PMN-MDSCs promoted CIA severity through modulating B cells
To verify that PMN-MDSCs played a proinflammatory or immunosuppressive role in CIA, we depleted PMN-MDSCs in CIA mice from day 32 (disease progression stage) using anti-Ly6G antibodies (1A8), and the control group used the isotype IgG Ab, The Ly6G Ab was effective in reducing the PMN-MDSC ratio and counts in the joints and spleens (Fig. S2A, S2B). PMN-MDSC depletion decreased the severity of arthritis, maintaining an average arthritis score of < 6 (Fig. 2A). Ly6G Ab alleviated joint swelling and decreased inflammatory cell infiltration and synovial hyperplasia compared to control isotype IgG (Fig. 2B). PMN-MDSCs depletion also decreased IgG and TNF-𝛂 in the plasma (Fig. 2C). And surprisingly, we found that the frequencies and absolute number of TNF-𝛂+B cells and Ki67+B cells performed by flow cytometry also decreased significantly in the joints and spleens after PMN-MDSCs were removed (Fig. 2D-2H). And the TNF-𝛂 transcriptional level in the joints and spleens detected by RT-qPCR also declined in Ly6G Ab group (Fig. S2C, S2D).
However, the anti-Ly6G antibody could deplete all cells with the surface marker Ly6G, and its potential neutralizing effect on neutrophils could not be ruled out. To rule out this possibility, PMN-MDSCs isolated from the spleens of CIA mice (clinical score >6) on day 35 were adoptively transferred to recipient CIA mice on days 21 and 28. The transferred PMN-MDSCs aggravated arthritis severity (Fig. 3A). The swelling of the joints was amplified and the pathology showed increased immune cell infiltration, inflammatory exudation, cartilage damage, and bone erosion (Fig. 3B). The concentration of IgG increased significantly in the plasma after PMN-MDSCs transfer, the TNF-𝛂 also increased slightly although had no significant difference (Fig. 3C). In addition, PMN-MDSCs transfer enhanced the frequencies and absolute number of TNF-𝛂+B cells and Ki67+B cells in the joints and spleens in vivo (Fig 3D-3H). Therefore, the proinflammatory role of PMN-MDSC in the pathogenesis of CIA were validated through selective depletion and adoptive transfer experiments, and B cell responses were modulated by PMN-MDSC.