ABSTRACT
Sarcoidosis of unknown etiology may result from exposure of a genetically
susceptible subject to a specific environmental agent(s)—possibly an infectious one, although none has been identified (1). Propionibacterium acnes is so far the only bacterium to be isolated from sarcoid lesions. Genomes of
P. acnes have been detected in large numbers in sarcoid lymph nodes by the quantitative polymerase chain reaction. By hybridization in situ,
P. acnes DNA were found in sarcoid granulomas. These results point to an etiological link between P. acnes and some cases of sarcoidosis. Host factors may be more critical than agent factors in the etiology of sarcoido-
sis, as already suggested from the phenomenon of the Kveim test. A recombinant trigger-factor protein, RP35, from P. acnes causes a cellular immune response in some patients with sarcoidosis, but not in subjects without
sarcoidosis. RP35 causes pulmonary granulomas in mice sensitized with
the protein and adjuvant. P. acnes is the most common bacterium commensal in lungs and mediastinal lymph nodes. Local and ectopic intracellular
proliferation of this endogenous bacterium may trigger sarcoid granuloma-
tous inflammation in genetically predisposed individuals.
