Mycobacterium leprae Immunostaining Pitfall.

Abstract

To the Editor: Leprosy is an infectious granulomatous disease caused by Mycobacteria leprae, an acid-fast bacillus, that can involve skin, peripheral nerves, and respiratory mucosa.1–3 It constitutes an important global health concern. The WHO reports that 202,256 new leprosy cases were registered globally in 2019, of which 14,893 were in children younger than 14 years.4 Early diagnosis and treatment of leprosy are necessary to prevent permanent disabilities.5,6 The diagnosis of leprosy involves consideration of clinical presentation, histopathological findings, and ancillary testing. Special stains, particularly FITE stains, which highlight the bacillus in tissue, are confirmed with molecular biology tools such as polymerase chain reaction amplification of the mycobacterial heat shock 65 gene (Hsp65) from DNA of lesional biopsies.7,8 In this study, we report an example of artifactual staining of lepra bacilli from a patient's skin biopsy with antibodies against Treponema pallidum, the microorganism that causes syphilis. We report this to alert the pathologist that a positive T. pallidum should be supported by a negative Fite-Faraco stain or acid-fast bacilli (AFB) stain, to prevent an erroneous diagnosis of syphilis. A 64-year-old South Asian woman presented to clinic with multiple enlarging, well-circumscribed erythematous plaques on her chest. The lesions extended to the trunk and had been present for 2 years. She was initially treated for dermatitis with topical steroids with no clinical improvement. Biopsy from a representative lesion showed dermal superficial and deep nonnecrotizing granulomas with epithelioid macrophages, plasma cells, and lymphocytes around blood vessels and dermal appendages (Fig. 1A). Several infectious etiologies were considered in the differential diagnosis including syphilis, deep fungal, and acid-fast infection. An IHC stain against T. pallidum (red chromogen) highlighted abundant positively staining organism with a rod-shaped morphology (Fig. 1B). An FITE acid-fast stain revealed abundant bacilli inside macrophages in the dermis forming clusters (globi) and extracellularly (Fig. 1C). Testing by polymerase chain reaction performed at a reference laboratory on formalin-fixed, paraffin-embedded tissue was positive for Mycobacterium leprae.FIGURE 1.: A, Dermal nonnecrotizing granulomas with inflammatory infiltrate (hematoxylin and eosin stain). B, IHC stain against Treponema pallidum highlighted abundant rod-shaped organism. C, Fite-faraco stain highlights globi of Mycobacteria leprae.This case highlights that M. leprae are recognized by the T. pallidum antibody in addition to previously reported other mycobacteria (M. tuberculosis and M. marinum).9 The list of shared antigens of M. leprae and T. pallidum includes 60/65 kDa and 70/70 heat shock protein and other proteins, lipids, carbohydrates, and their combinations.10 The antigen(s) of the M. leprae that reacts with T. pallidum antibody remains unknown. Cross-reactivity could be due to the way in which antibodies are produced in rabbits. Freund adjuvant is used to boost immune response in rabbits to the antigen of interest. Freund adjuvant may contain mycobacteria, which could lead to antibody production against these antigens.11 In addition to Mycobacteria, the T. pallidum antiserum cross-react with antigens of some spirochetes, Escherichia coli, and Helicobacter pylori.12–14 IHC has become increasingly popular to detect T. pallidum because it is more sensitive than traditional silver stains. The cross-reactivity of IHC for T. pallidum with Mycobacteria has been reported in few cases. Awareness of staining of M. leprae with antibodies against T. pallidum is important when interpreting results. Concurrent acid-fast and FITE stains should be performed when histopathologic or clinical suspicion for Mycobacterial infection exists.