Symptom pathogenesis during acute influenza: Interleukin‐6 and Other cytokine responses

In experimental human influenza infection initiated by nasal inoculation, the magnitude of viral replication, fever, and symptoms correlate with nasopharyngeal lavage fluid levels of various cytokines. Our aim was to assess these relationships in patients with naturally occurring acute influenza. Patients with culture‐positive influenza illness of less than 36 hr of duration were studied. Nasopharyngeal washing were collected at enrollment and on Day 2, 4, 6 and 8 for quantitative virus isolation and IL‐6, TNF‐α, INF‐α, INF‐γ and IL‐10 determinations. Blood samples collected at entry and on Day 2 and 6 were processed to assess plasma cytokines and circulating influenza RNA. Patients received either oseltamivir or placebo for 5 days. We assessed the correlation between nasopharyngeal lavage fluid or blood levels of cytokines before treatment and viral titers, symptom severity and fever. Sixteen adult subjects (median age of 22 years) were studied. In this small group of patients no significant differences between placebo and oseltamivir patients were found in viral replication or measures of cytokines. Thus the data for all 16 subjects were pooled for analysis. At entry, influenza A viruses were cultured from nasopharyngeal washes at a median titer of 4.8 log10TCID50/ml of wash. Viral titers correlated positively with symptom score (P= 0.006) and temperature values (P< 0.001). Viral titers, fever and symptoms were highest at enrollment and fell in parallel during the subsequent days. RT‐PCR assays failed to detect influenza RNA in the white blood cells from any patient. We observed a significant release, in both nasopharyngeal lavage fluid and in plasma, of IL‐6, TNF‐α, INF‐α, INF‐γ and IL‐10. At entry high IL‐6 levels were detected in the nasopharyngeal lavage fluid (median 10.3 pg/ml) and plasma (median 5.1 pg/ml) of all patients. We found a positive correlation between plasma IL‐6 levels and both symptom scores and temperature values (P< 0.05), as well as a positive correlation between nasopharyngeal lavage fluid levels of IL‐6 and TNF‐α and temperature (P< 0.05). We did not find significant associations between symptoms, fever and levels of INF‐α, INF‐γ or IL‐10. The magnitude of early decrease in viral titers correlated with initial levels of INF‐γ in nasopharyngeal lavage fluid (P< 0.05). Significant production of IL‐6, TNF‐α, INF‐α, INF‐γ and IL‐10 occurs in response to community acquired influenza A illness. As in experimental influenza, symptoms and fever in natural acute influenza correlate with the release of IL‐6. J. Med. Virol. 64:262–268, 2001. © 2001 Wiley‐Liss, Inc.