CD47 expression patterns in thymic epithelial tumors.

Abstract

8586 Background: Blockade of CD47, an immunoglobulin overexpressed on solid tumor cells that inhibits macrophage phagocytosis, is a promising anti-cancer immunotherapy which has not yet been explored in thymic epithelial tumors (TETs). TETs, including thymomas and thymic carcinomas, are rare tumors with limited immunotherapy treatment options due to the high rates of immune-related adverse effects observed with PD-1/PD-L1 checkpoint inhibitors. This study aimed to examine CD47 protein expression in TETs. Methods: A clinically annotated tissue microarray of 67 TETs consisting of 64 thymomas and 3 thymic carcinomas, as well as 14 thymic controls were included. Each sample with an average of 3 cores was stained for CD47 epithelial expression (rabbit monoclonal antibody SP279, Abcam, USA). Samples were scored for intensity as follows: 0 = none, 1 = weak, 2 = moderate, and 3 = strong. An H-score, defined as intensity x percentage of tumor involved, was also assigned and ranged from 0 to 300. Samples with an intensity score of < 2 or an H-score of < 150 were categorized as CD47low, while the rest as CD47high. Multivariate linear regression analysis accounted for WHO subtype, stage, resection status and presence of paraneoplastic syndrome (Prism 9, Graphpad). Results: Compared to normal thymic tissue, TETs were more frequently CD47 positive and had significantly higher levels of CD47 expression. CD47 was present in 91% of TETs, compared to 64.3% of normal thymus. Importantly, the level of expression was significantly higher in TETs by 16-fold (mean H-score 75.0 vs 4.6, p = 0.003). Among tumors, univariate analyses showed that higher CD47 expression was correlated with a lower stage (p = 0.032) and more complete resection (p = 0.058). A multivariate analysis accounting for these factors showed that CD47 expression by both H-score and intensity were each highly correlated with WHO histology subtype (p = 0.0005; p = 0.0017 respectively) with lower grade subtypes more frequently found in CD47high tumors. The most frequent subtype in CD47high, when compared to CD47low tumors, was AB (61.5% vs 13.7%) and the least frequent was B2 (0% vs 37.3%). Tumors with the highest grade (subtype C, thymic carcinomas) were exclusively CD47low. CD47high tumors were associated with an increased incidence of paraneoplastic syndromes (52.4% vs 12.0%, p = 0.0014). Conclusions: CD47 expression was found in the vast majority of TETs, and in significantly higher levels than normal thymic tissue. Among tumors, those with higher CD47 expression tended to have lower grade and stage, as well as higher frequency of paraneoplastic syndromes. This study is the first to examine CD47 expression in TETs. Given the prevalent expression of CD47 found in TETs and current available CD47 targeted agents, this study lends support for further investigation of this novel therapeutic approach.