Expression of the histiocytic marker PG-M1 in granuloma annulare and rheumatoid nodules of the skin.

BACKGROUND: The expression of PG-M1, the most specific histiocytic marker, has not yet been studied in granuloma annulare (GA) and other palisaded granulomas of the skin. We evaluated the reactivity of PG-M1 with a series of GA and rheumatoid nodules (RN) to establish the sensitivity and potential usefulness of this marker in the diagnosis and characterization of these entities. METHODS: Histological sections from 30 GA and 15 RN were immunostained with PG-M1. For comparison, additional sections were stained with KP-1 and lysozyme. The stains were recorded as negative, weakly positive (1+) and strongly positive (2+). RESULTS: PG-M1 stained all cases of GA (100%). KP-1 and lysozyme stained 26 (86%) and 18 (60%) GA cases, respectively. PG-M1 exhibited a significantly stronger staining intensity (1.8 +/- 0.07) when compared with KP-1 (1.4 +/- 0.13) (p = 0.018) and with lysozyme (0.9 +/- 0.15) (p < 0.0001). All RN were stained by PG-M1 (100%). KP-1 and lysozyme stained 14 (93%) and six (40%) RN cases, respectively. PG-M1 staining intensity (1.6 +/- 0.13) was slightly higher than that of KP-1 (1.4 +/- 0.18) (p = 0.27) and significantly higher than that of lysozyme (0.4 +/- 0.13) (p < 0.0001). CONCLUSIONS: PG-M1 is consistently and strongly expressed by the histiocytic population of GA and RN, being more sensitive and reliable than other histiocytic markers. We recommend its use in difficult cases in which the histiocytic nature of the lesion needs to be confirmed.

The histiocytic marker PG-M1 is helpful in differentiating histiocytes and histiocytic tumors from melanomas.

Previous studies have shown that immunohistochemical stains for histiocytes are immunoreactive for melanomas. Accordingly, their value in differentiating histiocytes and histiocytic lesions from melanomas was questioned. PG-M1, the most specific histiocytic marker, was not evaluated in these studies. Our aims were to assess the reactivity of PG-M1 with a series of primary cutaneous and metastatic melanomas and to establish the potential usefulness of this antibody in the differentiation between histiocytes and histiocytic tumors and melanomas. PG-M1 staining was performed in 50 primary cutaneous and metastatic melanomas. For comparison, additional sections were stained with KP-1 and lysozyme (commonly used as histiocytic markers) and with S-100 and HMB-45 (commonly used as melanoma markers). The intensity (1+, 2+) and extent (1+ to 4+) were recorded semiquantitatively. PG-M1 stained weakly (1+) and focally (2+) only four cases of melanoma (8%). In contrast, histiocytes were strongly reactive for PG-M1 in all cases, being readily differentiated from melanoma cells including the positive cases. KP-1 stained melanoma cells in 44 cases (88%), lysozyme in 11 cases (22%), S-100 in 50 cases (100%), and HMB-45 in 48 cases (96%). No changes were found after restaining of selected KP-1 and lysozyme positive melanomas using an endogenous avidin/biotin blocking kit. PG-M1 is helpful in discriminating histiocytes and histiocytic lesions from melanoma cells. We recommend its inclusion in any antibody panel put together to distinguish between them.