Killed Propionibacterium acnes enhances immunogenicity and tumor growth control of a dendritic-tumor cell hybrid vaccine in a murine melanoma model

Hybrid vaccines have been investigated in clinical and experimental studies once expresses total antigens of a tumor cell combined with the ability of a dendritic cell (DC) to stimulate immune responses. However, the response triggered by these vaccines is often weak, requiring the use of adjuvants to increase vaccine immunogenicity. Killed Propionibacterium acnes (P. acnes) exerts immunomodulatory effects by increasing the phagocytic and tumoricidal activities of macrophages, promoting DC maturation, inducing pro-inflammatory cytokines production and increasing the humoral response to different antigens. Here, we evaluated the effect of P. acnes on a specific antitumor immune response elicited by a hybrid vaccine in a mouse melanoma model. Hybrid vaccine associated with P. acnes increased the absolute number of memory T cells, the IFN-γ secretion by these cells and the IgG-specific titers to B16F10 antigens, polarizing the immune response to a T helper 1 pattern. Furthermore, the addition of P. acnes to a hybrid vaccine increased the cytotoxic activity of splenocytes toward B16F10 in vitro and avoided late tumor progression in a pulmonary colonization model. These results revealed the adjuvant effect of a killed P. acnes suspension, as it improved specific humoral and cellular immune responses elicited by DC-tumor cell hybrid vaccines.