Activated B Cells and Plasma Cells Are Resistant to Radiation Therapy.

Purpose: B cells play a key role in outcomes of cancer patients and responses to checkpoint blockade immunotherapies. However, the effect of radiation therapy on B cell populations is poorly understood. Here we characterize the effects of radiation on the development, survival, and phenotype of physiological B-cell subsets.
Methods and Materials: Naïve and immunized tumor bearing and nontumor bearing mice were treated with large-field or focal stereotactic radiation and distinct B-cell subsets of varying developmental stages were analyzed by flow cytometry and real-time reverse transcription polymerase chain reaction.
Results: We first report that focal stereotactic radiation is highly superior to large-field radiation at inducing tumor infiltration of B cells, CD8 ⁺  T cells, and macrophages. We observed that radiation affects B cell development in the bone marrow, increasing frequencies of early pro-B cells and late pro-B cells while inducing upregulation of programmed cell death protein 1. We then demonstrate that class switched B cells and plasma cells are highly resistant to radiation therapy compared with naïve B cells and upregulate activation markers programmed cell death 1 ligand 2 and major histocompatibility complex class II) after radiation. Mechanistically, radiation upregulates Xbp1 and Bcl6 in plasma cells, conferring radioresistance. Furthermore, using an immunization approach, we demonstrate that radiation enhances activation-induced cytidine deaminase mediated class switching and somatic hypermutation in primed B cells.
Conclusions: These data demonstrate that stereotactic radiation is superior to large-field radiation at inducing infiltration of immune cells into tumors and that plasma cells and class switched B cells are highly resistant to radiation therapy. These results represent the most comprehensive analysis of the effects of radiation on B cells to date and identify novel mechanisms by which radiation modulates immune cells within the tumor microenvironment.
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