Opportunities for Nitric Oxide in Potentiating Cancer Immunotherapy

Abstract

Despite nearly 30 years of development and recent highlights of nitric oxide (NO) donors and NO delivery systems in anticancer therapy, the limited understanding of exogenous NO's effects on the immune system has prevented their advancement into clinical use. In particular, the effects of exogenously delivered NO differing from that of endogenous NO has obscured how the potential and functions of NO in anticancer therapy may be estimated and exploited despite the accumulating evidence of NO's cancer therapy-potentiating effects on the immune system. After introducing their fundamentals and characteristics, this review discusses the current mechanistic understanding of NO donors and delivery systems in modulating the immunogenicity of cancer cells as well as the differentiation and functions of innate and adaptive immune cells. Lastly, the potential for the complex modulatory effects of NO with the immune system to be leveraged for therapeutic applications is discussed in the context of recent advancements in the implementation of NO delivery systems for anticancer immunotherapy applications. SIGNIFICANCE STATEMENT: Despite a 30-year history and recent highlights of nitric oxide (NO) donors and delivery systems as anticancer therapeutics, their clinical translation has been limited. Increasing evidence of the complex interactions between NO and the immune system has revealed both the potential and hurdles in their clinical translation. This review summarizes the effects of exogenous NO on cancer and immune cells in vitro and elaborates these effects in the context of recent reports exploiting NO delivery systems in vivo in cancer therapy applications.

Graphical abstract

Fig. 1

Direct effects of exogenous NO on cancer cell immunogenicity. Exogenous NO can suppress PD-L1 expression by cancer cells, which enhances the efficacy of anti-tumor CD8⁺ T cell immunity. It can also upregulate the PVR/CD155 expression by cancer cells, which induces NK cell responses. It can also stimulate the ATP release as well as CRT expression, which recruits APCs. These immunogenic effects on cancer cells mediated by the application of exogenous NO are dependent on the types of cancer cells and NO donors/delivery systems, as well as the dose of NO donors/delivery systems.

Fig. 2

Direct effects of exogenous NO on DCs. Exogenous NO improves differentiation of hematopoietic stem cells and monocytes to DCs. It enhances the endocytic functions of DCs, while hampering processing of intracellular antigen. Although exogenous NO itself has negligible effects on or inhibits the maturation of DCs, it can improve the effects of DC stimulating agents. It can also promote CCL19-mediated migration of DCs into the secondary lymphoid tissues. Although exogenous NO itself has an insignificant effect on or slightly enhances T cell proliferation, it can improve the efficacy of DC-stimulating agents on induction of T cell proliferation. These exogenous NO effects on DCs can be dependent on DC state as well as the types and dose of NO donors/delivery systems.

Fig. 3

Direct effects of exogenous NO on macrophages and B cells. (A) Exogenous NO promotes M1 polarization over M2, while the cotreatment with IFN-γ exerts the opposite effect. (B) Exogenous NO impairs the intracellular antigen processing functions of B cells by interfering with lysozyme activity.

Fig. 4

Direct effects of exogenous NO on T cells. (A) Exogenous NO nitrosylates the TCR of T cells, which suppress the proliferation of T cells in response to antigen recognition. (B) Low concentrations of exogenous NO promotes the differentiation of CD4⁺ T cells to a Th1 type, while high concentrations suppresses it. (C) Exogenous NO enhances the production of IL-4 from activated Th2 cells. (D) Exogenous NO suppresses Th17 differentiation from CD4⁺ T cells partially by expansion of NO T_regs. In addition, exogenous NO suppresses T_reg differentiation from CD4⁺ T cells and improves the proliferation of Th9 type cells.

Fig. 5

The effects of exogenous NO on CTLA-4 expression and its potential to potentiate the effects of aCTLA-4 immunotherapy. Exogenous NO induces not only the maturation and activation of DCs but also the elevated expression of CTLA-4 on DCs, macrophages, and MDSCs, which suppresses CD8⁺ T cell priming and expansion. Cotreatment with aCTLA-4 inhibits CTLA-4-mediated immunosuppression, which results in increased efficiency of DC-mediated CD8⁺ T cell instruction that is otherwise suppressed by CTLA-4 expressing immune cells. Figure adapted from Kim, Francis et al. (2022).