Characterization of Mucosa-Associated Invariant T (MAIT) Cell Dysregulation in the Lung Parenchyma of Smokers and COPD Patients

Abstract

Background

Mucosa-Associated Invariant T (MAIT) cells are innate-like, MR1-restricted T cells that are highly enriched in mucosal surfaces, where they play a pivotal role in maintaining antimicrobial defense and tissue homeostasis. While peripheral blood MAIT cell depletion is documented in chronic obstructive pulmonary disease (COPD), the specific phenotypic alterations, spatial distribution, and functional dysregulation of MAIT cells directly within human lung parenchyma—and how cigarette smoke drives these changes—remain poorly characterized.

Methods

Lung parenchymal tissue samples were obtained from patients undergoing lung volume reduction surgery or tumor resections with healthy margins. Cohorts included: patients with established COPD (GOLD Stages II–III, $n=15$), healthy current/former smokers without airflow limitation ($n=12$), and never-smokers ($n=10$). Parenchymal single-cell suspensions were analyzed via multicolor flow cytometry to quantify MAIT cell frequency ($\text{CD3}^+\text{CD161}^{\text{high}}\text{V}\alpha7.2^+$) and surface marker expression. Immunohistochemistry mapped spatial tissue distribution relative to alveolar structures. Isolated parenchymal MAIT cells were functionally challenged ex vivo with Haemophilus influenzae or cytokines ($\text{IL-12} + \text{IL-18}$) to measure intracellular interferon-gamma (IFN-$\gamma$), tumor necrosis factor-alpha (TNF-$\alpha$), and granzyme B (GZMB) expression.

Results

Flow cytometric analysis revealed a progressive, statistically significant decline in the frequency of parenchymal MAIT cells from never-smokers to smokers, reaching the lowest levels in COPD tissue ($p < 0.01$). Immunohistochemistry confirmed a marked loss of alveolar-resident MAIT cells, coupled with an architectural shift toward aggregate accumulation in the subepithelial space.

Phenotypically, remaining parenchymal MAIT cells in smokers and COPD patients displayed an exhausted profile, marked by significant upregulation of chronic activation and exhaustion check-points, including PD-1, TIM-3, and CTLA-4 ($p < 0.005$). Upon ex vivo bacterial or cytokine stimulation, parenchymal MAIT cells from COPD lungs exhibited severe functional impairment ("exhaustion"), producing significantly lower levels of defensive IFN-$\gamma$ and TNF-$\alpha$ compared to controls ($p < 0.001$). Strikingly, however, their baseline secretion of tissue-degrading granzyme B was paradoxically elevated, a feature that directly correlated with clinical metrics of lung hyperinflation and worsening emphysema severity (decreased $\text{DL}_{\text{CO}}$).

Conclusions

Cigarette smoke and COPD pathology significantly deplete and phenotypically exhaust alveolar-resident MAIT cells while skewing their functional behavior from antimicrobial protection toward pathogenic, granzyme B-driven tissue destruction. Restoring the homeostatic and antimicrobial properties of these mucosal T cells represents a novel therapeutic horizon for mitigating progressive parenchymal damage and reducing infection risk in COPD.

Keywords

Mucosa-Associated Invariant T (MAIT) Cells; Chronic Obstructive Pulmonary Disease (COPD); Lung Parenchyma; Tobacco Smoke; Exhaustion Markers; Mucosal Immunity.