RUNX1 overlapping RNA (RUNXOR) is a long non-coding RNA and plays a pivotal role in the differentiation of myeloid cells via targeting runt-related transcription factor 1 (RUNX1). We and others have previously reported that myeloid-derived suppressor cells (MDSCs) expand and inhibit host immune responses during chronic viral infections; however, the mechanisms responsible for MDSC differentiation and suppressive functions, in particular the role of RUNXOR-RUNX1, remain unclear. Here, we demonstrated that RUNXOR and RUNX1 expressions are significantly upregulated and associated with elevated levels of immunosuppressive molecules, such as arginase 1 (Arg1), inducible nitric oxide synthase (iNOS), signal transducer and activator of transcription 3 (STAT3), and reactive oxygen species (ROS) in MDSCs during chronic hepatitis C virus (HCV) infection. Mechanistically, we discovered that HCV-associated exosomes (HCV-Exo) can induce the expressions of RUNXOR and RUNX1, which in turn regulates miR-124 expression via STAT3 signaling, thereby promoting MDSC differentiation and suppressive functions. Importantly, overexpression of RUNXOR in healthy CD33+ myeloid cells promoted differentiation and suppressive functions of MDSCs. Conversely, silencing RUNXOR or RUNX1 expression in HCV-derived CD33+ myeloid cells significantly inhibited their differentiation and expressions of suppressive molecules and improved the function of co-cultured autologous CD4 T cells. Taken together, these results indicate that the RUNXOR-RUNX1-STAT3-miR124 axis enhances the differentiation and suppressive functions of MDSCs and could be a potential target for immunomodulation in conjunction with antiviral therapy during chronic HCV infection.

RUNX1 overlapping RNA (RUNXOR) is a long noncoding RNA and a key regulator of myeloid-derived suppressor cells (MDSCs) via targeting runt-related transcription factor 1 (RUNX1). We and others have previously reported MDSC expansion and inhibition of host immune responses during viral infections; however, the mechanisms regulating MDSC differentiation and suppressive functions, especially the role of RUNXOR-RUNX1 in the regulation of MDSCs in people living with HIV (PLHIV), remain unknown. In this study, we demonstrate that RUNXOR and RUNX1 expressions are upregulated in MDSCs that expand and accumulate in human PBMCs derived from PLHIV. We found that the upregulation of RUNXOR and RUNX1 is associated with the expressions of several key immunosuppressive molecules, including arginase 1, inducible NO synthase, STAT3, IL-6, and reactive oxygen species. RUNXOR and RUNX1 could positively regulate each other's expression and control the expressions of these suppressive mediators. Specifically, silencing RUNXOR or RUNX1 expression in MDSCs from PLHIV attenuated MDSC expansion and immunosuppressive mediator expressions, whereas overexpressing RUNXOR in CD33+ myeloid precursors from healthy subjects promoted their differentiation into MDSCs and enhanced the expression of these mediators. Moreover, loss of RUNXOR-RUNX1 function in MDSCs improved IFN-γ production from cocultured autologous CD4 T cells derived from PLHIV. These results suggest that the RUNXOR-RUNX1 axis promotes the differentiation and suppressive functions of MDSCs via regulating multiple immunosuppressive signaling molecules and may represent a potential target for immunotherapy in conjunction with antiviral therapy in PLHIV.