TREM1: A pivotal myeloid receptor target for treating autoimmune disorders and cancer.

• Gene Information: TREM1 is a transmembrane glycoprotein belonging to the immunoglobulin superfamily. It functions as a critical activating receptor on myeloid cells, essential for amplifying inflammatory cascades and modulating immune responses across various pathological conditions.
• Protein Expression: TREM1 is predominantly expressed on the surface of myeloid cells, including circulating neutrophils, inflammatory monocytes, and tumor-associated macrophages.
• Signaling Pathway: Upon ligand engagement, TREM1 associates with the ITAM-containing adapter DAP12 to recruit SYK kinase. This initiates downstream phosphorylation cascades driving NF-κB activation, which triggers massive pro-inflammatory cytokine release in autoimmunity while amplifying myeloid-driven suppressive networks in cancer.
• Therapeutic Targeting: TREM1 is a key target for therapeutic antagonists. In autoimmune diseases, blocking TREM1 attenuates pathological hyperinflammation and tissue damage. In oncology, neutralizing TREM1 reprograms tumor-associated macrophages to relieve immunosuppression, reinvigorating robust anti-tumor responses and enhancing immunotherapy efficacy.

TREM1

• The exons 1- 4 of mouse Trem1 gene that encode signal peptide and extracellular domain are replaced by human counterparts in B-hTREM1 mice.
• The genomic region of mouse Trem1 gene that encodes transmembrane domain and cytoplasmic portion is retained. The promoter, 5’UTR and 3’UTR region of the mouse gene are also retained. The chimeric TREM1 expression is driven by endogenous mouse Trem1 promoter, while mouse Trem1 gene transcription and translation will be disrupted.

• The mRNA expression of hTREM1 in B-hTREM1 mice(H/H) was similar to mTrem1 in C57BL/6 mice(+/+), demonstrating that introduction of hTREM1 in place of its mouse counterpart does not change the expression level of TREM1 mRNA.

Strain specific analysis of TREM1 gene expression in wild type C57BL/6 mice and homozygous  B-hTREM1 mice by RT-qPCR. Peripheral blood cell RNA was isolated from wild-type C57BL/6 mice (+/+) and homozygous B-hTREM1 mice(H/H), and then cDNA libraries were synthesized by reverse transcription, followed by qPCR with TREM1 primers.

• Mouse TREM1 was exclusively detectable in blood and tumor macrophage of wild-type mice.
• Human TREM1 was exclusively detectable in blood and tumor macrophage of homozygous B-hTREM1 mice.

Strain specific TREM1 expression analysis in homozygous B-hTREM1 mice by flow cytometry. MC38 cells were inoculated into wild-type C57BL/6(+/+) and homozygous B-hTREM1 mice(H/H). Blood and tumor were harvested when tumor volume reached approximately 500 mm³, and analyzed by flow cytometry with species-specific anti-TREM1 antibody.

* It is not clear what population of cells was stained with specific anti-hTREM1 in tumors of C57BL/6.

• Human TREM1 was exclusively detectable in tumor M-MDSC of homozygous B-hTREM1 mice.
• Mouse and human TREM1 were not detectable in blood monocyte of wild-type mice and homozygous B-hTREM1 mice.

Strain specific TREM1 expression analysis in homozygous B-hTREM1 mice by flow cytometry. MC38 cells were inoculated into wild-type C57BL/6(+/+) and homozygous B-hTREM1 mice(H/H). Blood and tumor were harvested when tumor volume reached approximately 500 mm³, and analyzed by flow cytometry with species-specific anti-TREM1 antibody.

* It is not clear what population of cells was stained with specific anti-hTREM1 in tumors of C57BL/6.

• Mouse TREM1 was exclusively detectable in blood and tumor neutrophil(G-MDSC) of wild-type mice.
• Human TREM1 was exclusively detectable in blood and tumor neutrophil(G-MDSC)  of homozygous B-hTREM1 mice.

Strain specific TREM1 expression analysis in homozygous B-hTREM1 mice by flow cytometry. MC38 cells were inoculated into wild-type C57BL/6(+/+) and homozygous B-hTREM1 mice(H/H). Blood and tumor were harvested when tumor volume reached approximately 500 mm3, and analyzed by flow cytometry with species-specific anti-TREM1 antibody.

* It is not clear what population of cells was stained with specific anti-hTREM1 in tumors of C57BL/6.

• Mouse and human TREM1 were not detectable in blood and tumor T cells of wild-type mice and homozygous B-hTREM1 mice. 

Strain specific TREM1 expression analysis in homozygous B-hTREM1 mice by flow cytometry. MC38 cells were inoculated into wild-type C57BL/6(+/+) and homozygous B-hTREM1 mice(H/H). Blood and tumor were harvested when tumor volume reached approximately 500 mm³, and analyzed by flow cytometry with species-specific anti-TREM1 antibody.

• Mouse and human TREM1 were not detectable in blood and tumor B cells of wild-type mice and homozygous B-hTREM1 mice.

Strain specific TREM1 expression analysis in homozygous B-hTREM1 mice by flow cytometry. MC38 cells were inoculated into wild-type C57BL/6(+/+) and homozygous B-hTREM1 mice(H/H). Blood and tumor were harvested when tumor volume reached approximately 500 mm³, and analyzed by flow cytometry with species-specific anti-TREM1 antibody.

