Variable GILT protein expression in melanoma cells of metastatic tumor specimens

Abstract

Melanoma is the sixth most common cancer in the US. Immunotherapies, such as anti-CTLA-4 and anti-PD-1, have transformed the treatment of advanced melanoma; however, a subset of patients fail to respond to immunotherapy. Thus, improved predictors of the anti-melanoma immune response and response to immunotherapy are needed. We have previously shown that 1) gamma-interferon-inducible lysosomal thiol reductase (GILT) is critical for MHC class II-restricted presentation of a melanoma antigen, 2) there is variable GILT protein expression in melanoma cells from primary and cutaneous metastatic melanoma specimens, and 3) high GILT mRNA expression in melanoma specimens is associated with improved overall survival. Here, we created a database of n = 43 metastatic melanoma patients (stage III or IV) annotated with clinical outcome. Formalin-fixed, paraffin-embedded sections of pre-treatment metastatic melanoma specimens were tested for GILT protein expression using immunohistochemistry and scored by a dermatopathologist and dermatologist. GILT expression in melanoma cells was detected in 65% of lymph node and distant organ metastases, consistent with our prior finding of GILT expression in 58% of cutaneous metastases. The frequency of GILT-staining melanoma cells was determined by the ratio of the area of tumor cells with positive GILT staining to the total area of tumor cells. The frequency of GILT-staining melanoma cells among specimens with positive GILT staining was 15.2% ± 24.7% (mean ± SD). The median frequency of GILT staining was used to divide the patients into no/low and high GILT expression groups. The median overall survival was 52.8 months in the no/low GILT expression group, while the median overall survival was not reached in the high GILT expression group (log-rank, p = 0.67). For the n = 34 patients who were treated with immunotherapy targeting CTLA-4 and/or PD-1, the median overall survival was 38.2 months in the no/low GILT expression group, while the median overall survival was not reached in the high GILT expression group (log-rank, p = 0.83). These data suggest that high GILT expression in melanoma cells may be associated with improved overall survival, although a statistically significant association of GILT with survival was not observed in the whole group or the subset of patients treated with immunotherapy. A limitation of the current analysis is that 65% of patients have incomplete clinical outcome data. This study demonstrates variable GILT protein expression in tumor cells in metastatic melanoma specimens. Updated clinical outcome data will allow us to better evaluate the association of GILT protein expression in melanoma cells with overall survival and response to immunotherapy in metastatic melanoma patients.