Defining the Immune Cell Infiltrate of Brain Metastases Before and After Stereotactic Radiosurgery

Abstract

Brain metastases are a significant source of morbidity and mortality among cancer patients. Stereotactic radiosurgery (SRS) offers excellent local control rates. However, some patients experience in-field recurrences and many develop distant brain failure, making long-term control of disease in the brain a challenge. The immune cell infiltrate of extracranial tumors has been studied and shown to correlate with clinical outcomes in numerous tumor histologies. Here, we aim to characterize the immune cell infiltrate of brain metastases.From September 2020 to January 2021, fresh brain metastasis samples were weighed and digested immediately after resection. Immune cells were isolated, washed and stained for the pan-leukocyte marker CD45, the cytotoxic T cell marker CD8, and the B cell marker CD19. Cells were analyzed by flow cytometry.Twenty-one brain metastases were analyzed (NSCLC, melanoma, breast, esophageal, renal cell carcinoma, and uterine). The median number of CD45+ lymphocytes per gram of tumor was 5.7 × 105 (range: 3.2 × 104 - 9.1 × 106); 19% of the CD45+ lymphocytes were CD8+. The median number of CD8+ T cells per gram of tumor was 1.1 × 105 (range: 5.6 × 103- 2.7 × 106). Most of these tumors were also infiltrated by B cells; the median number of CD19+ B cells per gram of tumor was 1.0 × 104 (range: 0- 4.9 × 105). One study patient had local recurrence after SRS. This patient underwent surgical resection of the dominant brain metastasis on presentation (prior to treatment). There was progression of a separate lesion after SRS, which was subsequently resected. The initial untreated tumor contained 1.7 × 106 CD45+ lymphocytes, 9.5 × 105 CD8+ T cells, and 1.8 × 104 CD19+ B cells per gram of tumor. After SRS, the recurrent tumor contained 2.1 × 105 CD45+ lymphocytes, 5.9 × 104 CD8+ T cells, and 6.4 × 103 CD19+ B cells per gram of tumor. Thus, the number and frequency of CD45+ CD8+ T cells decreased after SRS (5.9 × 104 vs 9.5 × 105 and 28% vs 56%, respectively).Brain metastases contain variable immune infiltration of cytotoxic T cells and B cells. SRS may decrease the density of lymphocytic immune cell infiltration in brain metastases. Given that CD8+ T cells comprise a smaller proportion of the post-radiation lymphocytic infiltrate, other populations, such as regulatory T cells, might be enriched after treatment.