The Immune Response in SAPHO Syndrome: Deficiency, Hyper- Responsiveness, or Both?

The pathophysiology of SAPHO syndrome still remains to be determined. However, like in other forms of spondylarthritides, this rare condition seems to result from the combination of genetic, environmental and immunological factors. Surely, SAPHO syndrome cannot be simply regarded as the adult form of the ‘caricatural’ DIRA (deficiency in interleukin-1 receptor antagonist) syndrome, although this purely genetic disease also causes multiple osteomyelitis and pustular rashes. An initial bacterial trigger, mainly represented by the cutaneous saprophyte Propionibacterium acnes, could take advantage of a selective deficiency of the innate immunity, implicating neutrophils. This could elicit thereafter a ‘hyperimmune’ reaction, as in other chronic inflammatory conditions like reactive arthritis, Crohn’s disease or hidradenitis suppurativa. The reported efficacy of either longterm antibiotic regimens (especially with azithromycin) or immunomodulatory biologic agents targetting TNF-α or IL-1 supports the concept of a post- or para-infectious hyperresponsiveness disorder, with a convincing rationale for ‘hybrid’ therapies.