Differential M y D 88/ IRAK 4 requirements for cross‐priming and tumor rejection induced by heat shock protein 70‐model antigen fusion protein

Priming of CD 8 + 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 CD 8 + 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 ( TLR s) interact with HSP and activate APC . The ability of HSP to activate APC through TLR s is, however, controversial because of the possibility of endotoxin contamination. Using a variety of TLR KO mice, we present evidence that TLR s ( TLR 2, 3, 4, 7, and 9) and their adaptor molecules M y D 88 and IRAK 4 are dispensable in cross‐priming by a mycobacterial HSP 70 – antigen (ovalbumin as a model antigen) fusion protein; in contrast, M y D 88/ IRAK 4, but not TLR s, 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 HSP 70 alone is insufficient for tumor rejection and that M y D 88/ IRAK 4‐dependent inflammatory stimulation, which might contribute to maintenance of the initially primed effector cells, is required to eradicate tumor burden. ( Cancer Sci 2012; 103: 851–859)