Abstract
Epigenetic modifications regulate gene expression in the host response to a diverse range of pathogens. The extent and consequences of epigenetic modification during macrophage responses to Streptococcus pneumoniae, and the role of pneumolysin, a key Streptococcus pneumoniae virulence factor, in influencing these responses, are currently unknown. To investigate this, we infected human monocyte derived macrophages (MDMs) with Streptococcus pneumoniae and addressed whether pneumolysin altered the epigenetic landscape and the associated acute macrophage transcriptional response using a combined transcriptomic and proteomic approach. Transcriptomic analysis identified 503 genes that were differentially expressed in a pneumolysin-dependent manner in these samples. Pathway analysis highlighted the involvement of transcriptional responses to core innate responses to pneumococci including modules associated with metabolic pathways activated in response to infection, oxidative stress responses and NFκB, NOD-like receptor and TNF signalling pathways. Quantitative proteomic analysis confirmed pneumolysin-regulated protein expression, early after bacterial challenge, in representative transcriptional modules associated with innate immune responses. In parallel, quantitative mass spectrometry identified global changes in the relative abundance of histone post translational modifications (PTMs) upon pneumococcal challenge. We identified an increase in the relative abundance of H3K4me1, H4K16ac and a decrease in H3K9me2 and H3K79me2 in a PLY-dependent fashion. We confirmed that pneumolysin blunted early transcriptional responses involving TNF-α and IL-6 expression. Vorinostat, a histone deacetylase inhibitor, similarly downregulated TNF-α production, reprising the pattern observed with pneumolysin. In conclusion, widespread changes in the macrophage transcriptional response are regulated by pneumolysin and are associated with global changes in histone PTMs. Modulating histone PTMs can reverse pneumolysin-associated transcriptional changes influencing innate immune responses, suggesting that epigenetic modification by pneumolysin plays a role in dampening the innate responses to pneumococci.
Highlights
Pneumolysin is one of the key virulence factors of S. pneumoniae, the leading cause of community-acquired pneumonia [1] and is present in the majority of clinical isolates causing invasive pneumococcal disease (IPD) [2, 3]
The probes coding for TNF, prostaglandin- endoperoxide synthase 2 (PTGS2) and nuclear receptor subfamily 4 group A member 3 (NR4A3) as well as the CXCL8 probe were amongst the 10 upregulated probes with the highest fold change
As we have previously identified changes in the oxidative stress response in alveolar macrophages in response to challenge with Streptococcus pneumoniae [29] we were interested in the fact that there were a number of terms relating to cellular responses to “stress” [17 in the S. pneumoniae challenge and 15 in the Dply mutant (Supplemental Table S3)] and in particular to oxidative stress responses
Summary
Pneumolysin (ply) is one of the key virulence factors of S. pneumoniae (the pneumococcus), the leading cause of community-acquired pneumonia [1] and is present in the majority of clinical isolates causing invasive pneumococcal disease (IPD) [2, 3]. Pneumolysin is a cholesterol-dependent cytolysin and part of a family of toxins expressed in Grampositive bacteria [4]. It is a 53 kDa protein that contains four domains [5]. It has been suggested that pneumolysin facilitates blood stream invasion by S. pneumoniae [3]. This highlights its importance as a key virulence factor of S. pneumoniae due to its role in the transmission of S. pneumoniae between hosts, in the progression from nasopharyngeal colonisation to invasive pneumococcal disease, the stimulation of inflammation and its cytotoxic effects [1]
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