Differential expression of MART-1, tyrosinase, and SM5-1 in primary and metastatic melanoma.

The new monoclonal antibody SM5-1 has been shown to have significant advantages in immunohistochemistry of melanoma over currently used antibodies such as HMB-45 or anti-S100. In this study we compared the immunohistological staining pattern of SM5-1 with that of the more recently described antibodies A103 (anti-MART-1) and T311 (anti-Tyrosinase) in 344 paraffin-embedded melanoma specimens, consisting of 101 primary melanomas (77 SSM, 16 NM, 6 ALM, 2 LMM) and 243 melanoma metastases. The overall reactivity of SM5-1 for all the specimens was 92% (318/344) compared with 83% (285/344) for MART-1 and 71% (245/344) for Tyrosinase. Staining of melanoma metastases with SM5-1 was found in 91% (222/243), but only in 77% (187/243) with A103 and 63% (154/243) with T311, respectively. Staining with SM5-1 was more homogenous with 196 of 243 (80%) of metastatic lesions showing 50% or more positively stained cells within the lesions, whereas A103 and T311 did so in 141 of 243 (58%) or 117 of 243 (48%) of the lesions. With regard to staining intensity of SM5-1, 157 of 243 (64%) showed a strong or very strong staining intensity, whereas A103 and T311 did so in 85 of 243 (35%) or 70 of 243 (29%) of the lesions. Staining intensity and percentage positivity correlated well for SM5-1, because from the 58 very strong positive metastases 55 showed staining in more than 75% of the cells within a lesion. Importantly, 52 of 56 MART-1-negative metastases and 81 of 89 Tyrosinase-negative metastases were positive for SM5-1. Thirty-eight metastases (15.6%) were negative for both A103 and T311. Of those, 35 (92.1%) were positive for SM5-1, demonstrating the value of SM5-1 in identifying melanoma-associated antigen-negative lesions. We conclude that SM5-1 could be of value in immunohistochemistry of melanoma.