Immune Exclusion Is Frequent in Small-Cell Carcinoma of the Bladder

Abstract

Small-cell cancer of the urinary bladder is a rare but highly aggressive disease. It is currently unclear whether immune checkpoint therapies that have been approved for urothelial carcinomas will also be efficient in small-cell carcinomas. In this study, we analyzed potential predictors of response including PD-L1 expression and the quantity and location of tumor-infiltrating lymphocytes (TILs) in 12 small-cell and 69 "classical" urothelial cancers by immunohistochemistry. The analysis revealed that small-cell carcinomas were characterized by the virtual absence of PD-L1 expression and an "immune-excluded" phenotype with only a few TILs in the center of the tumor (CT). In small-cell carcinomas, the average immune cell density in the CT (CD3: 159 ± 206, CD8: 87 ± 169 cells/mm²) was more than 3 times lower than that in the urothelial carcinomas (CD3: 625 ± 800, p < 0.001; CD8: 362 ± 626 cells/mm², p = 0.004) while there was no significant difference in the immune cell density at the invasive margin (IM) (small-cell carcinomas CD3: 899 ± 733, CD8: 404 ± 433 cells/mm²; urothelial carcinomas CD3: 1167 ± 1206, p = 0.31; CD8: 582 ± 864 cells/mm², p = 0.27). Positive PD-L1 staining was found in 39% of urothelial cancers, but in only 8% of small-cell bladder cancer cases (p = 0.04). Concordant with these data, a sharp decrease of PD-L1 positivity from >80% to 0% positive cells and of TILS in the CT from 466-1063 CD3-positive cells/mm² to 50-109 CD3-positive cells/mm² was observed in two cancers with clear-cut progression from "classical" urothelial to small-cell carcinoma. In conclusion, these data demonstrate that small-cell bladder cancer commonly exhibits an immune-excluded phenotype.

Figure 1

Representative pictures of CD3 staining (brown color) in (a, b) two cases of small-cell and (c, d) two cases of urothelial carcinoma of the bladder. Panels (a, c) show examples of the immune-desert phenotype and (b, d) examples of the immune-excluded phenotype. Arrowheads indicate the invasive margin. Asterisks label the center of the tumor, shown also in the insets at 400x magnification.

Figure 2

Comparison of CD3-, CD8-, and PD-1-positive cell densities at the invasive margin (IM) and center of the tumor (CT) between 12 small-cell carcinomas (SCCB) and 69 urothelial carcinomas of the bladder (UCB).

Figure 3

CD3 and PD-L1 staining of a sample from patient 4 that contained adjacent areas of small-cell and urothelial carcinoma of the bladder. Note the invasive margin (arrowhead) and the center of the tumor (asterisks). Insets show 400x magnification of the tumor bulk.