The animal protocol was performed according to the regulations of the Jilin University Animal Care and Use Committee. The related animal procedures also complied with the Guide for the Care and Use of Laboratory Animals (NIH Publication No. 85-23, revised 1996).

Anti-β-actin (clone: AC-15; Cat: A1978), anti-β-catenin (clone 7F7.2; Cat:04-958), DPI (Cat:4673-26-1), MitoTEMPO (cat:1334850-99-5), DCFH-DA (Cat: D6883), 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2-Htetrazolium bromide (MTT) (Cat: TOX1) and all secondary antibodies were purchased from Sigma-Aldrich (Shang Hai, China). Anti-GSDMD (clone: EPR19828; Cat: ab209845), anti-CD206(Cat: ab300621) and BrdU Cell Proliferation Assay Kit (Cat: ab287841) were purchased from Abcam (Shang Hai, China). Anti-TREM2 (D8I4C) rabbit mAb (91068) was purchased from Cell Signaling Technology (Shanghai, China). Mito-SOX (Cat: M36008) was purchased from Invitrogen (Shanghai, China). Anti-TREM2 neutralizing antibody (FAB17291A), mouse rIFN-_γ (Cat: NP_032363), and mouse rIL-4 (Cat: P07750) were purchased from R&D Systems (Shanghai, China). Alexa Fluor 647 Anti-Mouse F4/80 antibody (Clone: T45-2342), anti-mouse CD11b conjugated to APC-Cy7 (clone: M1/70), anti-mouse CD11c conjugated to FITC (clone: HL3), anti-murine Ly-6G conjugated to PE (clone: 1A8), and Anti-Mouse CD206 PE (Clone: Y17-505) were purchased from BD Bioscience (Shanghai, China). Other chemicals were purchased from Sigma-Aldrich (Shanghai, China) unless otherwise indicated.

B. abortus 2308 strain was a kind gift from Dr. Ding’s laboratory (China Institute of Veterinary Drug Control, Beijing, China). B. abortus 2308 ΔVirB10 strain was from Dr. Chen’s laboratory (Institute of Disease Control and Prevention, AMMS Beijing, China). The green fluorescent protein (GFP)-expressing B. abortus strain (Brucella-GFP) was previously generated by our lab (14). Brucella strains were grown on tryptic soy agar (TSA) or tryptic soy broth (TSB) plates. The number of B. abortus in the cultures was calculated by comparing the OD at 600 nm with a standard curve, whereas the actual concentration of inoculation was calculated by plating on TSA plates. All live B. abortus manipulations were performed in BSL3 (Biosafety Level 3) facilities based on the standard operating protocols approved by the Biosafety Committee of Jilin University.

The TREM2 gene from RAW264.7-derived cDNA was obtained using PCR and then inserted into the pCDNA3.1 vector through the XhoI and ApaI restriction sites. After the plasmid was verified by DNA sequencing, pCDNA3.1-TREM2 or empty pCDNA3.1 vector was transfected into RAW264.7 cell line using Lipofectamine 3000 (Sigma), and stable lines were generated by selection with 400 μg/ml G418 (Invitrogen) (15).

TREM2 knockout plasmid LentiCRISPR-Cas9-TREM2 targeting mouse TREM2 and non-targeting control plasmid LentiCRISPR-Cas9-NT using annealed single guide RNA (sgRNA) oligonucleotide 5^’-ACTGGTAGAGACCCGCATCA-3^’ and 5^’-GTACCATACCGCGTACCCTT-3^’, respectively. sgRNAs were inserted into the LentiCRISPRv2 vector (12). The lentivirus was produced by transiently transfecting HEK293T cells using the SuperFect transfection reagent (Qiagen, USA) with three plasmid systems (LentiCRISPR-Cas9-TREM2/CRISPR-Cas9-NT, psPAX2, and pMD2.G). The virus-containing supernatant was collected 72 h after transfection, filtered through 0.45 mm filters (Millipore, USA), and stored at −80°C. For transduction, lentivirus supernatants were mixed with DEAE-dextran (5 mg/mL) (Sigma-Aldrich) for 30 min and then added to RAW264.7 cells. After 3 days of infection, cells that had stably incorporated the lentiviral construct were selected for survival in the presence of puromycin (5 μg/mL) (Santa Cruz, USA) and analyzed by Western blotting to verify the overexpression or deletion of TREM2.

