The mechanisms that allow Mycobacterium tuberculosis (Mtb) to persist in human tissue for decades and to then abruptly cause disease are not clearly understood. Regulatory elements thought to assist Mtb to enter such a state include the heme two-component sensor kinases DosS and DosT and the cognate response regulator DosR. We have demonstrated previously that O₂, nitric oxide (NO), and carbon monoxide (CO) are regulatory ligands of DosS and DosT. Here, we show that in addition to O₂ and NO, CO induces the complete Mtb dormancy (Dos) regulon. Notably, we demonstrate that CO is primarily sensed through DosS to induce the Dos regulon, whereas DosT plays a less prominent role. We also show that Mtb infection of macrophage cells significantly increases the expression, protein levels, and enzymatic activity of heme oxygenase-1 (HO-1, the enzyme that produces CO), in an NO-independent manner. Furthermore, exploiting HO-1^+/+ and HO-1^-/- bone marrow-derived macrophages, we demonstrate that physiologically relevant levels of CO induce the Dos regulon. Finally, we demonstrate that increased HO-1 mRNA and protein levels are produced in the lungs of Mtb-infected mice. Our data suggest that during infection, O₂, NO, and CO are being sensed concurrently rather than independently via DosS and DosT. We conclude that CO, a previously unrecognized host factor, is a physiologically relevant Mtb signal capable of inducing the Dos regulon, which introduces a new paradigm for understanding the molecular basis of Mtb persistence.

Sorafenib, a multitargeted antiangiogenic tyrosine kinase inhibitor, is the standard of care for patients with advanced hepatocellular carcinoma (HCC). Cumulating evidence suggests that sorafenib differentially affects immune cells; however, whether this immunomodulatory effect has any impact on antitumor immune responses is unknown. Using an orthotopic mouse model of HCC and tumor‐free mice, we investigated the effects of sorafenib on antitumor immunity and characterized the underlying mechanisms. Sorafenib treatment inhibited tumor growth and augmented antitumor immune responses in mice bearing established orthotopic HCC. The tumor‐specific effector T cell functions were upregulated, while the proportion of PD‐1‐expressing CD8⁺ T cells and regulatory T cells (Tregs) was reduced in tumor microenvironment of sorafenib‐treated mice. Mechanistically, the sorafenib‐mediated effects on Tregs could be independent of its direct tumor‐suppressing activities. Sorafenib treatment reduced Treg numbers by inhibiting their proliferation and inducing apoptosis. Moreover, sorafenib inhibited the function of Tregs, characterized by diminished expression of Treg‐associated molecules important for their function and by their impaired suppressive capacity. These data reveal that sorafenib treatment enhanced functions of tumor‐specific effector T cells as well as relieved PD‐1‐mediated intrinsic and Treg‐mediated non‐cell‐autonomous inhibitions in tumor microenvironment leading to effective antitumor immune responses. In addition to the well‐known tumor‐inhibiting activity of sorafenib, its enhancement of antitumor immunity may also contribute to the clinical efficacy. Our findings uncover a previously unrecognized mechanism of action of sorafenib and indicate that sorafenib represents a potential targeted agent suitable to be combined with immunotherapeutic approaches to treat cancer patients.