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ABSTRACTChaperones are molecules found in all cells and are critical in stabilization of synthesized proteins, in repair/removal of defective proteins, and as immunodominant antigens in innate and adaptive immunity. Subjects with gingivitis colonized by the oral pathogen Porphyromonas gingivalispreviously demonstrated levels of anti-human chaperone Hsp90 that were highest in individuals with the best oral health. We hypothesized that similar antibodies to pathogen chaperones might be protective in periodontitis. This study examined the relationship between antibodies to P. gingivalisHtpG and clinical statuses of healthy and periodontitis-susceptible subjects. We measured the humoral responses (immunoglobulin G [IgG], IgA, and IgM) to peptides of a unique insert (P18) found in BacteroidaceaeHtpG by using a high-throughput, quantitative fluorescence enzyme-linked immunosorbent assay. Indeed, higher levels of IgG class anti-P. gingivalisHtpG P18 peptide (P< 0.05) and P18a, consisting of the N-terminal 16 amino acids of P18 (P< 0.05), were associated with better oral health; these results were opposite of those found with anti-P. gingivaliswhole-cell antibodies and levels of the bacterium in the subgingival biofilm. When we examined the same sera for IgA and IgM class antibodies, we found no significant relationship to subject clinical status. The relationship between anti-P18 levels and clinical populations and individual subjects was found to be improved when we normalized the anti-P18a values to those for anti-P18? (the central 16 amino acids of P18). That same ratio correlated with the improvement in tissue attachment gain after treatment (P< 0.05). We suggest that anti-P. gingivalisHtpG P18a antibodies are protective in periodontal disease and may have prognostic value for guidance of individual patient treatment.