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Events occurring during the initial phase of human immunodeficiency virus (HIV) infection are intriguing because of their dramatic impact on the subsequent course of the disease. In particular, the relationship between myeloid-derived suppressor cells (MDSCs) and HIV pathogenesis in primary infection remains unknown and the mechanism of MDSCs in HIV infection are incompletely defined.The frequency of MDSC expression in patients with primary HIV infection (PHI) and chronic HIV infection was measured, and the association with disease progression was studied. Programmed death-ligand 1 (PD-L1) and galectin-9 (Gal-9) expression on MDSCs was measured and in vitro blocking experiments were performed to study the role of PD-L1 in MDSCs' inhibition.We found increased levels of HLA-DR−/lowCD14−CD33+CD11b+ granulocytic(G)-MDSCs in PHI individuals compared with normal controls, which correlated with viral loads and was negatively related to CD4+ T-cell levels. When cocultured with purified G-MDSCs, both proliferation and interferon-γ secretion by T cell receptor (TCR)-stimulated CD8+ T cells from HIV-infected patients were significantly inhibited. We also demonstrated that PD-L1, but not Gal-9, expression on HLA-DR−/lowCD14−CD33+CD11b+ cells increased during HIV infection. The suppressive activity of G-MDSCs from HIV-infected patients was attenuated by PD-L1 blockade.We found a significant increase in G-MDSCs in PHI patients that was related to disease progression and PD-L1 was used by MDSCs to inhibit CD8+ T cells in HIV infection. Our data improve the understanding of HIV pathogenesis in PHI.