Loss of HDAC11 Promotes Myeloid-Derived Suppressor Cells Inhibition of T Cell Function in a Murine Lymphoma Microenvironment

Myeloid-derived suppressor cells constitute a heterogeneous population of immature myeloid cells derived from bone marrow that negatively regulate both innate and adaptive immunity in the tumor microenvironment. Previous work in our lab had demonstrated that MDSCs lacking histone deacetylase 11 (HDAC11) displayed an increase in suppressive activity against IFN-γ producing CD8⁺ T cells. The upregulated suppressive activity of HDAC11KO MDSCs was associated with a more aggressive tumor growth pattern when compared with wild type control mice.

Mechanistically we have found that tumor infiltrated HDAC11KO MDSCs isolated from lymphoma-bearing mice displayed up-regulation of expression and enzymatic activity of arginase 1 and Nos2, two enzymes that are crucial in regulating MDSCs suppressive function, when compare with wild type MDSCs. However, both arginase activity and NO production were at a similar level in the wild type and HDAC11KO MDSCs isolated from spleens of the same lymphoma-bearing mice. This finding suggests that HDAC11KO MDSCs are more suppressive within the tumor microenvironment. Moreover, the aberrant enzymatic activities of Arg1 and Nos2 in HDAC11KO MDSCs correlate with over-expression of the lineage-specific transcription factor C/EBPβ, which has been previously shown to be essential for the differentiation of functional MDSCs. Furthermore, ChIP analysis confirmed that HDAC11 is recruited to the C/EBPβ gene promoter where exerts a negative regulatory effect upon gene transcription.

Unlike MDSC's in which absence of HDAC11 is associated with a suppressive phenotype, T-cell lacking HDAC11 are hyper-reactive and endowed with strong antitumor activity. To assess which phenotype will be the dominant one in vivo, we performed adoptive immune cell transfer experiments of both MDSC and/or T-cells from either wild type or HDAC11 KO mice into C57BL/6 lymphoma-bearing animals. The transfer of HDAC11KO MDSCs was able to eliminate, at least partially, the anti-tumor effect elicited by the HDAC11KO T cells in the lymphoma microenvironment

Taken together, we have uncovered a previously unknown role for HDAC11 as a transcriptional regulator of MDSCs phenotype and function in a murine lymphoma model. A better understanding of this novel role of HDAC11 in myeloid biology will lead to targeted epigenetic therapies to manipulate the suppressive effect of these immunoregulatory cells in vivo.