Differential MyD88/IRAK4 requirements for cross-priming and tumor rejection induced by heat shock protein 70-model antigen fusion protein

Priming of CD8 ⁺ T cells requires two signals, one produced by T-cell receptor recognition of antigen, and a second that is often provided by the innate immune response. In this context, antigens non-covalently or covalently associated with heat shock proteins (HSP) are internalized and processed in antigen-presenting cells (APC) to be presented by MHC I molecules to CD8 ⁺ T cells, thus, signal 1 has been well characterized in this pathway of cross-presentation. Signal 2 is not fully understood, although there are reports that Toll-like receptors (TLRs) interact with HSP and activate APC. The ability of HSP to activate APC through TLRs is, however, controversial because of the possibility of endotoxin contamination. Using a variety of TLR KO mice, we present evidence that TLRs (TLR2, 3, 4, 7, and 9) and their adaptor molecules MyD88 and IRAK4 are dispensable in cross-priming by a mycobacterial HSP70-antigen (ovalbumin as a model antigen) fusion protein; in contrast, MyD88/IRAK4, but not TLRs, are required for tumor rejection induced by the same reagent. Our results indicate that HSP-mediated cross-priming uses a second signal produced by mechanisms other than TLR cascades. We hypothesize that efficient cross-priming by HSP70 alone is insufficient for tumor rejection and that MyD88/IRAK4-dependent inflammatory stimulation, which might contribute to maintenance of the initially primed effector cells, is required to eradicate tumor burden.