Bone marrow-derived macrophages (BMDMs) from C57BL/6 mice were purified and cultured as previously described (16). BMDM-M1, BMDM-M2, or TREM2^−/− BMDM-M2 was derived from BMDM that was stimulated with 10 ng/mL of mouse rINFγor rIL-4 overnight prior to experiment and kept throughout the experiments, respectively (4). RAW-M2-NT, RAW-M2-ΔTREM2, RAW-M2-TREM2, or RAW-M2-vector were generated from RAW264.7, derivative cell lines that were stimulated with 10 ng/mL of mouse rIL-4 overnight prior to the experiment and maintained throughout the experiments (17). Cells were then infected with B. abortus 2308 or ΔVirB2 strains in triplicate wells of a plate at a multiplicity of infection (MOI) of 100:1 by centrifuging B. abortus into cells at 400g at 4°C for 10 min. After 15 min of infection in an atmosphere containing 5% CO₂ at 37°C, αMEM medium was used to wash the cells three times to remove extracellular B. abortus, and the cells were infected for another 60 min in αMEM with 40 μg/ml gentamicin to kill extracellular Brucella. To investigate Brucella intracellular growth, infected cells were lysed with phosphate-buffered saline (PBS) with 0.1% Triton X-100 at the indicated time points, and serial dilutions of lysates were plated immediately onto TSA plates to count CFUs. When infection was performed in the presence of DPI or Mito-TEMPO, cells were pretreated for 3 h before infection. To block TREM2 signaling, the cells were pretreated with an anti-TREM2 neutralizing antibody for 20 min at room temperature before infection.

RAW-M2 derivative cell lines at a density of 50,000 cells/well were infected with Brucella-GFP, as described above. For the opsonized samples, B. abortus was pretreated with 20% mouse serum for 30 min before infection. After 4 h of infection, cells were scraped from the wells. PBS was used to wash the cells three times to take dead cells. The pellet was resuspended in PBS. Phagocytosis was measured by flow cytometry through analyzing the GFP signal.

The supernatant of each cell sample was collected at certain time points after B. abortus challenge. Nitric oxide (NO) content was measured by analyzing its stable product, nitrite, using Griess reagent (Sigma), as previously described (13). Data are expressed as micromoles of nitrite (mean ± SEM).

Intracellular and mitochondrial ROS levels were detected using DCFH-DA and Mito-SOX, respectively. Briefly, macrophages at a density of 1 × 10⁵ cells/well in a 24-well plate at the indicated time points were incubated with 10 μM DCFH-DA or 5 μM Mito-SOX at 37°C for 30 min in the dark. Cells were washed three times with PBS, and the fluorescence intensity was measured using Zeiss LSM 800 confocal microscopy or flow cytometry.

The concentration of lactate dehydrogenase (LDH) released from the macrophages was evaluated using a commercial kit of LDH assay kit (Roche) according to the manufacturer’s procedure. In brief, supernatants of macrophages were collected at the indicated infection time points, the kit reagent was added to an aliquot of supernatant (50 μl) and incubated for 45 min, the reaction was halted, and the absorbance was analyzed at 560 nm using an ELISA plate reader. LDH released from macrophages was expressed as a percentage of total LDH activity. The total LDH was determined by lysing the cultures with Triton X-100 (13, 18).

BMDM-M2 macrophage viability was measured using an MTT assay. Briefly, BMDM-M2 macrophages were cultured at a density of 2 × 10⁴ cells/well in a 96-well plate, treated with DMSO, Mito-TEMPO + Baf A1, Baf A1 or Baf A1+DPI, and then infected with B. abortus for 24 h. For TREM2^−/− BMDM-M2, M2 macrophages were cultured at a density of 2 × 10⁴ cells/well in a 96-well plate and infected with B. abortus for 24 h. MTT solution was added to each well at a final concentration of 0.5 mg/ml and the macrophages were incubated for another 4 h. Based on the manufacturer’s protocol, absorbance was measured at 570 nm using an automated ELISA plate reader (19).

For each real-time reaction, reverse-transcribed cDNA products (2 μl) were amplified by PCR in a total volume of 25 μl with 10 pmol each of the related forward and reverse primers (see Supplementary Table ). B-actin served as the control. The fold change in mRNA levels in cells was determined using the comparative threshold cycle (Ct) method (14).

