Bacillus Calmette–Guérin (BCG) has been the standard of care for the treatment of high-risk, non-muscle-invasive bladder cancer (NMIBC) for decades, but 49.6% of high-risk and very-high-risk patients will experience progression to muscle-invasive disease in five years. Furthermore, cytology and cystoscopy entail a high burden for both patients and health care systems due to the need for very long periods of follow-up. Subsequent adjuvant treatment using intravesical immunotherapy with BCG has been shown to be effective in reducing tumor recurrence and progression, but it is not free of severe adverse effects that ultimately diminish patients’ quality of life. Because not all patients benefit from BCG treatment, it is of paramount importance to be able to identify responders and non-responders to BCG as soon as possible in order to offer the best available treatment and prevent unnecessary adverse events. The tumor microenvironment (TME), local immune response, and systemic immune response (both adaptive and innate) seem to play an important role in defining responders, although the way they interact remains unclear. A shift towards a proinflammatory immune response in TME is thought to be related to BCG effectiveness. The aim of this review is to collect the most relevant data available regarding BCG’s mechanism of action, its role in modulating innate and adaptive immune responses and the secretion of certain cytokines, and their potential use as immunological markers of response; the aim is also to identify promising lines of investigation.

Bladder cancer (BC) is the second most frequent cancer of the genitourinary system. The most successful therapy since the 1970s has consisted of intravesical instillations of Bacillus Calmette–Guérin (BCG) in which the tumor microenvironment (TME), including macrophages, plays an important role. However, some patients cannot be treated with this therapy due to comorbidities and severe inflammatory side effects. The overexpression of histone deacetylases (HDACs) in BC has been correlated with macrophage polarization together with higher tumor grades and poor prognosis. Herein we demonstrated that phenylbutyrate acid (PBA), a HDAC inhibitor, acts as an antitumoral compound and immunomodulator. In BC cell lines, PBA induced significant cell cycle arrest in G1, reduced stemness markers and increased PD-L1 expression with a corresponding reduction in histone 3 and 4 acetylation patterns. Concerning its role as an immunomodulator, we found that PBA reduced macrophage IL-6 and IL-10 production as well as CD14 downregulation and the upregulation of both PD-L1 and IL-1β. Along this line, PBA showed a reduction in IL-4-induced M2 polarization in human macrophages. In co-cultures of BC cell lines with human macrophages, a double-positive myeloid–tumoral hybrid population (CD11b⁺EPCAM⁺) was detected after 48 h, which indicates BC cell–macrophage fusions known as tumor hybrid cells (THC). These THC were characterized by high PD-L1 and stemness markers (SOX2, NANOG, miR-302) as compared with non-fused (CD11b⁻EPCAM⁺) cancer cells. Eventually, PBA reduced stemness markers along with BMP4 and IL-10. Our data indicate that PBA could have beneficial properties for BC management, affecting not only tumor cells but also the TME.