A case of mistaken identity: HSPs are no DAMPs but DAMPERs with autoimmune diseases. And as an aside, direct polyclonal stimulation of mouse B cells with human HSP60 induced production of IgG3. These activated B cells were also found to stimulate a Th0 profile in T cells as opposed to a typical Th1, as indicated by their production of interleukin (IL)-10 and interferon gamma (INF-γ; Cohen-Sfady et al. 2005). In addition, T cells reactive to HSPs that have been reported were typically not pro- inflammatory, regardless of whether they were stimulated with mammalian (self) HSP or microbial HSPs. In stark contrast, many antibodies or responding T cells were found to have anti-inflammatory activities (Moudgil et al. 1997; Ulmansky et al. 2002; van Eden et al. 2005). These were demonstrated by their abilities to suppress an inflammatory disease or to produce anti-inflammatory cytokines such as IL-10 (Prakken et al. 2001). In other words, despite the fact that responses to HSPs may be of fundamental interest to the immune system, the phenotypic characteristic of HSP immune reactivity does not reflect a typical DAMP- associated pro-inflammatory response. Be that as it may, one cannot exclude that the dominant immunogenicity of HSP stems from their interaction with TLRs or other innate receptors. However, there remain other possibilities which may explain the potent immuno- genicity of HSPs equally well (Table 1). Firstly, the more practical explanation is exposure. The immune system is in continuous contact with conserved microbial HSPs. The symbiotic relationship between the gut and intestinal microbiota ensures the regular immunological surveillance of our resident flora. Be it benign colonization by commensals or true invasion by pathogens, it will impose stress on every intruder, which will lead to an upregulation of microbial HSP and provide ample and repetitive opportunity for our immune system to respond to HSP. And logically, the immune response would most frequently encounter the most conserved epitopes. In this response, the expansion of T cells specific for conserved epitopes increases the likelihood of cross-reactivity with self-HSP. Secondly, given their chaperone activities, an alternative explanation is that HSPs are hitchhiking with other antigens for uptake and processing by antigen-presenting cells. In this manner, they may frequent the priority lane in the antigen processing system. A third possibility is that HSPs have an organized priority status for the immune system. Cohen has argued that HSPs are examples of immune- Table 1 Mechanisms leading to the HSP immunodominance Repeated contact leads to a focus on conserved regions Hitchhiking in the antigen uptake and processing machinery Built-in feature of the immune system (homunculus idea) Triggering of innate TLR receptors? 283 privileged antigens, so-called homunculus antigens. This would imply that the reactivity to such molecules is a genetically determined inherent quality of the immune system (Cohen 2007). Thus, there are more ways to explain the exceptional immunogenicity of HSP, which do not depend on immediate interaction with TLRs or other receptors of the innate immune system. In conclusion, although the high immunogenicity may suggest innate receptors to be involved, there are ample alternative options to explain this aspect of HSP. A primary feature of chaperones is their capacity to bind peptides. HSP70 has peptide-binding domains for exposed hydrophobic residues of substrate proteins. When peptide- loaded HSP70 is delivered to dendritic cells (DCs), the peptide is rerouted through the major histocompatibility complex (MHC) class I pathway antigen presentation, via cross-presentation, for display to CD8+ cytotoxic T cells (Rock and Shen 2005). In a number of studies on cross- presentation by HSP, evidence was provided for a stimula- tory effect on DC maturation and activation (Binder 2009). With respect to accompanying DC maturation, controversy has arisen regarding the role of microbial contaminants in this DC stimulatory effect as some authors have presented evidence that lipopolysaccharide (LPS; TLR4), lipoprotein (TLR2), or flagellin (TLR5) were the active compounds here (Tsan and Gao 2009). In one study, purposeful removal of contaminating LPS did not prevent peptide presentation, but did abrogate activation of human DCs in association with human HSP70 (Bausinger et al. 2002). In juxtaposition, peptides fused to HSP60 also induced CD8+ cytotoxic T cells (Cho et al. 2000). This fusion product was stimulatory to DCs, leading to upregulation of MHC (class I and II) and co-stimulatory molecules (B7.2), whereas unexpectedly HSP60 alone was not stimulatory. Again, this led to a suspected role of contaminants in the fusion proteins. Thus, although cross-presentation mediated by HSP seems real, the DC stimulating activities may be uncertain, to say the least. A balanced review which, overall, argues against endotoxin being the “active” ingredient in many HSP effects is however Henderson et al. (2010). The difficulty of the “one molecule–one receptor–one functional response” idea DAMPs are generally regarded as being intracellular proteins that are hidden from recognition by the immune system until exposed by cell damage, such as necrosis. Presumably, this release of DAMPs into the extracellular fluid begins an inflammatory response. Several receptors are purported to mediate DAMP recognition (e.g., receptor for advanced glycation end products (RAGE) and TLR9) and subsequently propagate an inflammatory response.