BMDM-M2 macrophage proliferation was analyzed using the BrdU incorporation assay. Based on the protocol of the BrdU cell proliferation ELISA kit (Abcam), an assay was performed to measure macrophage proliferation. Briefly, BMDM-M2 macrophages were cultured at a density of 2 × 10⁴ cells/well in a 96-well plate and treated with DMSO, Mito-TEMPO + Baf A1, Baf A1, anti-TREM2, or isotype antibody, and then infected with B. abortus for 48 h in the presence of BrdU. For TREM2^−/− BMDM-M2, M2 macrophages were cultured at a density of 2 × 10⁴ cells/well in a 96-well plate, and then infected with B. abortus for 48 h in the presence of BrdU. M2 macrophages were then fixed, permeabilized, and denatured to enable the detection of BrdU incorporated with anti-BrdU antibody. After incubation with horseradish peroxidase-conjugated secondary antibody, an ELISA plate reader was used to quantify the colored reaction product at a dual wavelength of 450/550 nm. To eliminate the possibility that the results were affected by potential changes in viable cells, BrdU-positive ELISA signals were normalized to the total number of cells (19).

Macrophages were lysed in ice-cold RIPA (radioimmunoprecipitation) mild detergent buffer. The protein content of the cells was analyzed using a BCA protein evaluation kit (Pierce, USA). Approximately 20 mg of protein from the lysates was loaded onto SDS–PAGE and then transferred to a PVDF membrane. Membranes were incubated with the primary antibody at 4°C overnight and then washed with PBS three times at room temperature, followed by incubation with a suitable secondary antibody for 2 h at room temperature. Protein bands on the membrane were visualized using ECL reagent (GE Healthcare, Shanghai, China). The band densities of the membranes were analyzed by densitometry (model GS-700, Imaging Densitometer; Bio-Rad). The background of the visualized bands was subtracted from that of the control area (15).

Animal experiments were approved by the Institutional Animal Care and Use Committees of Jilin University (Changchun, China) and performed according to institutional guidelines. Five mice from each group (C57BL/6) were infected i.p. with 1 × 10⁶ B. abortus. At 3, 9, and 30 days after infection, the mice were euthanized by CO₂ asphyxiation, and the spleens were collected aseptically at necropsy. When indicated, mice were injected intraperitoneally (i.p.) with DMSO or Baf A1 (1 mg/kg body weight daily) from 18 dpi to 30 dpi. Spleen samples were also collected for the spleen CFU assay, myeloperoxidase (MPO) activity assay, gene expression analysis, immunohistochemistry analysis, and flow cytometry (20). TREM2^−/− mice with C57BL/6 background were generously provided by Dr. Shuangxi Wang (Shandong University, China). Five mice from each group (wild-type or TREM2^−/−) were infected i.p. with 1 × 10⁶ B. abortus. Thirty days after infection, mice were euthanized by CO₂ asphyxiation, and the spleen was collected aseptically at necropsy for spleen CFU assay or CD11b⁺ CD206⁺ assay by flow cytometry. Serum TNFα levels were analyzed using an ELISA kit from eBioscience (Shanghai, China) according to the manufacturer’s instructions.

Flow cytometry analysis for the detection of M2 macrophages, DC, and neutrophils was performed in the spleen cells of C57BL/6 or TREM2^−/− mice uninfected (NI) or infected with B. abortus for 9 or 30 days. Briefly, mice were inoculated intraperitoneally (i.p.) with 1 × 10⁶ CFU of B. abortus and sacrificed 9 or 30 days post-infection. The spleen cells were harvested and washed twice with sterile PBS. After washing, the cells were adjusted to 1 × 10⁶ cells in a 96-well plate, centrifuged at 1,500 rpm for 5 min at 4 ˚C and washed with PBS/BSA (PBS containing 1% bovine serum albumin). The cells were blocked with an anti-CD16 antibody for 20 min at 4 ˚C in PBS/BSA. The cells were then incubated for 20 min at 4 ˚C with a cocktail of anti-F4/80 Alexa Fluor 647, anti-CD206 PE, anti-CD11b APC-Cy7, anti-CD11c FITC, and anti- Ly-6G PE. Finally, the cells were washed twice with PBS/BSA, resuspended in PBS, and detected using Attune Acoustic Focusing equipment (Thermo Fisher Scientific). The results were analyzed using the FloWJov10 software (BD Biosciences) (18).

