The intraprostatic immune environment after stereotactic body radiotherapy is dominated by myeloid cells

Abstract

Background: Hundreds of ongoing clinical trials combine radiation therapy, mostly delivered as stereotactic body radiotherapy (SBRT), with immune checkpoint blockade. However, our understanding of the effect of radiotherapy on the intratumoral immune balance is inadequate, hindering the optimal design of trials that combine radiation therapy with immunotherapy. Our objective was to characterize the intratumoral immune balance of the malignant prostate after SBRT in patients.

Methods: Sixteen patients with high-risk, non-metastatic prostate cancer at comparable Gleason Grade disease underwent radical prostatectomy with (n = 9) or without (n = 7) neoadjuvant SBRT delivered in three fractions of 8 Gy over 5 days completed 2 weeks before surgery. Freshly resected prostate specimens were processed to obtain single-cell suspensions, and immune-phenotyped for major lymphoid and myeloid cell subsets by staining with two separate 14-antibody panels and multicolor flow cytometry analysis.

Results: Malignant prostates 2 weeks after SBRT had an immune infiltrate dominated by myeloid cells, whereas malignant prostates without preoperative treatment were more lymphoid-biased (myeloid CD45⁺ cells 48.4 ± 19.7% vs. 25.4 ± 7.0%; adjusted p-value = 0.11; and CD45⁺ lymphocytes 51.6 ± 19.7% vs. 74.5 ± 7.0%; p = 0.11; CD3⁺ T cells 35.2 ± 23.8% vs. 60.9 ± 9.7%; p = 0.12; mean ± SD).

Conclusion: SBRT drives a significant lymphoid to myeloid shift in the prostate-tumor immune infiltrate. This may be of interest when combining SBRT with immunotherapies, particularly in prostate cancer.

Figure 1.. Prostate SBRT drives a lymphoid-to-myeloid shift in the balance of infiltrating immune cells.

Polar graph illustrating the mean log2-fold differences in prostate tumor infiltrates following pre-operative, hypofractionated SBRT (n=9, solid red line) compared to unirradiated prostate tumors (n=7, black, dotted line). All immune subpopulations were gated on CD45⁺. Data are log2[mean value irradiated/mean value unirradiated]. Myeloid subsets highlighted in green. Underlined subsets indicate significant differences between irradiated vs non-irradiated based on adjusted p value = 0.2 (false discovery rate threshold).

Figure 2.. Irradiated prostate cancer patients cluster into treatment cohorts according to immune cell infiltrates.

Heatmap showing the SBRT-treated patient cohort having greater inter-patient variability and a rise in myeloid lineage (green) dominance compared to the unirradiated patient cohort that is overall more uniform and dominated by lymphoid subsets. Heatmap was constructed by normalizing each immune cell subset across the all subjects. Values are z-scores for each immune subset in each patient.