Immunohistochemical analysis of the tumor microenvironment in acral lentiginous melanoma (ALM).

e21588 Background: Acral lentiginous melanoma (ALM) is a rare subtype of melanoma occurring on the palms, soles and nail beds. Molecularly, ALM is characterized by a higher frequency of chromosomal structural and copy number variants, but lower tumor mutational burden (TMB). Meanwhile, immune checkpoint inhibitors (ICI) are less efficacious in this subtype. In order to better understand these differences in outcomes to ICI, we conducted an immunohistochemical analysis of ALM to explore putative immune biomarkers. Methods: Clinically annotated samples of ALM were identified from patients treated at the USC Keck School of Medicine between 2010-2018. Immunohistochemistry stains included CD8, PD-L1, CD56, CD68, FOXP3, CTLA-4, HLA-ABC and HLA-G, and were characterized based on the intensity of staining. The percentage of tumor cells and infiltrating immune cells (IC) within the tumor microenvironment for the aforementioned antigens were recorded. Data are reported as percent of total cells ( > 1%, > 5%, > 25%, > 50%) expressing marker in the entire sample. Results: A total of 28 ALM specimens from 21 different patients were analyzed, including 19 primary and 9 metastatic tumors. The median age of all patients was 67 years (range 32-92); the majority were men (66%) and Hispanic (66%). Primary ALM tumors were located more often on the plantar foot (62%), compared to the plantar hand (29%). The median tumor thickness was 2.8 mm (range 0.25 – 6.5 mm). Among all patients, 57% had localized disease, while 19% had regional metastases and 24% had distant metastases. CD8+ TIL cells were observed in 95% (18/19) of samples, with 68.4% (13/19) showing > 50% CD8+ staining on IC. PD-L1 expression > 1% on tumor cells was noted in 37% (10/27) of samples, and 19% (5/27) on IC; PD-L1 > 5% was noted in only 18.5% (5/27) on tumor cells and on 11% (3/27) on IC. Both CD56 and CD68 were noted to have at least 1% expression on IC among nearly all (94%) of samples analyzed (7/8 and 9/9, respectively). FoxP3 expression > 1% was noted on IC in 92.3% (24/26) of samples, and > 5% in 50% (13/26). CTLA-4 expression > 1% was observed among 81.5 % (22/27) of samples, while 63% (17/27) had expression > 5%; CTLA-4 expression > 50% was noted among 100% on melanophages (7/7 samples). HLA-ABC expression was detected in 100% of samples analyzed (10/10). The percentage of samples with HLA-G expression > 1% was 60.7% (17/28), while 28.6% (8/28) demonstrated > 5% expression. Further correlations with clinical outcomes will be reported. Conclusions: Our study suggests a unique immune tumor microenvironment in ALM, characterized by low PD-L1 expression, despite infiltration by CD8+, as well as CD56 and CD68 immune cells. Furthermore, the presence of FOXP3+ Tregs and HLA-G expression, both which mediate immune response evasion, may explain the poor outcomes of ICI therapy among these patients and warrant further investigation.