Mouse spleens were fixed with formalin, embedded in paraffin, and cut into 6-um thick sections. Spleen tissues were stained with hematoxylin–eosin (HE). The slides were analyzed for histopathology. Immunohistochemical (IHC) staining of the spleen was performed for CD206 (Abcam, Shanghai, China) according to the manufacturer’s protocols. Representative images were obtained using an Olympus microscope, and brightness was adjusted.

All values are expressed as the means ± SD. Data were analyzed using one-way ANOVA followed by Bonferroni correction. For statistical comparisons between two groups, we used an unpaired Newman–Student t-test. All statistical analyses were performed using the SPSS software (version 10.0). Statistical significance was set at P <0.05.

Bone marrow derived macrophages (BMDMs) were infected with B. abortus and followed for 48 h. TREM2 mRNA levels remained unchanged in the BMDM ( Figure 1A ). BMDM polarized into M1 and infected with B. abortus also had similar TREM2 expression as uninfected cells ( Figure 1B ). However, BMDM polarized into M2, and infected with B. abortus had significantly increased TREM2 mRNA and protein levels at 24 h and 48 h after infection ( Figures 1C, D ). Moreover, upregulation of TREM2 required live bacteria, as heat-killed B. abortus failed to induce TREM2 expression in BMDM-M2 cells ( Figure 1E ). These data suggest that an active bacteria-driven process is required to stimulate TREM2 expression in M2 cells. As the Brucella effectors secreted by T4SS is involved in regulating signaling pathways of host cells (21), we examined the expression of TREM2 in BMDM-M2 infected with B. abortus mutant with a mutation in T4SS (deletion of the critical VirB2 subunit, ΔVirB2). We found that ΔVirB2 B. abortus infection failed to increase TREM2 expression in M2 ( Figure 1E ). Suppression of VirB by bafilomycin A1(Baf A1) also prevented the upregulation of TREM2 expression compared with in the DMSO control in M2 cells ( Figure 1F ). Taken together, these data show that B. abortus infection triggers an upregulation of TREM2 expression in M2 macrophages in a T4SS-dependent manner.

Next, we used mouse macrophages RAW264.7 cells to investigate the role of TREM2 in M2 in mediating phagocytosis of B. abortus due to its functional similarity to BMDM ( Figure 2A ). We generated RAW264.7 derivative cell lines that overexpressed TREM2 (RAW-TREM2) or deleted TREM2 (RAW-ΔTREM2). For controls, RAW264.7 cells expressing endogenous TREM2 were transduced with an empty expression vector (RAW-Vector) or a vector expressing a non-target sgRNA sequence (RAW-NT). All cell lines were polarized into M2 (RAW-M2) with mouse IL-4, based on previous methods (17, 22). The overexpression or deletion of TREM2 in RAW-M2 cell lines was confirmed by western blotting ( Figure 2B ) and then infected with unopsonized GFP-expressing B. abortus. TREM2-deficeincy reduced the phagocytosis of B. abortus compared to RAW-M2-NT macrophages, whereas overexpression of TREM2 significantly increased the phagocytosis of B. abortus compared to RAW-M2-vector macrophages ( Figure 2C ). When B. abortus was opsonized, phagocytosis of B. abortus by TREM2 overexpression-M2 was enhanced compared to RAW-M2-vector M2, but TREM2-deficeincy did not decrease phagocytosis of B. abortus ( Figure 2D ), suggesting that TREM2 functions as a nonopsonic phagocytic receptor for B. abortus. To confirm that TREM2 contributes to phagocytosis by B. abortus, anti-TREM2 blocking antibody was used at various concentrations (1 ug/mL to 10 ug/mL) to treat M2 prior to infection with B. abortus. We found that blocking TREM2 with an anti-TREM2 antibody significantly diminished the ability of M2 to phagocytose B. abortus at 5 mg/mL or 10 mg/mL ( Figure 2E ). Together, our data suggest that TREM2 is involved in the phagocytosis of B. abortus in M2.

