Utility of IL-36 immunostaining in distinguishing psoriasis from pityriasis rosea and pityriasis lichenoides.

BACKGROUND: Psoriasis is an inflammatory skin disease driven by upregulation of cytokines in the Th17 pathway, including interleukin-36 (IL-36). Previous studies have highlighted the utility of IL-36 immunostaining for psoriasis compared to spongiotic dermatitis and other psoriasiform dermatoses; however, no study has examined the role of IL-36 staining in distinguishing psoriasis from pityriasis rosea (PR) and pityriasis lichenoides (PL), known histologic mimickers of psoriasis. METHODS: We compared the immunostaining pattern of IL-36 for 21 PR cases, 22 PL cases, and 10 psoriasis cases. We graded the immunostaining as 0, negative; 1, focal weak; 2, diffuse weak; 3, focal, strong; or 4, diffuse strong. We further categorized stains as negative (0-2 score) or positive (3-4 score) and utilized Fisher's exact test to compare the immunostaining pattern of these entities. RESULTS: All psoriasis specimens were positive for IL-36, whereas all PR specimens were negative (p = 0.00000002). Twenty PL specimens were negative (p = 0.000001). Nine of 10 pityriasis lichenoides et varioliformis acuta cases were negative (p = 0.00012), and 11 of 12 cases of pityriasis lichenoides chronica were negative (p = 0.00003). CONCLUSIONS: Our findings highlight the potential role of IL-36 immunostaining in distinguishing psoriasis from other psoriasiform dermatoses, including PR and PL.

Recurrent inverse necrotizing infundibular crystalline folliculitis.

Necrotizing infundibular crystalline folliculitis (NICF) is a rare type of necrotizing folliculitis. The disease typically manifests as folliculocentric papules arising in a seborrheic distribution. Only 23 cases exist in the literature. Most reported cases have arisen spontaneously, but a small number of drug-induced cases in the setting of epidermal-derived growth factor, vascular endothelial-derived growth factor, or PD-1 inhibitor therapy have been reported. Colonization by bacteria and/or yeast occurs frequently. The etiology remains unknown, but some suggest a complex interplay with an aberrant microbiome, sebaceous gland dysfunction, and perturbed EGFR signaling in follicular infundibula. Histopathologic findings include rupture of follicular epithelium, neutrophilic inflammation, and nodular cup-shaped crystal deposits. We present a case of spontaneous, recurrent NICF in an inverse pattern in the inguinal region.

Interleukin 36 expression in psoriasis variants and other dermatologic diseases with psoriasis-like histopathologic features.

BACKGROUND: Elevated epidermal interleukin (IL)-36 expression distinguishes psoriasis from eczematous dermatitis, but other psoriasiform dermatitides (PDs) have not been thoroughly investigated for IL-36 expression. In this study, we assess the IL-36 staining pattern (IL36-SP) in psoriasis variants and other PDs including lichen simplex chronicus (LSC), prurigo nodularis (PN), lichen planus (LP), tinea, pityriasis rubra pilaris (PRP), mycosis fungoides (MF), pemphigus foliaceus (PF), acute generalized exanthematous pustulosis (AGEP), impetigo (IMP), and syphilis (SY). METHODS: IL-36 immunostaining was performed on 307 cases of psoriasis and various PDs. IL36-SP in the upper epidermis was graded on a scale of 0-4. RESULTS: High IL36-SP occurred in all variants of psoriasis, as well as in AGEP, PRP, PN, tinea, IMP, and LP (P > 0.05). SY, PF, LSC, and MF showed a lower IL36-SP (P ≤ 0.05) compared with psoriasis. CONCLUSION: All variants of psoriasis exhibit high IL36-SP. IL-36 staining can assist in differentiating MF, PF, SY, and LSC from psoriasis, particularly MF and LSC, which have consistent low IL-36 expression. AGEP, PRP, tinea, IMP, PN, and LP exhibit high IL-36 expression similar to psoriasis, indicating Th17 activation in these diseases.

Histopathologic and genetic findings in atypical spindle cell/pleomorphic lipomatous tumors and atypical pleomorphic fibromas.

