[1935] Is the Anti-S-100 Antibody Decreasing Its Affinity for Epidermal Melanocytes? A Possible Diagnostic Pitfall.

Felipe J Solano, Jyoti Kapil, Michael T Deavers, Michael S McLemore, Victor G Prieto. University of Texas Health Science Center at San Antonio; George Washington University Medical Center, Washington, DC; University of Texas M.D. Anderson Cancer Center, Houston

Background: Immunohistochemical (IHC) staining with anti-S-100 is used as a reliable marker of melanocytic differentiation. At MD Anderson Cancer Center (MDACC), IHC technique employs a monoclonal antibody derived from murine species stimulated with the purified bovine S-100 protein immunogen (clone 15E2E2), used in a 1:900 dilution to stain paraffin-embedded tissue following a standard protocol. Based on our experience at MDACC, anti-S-100 IHC frequently fails to label epidermal melanocytes while strongly labeling epidermal and dermal dendritic cells and nerves. Our objective is to determine whether epidermal melanocytic staining affinity with anti-S-100 is preserved in dermatopathology cases.
Design: We searched the Pathology files at MDACC for recent inside and outside consultation cases in dermatopathology in which anti-S-100 IHC was performed and available for review. Epidermal melanocytes labeling with anti-S100 were evaluated in each case for intensity (0: negative, 1: weak, 2: moderate to marked), and for percentage of cells staining (0: negative, 1: 1-25%, 2: 26-75%, 3: >75 %). For one outside case, anti-S-100 IHC was repeated, and for another, the anti-S-100 was performed in-house.
Results: From 3/1/2010 to 9/24/2010, 28 skin cases (18 inside, 10 outside cases) were identified (see Table 1). All cases had squamous epithelium with normal melanocytes, as well as dermal dendritic cells and nerves to serve as an internal positive control. In a majority of cases, additional IHC had previously been performed which highlighted melanocytes (i.e., anti-MART1, HMB45, anti-tyrosinase, or MITF).

Table1: Intensity and Percentage of Anti-S-100 IHC Staining in Both Inside and Outside Skin Cases at MDACC

Intensity/Percentage of Staining with Anti-S-100:	0/0	1/1	2/1	1/2	2/2	2/3
Number of Inside Cases:	4	9	2	1	3	1
Number of Outside Cases:	1	4	0	1	1	2

Conclusions: This study suggests that current antibodies and IHC techniques employed to detect S-100 expression may have a relatively lower sensitivity for melanocytes than other cells (e.g., Langerhans cells, nerves). When analyzing anti-S100 IHC, it is necessary to determine if the internal control melanocytes, when available, are labeled before considering that the slide is appropriate for interpretation.
Category: Techniques

Monday, February 28, 2011 1:00 PM

Poster Session II # 260, Monday Afternoon