Bacillus Calmette-Guerin (BCG) is the standard of care for intravesical therapy 
for carcinoma in situ and non-muscle invasive, nonmetastatic human urothelial 
carcinoma. Although the responsiveness to this immunotherapeutic is believed to 
be linked with (i) a high number of somatic mutations and (ii) a large number of 
tumor-infiltrating lymphocytes, recent findings of the roles that inhibitory 
immune receptors and their ligands play in tumor evasion may provide insights 
into the limitations of the effectiveness of BCG and offer new targets for 
immune-based therapy. In this study, an aggressive, bioluminescent orthotopic 
bladder cancer model, MB49 tumor cells transfected with luciferase (MB49(luc)), 
was used to study the antitumor effects of avelumab, an antibody to PD-L1. 
MB49(luc) murine tumor cells form multifocal tumors on the mucosal wall of the 
bladder reminiscent of non-muscle invasive, nonmetastatic urothelial carcinomas. 
MB49(luc) bladder tumors are highly positive for the expression of PD-L1, and 
avelumab administration induced significant (P < 0.05) antitumor effects. These 
antitumor effects were more dependent on the presence of CD4 than CD8 T cells, 
as determined by in vivo immune cell depletions. The findings suggest that in 
this bladder tumor model, interruption of the immune-suppressive PD-1/PD-L1 
complex releases a local adaptive immune response that, in turn, reduces tumor 
growth. This bladder tumor model can be used to further identify host antitumor 
immune mechanisms and evaluate combinations of immune-based therapies for 
carcinoma in situ and non-muscle invasive, nonmetastatic urothelial carcinoma, 
to provide the rationale for subsequent clinical studies. Cancer Immunol Res; 
4(5); 452-62. ©2016 AACR.