Secondary respiratory bacterial infections following trauma, sometimes caused by emergent intubation, can be lethal. Wanke-Jellinek et al. (p. 767) used an outbred mouse painless burn-injury model followed by severe Streptococcus pneumoniae respiratory infection to determine if administration of the TLR9 ligand CpG-oligodeoxynucleotide (ODN) could be beneficial for early treatment of secondary infection following trauma. Ninety-two percent of mice subjected to burn-injury before S. pneumoniae infection (Burn-Infected group) succumbed to infection; treatment with CpG-ODN 2 h after burn-injury (Burn-CpG-Infected group) restored mortality to the 70% rate observed in the group that did not receive burn-injury (CTL-Infected group). Unsure of what immune changes mediated the protective effect of CpG-ODN, the authors used mass cytometry by time-of-flight (CyTOF) to evaluate many cell types and functionality markers at once. Burn-Infected mice had more macrophages and dendritic cells, and higher expression of proinflammatory cytokines such as IFNγ, than CTL-Infected mice 1 d after S. pneumoniae infection. Burn-CpG-Infected mice had fewer inflammatory changes in comparison with Burn-Infected mice, and also had increased early IL-17A expression in γδ T cells. Relative to Burn-Infected mice 3 d postinfection, Burn-CpG-Infected mice had an increase in PD-L1 on CD11c+ macrophages and dendritic cells, and CCR6 expression was increased on multiple cell types. Secreted cytokines were measured by Luminex, revealing transiently higher levels of IL-6 and IFNγ in the lungs of Burn-CpG-Infected mice relative to Burn-Infected mice. Depletion of γδ T cells prior to burn-injury in Burn-CpG-Infected mice led to less proinflammatory cytokine production, including less IL-6 and IL-17A, as well as increased bacterial burden, indicating that γδ T cells are contributing to the protective effects observed. These studies highlight how new technology like CyTOF can be used to probe intriguing biological phenomenon, such as the protection conveyed by CpG-ODN treatment in this model.

It has been observed that less developed countries have lower incidences of allergies and autoimmunity, possibly due to altered immune states associated with endemic helminth infection. In particular, multiple sclerosis (MS) patients infected with helminths have been found to have increased regulatory T cells (Treg cells) and decreased disease severity. Similarly, mice infected with the helminth Fasicola hepatica also had increased Treg cells and diminished clinical severity in experimental autoimmune encephalomyelitis (EAE). As safety concerns may impede development of live helminth treatment for MS, Finlay et al. (p. 703) investigated the effect of F. hepatica excretory-secretory products (FHES) on development of EAE. FHES treatment delayed onset and lessened severity of disease, even when administered 5 d after EAE induction. Relative to control mice with EAE, FHES-treated mice had fewer neutrophils and T cells in the brain, including IFNγ- and IL-17–producing CD4+ T cells. Somewhat surprisingly, Treg cell numbers were unaffected by FHES treatment, and depleting Treg cells with anti-CD25 Ab did not reverse the protective effect of FHES. IL-4−/− and IL-10−/− mice also benefited from FHES treatment, further suggesting that the diminished and delayed EAE symptoms were not due to CD4+ T cell skewing to Th2 or Treg phenotypes. Interestingly, increased eosinophil numbers in the lymphoid organs, blood, and brain were observed with FHES treatment. Produced following helminth infection, IL-33 induces IL-5, and both are important cytokines for eosinophil differentiation. IL-33−/− mice or mice treated with anti-IL-5 Ab did not have any FHES-induced eosinophilia or protection from EAE. Treatment of naive mice with IL-33109-266 induced serum IL-5 and eosinophils in the peritoneum, and continuous cytokine treatment starting the day before EAE induction delayed disease onset and decreased disease severity, an effect that was negated by IL-5 neutralization. These results demonstrate that FHES can protect against EAE in an IL-5– and eosinophil-dependent manner, and may pave the way for future immunomodulatory therapeutics for the treatment of MS.

It has been shown that γδ T cell homeostasis depends on IL-7 and IL-15, but the distinction between subsets such as conventional γδ T cells and innate-like IL-17–producing (γδT-17) and IFNγ-producing (γδT-IFNγ) γδ T cells was not appreciated at the time those studies were conducted. Corpuz et al. (p. 645) investigated which homeostatic cytokines support these different γδ T cell subsets. In vivo cytokine treatment of mice followed by analysis of proliferation and expression of anti-apoptotic molecules indicated that γδT-IFNγ cells depended on IL-15 and γδT-17 cells depended on IL-7, although both subsets proliferated moderately in response to the alternate cytokine. IL-15−/− mice had increased γδT-17 cells and almost a complete loss of γδT-IFNγ cells. IL-7−/− mice have a defect in γδ T cell thymic development, so to directly compare homeostatic mechanisms, adoptive transfer experiments were conducted. Homeostatic proliferation is observed following transfer of cells into lymphopenic hosts, due to the abundant availability of cytokines. Conventional γδ T cells proliferated more slowly than innate-like γδ T cells in sublethally irradiated hosts, and proliferated slightly less in IL-15−/− hosts and even less in IL-7−/− hosts. γδT-17 cells proliferated equally well in sublethally irradiated wild-type and IL-15−/− mice, but were nearly absent in IL-7−/− hosts. Although γδT-IFNγ cells were able to proliferate in response to IL-15, there was only a slight reduction of proliferation in IL-15−/− hosts, but the proliferation observed was abrogated by IL-7 blockade. These results indicate that different γδ T cell subsets have different cytokine survival requirements in the periphery.

Hypercapnia, the elevation of carbon dioxide (CO2) partial pressure in the blood or tissue, can occur in chronic lung infections and chronic obstructive pulmonary disease and is associated with increased susceptibility to bacterial infection and the inhibition of immune functions, including the expression of inflammatory cytokines and NF-κB activation. Little is known about the molecular mediators involved in immune suppression under hypercapnic conditions, prompting Helenius et al. (p. 655) to perform RNA interference (RNAi) screens to identify CO2-sensitive genes that may regulate antibacterial immune responses. The authors conducted a genome-wide RNAi screen of Drosophila S2* cells to identify genes that upregulate the antimicrobial peptide gene Dipt in response to bacterial peptidoglycan stimulation. Further analyses revealed 16 CO2-sensitive genes with human orthologs, including the transcription factor zfh2, chromatin-associated proteins, and regulators of signal transduction. Zfh2 was found to be expressed in the fat body, the major immune and metabolic organ in Drosophila. Compared with wild-type (WT) flies, those with a partial loss-of-function mutant zfh2 maintained in normal air had comparable mortality after Staphylococcus aureus infection but relatively reduced mortality when kept in an environment with elevated CO2 for 48 h prior to bacterial infection. Flies expressing a short hairpin RNA construct specifically targeted to zfh2 in the fat body also had reduced mortality and lower bacterial burden after S. aureus infection under conditions of elevated CO2. However, flies expressing fat body–specific zfh2 kept in normal air had greater mortality after S. aureus infection relative to WT flies, suggesting that inhibition of zfh2 may only be beneficial under hypercapnic conditions. Corroborative data from cultures of fat bodies subjected to zfh2 small interfering RNA knockdown in elevated CO2 conditions, but not in normal air, showed improved induction of the antimicrobial peptide gene Dipt relative to control fat bodies. Together, these results suggest that zfh2 may be a key regulator of immune responses under hypercapnic conditions.