In a recent study published in the journal Nature, researchers explored the factors influencing cytokine release, a critical component of the host immunological response.
The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic emphasized the wide variation in immunological responses between populations, with age, sex, and genetic variables all playing vital roles. However, therapy and vaccine development often disregard immunological diversity. The Milieu Intérieur research project has contributed to understanding immune homeostasis by quantitatively evaluating the impacts of age, gender, cellular composition, and genetics on immune-related gene transcript levels and those of age, gender, smoking, and cytomegalovirus (CMV) infections on leukocyte distribution in blood. Further study might help us better understand the elements that influence immune responses and how they affect clinical outcomes.
Study: Smoking changes adaptive immunity with persistent effects. Image Credit: NeydtStock / Shutterstock
About the study
In the present study, researchers investigated environmental variables associated with cytokine responsiveness to immunological activation.
The team measured the levels of several cytokines [C‐X‐C motif chemokine ligand 5 (CXCL5), colony-stimulating factor 2 (CSF2), interferon-gamma (IFNγ), interleukin-1 beta (IL-1β), IL-2, 6, 8, 10, 12p70, 13, 17, 23, and tumor necrosis factor (TNF)] after 22 hours of whole-blood stimulations with 11 immunological agonists for 1,000 Milieu Intérieur project donors and in an unstimulated (control) condition. They categorized the stimulations as microbial, viral, T-lymphocyte activated, and cytokines.
Heat maps and principal component analyses (PCA) of 13 cytokine molecules investigated in 12 immunological stimulations revealed the individual cytokines generated by every independent condition. The team performed hierarchical clustering evaluations of log mean variations in cytokine levels to identify groups corresponding to stimulation types.
The researchers compiled 136 environmental, socio-demographic, nutritional, and clinical variables from the digital case report forms and tested for their relationships with cytokines induced in every stimulation using likelihood ratio tests (LRTs) with age, experimental batch, and gender as covariates. They also investigated human leukocyte antigen (HLA) as a predictor of immune response variability, particularly in antigen-specific responses. The team investigated whether smoking-cytokine correlations continued when particular subsets of circulating immune cells were included in their models, as these cells are related to cytokine elevations. They evaluated the biological impact of smoking on cytokine production, calculating the effect sizes for the smoking variables in the linear models and assessing the influence of 326 soluble proteins in sera obtained from 400 donors.
The researchers investigated whether epigenetic pathways contribute to the impact of smoking on adaptive immune responses. They analyzed deoxyribonucleic acid (DNA) methylation at more than 850,000 CpG sites and investigated whether the levels may explain the association between smoking and cytokine levels following SEB stimulation. The study was especially well-suited to identifying response protein quantitative trait loci (pQTLs) since it tested 5,699,237 high-quality imputed single nucleotide polymorphisms (SNPs) for relationships with the cytokines elicited by each stimulation.
Results
The team identified smoking, CMV latent infection, and body mass index (BMI) as the most significant drivers of cytokine response variability. Smoking impacts innate and adaptive immune responses, with the influence on innate responses diminishing after quitting and associated with serum carcinoembryonic antigen-related cell adhesion molecule 6 (CEACAM6) levels. However, the impact on adaptive responses lasts long after smoking cessation and is associated with epigenetic memory.
The study highlighted eleven factors related to one or more cytokines in the immune stimulations, with BMI being the most prevalent. Smoking-related factors were related to interleukin-2 and interleukin-13 (adaptive immunity) in Staphylococcus aureus enterotoxin B superantigen (SEB), anti-cluster of differentiation 3 (anti-CD3) and anti-CD28 immune stimulations, and CXCL5 following Escherichia coli infections or innate immunological stimulations. The findings indicate that smoking causes inflammation and reduces immunity against bacterial infections.
Cytomegalovirus latent infection was associated with TNF, CSF2, and IFNγ cytokines secreted by adaptive immune cells. BMI-related factors were related to CXCL5 following Bacillus Calmette-Guérin (BCG) immune stimulation, and interleukin-2 following SEB stimulation demonstrated obesity dysregulation. The team found no significant association between major histocompatibility complex (MH) class II, DQ beta 1, and HLA.DBQ1.1P, and IL-6 in the control condition.
The study found 2,416 CpG locations related to smoking in the Milieu Intérieur sample, with 129 significantly associated with IL-2 in SEB stimulation. However, 11 CpGs abolished the relationship between smoking and IL-2 and IL-13. Current smokers had lower DNA methylation than non-smokers, but former smokers had an intermediate methylation level. The number of years smoked, total cigarettes smoked, and IL-2 levels in SEB stimulation were adversely linked with DNA methylation, although the number of years after smoking typically correlated positively.
Overall, the study findings identified three novel factors, i.e., smoking status, CMV latent infection, and BMI, associated with variability in cytokine secretion following immunological stimulation. These characteristics may have clinical consequences for the risk of contracting infections, cancer, or autoimmune diseases. Smokers have a heightened inflammatory response after bacterial activation, which promptly decreases after quitting. However, the impacts on adaptive immunity last for years after stopping. The link between smoking and long-lived B and T cell subsets and DNA methylation offers a potential for long-term consequences in the adaptive response.
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