Cell wall arabinogalactan is responsible for Fungitell® cross reactivity in nocardiosis

Nocardiosis is a serious infection in immunosuppressed patients, especially transplant recipients. The slow-growing phenotype of the bacterium and the variety of symptoms complicate diagnosis and delay antimicrobial therapy, which results in high mortality rates despite effective treatments. Incidentally, some nocardiosis patients test positive in fungal diagnostics that detect (1,3) β−D-glucan (the Fungitell® assay), but the basis for this cross-reactivity remains unknown. We demonstrate that nocardial cell wall arabinogalactan is a cryptic antigen responsible for cross reactivity in the Fungitell® assay and that this antigen is revealed in vivo following bacterial cell lysis. We further show that the reactivity results from β-glucose substitution of the galactan domain, a modification specific to Nocardia, and identify the optimal antigen as a tetramer of the trisaccharide repeating unit. By providing structural evidence for Fungitell® cross-reactivity during nocardiosis, this work paves the way for developing specific diagnostic tools that are presently lacking.