RAW- M2-NT, RAW-M2-ΔTREM2, RAW-M2-Vector, or RAW-M2-TREM2+ cells were infected with B. abortus for 48 h, and bacterial intracellular growth was determined by CFU assay. We observed that B. abortus survival was reduced in RAW-M2-ΔTREM2 compared to RAW- M2-NT, whereas overexpression of TREM2 increased intracellular B. abortus growth compared to RAW-M2-Vector ( Figure 3A ). To determine whether B. abortus exploits TREM2 to promote its intracellular survival, DMSO or Baf A1- treated M2, which have the same levels of TREM2 protein at 4 h post-infection ( Figure 1 ), were used to determine the role of TREM2 in mediating intracellular bacterial survival during infection. As shown in Figure 3B , phagocytosis was similar for all conditions at 4 h post-infection, but the number of B. abortus in DMSO (or mock)-treated M2 was significantly increased at 24 h or 48 h post-infection compared to Baf A1- treated M2. These data indicate that upregulation of TREM2 expression is required for the increased intracellular survival of B. abortus.

As TREM2-mediated antibacterial defense is associated with intracellular NO or ROS production (9, 12, 13), we investigated whether NO or ROS production driven by TREM2 was responsible for the B. abortus survival in M2. The intracellular NO and ROS levels of DMSO or Baf A1-treated B. abortus-infected M2 macrophages or B. abortus-infected TREM2^−/− M2 macrophages were measured. We did not find significant changes in NO production in the experimental groups ( Figure 3C ; Supplementary Figure S1A ), suggesting that NO was not involved in TREM2-mediated Brucella intracellular survival. However, M2 intracellular and mitochondrial ROS were significantly inhibited by B. abortus at 24 h and 48 h post-infection ( Figures 3D, E ; Supplementary Figures S1B, C ). Baf A1-treatment or TREM2 deficiency prevented the suppression of ROS compared to mock and DMSO treated or uninfected M2 ( Figures 3D, E ; Supplementary Figures S1B, C ). Interestingly, inhibition of cellular ROS production with the intercellular ROS inhibitor DPI ( Figure 3F ), but not mitochondrial ROS ( Figure 3G ), inhibition with Mito-TEMPO, increased the intracellular survival of B. abortus in M2. Using an antibody-mediated neutralization approach, we confirmed that neutralization of surface TREM2 in B. abortus-infected M2 cells, inhibited the intracellular survival of B. abortus ( Figure 3H ; Supplementary Figure S1D ). Taken together, our data demonstrate that B. abortus might exploit TREM2 to suppress intracellular ROS to promote its survival within M2.

As overexpression of TREM2 suppressed mitochondrial ROS generation and has been shown to protect macrophages from pyroptosis during E. coli pulmonary infection (23), we investigated whether upregulation of TREM2 expression protects B. abortus-infected M2 from pyroptosis during Brucella infection. M2 were infected with B. abortus (B.a.) for 24 h in the presence of Baf A1, Baf A1 + DPI or Baf A1 + Mito-TEMPO and supernatant was harvested and submitted to western blotting (18) ( Figure 4A ). Baf A1 or Baf A1 + DPI-treated infected M2 were able to cleave GSDMD into its active GSDMD-N subunit compared to DMSO-treated infected M2. GSDMD cleavage was inhibited in Baf A1 + Mito-TEMPO-treated M2 cells ( Figure 4A ). We also found that B. abortus triggered a higher percentage of LDH release in Baf A1-treated M2 macrophages than in DMSO-treated M2 macrophages. Inhibition of mROS generation with Mito-TEMPO, but not intracellular ROS generation with DPI, suppress LDH release in M2 ( Figure 4B ). Consistent with GSDMD cleavage and LDH release, cell survival was suppressed significantly in Baf A1or Baf A1 + DPI-treated M2 compared to DMSO or Baf A1 + Mito-TEMPO-treated M2, as examined by MTT assay ( Figure 4C ). Using M2 macrophages from TREM2^−/− mice, we confirmed that TREM2 suppressed LDH release and increased cell survival by inhibiting mitochondrial ROS production ( Supplementary Figures S2A, B ). These data suggest that B. abortus can exploit TREM2 to prevent M2 pyroptosis by inhibiting mitochondrial ROS production.