BACKGROUND: Spindle cell lipomas, pleomorphic lipomas (SCL/PLs), and pleomorphic fibromas (PF) are tumors with loss of retinoblastoma (RB). The latest World Health Organization classification includes a category of atypical spindle cell/pleomorphic lipomatous tumors (ASPLT), which encompasses tumors in this spectrum that show atypical histopathologic features. We have observed PFs that show similar atypical features. METHODS: Cases of SCL/PL and PF with atypical features were collected from tissue archives between 2010 and 2019. Genetic alterations were investigated using array comparative genomic hybridization (aCGH). RESULT: Of 15 cases found, most tumors were dermal based with fibrocytic or fibroadipocytic appearance and occasional lipoblasts. All cases had a high proliferation index with atypical mitotic figures in 71% of cases. Chromosome 13q loss was present in all cases with CGH data. Additional recurrent chromosomal losses included 17p, 16q, 17q, 20p, 4, and 10. No recurrence was found in limited follow-up. CONCLUSIONS: ASPLTs are characterized by loss of RB, prominent nuclear pleomorphism, mitotic activity including atypical mitotic figures, and genomic instability with multiple chromosomal aberrations. A similar group of tumors with these histopathologic features lacks lipomatous differentiation, and we propose the diagnosis of atypical PF as a fibromatous variant of ASPLT. Limited clinical follow-up appears benign.

Primary Cutaneous Malignant Perivascular Epithelioid Cell Tumor Mimicking Undifferentiated Pleomorphic Sarcoma: A Report of a Rare Entity.

ABSTRACT: Primary cutaneous malignant perivascular epithelioid cell tumor (PEComa) is a rare and potentially aggressive neoplasm. In this article, we report the case of a 34-year-old man who initially presented with a 3-cm mass involving the skin and soft tissue of the right shoulder that, over 3 months, enlarged to 12 cm. Histologic examination of the mass revealed an infiltrative neoplasm with features resembling an undifferentiated pleomorphic sarcoma, including sheets of pleomorphic cells with abundant atypical mitoses and necrosis. Immunohistochemical evaluation showed features suggestive of PEComa. Next-generation sequencing revealed pathogenic homozygous deletions of TSC2 and TP53 genes and numerous large-scale copy number changes. Taken together, the findings supported malignant PEComa. This case demonstrates only the seventh example of malignant cutaneous PEComa. Although cutaneous PEComa is chiefly a benign mesenchymal neoplasm, in rare cases, it can rapidly transform into a malignant and infiltrative sarcoma, requiring prompt surgical management.

Impact of second-opinion dermatopathology reviews on surgical management of malignant neoplasms.

BACKGROUND: Second-opinion review is linked to error reduction and treatment changes in anatomic pathology. OBJECTIVE: We sought to establish the rate of diagnostic discrepancy identified by second-opinion dermatopathologic review and the effect on surgical treatment. METHODS: Cases referred for treatment of a malignant neoplasm diagnosed by an outside pathologist were reviewed. The external and internal second-opinion dermatopathologic reports were compared. Discordance in diagnosis, subtype, and treatment change owing to second-opinion review was recorded. The referring pathologist's level of dermatopathologic training was also documented. RESULTS: A total of 358 cases were included. Dermatopathologic second-opinion diagnosis was discordant with the outside diagnosis in 37 of 358 cases (10.3%). In 32 of 358 cases (8.9%), second-opinion review resulted in a change in treatment, with 28 of 32 (87.5%) of these changes resulting in cancelled surgery. Dermatologists without dermatopathologic fellowship training had the highest rate of discordant diagnoses compared with pathologists and dermatopathologists. LIMITATIONS: This was a retrospective study at a tertiary care facility. CONCLUSION: Second-opinion dermatopathologic review is associated with identification of discordant diagnoses and a substantial influence on treatment, with both cancellation of surgery and augmented management. Secondary pathologic review should be considered in high-volume surgical practices.

The utility of PRAME staining in identifying malignant transformation of melanocytic nevi.

