Immunohistochemical expression of melanocytic antigen PNL2, Melan A, S100, and PGP 9.5 in equine melanocytic neoplasms.
Authors: Ramos-Vara J A, Frank C B, DuSold D, Miller M A
Journal: Veterinary pathology
Summary
# Editorial Summary: Immunohistochemical markers for equine melanocytic neoplasms Distinguishing melanocytic tumours from other equine neoplasms can be diagnostically challenging, particularly when dealing with poorly differentiated lesions or ambiguous histological findings. Ramos-Vara and colleagues conducted an immunohistochemical study on 50 formalin-fixed equine melanomas alongside 62 non-melanocytic tumours and normal tissue samples to evaluate the diagnostic utility of four antibodies: PNL2, Melan A, S100 protein, and PGP 9.5. All 50 melanocytic neoplasms expressed PNL2, PGP 9.5, and S100 protein, yet notably none showed expression of Melan A; critically, PNL2 staining was absent in all non-melanocytic tumours tested and in normal tissues except the epidermis, making it substantially more specific than the comparator markers. Whilst S100 and PGP 9.5 remain useful, their lower specificity (as they can be expressed in other neural and neuroendocrine tissues) limits their utility in diagnostic panels. For equine practitioners, these findings suggest that PNL2 antibody represents a valuable addition to immunohistochemical protocols for confirming melanoma diagnosis, particularly when histological assessment alone is inconclusive—a practical advantage given the prevalence of melanocytic tumours in grey horses and the need for reliable differentiation from sarcoids and other common equine neoplasms.
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Practical Takeaways
- •PNL2 is a reliable immunohistochemical marker for confirming equine melanoma diagnosis with better specificity than conventional S100 staining
- •Melan A antibody should not be used for equine melanoma diagnosis as it does not cross-react with equine melanomas, despite effectiveness in human pathology
- •Pathologists should request PNL2 immunostaining when diagnosing suspected equine melanocytic tumors for improved diagnostic accuracy
Key Findings
- •PNL2 antibody detected in all 50 equine melanocytic neoplasms with no expression in 62 non-melanocytic tumors, demonstrating high sensitivity and specificity
- •PNL2 is more specific than S100 protein or PGP 9.5 for identifying equine melanocytic neoplasms
- •Melan A monoclonal antibody showed no reactivity with any equine melanomas, unlike its use in human melanoma diagnosis
- •PNL2 expression was restricted to epidermis in normal tissues, indicating minimal background staining