Heat shock proteins and cancer vaccines: developments in the past decade and chaperoning in the decade to come

Abstract

Molecular chaperone-peptide complexes extracted from tumors (heat shock protein [HSP] vaccines) have been intensively studied in the preceding two decades, proving to be safe and effective in treating a number of malignant diseases. They offer personalized therapy and target a cross-section of antigens expressed in patients' tumors. Future advances may rely on understanding the molecular underpinnings of this approach to immunotherapy. One property common to HSP vaccines is the ability to stimulate antigen uptake by scavenger receptors on the antigen-presenting cell surface and trigger T-lymphocyte activation. HSPs can also induce signaling through Toll-Like receptors in a range of immune cells and this may mediate the effectiveness of vaccines.

Figure 1. Functional domains in heat shock protein A and heat shock protein C family members

Heat shock protein (HSP)A (Hsp70, large HSP) and HSPC (Hsp90, GP96) each possess two major domains including a nucleotide (ATP, ADP) binding domain and a polypeptide (substrate) binding domain. When ATP binds to the HSP, the polypeptide-binding domain opens and cargo dissociates. When ADP binds to the nucleotide-binding domain, polypeptide cargo is bound and in this form HSP may capture and chaperone tumor antigens. These two domains, although similar in function, may differ considerably between individual HSPs. Pi: Phosphate; S: Substrate.

Figure 2. Heat shock protein vaccines prepared from tumor lysates

Heat shock protein (HSP)A and HSPC family members chaperone intracellular antigens in tumor cells (see Figure 1). When tumor cells are lysed, HSP-peptide complexes (HSP.PC) can be isolated biochemically and form the basis for development of HSP vaccines. The vaccines are then injected into the periphery of tumor bearing hosts where they encounter APCs. APCs are able to take up the HSP.PC and transport them to the sites of antigen processing where the peptide antigens are proteolytically processed, loaded onto MHC class I molecules and presented on the APC surface. APCs (usually dendritic cells [DCs]) then traffic to the efferent lymph nodes where they encounter cognate CD8⁺ lymphocytes that recognize the tumor antigen-MHC class I complex on the DC surface through their T-cell receptors. T cells then become activated into proliferating CTLs and traffic to the tumor where, under the correct inflammatory circumstances they can extravasate, enter the tumor milieu and kill tumor cells expressing surface MHC class I–antigen complexes with high specificity. Ag: Antigen; APC: Antigen-presenting cell; CTL: Cytotoxic T lymphocyte.

Figure 3. Heat shock protein receptors and Toll-like receptor 4 signaling

TLR-4 (and other TLR family members) can be activated in antigen-presenting cells and other immune cells by extracellular HSP either directly or indirectly through a primary receptor such as SREC-I or LOX-1. Ligand binding then triggers the complex TLR signaling cascade. We show a highly simplified cartoon. Ligand-activated TLR-4 engages a range of intracellular adaptor proteins (Myd88, TRAM, MAL, TRIF shown here) that regulate the various arms of TLR-4 signaling. Here we show activation of transcription factors NF-κB and IRF3. NF-κB complex proteins p65 and p50 can bind the promoters of cytokine genes, co-activators and scavenger receptors to modulate the phenotype of the antigen-presenting cell and influence other cells through receptor expression and cytokine signaling. TLR-4 activation can influence other transcriptional events such as activation of IFN-β through IRF3. HSP: Heat shock protein; IRF: Interferon regulatory factor; TLR: Toll-like receptor.

Figure 4. Hsp90 and antigen cross-presentation

Hsp90–peptide complexes bind to SREC-I and this receptor causes uptake of complexes into early endosomes. Peptides can then be processed within the endosome by proteases such as cathepsin S, encounter MHC class I molecules in recycling endosomes and transit to the cell surface. Polypeptides can also exit the endosome, be processed through the proteasome and enter the conventional pathway of antigen presentation requiring transporter associated with antigen presentation. We found that larger antigens such as full length Ova tend to utilize the latter pathway for processing. Hsp90 appears to chaperone peptides through the receptor binding step until the early endosome stage. ER: Endoplasmic reticulum.

Figure 5. Heat shock protein co-activation and co-repression

Activation of a cytotoxic T lymphocyte response by APCs requires encounter of MHC class I–peptide complexes on APCs with the TCR. However, mature dendritic cells also express coreceptors that are usually required for full activation. These include Toll-like receptor-induced immunoglobulin family members CD80 and CD86 that encounter CD28 on the T-cell surface (arrows). However, T cells also express an inhibitory co-receptor CTLA-4 that can be induced by TCR ligation and inhibits cytotoxic T lymphocyte function by multiple mechanisms. Treg cells also express CTLA-4, which can disrupt APC–dendritic cell interaction by antagonizing the functions of CD80 and CD86 and blocking T-cell activation. HSPs may influence this process by altering co-activator expression or binding and triggering Tregs. APC: Antigen-presenting cell; CTLA: Cytotoxic T-lymphocyte antigen; HSP: Heat shock protein; TCR: T-cell receptor.