BACKGROUND: PReferentially expressed Antigen in MElanoma (PRAME) immunohistochemical (IHC) staining is used to aid melanoma diagnosis. PRAME expression in nevus-associated melanoma (NAM) has not been evaluated. METHODS: PRAME IHC was applied to cases of NAM; staining for each population of melanocytes (benign and malignant) was graded based on the percentage of labeled cells. No labeling was graded 0, 1% to 25% labeling was 1+, 26% to 50% was 2+, 51% to 75% was 3+, and >76% was 4+. RESULTS: Thirty-six cases were reviewed. Sixty-seven percent (24/36) of melanomas were PRAME positive (4+) while no (0/36) nevi showed 4+ positivity. Eighty-one percent (29/36) of nevi were completely PRAME negative compared to 17% (6/36) of melanomas. In 67% of cases (24/36) PRAME differentiated between benign and malignant melanocyte populations. CONCLUSIONS: We identified a high rate (67%) of differential PRAME staining in adjacent benign and malignant melanocyte populations in NAM. In PRAME positive (4+) melanomas, PRAME differentiates 100% (24/24) of benign and malignant melanocyte populations. When 4+ staining is used as the threshold for positivity, PRAME staining has a sensitivity of 67% (24/36) and a specificity of 100% (36/36). These results support PRAME IHC can assist in distinguishing melanocyte populations in melanoma arising within nevi.

Spitz melanoma is a distinct subset of spitzoid melanoma.

Melanomas that have histopathologic features that overlap with those of Spitz nevus are referred to as spitzoid melanomas. However, the diagnostic concept is used inconsistently and genomic analyses suggest it is a heterogeneous category. Spitz tumors, the spectrum of melanocytic neoplasms extending from Spitz nevi to their malignant counterpart Spitz melanoma, are defined in the 2018 WHO classification of skin tumors by the presence of specific genetic alterations, such as kinase fusions or HRAS mutations. It is unclear what fraction of "spitzoid melanomas" defined solely by their histopathologic features belong to the category of Spitz melanoma or to other melanoma subtypes. We assembled a cohort of 25 spitzoid melanomas diagnosed at a single institution over an 8-year period and performed high-coverage DNA sequencing of 480 cancer related genes. Transcriptome wide RNA sequencing was performed for select cases. Only nine cases (36%) had genetic alterations characteristic of Spitz melanoma, including HRAS mutation or fusion involving BRAF, ALK, NTRK1, or MAP3K8. The remaining cases were divided into those with an MAPK activating mutation and those without an MAPK activating mutation. Both Spitz melanoma and spitzoid melanomas in which an MAPK-activating mutation could not be identified tended to occur in younger patients on skin with little solar elastosis, infrequently harbored TERT promoter mutations, and had a lower burden of pathogenic mutations than spitzoid melanomas with non-Spitz MAPK-activating mutations. The MAPK-activating mutations identified affected non-V600 residues of BRAF as well as NRAS, MAP2K1/2, NF1, and KIT, while BRAF V600 mutations, the most common mutations in melanomas of the WHO low-CSD category, were entirely absent. While the "spitzoid melanomas" comprising our cohort were enriched for bona fide Spitz melanomas, the majority of melanomas fell outside of the genetically defined category of Spitz melanomas, indicating that histomorphology is an unreliable predictor of Spitz lineage.

Clinicopathologic overlap of psoriasis, eczema, and psoriasiform dermatoses: A retrospective study of T helper type 2 and 17 subsets, interleukin 36, and ß-defensin 2 in spongiotic psoriasiform dermatitis, sebopsoriasis, and tumor necrosis factor α inhibitor-associated dermatitis.

BACKGROUND: T helper (Th) type 17 and Th2 cells mediate psoriasis and eczema, respectively. Some dermatoses exhibit overlapping clinicopathologic features, and their immunopathology is relatively unexplored. OBJECTIVE: To determine whether Th17 and Th2 subsets and interleukin (IL) 36 and ß-defensin 2 (BD-2) markers of IL-17 signaling expression can discriminate between biopsy samples of psoriasis and eczematous/spongiotic dermatitis and to use those markers to immunophenotype cases with clinicopathologic overlap. METHODS: A retrospective study was performed on biopsy samples of psoriasis, eczema/spongiotic dermatitis, sebopsoriasis, tumor necrosis factor α inhibitor-associated psoriasiform dermatitis, and ambiguous cases diagnosed as spongiotic psoriasiform dermatitis. Dual CD4/GATA3 and CD4/RORC, IL-36, and BD-2 immunohistochemistry was performed. RESULTS: IL-36 and BD-2 were strongly expressed in biopsy samples of psoriasis compared with eczema/spongiotic dermatitis. No significant differences were observed in the percentages of Th2 and Th17 cells between disease types. Strong expression of IL-36 and BD-2 was observed in a subset of spongiotic psoriasiform dermatitis, sebopsoriasis, and tumor necrosis factor α inhibitor-associated psoriasiform dermatitis biopsy samples. LIMITATIONS: This was an exploratory study with a small sample size. No multiple testing adjustment was done. Clinical follow-up was limited. CONCLUSIONS: In cases with clinicopathologic overlap between psoriasis and spongiotic dermatitis, IL-36, and to a lesser extent BD-2, may be used to assess for a psoriasis-like/IL-17 phenotype, which could inform therapeutic clinical decisions.

