Involvement of cytotoxic T cells and interferon-gamma
Dr. Facciabene and colleagues found that the irradiated and unirradiated B16 OVA melanoma tumors treated with the radiotherapy-vancomycin combination were infiltrated by CD3+ and CD8+ T cells.
The investigators selectively depleted CD8+ T cells by pretreating the mice with an anti-CD8 monoclonal antibody. Depletion of CD8+ cells prior to administering radiotherapy plus vancomycin abrogated the antitumor effects of the combination treatment, demonstrating that the CD8+ T cells were required.
To characterize the antigen specificity of the tumor-infiltrating CD8+ T cells, Dr. Facciabene and colleagues used OVA MHC class 1 tetramer. Tumors from mice treated with vancomycin alone, radiotherapy alone, or the combination were dissected. Individual dendritic cells were assayed for OVA tetramer by flow cytometry.
The investigators found that tumors from mice treated with radiotherapy plus vancomycin had a significantly higher number of OVA-specific CD8+ T cells, in comparison with untreated tumors or tumors treated with either vancomycin alone or radiotherapy alone. Since antibody that impaired recognition of MHC class I peptides by T cells ablated the effect, it was clear that antigen recognition was vital.
Interferon-gamma (IFN-gamma) is known to play a critical role in both differentiation and effector functions of CD8+ cytolytic T cells in the antitumor immune response. To determine whether IFN-gamma is involved in the antitumor effects of the radiotherapy-vancomycin combination, the investigators measured intratumoral expression of IFN-gamma in the tumors 5 days after radiotherapy.
IFN-gamma messenger RNA expression levels were significantly elevated in the combination treatment group when compared with either treatment alone. In B16-OVA melanoma–challenged knockout mice, the enhancement of the radiotherapy effects by vancomycin was ablated.
The investigators concluded that vancomycin remodels the tumor microenvironment and increases the functionality of tumor-infiltrating, tumor-specific, CD8+ T cells. Furthermore, IFN-gamma is required to augment the radiotherapy-induced immune effect against the tumor.
Potential biochemical mediators of immune effects
The gut microbiota aid host digestion and generate a large repertoire of metabolites after defermentation of fiber. Short-chain fatty acids (SCFAs) constitute the major products of bacterial fermentation.
Acetic acid, propionic acid, and butyric acid represent 95% of total SCFAs present in the intestine. SCFAs are known to directly modulate cytokine production and dendritic cell function.
In their study, Dr. Facciabene and colleagues focused on butyric acid. Using mass spectroscopy, they demonstrated that vancomycin treatment reduces butyrate concentrations in tumor and tumor-draining lymph nodes by eradicating the major families of SCFA-producing Clostridia species.
To test whether supplementing butyrate could influence the synergy of the radiotherapy-vancomycin combination in vivo, the investigators added sodium butyrate to the mice’s drinking water when starting vancomycin treatment. The team then challenged the mice with B16-OVA tumors and treated them with radiotherapy.
In agreement with the group’s prior findings, vancomycin enhanced the tumor-inhibitory effects of radiotherapy, but dietary butyrate inhibited the benefit. The investigators found a significant decrease in the population of B16-OVA–presenting dendritic cells in the lymph nodes of mice receiving the supplemental butyrate.
Dr. Facciabene said these findings were supported by a recent publication. The authors observed that butyrate inhibited type I IFN expression in dendritic cells and radiotherapy-induced, tumor-specific cytotoxic T-cell immune responses without directly protecting tumor cells from the cytotoxic effects of radiotherapy.