Rhesus Macaque Myeloid-Derived Suppressor Cells Demonstrate T Cell Inhibitory Functions and Are Transiently Increased after Vaccination.

Myeloid-derived suppressor cells (MDSCs) are major regulators of T cell responses in several pathological conditions. Whether MDSCs increase and influence T cell responses in temporary inflammation, such as after vaccine administration, is unknown. Using the rhesus macaque model, which is critical for late-stage vaccine testing, we demonstrate that monocytic (M)-MDSCs and polymorphonuclear (PMN)-MDSCs can be detected using several of the markers used in humans. However, whereas rhesus M-MDSCs lacked expression of CD33, PMN-MDSCs were identified as CD33 low-density neutrophils. Importantly, both M-MDSCs and PMN-MDSCs showed suppression of T cell proliferation in vitro. The frequency of circulating MDSCs rapidly and transiently increased 24 h after vaccine administration. M-MDSCs infiltrated the vaccine injection site, but not vaccine-draining lymph nodes. This was accompanied by upregulation of genes relevant to MDSCs such as arginase-1, IDO1, PDL1, and IL-10 at the injection site. MDSCs may therefore play a role in locally maintaining immune balance during vaccine-induced inflammation.