Patch-type granuloma annulare: An institution-based study of 23 cases.

BACKGROUND: Granuloma annulare (GA) is a skin disorder of uncertain etiology. Patch (type) GA is an uncommon variant of GA with a paucity of data characterizing it. We describe the features of 23 cases of patch GA. METHODS: The archives of dermatopathology were searched for cases of patch GA. The clinical history and morphology for each patient were reviewed. Only cases with patch clinical morphology were included. The clinical and histopathologic features were assessed including the pattern of granulomatous inflammation and presence of other inflammatory cell types. RESULTS: Most patients were female (19/23) with erythematous patches on the trunk and proximal extremities. The most common clinical differential diagnosis included mycosis fungoides (MF), morphea and contact dermatitis. Dyslipidemia was the most common comorbidity (30%), followed by diabetes (15%) and hypertension (15%). Histopathologic features included interstitial lymphocytes and histiocytes with dermal mucin. Two cases showed focal palisaded granulomas. Eosinophils and plasma cells were present in 1/3 of cases. CONCLUSION: Patch GA is an uncommon GA variant with an interstitial granulomatous histopathologic pattern that predominantly affects women over 50. It can mimic interstitial MF and early morphea both clinically and histopathologically. Awareness of this GA variant can help prevent misdiagnosis and inappropriate treatment for these patients.

Diagnosing Calciphylaxis: A Review With Emphasis on Histopathology.

Calciphylaxis is a cutaneous vasculopathy with high morbidity and mortality characterized by vascular intimal fibrosis, calcification, stenosis, thrombosis, and eventual tissue death due to ischemia. Histopathologic diagnosis is often difficult, frequently necessitating multiple tissues samples due to lack of specific histopathologic features and subtle changes on biopsies of early lesions. In this study, we review the reported clinical and histopathologic features of calciphylaxis, correlating them with relevant imaging, ancillary studies, and pathophysiology. Although many histopathologic changes seen in calciphylaxis are also reported in other conditions (eg, Mönckeberg sclerosis, lupus panniculitis, pancreatic panniculitis, and peripheral artery disease), calcification of subcutaneous small vessels, thrombosis and/or ischemic changes, pseudoxanthoma elasticum-like changes in the subcutis, and perieccrine calcification may serve as helpful clues. von Kossa and Alizarin red stains can assist in the identification of subtle calcification. Netlike calcification of the affected blood vessels on imaging further supports the diagnosis. Studies into the pathophysiology of calciphylaxis are ongoing and will hopefully facilitate the development of additional diagnostic adjuncts to increase sensitivity and specificity for the diagnosis of this disease.

Expression of programmed cell death ligand 1 and programmed cell death 1 in cutaneous warts.

BACKGROUND: Cutaneous warts have high prevalence and cause significant morbidity. Understanding the mechanisms by which warts evade the immune system could lead to targeted and improved treatments. OBJECTIVE: To determine whether cutaneous warts express programmed cell death ligand 1 (PD-L1) and to characterize the expression of programmed cell death 1 (PD-1) within the immune infiltrate of inflamed lesions. METHODS: In total, 44 biopsies of cutaneous warts were retrieved from the Department of Dermatopathology archives of the University of California, San Francisco. Biopsies were stained with hematoxylin and eosin and PD-L1 monoclonal antibody, and biopsies of inflamed lesions were stained with PD-1 monoclonal antibody. RESULTS: PD-L1 was expressed on keratinocytes in cases of verrucae vulgares (12/30, 40%) and myrmecia (7/14, 50%) and was associated with an interface inflammatory reaction. PD-1 was expressed by the inflammatory infiltrate in verrucae vulgares (21/24, 88%) and myrmecia (5/8, 63%). LIMITATIONS: This was a retrospective observational study conducted at a single institution. CONCLUSION: Many cutaneous warts express PD-L1, suggesting that human papillomavirus might use this pathway to promote immune dysfunction. This discovery helps explain the recalcitrance of warts to current therapies and provides a rationale for investigating anti-PD-1 immunotherapy as a potential treatment for warts.

