CD70 in Thymic Squamous Cell Carcinoma: Potential Diagnostic Markers and Immunotherapeutic Targets

Abstract

CD70 - a ligand protein of CD27 on lymphocytes - is expressed in a large spectrum of malignancies. It is an attractive target for antibody-based therapy and several clinical trials are currently being conducted. However, there is no evidence regarding the expression of CD70 and its relationship with expression of programmed death ligand-1 (PD-L1) and CD27+ tumor-infiltrating lymphocytes (TIL) in formalin-fixed paraffin-embedded (FFPE) tissues of thymic tumors. FFPE tissues of thymic squamous cell carcinoma (TSCC) (operative specimens, n = 31; biopsy specimens, n = 11), thymoma (n = 60), thymic carcinoid (n = 3), and lung squamous cell carcinoma (LSCC) (n = 30) were analyzed immunohistochemically. Immunoreactivity for CD70 was semi-quantitatively scored according to the proportion of positive tumor cells. Moreover, the densities of CD27-positive intratumoral TIL (iTIL) and stromal TIL of TSCC were assessed and survival was compared. Most TSCC cases (87%; 27/31) were CD70-positive. In contrast, all thymoma and thymic carcinoid cases were CD70-negative. In LSCC cases, CD70-positivity was significantly lower than TSCC cases (20%; 6/30). Biopsy and resected specimens obtained from the same patients demonstrated a consistent staining pattern (6/6 patients). The proportion of CD70-positive TSCC was comparable with those of CD5 (87%) and CD117 (90%). Correlation between CD70 and PD-L1 expression score was observed. There was no significant difference in survival between the CD70-high and CD70-low expression groups. Meanwhile, patients with CD27-positive iTIL-high tumors exhibited better survival than those with iTIL-low tumors. This tendency was weaker in the CD70-high subset. CD70 immunohistochemistry is useful in diagnosing TSCC. CD70 may prevent anti-tumor immunity via CD27. Immunotherapy targeting the CD70-CD27 axis may be a promising option for the treatment of TSCC.

Keywords: CD27; CD70; immunohistochemistry; thymic carcinoma; tumor-infiltrating lymphocyte.

Figure 1

Normal adult thymus tissue stained with anti-CD70 antibody. Thymic medullary epithelial cells or dendritic cells were positive for CD70 (A, B). Immunostaining of CD70 in thymic squamous cell carcinoma (TSCC) (C). CD70-positivity was scored according to the proportion of positive tumor cells (score 0: <1%; score 1: 1–24%; score 2: 25–49%; score 3: 50–74%; score 4: 75–100%). Schematic summary of immunohistochemical analysis (D).The numbers indicate the positive cases of each staining. The numbers in the parentheses are the total number of the positive cases. Immunostaining of CD70 (score 4), CD5 and CD117 in TSCC (E). In contrast, thymoma showed the absence of CD70 expression (F). The CD70 score in lung squamous cell carcinoma was relatively low.

Figure 2

CD70 expression analysis using real-time reverse transcriptase polymerase chain reaction (RT-PCR). A, type A thymoma; B3, type B3 thymoma; C, thymic squamous cell carcinoma; TSCC, thymic squamous cell carcinoma.

Figure 3

CD27 staining of thymic squamous cell carcinoma (A). CD27-positive intratumoral tumor-infiltrating lymphocytes (iTIL, arrow) and stromal TIL (sTIL, arrowhead). Overall survival of patients with thymic carcinoma according to the CD70 expression status (B), the number of CD27-positive iTIL (C) and sTIL (D). Stratified survival curve in the CD70-low group (E) and CD70-high group (F). x-axis: days.