Cross‐Reactivity of Treponema pallidum Polyclonal Antibody With Mycobacterium Species

INTRODUCTION: Mycobacteria are typically identified using the acid-fast staining technique. Prior reports have observed cross-reactivity of Treponema pallidum immunohistochemistry (IHC) with Mycobacterium species, particularly Mycobacterium leprae. Further characterization of this cross-reactivity is needed across a broader range of Mycobacterium species, including its extent, strength, and quality. MATERIALS AND METHODS: A retrospective review was conducted to identify all cases of mycobacterial infection at a single institution. Cases with available histopathologic slides were included. Cases in which mycobacteria were not identified on histologic examination were excluded. T. pallidum IHC was performed for each case that met inclusion criteria, and staining patterns were recorded. RESULTS: Of the 55 cases of mycobacterial infection identified, a total of 19 cases met the inclusion criteria: 10 cases of M. leprae, 6 cases of M. chelonae, 2 cases of M. tuberculosis, and 1 case of M. avium. IHC for T. pallidum showed cross-reactivity in all 10 M. leprae cases (100%), 3 of 6 M. chelonae cases (50%), 1 of 2 M. tuberculosis cases (50%), and no staining in the single M. avium case. Cases of M. leprae and M. chelonae showed a consistent strong granular and clumped staining pattern. CONCLUSIONS: This study demonstrates that T. pallidum IHC can highlight diverse Mycobacterium species, a cross-reactivity that pathologists should recognize to avoid false-positive interpretations. In the appropriate clinical context, this cross-reactivity may be utilized to further support the diagnosis of leprosy, given the consistently strong staining observed with M. leprae.