Triggering Receptor Expressed on Myeloid Cells-2 Expression Tracks With M2-Like Macrophage Activity and Disease Severity in COPD

Abstract

Background: Cell and animal models show a key role for Triggering Receptor Expressed on Myeloid Cells (TREM)-2 in chronic airway disease after viral infection, but comparable evidence in humans still needs to be established.

Methods: Lung tissue samples were obtained from lung transplant recipients with Global Initiative for Chronic Obstructive Lung Disease (GOLD) stage IV COPD (n = 16), nontransplantable donor lung tissues (n = 7), and resected lung tissues from patients at risk or with GOLD stage I through IV (n = 55) and were assessed for TREM-2 and TREM-1 messenger RNA (mRNA), protein expression, and other markers of a type 2 immune response.

Results: TREM2 (but not TREM1) mRNA levels were increased in GOLD stage IV COPD lung tissues compared with non-COPD lung tissues. TREM2 mRNA was coexpressed with its signaling molecule DAP12 and the macrophage marker CD68 and M2-macrophage markers CD206 and CHIT1. TREM-2 protein was also increased in COPD lung tissues and was localized to CD14⁺ macrophages by flow cytometry and CD68⁺ and CCR2⁺ macrophages by tissue immunostaining. In lung samples from patients at risk and with GOLD stage I through IV COPD, TREM2 but not TREM1 mRNA levels were also increased, and the ratio of TREM2/TREM1 mRNA levels was associated with increases in CHIT1 mRNA and decreases in FEV₁ and FEV₁/FVC.

Conclusions: TREM-2 expression is increased in lung macrophages in COPD, particularly in comparison with TREM-1. Therefore, TREM-2 levels and the ratio of TREM-2/TREM-1 signifies M2 activation in COPD lung tissues and may help to guide therapeutics directed against the type 2 immune response in patients with this disease.

Keywords: COPD; Triggering Receptor Expressed on Myeloid Cells-2; macrophages; translating basic research; type 2 inflammation.

Figure 1

A-C, TREM2 messenger RNA (mRNA) expression is increased in COPD in cohort 1. A, Levels of TREM1 and TREM2 mRNA in lung tissues from patients with Global Initiative for Chronic Obstructive Lung Disease stage IV COPD and non-COPD. B, Corresponding analysis of TREM2 vs DAP12, CD68, and CD206 mRNA levels in lung tissues from patients with COPD and non-COPD. C, TREM1 and TREM2 mRNA expression normalized to GAPDH mRNA level in lung tissues from patients with COPD and non-COPD. Each dot represents the value for a single subject. Boxes demonstrate the median (horizontal line) and 25th to 75th percentiles, and whiskers represent the range. r values are from Spearman correlation, and P values are from Mann-Whitney U test. CD68 expression was not significantly different between control and COPD tissues. NS = not significant.

Figure 2

A-C, TREM-2 protein expression is increased in COPD in cohort 1. A, Levels of TREM-1 and TREM-2 protein in lung tissues from patients with COPD and non-COPD measured by enzyme-linked immunosorbent assay. Each dot represents the value for a single subject. Boxes demonstrate the median (horizontal line) and 25th to 75th percentiles, and whiskers represent the range. P values are from Mann-Whitney U test. B, Immunostaining for TREM-2 (red) and TREM-1, CD68, and CCR2 (green) in lung tissues patients with Global Initiative for Chronic Obstructive Lung Disease stage IV COPD. Bars = 50 μm. C, Flow cytometry analysis of TREM-2 on CD14+ myeloid cells in lung tissues from patients with COPD and non-COPD. Gating strategy is noted in the first panel. Results from CD14-positive lung cells labeled with anti-TREM2 (gray) and isotype control cells (red) are representative of five patients with COPD and four patients with non-COPD examined overall, respectively. TREM = Triggering Receptor Expressed on Myeloid Cells. See Figure 1 legend for expansion of other abbreviation.

Figure 3

A-F, TREM2 messenger (mRNA) expression is increased in COPD in cohort 2. A, Levels of TREM1 and TREM2 mRNA (normalized to GAPDH) in lung tissues from patients with Global Initiative for Chronic Obstructive Lung Disease (GOLD) stage IV COPD and never smokers. B, Levels of TREM1 and TREM2 mRNA normalized to CD68 mRNA level in lung tissues from patients with GOLD stage IV COPD and never smokers. D, Levels of TREM2/TREM1 mRNA ratio in patients with GOLD stage IV COPD and never smokers. E, Analysis of TREM2/TREM1 mRNA ratio in lung tissue from all subjects in cohort 2 using the receiver operating characteristic curve to compare values in patients with no COPD or GOLD stage I or II COPD vs GOLD stage III or IV COPD. F, Analysis of TREM2/TREM1 mRNA ratio vs FEV₁ percent predicted or FEV₁/FVC in all subjects from cohort 2. In (A-F), each dot represents the value for a single subject. Boxes demonstrate the median (horizontal line) and 25th to 75th percentiles, and whiskers represent the range. r values are from Spearman correlation, and P values are from Mann-Whitney U test. CD68 expression was not significantly different between never smokers and GOLD stage IV COPD in (B), and did not correlate with FEV₁ percent predicted or FEV₁/FVC in (F). AUC = area under the curve; NS = not significant.