• Under LPS-stimulated conditions, human TREM1 could be activated effectively and then induced rapid phosphorylation of Syk in B-hTREM1 mice.
• Human TREM1 can be efficiently activated and cleaved in B-hTREM1 mice.

Analysis of the hTREM1 function in B-hTREM1 mice. Wild-type C57BL/6 mice(+/+) and B-hTREM1 mice(H/H) were randomly grouped and injected intraperitoneally (i.p.) with LPS from E.coli(500 ug/mouse). Serum and peritoneal macrophages were collected after LPS administration. (A) The phosphorylation of Syk. (B) the release of soluble TREM1. ND, not detected.

• Human TREM1 can be activated by its ligand PGLYRP1 in B-hTREM1 mice and that cross-reactivity exists between human and mouse ligand-receptor interactions. 

Bone marrow-derived macrophages from B-hTREM1 mice can be activated by its ligand PGLYRP1. Bone marrow-derived macrophages induced with M-CSF from wild-type C57BL/6 and B-hTREM1 mice were stimulated with m/hPGLYRP1 under the indicated conditions for 24 hours, and the levels of mouse TNF-α in the supernatant were measured. The results show that PGN alone, co-stimulation with m/hPGLYRP1 and PGN, and pre-coated m/hPGLYRP1 all significantly activated macrophages. Values are expressed as mean ± SEM. Significance was determined by two-way ANOVA test.

• The percentages of T cells, B cells, NK cells, DCs, granulocytes, monocytes, and macrophages in homozygous B-hTREM1 mice are similar to those in C57BL/6 mice.
• Humanization of TREM1 does not affect normal immune cell development or distribution.

Analysis of leukocyte subpopulations by flow cytometry in immune organs and blood. Splenocytes, peripheral blood, and lymph nodes were isolated from female C57BL/6 and B-hTREM1 mice (female, 6-week-old, n = 3). Single live cells were gated on the CD45⁺ population and analyzed by flow cytometry as indicated. Values are expressed as mean ± SEM.

• The proportions of CD4⁺ T cells, TREM1⁺ T cells, and Tregs in homozygous B-hTREM1 mice are comparable to those in C57BL/6 mice.
• Humanization of TREM1 does not affect normal T cell development, differentiation, or distribution.

Analysis of T-cell subpopulations by flow cytometry in immune organs and blood. Splenocytes, peripheral blood, and lymph nodes were isolated from female C57BL/6 and B-hTREM1 mice (female, 6-week-old, n=3). Single live cells were gated on the T-cell population and analyzed by flow cytometry as indicated. Values are expressed as mean ± SEM.

Growth curve of B-hTREM1 mice. Three-week-old mice were grouped by sex (15 males and 15 females). Body weight was measured weekly for 12 weeks on the same day each week. The minimum and maximum body weights shown in the table were calculated from the mean ± SD.

• No significant differences were observed compared with wild-type mice.

Complete blood count (CBC) of B-hTREM1 mice. Values are expressed as mean ± SD.

• No significant differences were observed compared with wild-type mice.

Complete blood count (CBC) of B-hTREM1 mice. Values are expressed as mean ± SD.

• No significant differences were observed compared with wild-type mice.

Complete blood count (CBC) of B-hTREM1 mice. Values are expressed as mean ± SD.

• No significant differences were observed compared with wild-type mice.

Blood biochemical parameters of B-hTREM1 mice are shown. Values are expressed as mean ± SD.

• No significant differences were observed compared with wild-type mice.

Blood biochemical parameters of B-hTREM1 mice are shown. Values are expressed as mean ± SD.

• No significant differences were observed compared with wild-type mice.

Blood biochemical parameters of B-hTREM1 mice are shown. Values are expressed as mean ± SD.

• No abnormalities were observed.

Organs of B-hTREM1 mice (7-week-old, n = 5).

• No abnormalities were observed.

Average weights of major organs in B-hTREM1 mice.

No abnormalities were observed.

Average weights of major organs in B-hTREM1 mice.

• No obvious abnormalities were observed in any organs examined (heart, liver, spleen, lung, kidney, brain, stomach, small intestine, large intestine, testis, uterus , ovary).

Histopathological analysis of organs in B-hTREM1 mice. Major organs from B-hTREM1 mice were collected at 15 weeks of age and analyzed by H&E staining (male, n = 5; female, n = 5).

Experimental schedule for antitumor activity of anti-mouse PD-1 antibody combined with anti-human TREM1 antibody in B-hTREM1 mice. 5×10⁵ MC38 cells were subcutaneously implanted into B-hTREM1 mice (female, 9–week-old, n=5). Mice were grouped when tumor volume reached approximately 100 mm³, at which time they were injected intraperitoneally with anti-mouse PD-1 antibodies.

• Combination of anti-hTREM1 and anti-mPD-1 antibody shows more inhibitory effects than the individual treatments, demonstrating that B-hTREM1 mice provide a powerful preclinical model for the in vivo evaluation of anti-human TREM1 antibodies.

Antitumor activity of anti-mouse PD-1 antibody combined with anti-human TREM1 antibody in B-hTREM1 mice. (A) Anti-mouse PD-1 antibody combined with anti-human TREM1 antibody inhibited the growth of MC38 tumors in B-hTREM1 mice. (B) Body weight changes during treatment. Values are expressed as mean ± SEM.