DICERing macrophages for reprogramming TAMs

Macrophages are cells of the innate immune system that respond to external stimuli by rapidly adapting their transcriptional programs and behavior. Upon exposure to inflammatory signals or pathogen-associated molecules, macrophages activate a proinflammatory program (called “classical” or M1 activation) that enhances their direct microbicidal functions and, arguably, their ability to stimulate the immune system. Vigorous M1 activation of macrophages, as seen during acute inflammation, also promotes cell death and tissue damage. However, macrophages can likewise acquire other activation states. For example, “alternatively” (or M2) activated macrophages tune down acute inflammation and promote tissue remodeling and repair. In the context of cancer, macrophages are traditionally regarded as M2-polarized and, indeed, are immunosuppressive and ostensibly pro-tumoral. However, increasing evidence indicates that, in both human and murine tumors, tumor-associated macrophages (TAMs) display more nuanced M1and M2-like traits, which are modulated by the highly variegated repertoire of signals present in distinct – and rapidly evolving – tumor microenvironments. Accordingly, TAMs can both limit and facilitate tumor growth. There is also evidence for the existence of molecularly and functionally distinct TAM subsets, some of which have more marked pro-tumoral functions. For example, perivascular M2-like TAMs (also known as TIE2expressing macrophages) facilitate angiogenesis and cancer cell dissemination by increasing vascular permeability. Tumors also contain macrophages that express an attenuated M1 (or M1-like) phenotype. However, their potential anti-tumoral and immunostimulatory functions may be blunted by the strongly immunosuppressive signals that emanate from other tumor-associated cells, including M2 TAMs. Thus, interventions aimed to broadly (re)programming TAMs toward an M1 phenotype may curb cancer-associated immunosuppression and help energize anti-tumor immunity. We recently showed that myeloid-specific inactivation of the microRNA (miRNA)-processing enzyme, DICER, promotes the functional programming of TAMs to an M1-like phenotype that can delay, or even prevent, tumor growth in mice. miRNAs are small non-coding RNAs that finely tune gene expression posttranscriptionally. Conditional deletion of Dicer significantly abated miRNA levels in TAMs, but did not obviously influence endogenous miRNA levels in other tumor-infiltrating myeloid cells. When challenged with tumors, mice lacking DICER in macrophages displayed varying degrees of tumor growth retardation, depending on the tumor type. Intriguingly, in each tumor model analyzed, the DICER-deficient TAMs had acquired a strongly M1-like phenotype, characterized by enhanced expression of proinflammatory cytokines and T-cell–recruiting chemokines (Fig. 1). Likely as a consequence of this phenotypic switch in the TAMs, the tumors attracted more cytotoxic T lymphocytes (CTLs), which are key effectors of anti-tumor immunity. Accordingly, the elimination of CTLs rescued tumor growth in the DICER-deficient mice. Although DICER inactivation was sufficient to initiate M1 programming of macrophages in vitro, the elimination of CTLs attenuated M1 TAM activation, suggesting that a cross talk between the DICER-deficient TAMs and the recruited CTLs had reinforced and sustained M1 macrophage programming in the tumors. Furthermore, the pharmacological neutralization of CTL-derived interferon-g (IFNg) impeded M1 programming of DICER-deficient TAMs. Together, these results imply that DICER and miRNA activity in TAMs may function to limit M1 macrophage activation in response to CTL-derived IFNg. Of note, IFNg is a prototypical T helper-1 (Th1) and M1-activating cytokine, as opposed to interleukin-4 (IL4), which is a Th2 cytokine that instead promotes M2 macrophage activation. Suppressing DICER activity may, therefore, make the TAMs more sensitive to IFNg, and possibly other M1polarizing stimuli. These may include various Th1 cytokines and pro-inflammatory mediators expressed endogenously in tumors, but also exogenously (therapeutically) administered agents that may serve to activate macrophages and stimulate anti-tumor immunity. Importantly, DICER inactivation in TAMs greatly enhanced the efficacy of cancer immunotherapies, namely PD1 checkpoint blockade and CD40 agonistic antibodies, leading to complete tumor regressions in mice.