Distinguishing histopathologic features of acantholytic dermatoses and the pattern of acantholytic hypergranulosis.

BACKGROUND: Acantholysis can be seen in multiple skin diseases. Adnexal acantholysis has been regarded as a feature distinguishing pemphigus vulgaris (PV) from acantholytic conditions. METHODS: A retrospective review of the histopathologic features of diseases with acantholysis including PV, pemphigus foliaceus (PF), Hailey-Hailey disease (HHD), Darier disease (DD), Grover disease, and pityriasis rubra pilaris (PRP) was performed. RESULTS: Biopsies of PV (n = 49), HHD (n = 27), DD (n = 25), Grover disease (n = 65), and PRP (n = 33) showed suprabasilar acantholysis. Acantholysis was limited to the lower epidermis in PV and PRP, and involved all epidermal layers in HHD, DD, and Grover disease. Acantholysis in PF (n = 38) mainly involved the upper epidermis. Follicular acantholysis occurred more frequently in PV and PF (P < 0.0001). Eccrine acantholysis was found in PV (42%), HHD (18%), PF (13%), and DD (4%). Grover disease, DD, and HHD had greater dyskeratosis (P < 0.0001). Neutrophils were more common in PV, PF, and HHD, while eosinophils were more common in Grover disease and DD. A pattern termed acantholytic hypergranulosis occurred predominantly in PF. CONCLUSION: Adnexal acantholysis does not reliably distinguish PV from PF. The level of acantholysis, degree of dyskeratosis, and acantholytic hypergranulosis are distinguishing features between the two types of pemphigus and other acantholytic disorders.

Loss of ZNF750 in ocular and cutaneous sebaceous carcinoma.

BACKGROUND: Sebaceous carcinoma (SeC) is an uncommon malignancy arising from sebaceous glands of the conjunctiva and skin. Recurrent mutations in the ZNF750 were recently identified in ocular SeC. We assessed whether ZNF750 loss is a specific feature of ocular SeC or a general feature of sebaceous tumors. METHODS: Immunostaining for ZNF750 expression was performed in 54 benign and malignant sebocytic proliferations. Staining for ZNF750 was scored on a three-tier scale: positive (>75%), partially positive (5%-74%), and negative (<5%). RESULTS: ZNF750 expression was negative in 4/11 ocular SeC, and partially positive in 4/11 ocular SeC and 6/13 cutaneous SeC. No extraocular tumors were negative. No loss was found in sebaceous adenoma or sebaceous hyperplasia. In nine previously sequenced ocular SeCs, two lacked detectable somatic mutations in ZNF750, but showed complete loss of staining, indicating non-mutational inactivation of ZNF750. CONCLUSION: We show complete loss of the ZNF750 epidermal differentiation regulator in about half of ocular SeC, highlighting the most common genetic defect in this cancer type. Loss of ZNF750 expression is seen even in tumors without truncating mutations and reduced in many of the remaining ocular and cutaneous SeC. In contrast, no ZNF750 loss was detected in benign sebaceous proliferations.

Brief Report: Interleukin-17A-Dependent Asymmetric Stem Cell Divisions Are Increased in Human Psoriasis: A Mechanism Underlying Benign Hyperproliferation.

The balance between asymmetric and symmetric stem cell (SC) divisions is key to tissue homeostasis, and dysregulation of this balance has been shown in cancers. We hypothesized that the balance between asymmetric cell divisions (ACDs) and symmetric cell divisions (SCDs) would be dysregulated in the benign hyperproliferation of psoriasis. We found that, while SCDs were increased in squamous cell carcinoma (SCC) (human and murine), ACDs were increased in the benign hyperproliferation of psoriasis (human and murine). Furthermore, while sonic hedgehog (linked to human cancer) and pifithrinα (p53 inhibitor) promoted SCDs, interleukin (IL)-1α and amphiregulin (associated with benign epidermal hyperproliferation) promoted ACDs. While there was dysregulation of the ACD:SCD ratio, no change in SC frequency was detected in epidermis from psoriasis patients, or in human keratinocytes treated with IL-1α or amphiregulin. We investigated the mechanism whereby immune alterations of psoriasis result in ACDs. IL17 inhibitors are effective new therapies for psoriasis. We found that IL17A increased ACDs in human keratinocytes. Additionally, studies in the imiquimod-induced psoriasis-like mouse model revealed that ACDs in psoriasis are IL17A-dependent. In summary, our studies suggest an association between benign hyperproliferation and increased ACDs. This work begins to elucidate the mechanisms by which immune alteration can induce keratinocyte hyperproliferation. Altogether, this work affirms that a finely tuned balance of ACDs and SCDs is important and that manipulating this balance may constitute an effective treatment strategy for hyperproliferative diseases. Stem Cells 2017;35:2001-2007.

