PD-L1 expression and the immune microenvironment in primary invasive lobular carcinomas of the breast

Tumor-infiltrating lymphocytes and immune checkpoint proteins such as PD-L1 are potential prognostic factors and therapeutic targets in breast cancer. Most studies characterizing the breast tumor immune microenvironment have focused on ductal carcinomas. Here we investigate the tumor microenvironment of primary invasive lobular carcinomas. Previously constructed tissue microarrays of 47 lobular carcinomas were labeled by immunohistochemistry for PD-L1, CD8, CD20, and FoxP3. The stromal immune infiltrate density was qualitatively scored as a percentage of tumor area: 1+ (<5%); 2+ (5-10%); 3+ (10-15%); or 4+ (>50%). The average immune cell subtype per high-power field was quantitatively scored. The percentage PD-L1 labeling on tumor-infiltrating lymphocytes was scored as none, focal (<5%), moderate (10-24%), or diffuse (50-100%). The percentage of membranous carcinoma cell PD-L1 labeling was also recorded, with <5% considered negative. All lobular carcinomas contained PD-L1⁺ tumor-infiltrating lymphocytes with the majority showing 1+ immune infiltrates with focal-moderate PD-L1 labeling. PD-L1 was expressed by tumor cells in 17% of lobular carcinomas. In contrast to ductal carcinomas, there was no correlation between the immune infiltrate density, the PD-L1 expression by lobular carcinoma cells, tumor grade, or the expression of estrogen receptor or human epidermal growth factor receptor-2. However, both the tumor-infiltrating lymphocyte density and the average CD8⁺ T-cell counts correlated with immune cell PD-L1 status (P=0.004 and 0.03, respectively). Similar to breast ductal carcinomas, PD-L1⁺ lobular breast carcinomas had higher numbers of PD-L1⁺ tumor-infiltrating lymphocytes (63%) than PD-L1^- lobular carcinomas (23%; P=0.04). These data show that a subset of primary breast lobular carcinomas both express PD-L1 on tumor cells and contain PD-L1⁺ tumor-infiltrating lymphocytes, suggesting the possibility of both constitutive and adaptive PD-L1 expression. Together, these results support immunotherapy as a potential treatment for a subset of patients with primary invasive lobular breast carcinomas.