and LPS plus IFN--primed cells were administrated with luteolin (Sigma, USA) at 2.5 and 5.0 108 mol/L for 24 h. Following treatment, cells were added with 20 μL MTT solution (5 mg/mL; 109 Sigma, MO. USA). 4 h later, culture medium was removed and crystals were dissolved with 150 110 μL/well DMSO. The absorption values were measured at 570 nm. 111
The morphology of polarized BMDM. BMDM in logarithmic stage were cultivated in 6-well 112 plates, and incubated overnight followed by treatment with LPS (20 ng/mL) plus IFN- (10 ng/mL) 113 or IL-4 (20 ng/mL). Simultaneously, the cells stimulated with LPS+IFN- were co-cultured with 114 specified concentrations of luteolin for 24 h. The cell morphology was observed under an inverted 115 microscope (Olympus, Japan). Cells and culture supernatants were collected for subsequent 116 mRNA and protein analysis. 117
Quantitative real-time PCR (qPCR). BMDM were stimulated with LPS plus IFN- or IL-4 118 respectively, following dosing stimuli, cells were collected and total RNA was extracted by Trizol 119 (Ambion Life Technology, USA) and reverse-transcribed into cDNA. With GAPDH as the 120 internal reference, qPCR was carried out using SYBR Green Master Mix (Toyobo, Japan) 121 according to primer sequences with Quant studio DX real-time quantitative PCR employed 122 biosystems (Life Technologies, USA). Relative gene expression was calculated using the 2 −ΔΔct 123 comparative method. Primer Sequences were obtained from Generay Biotech Co. Ltd. (Shanghai, 124 China) and listed in Table 1 . 125 antibodies with shaken at room temperature for 30 min. Immunoreactive proteins were exposed 145 and developed using ECL (Beyotime, China), and -actin was used as an internal reference to 146 calculate the relative expression of protein. corresponding LPS+IFN--treated group, there was a statistical difference (Fig. 6) . 220 group) in M1-polarized BMDM in a concentration-dependent pattern (Fig.7) . 237 Immunoblotting assay and densitometry analysis of STAT proteins revealed that M1-polarized lowly expresses p-STAT1 and highly expresses p-STAT6. Predominantly, after luteolin 250 contribution, p-STAT1 expression was depressed while p-STAT6 was strengthened in protein 251 pathway of M1-polarized BMDM (Fig. 8) . crucial cytokines IL-6 and TNF- generated abundantly by IL-1 stimulation or autocrine from 299 activated "mononuclear-macrophage system", are elevated not only in bacterial infection but also 300 during viral infection (23-24). More importantly, they are most strong pro-inflammatory agent 301 causing "cytokine storm". Studies have shown that patients with severe COVID-19 characterized 302 by a "cytokine storm" inexorably exhibited high levels of IL-6 and TNF-in serum, and IL-6 or 303 TNF- antagonist seems to be very promising for severe COVID-19 cases (25-26). 304
To our knowledge, IL-4 or IL-13 can induce M2-type polarization of macrophages, and M2-type anti-inflammatory factors such as IL-10, Arg1 and CD206 are up-modulated. In this 306 regard, IL-4 binds to its receptor to activate JAK to further phosphorylate STAT6 and enhance the 307 Arg1 activity. Arg1 and iNOS are important hallmarkers of M2/M1 type macrophage polarization, 308
respectively. Under normal circumstances, the activities of Arg1 and iNOS are strictly regulated 309 by macrophages and maintain a dynamic equilibrium. When M2 polarization occurs, Arg1 310 competes for iNOS to decompose substrate arginine, thus benefit for tissue regeneration. 311
Moreover, Arg1 is also inseparable from M2 macrophage properties in playing immune memory 312 function to eliminate infectious agents. CD206, so called mannose receptor, is a membrane surface 313 marker of M2 cells, which can specifically recognize antigens to clear pathogens, promote 314 angiogenesis and repress immune response (27). Another M2-type anti-inflammatory factor, IL-10, 315 on the one hand, enhances the sensitivity of macrophages to IL-4 and IL-13 by increasing the 316 abundance of IL-4 receptors on the macrophage surface, which contributes to M2-type 317 polarization of macrophages. On the other hand, it can synergize with IL-4 to inhibit 318 pro-inflammatory cytokines IL-1 and TNF- to reduce inflammation. In the light of preliminary 319 data, IL-10 displayed higher levels in patients with sepsis and serious COVID-19 (28). All these 320 indicate that when inflammation is motivated, an intricate network is formed between the 321 pro-inflammatory mediators and activated STAT1, eliciting "inflammatory storm." Nevertheless, 322 upon luteolin contribution, a complex network is also formed between anti-inflammatory 323 mediators and activated STAT6, further facilitating the expression of anti-inflammatory factors 324 which resist the formation of pro-inflammatory factors and alleviate inflammation accordingly. 325
Herbal compound Physalin D can repolarize M1 toward M2 polarization in BMDM through 326 STAT1 suppression and STAT6 activation (29), which consist with our study.
Altogether, BMDM polarization mechanism is complex and involves many protein 328 pathways. Only by actively exploring the regulation of BMDM polarization and maintaining the 329 balance of inflammatory mediators can maintain the physical stable. In this investigation, 330 LPS/IFN- induced M1 polarization and IL-4 induced M2 polarization of BMDM. After being 331 treated with herbal compound luteolin, the M1 polarized BMDM showed lowered M1-type 332 pro-inflammatory factors and elevated M2-type anti-inflammatory factor, and that signaling 333 protein p-STAT1 was down-regulated and P-STAT6 was up-regulated. That is, the macrophage 334 population underwent a transformation from a pro-inflammatory M1-phenotype to an 335 anti-inflammator M2-phenotype. Simultaneously, inflammatory factors analogously altered from 336 pro-inflammatory to anti-inflammatory. In light of these findings, our research provide a novel 337 insight into the role of luteolin to be a candidate for controlling macrophage phenotype to treat 338 infectious disease. 339
We acknowledge the support of their staff and Facility of department of Endocrine and 341
Metabolism in accomplishing this work. We also thank Dr. 