M2 were infected with B. abortus for 48 h in the presence of Baf A1, Baf A1 + DPI, DMSO or Baf A1 + Mito-TEMPO and a BrdU incorporation assay was performed to analyze M2 proliferation. Infection alone (DMSO) significantly increased M2 proliferation compared to Baf A1or Baf A1 + DPI treatment ( Figure 5A ). Interestingly, Baf A1 + Mito-TEMPO treatment also led to a significant change in M2 proliferation. The β-catenin signaling pathway is known to promote cell growth and is downstream of TREM2 (19, 24, 25). Therefore, we analyzed β-catenin expression in cell lysates from the treated cells. In line with these results, we found that infection alone (DMSO) or Baf A1 + Mito-TEMPO treatment increasedβ-catenin expression at 48 h post-infection; whereas β-catenin expression remained unchanged when B. abortus infected-M2 were treated with Baf A1or Baf A1 + DPI treatment ( Figure 5B ). TREM2 deficiency or blocking of TREM2 with an anti-TREM2 antibody significantly diminished M2 proliferation compared to WT cells or isotype antibody treatment ( Figure 5C ; Supplementary Figure S3A ). Western blot data confirmed the accumulation of β-catenin in isotype antibody-treated cells or WT cells and loss of β-catenin in anti-TREM2 antibody-treated cells or TREM2^−/− cells ( Figure 5D ; Supplementary Figure S3B ). Taken together, these results indicate that upregulation of TREM2 expression triggered by B. abortus promotes M2 proliferation by inhibiting mitochondrial ROS production, which leads to activation of the β-catenin signaling pathway.

To further validate the above results, we analyzed transcripts of the TREM2 and M2 marker Arg1 in splenic CD11b⁺ cells of C57BL/6 mice infected intraperitoneally with B. abortus for 3, 9, and 30 days. Our results showed that transcription of TREM2 was two times higher at 3 days post-infection (dpi), three times higher at 9 dpi, and eight times higher at 30 dpi, compared to uninfected conditions ( Figure 6A ). The expression level of the Arg1 gene was increased significantly at chronic infection (30 dpi) compared to acute infection (3 dpi–9 dpi) ( Figure 6B ). In line with the increase in Arg1 gene expression, B. abortus infection led to a significant increase in the number of CD11b⁺ CD206⁺ M2 cells in the spleen during chronic infection ( Figure 6C ). TREM2 expression was significantly decreased in splenic CD11b⁺ cells at 30 dpi when mice were injected intraperitoneally with Baf A1 ( Figure 6D ). Through flow cytometry analysis, we observed that Baf A1 treatment or TREM2 deficiency could lead to a significant decrease in CD11b⁺ CD206⁺ M2 numbers in the spleen compared to mock or DMSO treatment or wild-type mice ( Figure 6E ; Supplementary Figure S4 ). The immunohistochemistry data showed that CD206 positive cells were abundant in the spleen tissue of infected mice, but Baf A1 treatment significantly reduced CD206 positive cells at 30 dpi ( Figure 6F ). These results demonstrate that upregulation of TREM2 expression is important for sustaining abundant M2 macrophages during chronic brucellosis in vivo.

C57BL/6 or TREM2^−/− mice were infected with B. abortus for 30 days, and the bacterial CFU in the spleens were evaluated. Injection of B. abortus-infected mice with Baf A1 or TREM2 deficiency significantly decreased bacterial numbers in the spleen compared to DMSO or mock treatment ( Figure 7A ; Supplementary Figure S5A ). Anti-inflammatory cytokine IL-10 levels increased with infection, but significantly decreased in the serum of Baf A1 treated-mice at 30 dpi ( Figure 7B ). Treatment of B. abortus-infected mice with Baf A1or B. abortus-infected TREM2^−/− mice also led to a significant increase in TNF-α production compared to that in control infected mice ( Figure 7C ; Supplementary Figure S5B ). In line with the above results, Baf A1 treated-mice also exhibited increased induction of proinflammatory cytokine transcription, including IL-6, TNF-α, and IFN-γ, in the spleen ( Figures 7D–F ).

To further evaluate the role of TREM2 in promoting Brucella growth within the spleen, we assessed the recruitment of immune cell populations into the spleen using flow cytometry after 30 dpi. The infected mice showed an increase in the number of macrophages (CD11b⁺F4/80⁺) ( Figure 7G ), dendritic cells (CD11b⁺CD11c⁺) ( Figure 7H ), and neutrophils (CD11b⁺LY6G⁺) ( Figure 7I ). Baf A1-treatment markedly increased these populations compared with DMSO-treated mice. Myeloperoxidase (MPO) activity in spleen cell homogenates was tested to corroborate the changes in neutrophil recruitment ( Figure 7J ). Taken together, these data suggest that TREM2 promotes anti-inflammatory M2 macrophage infiltration and proliferation, suppresses the recruitment of pro-inflammatory macrophage, dendritic cell, and neutrophil recruitment and allows chronic infection with B. abortus.