282 tors. At this moment, however, DAMPs is more known as “damage”-associated molecular patterns. The latter was proposed for the situation in clinical organ transplantation, where the transplant does not reflect a danger but rather a benefit to the recipient. The upregulated HSPs resulting from reperfusion injury were therefore viewed as a typical DAMP in the sense of “damage” and not danger-associated molecular pattern (Land 2005). In the more narrow definition, DAMPs are intracellular nonmicrobial molecules normally hidden from recognition by the immune system. They are released during necrotic and not apoptotic cell death, after which the innate immune system responds both directly with inflammation and indirectly by recruiting reinforcements from the adaptive immune system (Kono and Rock 2008). Kono and Rock have attempted to outline four criteria in terms of biological outcomes that must be fulfilled for a molecule to be classified as a DAMP (Kono and Rock 2008): 1. A DAMP should be active as a highly purified molecule 2. The biological effect should not be owing to contam- ination with microbial molecules. Caution is particu- larly warranted if the putative DAMP is found to work through receptors for PAMPs such as toll-like receptors (TLRs). 3. The DAMP should be active at concentrations present in pathophysiological situations 4. Selective elimination or inactivation of the DAMP should ideally inhibit the biological activity of dead cells in in vitro or in vivo assays. In the case of HSPs, the first two criteria are already problematic. Given their chaperone nature, HSPs readily engage other molecular structures, and experience has taught us that recombinant HSPs are easily contaminated with microbial products when produced in microbial expression systems. Criterion 4, by the authors’ own admission, is an ideal effect, and in most cases, this criterion is not fulfilled. To put it bluntly, it seems questionable whether or not there exists any single DAMP which meets all of these criteria. Our brief analysis of the current literature on DAMPs suggested that convincing evidence for single DAMP molecules initiating pro- inflammatory responses, as proven by the above criteria, may be lacking so far. Microbial HSPs are immunodominant and induce cross-reactivity with self-HSP One of the remarkable characteristics of HSPs is their immunodominance. HSP60 was already known as the “common antigen” of gram-negative bacteria before the actual recognition of its molecular identity. Immunization W. van Eden et al. with gram-negative bacteria led to the production of high levels of antibodies specific for this common antigen (Thole et al. 1988). Since the mycobacterial HSP60 molecule was cloned (Thole et al. 1985), Kaufmann and others have performed immunization studies with myco- bacteria and with recombinant mycobacterial HSP60 demonstrating the resulting frequency of HSP60-specific T cells can be very high. After immunization with Mycobacterium tuberculosis, approximately one out of five of the M. tuberculosis-reactive T cells recognizes HSP60. Immunization with recombinant HSP60 in an adjuvant also stimulated an impressively high number of M. tuberculosis- reactive T cells (1 out of 2,000) (Kaufmann et al. 1987). Since then, many others have shown the dominant immuno- genicity of microbial HSPs (Zugel and Kaufmann 1999). Given the conserved nature of HSPs, this high level of immunogenicity is not immediately understood. One would predict a low degree of immunogenicity for molecules having extensive sequence similarities with self antigens. A healthy immune system is not supposed to respond to self antigens or their closely related homologs. Paradoxically, when mapping T cell epitopes for mycobacterial HSP60 in Lewis rats, defined epitopes included many conserved sequences, and the responding T cells were found to cross- respond to homologous self-peptide derived from mammalian HSP60 (Anderton et al. 1995). Since then, self-HSP cross- reactive T cells have been identified in many models and even in human cord blood (Durai et al. 2004; Paul et al. 2000). In children suffering from juvenile idiopathic arthritis, their strongest T cell responses to self-HSP60 were detected in patients during remitting phases of their disease, sugges- tive of an anti-inflammatory mechanism resulting from self- HSP-directed T cell reactivity (de Graeff-Meeder et al. 1995; de Kleer et al. 2003; Prakken et al. 1996). Furthermore, autoantibodies against HSP have been found to be prevalent in healthy subjects. For HSP60, this was summarized by Quintana and Cohen (2011). Similar patterns of autoantibodies are observed for self-HSP70, for example in diabetes and atherosclerosis (Abulafia- Lapid et al. 2003). Interestingly, antigen microarray analysis for multiple sclerosis (MS) patients revealed that HSP70 autoantibodies were a characteristic antibody signature of patients with a remitting and relapsing form of the disease. These antibodies were much less abundant in patients with progressive forms of MS, again associat- ing HSP70 immune reactivity with disease remission (Quintana et al. 2008). The immunodominance of HSPs may not depend on interactions with innate receptors There are no reports of peculiar pro-inflammatory isotype (e.g., IgG2a) profiles for HSP-binding antibodies in patients