Merkel cell carcinoma: An update and review: Pathogenesis, diagnosis, and staging.

Merkel cell carcinoma (MCC) is an uncommon primary cutaneous neuroendocrine cancer. It most commonly presents as an indurated plaque or nodule on sun-damaged skin in elderly patients and is characterized by high rates of local recurrence and nodal metastasis. Survival at 5 years is 51% for local disease and as low as 14% for distant disease, which underscores the aggressive nature of this tumor and challenges in management. Advances in immunology and molecular genetics have broadened our understanding of the pathophysiology of MCC and expanded our therapeutic arsenal. With this comprehensive review, we provide an update of MCC epidemiology, pathogenesis, clinical presentation, diagnostic evaluation and prognostic markers. The second article in this continuing medical education series explores the evolving landscape in MCC management.

Human polyomavirus 6 and 7 are associated with pruritic and dyskeratotic dermatoses.

BACKGROUND: Human polyomavirus (HPyV)6 and HPyV7 are shed chronically from human skin. HPyV7, but not HPyV6, has been linked to a pruritic skin eruption of immunosuppression. OBJECTIVE: We determined whether biopsy specimens showing a characteristic pattern of dyskeratosis and parakeratosis might be associated with polyomavirus infection. METHODS: We screened biopsy specimens showing "peacock plumage" histology by polymerase chain reaction for HPyVs. Cases positive for HPyV6 or HPyV7 were then analyzed by immunohistochemistry, electron microscopy, immunofluorescence, quantitative polymerase chain reaction, and complete sequencing, including unbiased, next-generation sequencing. RESULTS: We identified 3 additional cases of HPyV6 or HPyV7 skin infections. Expression of T antigen and viral capsid was abundant in lesional skin. Dual immunofluorescence staining experiments confirmed that HPyV7 primarily infects keratinocytes. High viral loads in lesional skin compared with normal-appearing skin and the identification of intact virions by both electron microscopy and next-generation sequencing support a role for active viral infections in these skin diseases. LIMITATION: This was a small case series of archived materials. CONCLUSION: We have found that HPyV6 and HPyV7 are associated with rare, pruritic skin eruptions with a distinctive histologic pattern and describe this entity as "HPyV6- and HPyV7-associated pruritic and dyskeratotic dermatoses."

Histopathologic features of cutaneous leishmaniasis and use of CD1a staining for amastigotes in Old World and New World leishmaniasis.

BACKGROUND: Positive CD1a staining of Leishmania has been reported in Old World leishmaniasis, but the sensitivity of such staining for other Leishmania species is unknown. METHODS: A retrospective review was done on skin biopsies of proven cutaneous leishmaniasis based on histology, immunohistochemistry, culture and/or polymerase chain reaction (PCR). We assessed the pattern of inflammation present and assessed for CD1a (MTB1 clone) positivity in amastigotes. Patients without a clearly documented travel history to delineate Old vs New World leishmaniasis and cases without tissue for CD1a staining were excluded. RESULTS: Various patterns of granulomatous inflammation were observed including sarcoidal (31%), diffuse (25%), suppurative and granulomatous (25%), palisaded (13%) and lichenoid (6%). CD1a staining was positive in amastigotes in 9 of 16 cases (56%). Five of 7 (71%) cases of Old World disease were CD1a positive, while 4 of 9 cases (44%) of New World cases were positive. CONCLUSIONS: Multiple patterns of granulomatous inflammation occur in cutaneous leishmaniasis. Our results confirm CD1a (MTB1 clone) can be a diagnostic adjunct to highlight amastigotes in biopsies of cutaneous leishmaniasis, with variable positivity in both Old World and New World forms of the disease. As 44% of cases were CD1a negative in our cohort, there are significant limitations to this screening approach.