IL-32B is the predominant isoform expressed under inflammatory conditions in vitro and in COPD

Abstract

Background: The IL-32 cytokine family is associated with chronic inflammatory diseases such as Chronic Obstructive Pulmonary Disease. However, this unique family is comprised of several alternative splicing isoforms and their expression pattern or responses to inflammatory stimuli are unclear. We aimed to determine which IL-32 isoforms are expressed in various cell types and in chronic airway disease, as well as their response to inflammatory stimuli. Methods: We used several cell lines, human airway and lung primary cells from normal or chronic airway diseases to determine the basal expression of IL-32 isoform members and their response to inflammatory cytokine stimulations in cultured condition. We used real-time PCR to determine the gene expression of different IL-32 isoforms. We further applied hierarchical cluster analysis of the members of IL-32 based on their expression and response to inflammatory stimuli in different cell types. Finally, we analyzed IL-32 expression from published microarray data of different pulmonary diseases. Results: By real-time PCR analysis, we found that IL-32B isoform expression is significantly higher in Chronic Obstructive Pulmonary Disease, but not asthma, compared to normal lungs. Analysis of basal expression of IL-32 isoforms revealed that cell lines from various lineages including NK cells, monocytes, epithelia, and human lung tissues have varying levels of IL-32 isoform expression. We also found that most cell types examined expressed IL-32B as the predominant isoform while peripheral blood mononuclear cells expressed IL-32A as the major isoform. Hierarchical clustering of IL-32 isoform based on responsiveness to inflammatory cytokines revealed that IL-32A, C, and D are regulated similarly in most of the cell lines when stimulated by TNF-α, IL-1β, and IL-6. By contrast, IL-32B and G did not share induction patterns with the other isoforms. Conclusions: Our findings indicate that the specific isoform, IL-32B, but not pan-IL-32 isoforms, is elevated in Chronic Obstructive Pulmonary Disease, and that despite low basal expression, IL-32B is highly inducible by inflammatory cytokines in airway epithelial cells. The data also support that IL-32B is the predominant isoform and may be most relevant to lung inflammation.