Differences in systemic adaptive immunity contribute to the 'frequent exacerbator' COPD phenotype

Abstract

Background: Some COPD patients are more susceptible to exacerbations than others. Mechanisms underlying these differences in susceptibility are not well understood. We hypothesized that altered cell mediated immune responses may underlie a propensity to suffer from frequent exacerbations in COPD.

Methods: Peripheral blood mononuclear cells (PBMCs) were obtained from 24 stable COPD patients, eight frequent exacerbators (≥3 diary-card exacerbations/year) and 16 infrequent exacerbators (< 3 diary-card exacerbations/year). Detailed multi-parameter flow cytometry was used to study differences in innate and adaptive systemic immune function between frequent and infrequently exacerbating COPD patients.

Results: The 24 COPD patients had a mean (SD) age of 76.3 (9.4) years and FEV₁ 1.43 (0.60)L, 53.3 (18.3)% predicted. PBMCs of frequent exacerbators (FE) contained lower frequencies of CD4+ T central memory cells (CD4+ Tcm) compared to infrequent exacerbators (IE) (FE = 18.7 %; IE = 23.9 %; p = 0.035). This observation was also apparent in absolute numbers of CD4+ Tcm cells (FE = 0.17 × 10^6/mL; IE = 0.25 × 10^6/mL; p = 0.035). PBMCs of FE contained a lower frequency of CD8+ T effector memory cells expressing HLA-DR (Human Leukocyte Antigen - D Related) compared to IE COPD patients (FE = 22.7 %; IE = 31.5 %; p = 0.007).

Conclusion: Differences in the adaptive systemic immune system might associate with exacerbation susceptibility in the 'frequent exacerbator' COPD phenotype. These differences include fewer CD4+ T central memory cells and CD8+ T effector memory cells.

Trial registration: Not applicable.

Keywords: Adaptive immunity; COPD; COPD exacerbation; Flow cytometry; Respiratory immunology.

Fig. 1

Gating strategy for adaptive immune cells. Lymphocytes were gated based on their characteristic scatter patterns. Lymphocytes were then classified based on CD3 expression to identify T cells, which were divided into CD4⁺ and CD8⁺ T cells, prior to separation into their four main subsets of naive, central memory (Tcm), effector memory (Tem), and end-stage T cells (Tend); this separation was based on the expression of CD45R0 and CD62L. Tcm cells: CD45R0+/CD62L+, Tem cells: CD45R0+/CD62-, Naive T cells: CD45R0-/CD62L+, and Tend cells: CD45R0-/CD62L-. The CD4+ and CD8+ T cells were subsequently analysed (including each of the main subsets) for expression of the markers CD127, HLA-DR, CD25 and PD-1. Regulatory T-cells were identified based on the high expression of CD25 and low expression of CD127 in CD4+ T cells

Fig. 2

Representative plots of CD4+ T central memory cells in patients with frequent exacerbations and in patients who exacerbate infrequently. CD4+ Tcm cells were identified as CD45R0+ and CD62L+ (top-right gate). The left plots present the memory profile belonging to a COPD patient susceptible to frequent exacerbations and the right plots are from patients who exacerbate infrequently. Tem = CD4+ T effector memory (bottom-right gate)

Fig. 3

Susceptibility to Exacerbation in COPD is associated with reduced CD4+ Tcm and CD8+ Tem HLA-DR+. Graphs show the frequency (left) and absolute number (right) of CD4+ Tcm cells in frequent exacerbators (FE) and infrequent exacerbators (IE). Horizontal lines indicate the mean. Each dot represents an individual patient

Fig. 4

Principal component analysis. Graph presenting the results of the principal component analysis; this analysis was unable to concretely stratify patients based on frequency of exacerbations, which suggests high heterogeneity. a Graph presenting the percentage of variance explained, bars represent each separate principal component (PC), the line represents the cumulative percentage of these PCs. b-d The bar graphs present the principle component analysis parameter loadings for the first three PCs. The bars illustrate the weighting coefficients, which demonstrates the contribution each component (in this case: cell types) has in relation to their respective PC