Secondary syphilis in HIV with CD8-predominant epidermotropic infiltrate mimicking cutaneous T-cell lymphoma.

Background: Syphilis and mycosis fungoides are both classically referred to as "great imitators" of skin disease, and their differential diagnosis is particularly challenging in people living with HIV (PLWH). Secondary syphilis may produce a plasma-cell-poor, epidermotropic, CD8-predominant, highly proliferative infiltrate that closely resembles aggressive cutaneous T-cell lymphoma (CTCL). Case presentation: A 49-year-old man who has sex with men (MSM), with poorly controlled HIV (CD4 340 cells/µL, viral load 6220 copies/mL, ART discontinued two years earlier), presented with widespread coalescing copper-red macules, papules and infiltrated plaques in a corymbriform-like configuration. A skin biopsy and syphilis serology were ordered simultaneously as part of the initial diagnostic work-up. The nontreponemal test (VDRL) was strongly reactive at 1:128 and the treponemal test was reactive; however, biopsy demonstrated an epidermotropic CD3⁺/CD8⁺ infiltrate with Ki-67 ≈ 80% and admixed plasma cells, initially interpreted as suggestive of an aggressive epidermotropic CD8⁺ CTCL, and the strongly reactive serology was initially regarded as an incidental coinfection. Recognition of the plasma cells and of features atypical for CTCL (endothelial swelling, absence of necrotic ulcerated nodules, sub-acute course) prompted a Warthin-Starry stain on the same biopsy, which identified Conclusions: An epidermotropic CD8-predominant infiltrate with high Ki-67 in an HIV-coinfected patient does not exclude secondary syphilis. Careful integration of clinical, histopathologic, immunohistochemical and serologic data - and in situ demonstration of