Heat-shock proteins (HSPs) are the most abundant and ubiquitous soluble intracellular proteins. In single-cell organisms, invertebrates and vertebrates, they perform a multitude of housekeeping functions that are essential for cellular survival. In higher vertebrates, their ability to interact with a wide range of proteins and peptides--a property that is shared by major histocompatibility complex molecules--has made the HSPs uniquely suited to an important role in organismal survival by their participation in innate and adaptive immune responses. The immunological properties of HSPs enable them to be used in new immunotherapies of cancers and infections.

Heat shock proteins are abundant soluble intracellular proteins, present in all cells. Members of the heat shock protein family bind peptides including antigenic peptides generated within cells. Heat shock proteins also interact with antigen presenting cells through CD91 and other receptors, eliciting a cascade of events including re-presentation of heat shock protein-chaperoned peptides by MHC, translocation of NF kappa B into the nuclei and maturation of dendritic cells. These consequences point to a key role of heat shock proteins in fundamental immunological phenomena such as activation of antigen presenting cells, indirect presentation (or cross-priming), and chaperoning of peptides during antigen presentation. Heat shock proteins appear to have been involved in innate immune responses since the emergence of phagocytes in early multicellular organisms and to have been commandeered for adaptive immune responses with the advent of specificity. These properties of heat shock proteins also allow them to be used for immunotherapy of cancers and infections in novel ways.

gp96 is an endoplasmic reticulum chaperone for cell-surface Toll-like receptors (TLRs). Little is known about its roles in chaperoning other TLRs or in the biology of macrophage in vivo. We generated a macrophage-specific gp96-deficient mouse. Despite normal development and activation by interferon-gamma, tumor necrosis factor-alpha, and interleukin-1beta, the mutant macrophages failed to respond to ligands of both cell-surface and intracellular TLRs including TLR2, TLR4, TLR5, TLR7, and TLR9. Furthermore, we found that TLR4 and TLR9 preferentially interacted with a super-glycosylated gp96 species. The categorical loss of TLRs in gp96-deficient macrophages operationally created a conditional and cell-specific TLR null mouse. These mice were resistant to endotoxin shock but were highly susceptible to Listeria monocytogenes. Our results demonstrate that gp96 is the master chaperone for TLRs and that macrophages, but not other myeloid cells, are the dominant source of proinflammatory cytokines during endotoxemia and Listeria infections.

Srivastava et al. define a new and improved way to predict immunoprotective cancer neoepitopes based in part on the difference in MHC-binding scores between the mutant epitope and its wild-type counterpart. Remarkably, all neoepitopes that elicited tumor regression bound to class I MHC molecules with very low affinity.

Heat shock protein (HSP) preparations derived from cancer cells and virus-infected cells have been shown previously to elicit cancer-specific or virus-specific immunity. The immunogenicity of HSP preparations has been attributed to peptides associated with the HSPs. The studies reported here demonstrate that immunogenic HSP–peptide complexes can also be reconstituted in vitro. The studies show that (a) complexes of hsp70 or gp96 HSP molecules with a variety of synthetic peptides can be generated in vitro; (b) the binding of HSPs with peptides is specific in that a number of other proteins tested do not bind synthetic peptides under the conditions in which gp96 molecules do; (c) HSP–peptide complexes reconstituted in vitro are immunologically active, as tested by their ability to elicit antitumor immunity and specific CD8+ cytolytic T lymphocyte response; and (d) synthetic peptides reconstituted in vitro with gp96 are capable of being taken up and re-presented by macrophage in the same manner as gp96– peptides complexes generated in vivo. These observations demonstrate that HSPs are CD8+ T cell response–eliciting adjuvants.