MHC I cross-priming of CD8 CTL in the absence of lymph nodes provides a novel mechanism for local, tissue-based CTL generation at the site of gp96 release. This pathway may constitute a critically important, early detection, and rapid response mechanism that is operative in parenchymal tissues for effective defense against tissue damaging antigenic agents. C ross-priming of CD8 cells (1) by bone marrow-derived APCs is critical for the induction of cytotoxicity against intracellular pathogens and tumors (2-6), but the physical nature of the cross-priming Ag taken up by APCs and crosspresented by MHC I is controversial (7-10). Heat shock proteins chaperone peptides that can be taken up by APCs and cross-presented to CD8 cells (11-15). Exogenous heat shock protein (HSP) 3 are actively captured by CD91 and LOX-1 on dendritic cells (DC) (16 -18) and elicit peptide-specific immune responses (19) by delivering the chaperoned peptide to the MHC class I pathway to be cross-presented to CD8 ϩ CTL (7, 15, 20, 21) . Cross-priming by HSP-gp96 is associated with TLR2 and TLR4 stimulation and DC maturation resulting in a CD8 CTL-biased response (15, (22) (23) (24) . We have previously developed a model of tumor cell-secreted gp96 for use as tumor vaccine. Immunization with gp96-secreting tumor cells generated tumor-specific and surrogate Ag-specific immunity that was independent of CD4 cells (25, 26) . Using this immunization method to quantitate CD8 responses, we report here that minute, femtomolar amounts of gp96-chaperoned Ag are sufficient for cognate CD8 cross-priming locally at the site of gp96 release independent of lymph nodes and CD4 cells.