In man, hsp90, hsp70, hsp60/Chaperonin and hsp40 families have been characterized.[8] In prokaryotes, GroEL (hsp60) and DnaK (hsp70) are the main hsp families. Stress proteins are ubiquitous and can be detected readily in normal human plasma samples.[9]

Absolute levels of extracellular hsp vary markedly between individuals. For example, reported levels for human plasma hsp60 range between < 1 ng/ml and 1 mg/ml[9] and between 100 pg/ml and 160 ng/ml for CDK inhibitor serum hsp72.[10] Levels of hsp are dynamic during normal physiological activities; exercise increases hsp72 levels in serum by fourfold to eightfold.[11] Therefore, extracellular hsp are continuously present in the circulation of normal individuals and can be increased transiently by several fold without apparent pathology. In addition to functioning as intracellular protein chaperones, hsp modulate the immune system by stimulating both innate and adaptive responses. The term ‘chaperokine’ has been used to describe the dual activity of hsp functioning as both chaperone and cytokine.[12] Once released from a host or pathogen cell, hsp bind to this website cellular receptors to trigger an

innate immune response, including maturation of DC and secretion of pro-inflammatory cytokines and chemokines, for example RANTES (Regulated on Activation Normal T-cell Expressed and Secreted), through Toll-like receptor activation.[13] Processing of cargo proteins carried by hsp occurs, leading to antigen presentation on MHC. Hence hsp link the innate and acquired immune responses to pathogens and have the potential to function as vaccine aminophylline adjuvants in infections and cancer.[14] For

example, hsp70 is an effective and safe adjuvant in neonatal mice and functions effectively via mucosae to generate protective cell-mediated immune responses against herpes simplex virus type-1.[15] Moreover, modified hsp are also capable of inducing cytokine responses. For example, a fusion protein containing Bacillus Calmette–Guérin (BCG)-derived hsp70 and Mycobacterium leprae-derived major membrane protein binds to human DC stimulating production of interleukin-12 p70 through Toll-like receptor 2.[16] Dendritic cells and other cell types possess multiple receptors that bind hsp but the identities and functions of those proposed to modulate the immune system in vivo are not fully understood.[17] The expression profile of these receptors is broad, including, but not limited to, multiple immune, epithelial, endothelial and fibroblast cells and multiple cell types of the central nervous system. Receptors for which evidence supports a role in hsp binding and their distribution on immune cells are shown (Table 2). The relative contribution made by each receptor type to the binding and internalization of hsp by DC is poorly understood.