Novel Combination Strategies to Enhance Immune Checkpoint Inhibition in Cancer Immunotherapy: A Narrative Review

Programmed cell death protein-1 (PD-1) is an immune checkpoint receptor that induces and maintains tolerance of T cells, invariant natural killer T (iNKT) cells, and natural killer (NK) cells, among other lymphocytes. Immune checkpoint inhibition by PD-1 blockade restores the lymphocytic immunostimulatory phenotype and has been successful in the treatment of various malignancies. However, while immune checkpoint blockade has been shown to provide robust antitumor treatment outcomes, its overall response rate remains low in a significant portion of cancer patients. An essential unmet need in cancer therapy is the development of novel pharmacologic strategies designed to lower rates of resistance associated with immune checkpoint blockade. Therefore, efforts that seek to enhance the efficacy of PD-1 inhibition possess profound immunotherapeutic potential. Here, three promising combination strategies that harness the antitumor effects of immune checkpoint inhibitors (ICIs) together with non-ICI antitumor therapeutic agents are reviewed. These agents include (1) ABX196, a potent inducer of iNKT cells, (2) chimeric antigen receptor (CAR)-T cell therapy, and (3) NK cell therapy. A comprehensive literature search was conducted using the PubMed and ClinicalTrials.gov databases for scientific articles and active trials, respectively, pertaining to immune checkpoint inhibition, iNKT cells, CAR-T cells, and NK cell immunotherapy. Preliminary clinical and preclinical data suggest that these combination treatment regimens greatly suppress tumor growth and may serve as innovative methods to enhance and optimize anticancer immunotherapy.

Niche-Specific Reprogramming of Epigenetic Landscapes Drives Myeloid Cell Diversity in Nonalcoholic Steatohepatitis

Tissue-resident and recruited macrophages contribute to both host defense and pathology. Multiple macrophage phenotypes are represented in diseased tissues, but we lack deep understanding of mechanisms controlling diversification. Here, we investigate origins and epigenetic trajectories of hepatic macrophages during diet-induced non-alcoholic steatohepatitis (NASH). The NASH diet induced significant changes in Kupffer cell enhancers and gene expression, resulting in partial loss of Kupffer cell identity, induction of Trem2 and Cd9 expression, and cell death. Kupffer cell loss was compensated by gain of adjacent monocyte-derived macrophages that exhibited convergent epigenomes, transcriptomes, and functions. NASH-induced changes in Kupffer cell enhancers were driven by AP-1 and EGR that reprogrammed LXR functions required for Kupffer cell identity and survival to instead drive a scar-associated macrophage phenotype. These findings reveal mechanisms by which disease-associated environmental signals instruct resident and recruited macrophages to acquire distinct gene expression programs and corresponding functions.

Liver-Derived Signals Sequentially Reprogram Myeloid Enhancers to Initiate and Maintain Kupffer Cell Identity

Tissue environment plays a powerful role in establishing and maintaining the distinct phenotypes of resident macrophages, but the underlying molecular mechanisms remain poorly understood. Here, we characterized transcriptomic and epigenetic changes in repopulating liver macrophages following acute Kupffer cell depletion as a means to infer signaling pathways and transcription factors that promote Kupffer cell differentiation. We obtained evidence that combinatorial interactions of the Notch ligand DLL4 and transforming growth factor-b (TGF-β) family ligands produced by sinusoidal endothelial cells and endogenous LXR ligands were required for the induction and maintenance of Kupffer cell identity. DLL4 regulation of the Notch transcriptional effector RBPJ activated poised enhancers to rapidly induce LXRα and other Kupffer cell lineage-determining factors. These factors in turn reprogrammed the repopulating liver macrophage enhancer landscape to converge on that of the original resident Kupffer cells. Collectively, these findings provide a framework for understanding how macrophage progenitor cells acquire tissue-specific phenotypes.

Deciphering liver environmental signaling pathways for Kupffer cell identity

Functional specialization of tissue resident macrophages occurs through environmental signals controlling activity and/or expression of transcription factors. Kupffer cells are resident macrophages in the hepatic sinusoids and have critical roles in the innate immune response and iron metabolism. Here, we characterize transcriptomic and epigenetic changes in repopulating liver macrophages following acute Kupffer cell depletion as a means to infer signaling pathways and transcription factors that promote Kupffer cell differentiation. Nr1h3 encoding LXRα is rapidly and highly induced in repopulating liver macrophages, suggesting its induction plays a crucial role in Kupffer cell differentiation. Restricted deletion of Nr1h3 in Kupffer cells reveal that it is required for shaping the Kupffer cell-specific enhancer landscape. Further, we obtain evidence that combinatorial interactions of DLL4 and TGF-β/BMP produced by sinusoidal endothelial cells and endogenous LXR ligands are required for the induction and maintenance of Kupffer cell identity. DLL4 regulation of RBPJ through Notch signaling plays a key role in activating poised enhancers to rapidly induce LXRα and other Kupffer cell lineage-determining factors. These factors in turn reprogram the repopulating liver macrophage enhancer landscape to converge on that of the original resident Kupffer cells. Using molecules which mimic these liver environment signals, we show that it is possible to induce Kupffer cell-specific genes in mouse bone marrow progenitor cells and human monocytes in vitro. Collectively, these findings provide a framework for understanding how macrophage progenitor cells acquire tissue-specific phenotypes.

Exploiting altered enhancer landscapes to decode pathogenic changes in gene expression of diverse hepatic macrophages

Kupffer cells have specialized roles supporting the environment of the liver during homeostasis and disease. However, the key regulatory elements governing these behaviors are unknown. Using scRNA-seq, we found diversification of Kupffer cells and recruitment of additional macrophage subtypes during nonalcoholic steatohepatitis (NASH). A significant source of macrophage heterogeneity during NASH was traced to Cx3cr1 expressing monocytes. Further, macrophage subsets were localized in distinct niches, suggesting environmental specification as a key determinant of macrophage heterogeneity. We profiled chromatin accessibility of the major NASH associated macrophage populations to identify transcription factors governing their environmental specification. These results predict greater NFκB, RUNX, and AP1 activity in recruited hepatic macrophages compared to Kupffer cells. Surprisingly, we found minimal significant chromatin accessibility changes comparing Kupffer cells from healthy mice to mice with NASH, even though several thousand genes were differentially expressed. Instead, NASH led to altered chromatin activity, as measured by H3K27ac ChIP-seq, at Kupffer cell enhancer regions. A binding element for LXR (liver X receptor) was the top transcription factor motif identified in Kupffer cell enhancers with reduced activity during NASH. Furthermore, LXRα was required to maintain expression of a unique gene signature defining healthy Kupffer cells. Thus, our studies establish for the first time gene regulatory events controlling diverse hepatic macrophages during homeostasis and NASH.