Enhancing production of herpes simplex virus type 1 in oral epithelial cells by co-infection with Aggregatibacter actinomycetemcomitans

The association between herpetic/bacterial co-infection and periodontal diseases has been reported. However, how interactions between herpesviruses and periodontal bacteria dampen periodontal inflammation is still unclear. This study determined effects of co-infection with oral bacteria, including Streptococcus sanguinis, Fusobacterium nucleatum or Aggregatibacter actinomycetemcomitans, in herpes simplex virus type 1 (HSV-1)-infected oral epithelial cells. Cell viability was determined by detection the activity of mitochondrial dehydrogenase. Viral production was measured using the plaque assay. Levels of bacterial and viral DNA were determined by real-time polymerase chain reaction. Secretion of interleukin (IL)-6 and IL-8 was measured using the enzyme-linked immunosorbent assay. Viability was not further reduced by bacterial co-infection in HSV-1-infected cells. Co-infection with HSV-1 and S. sanguinis or F. nucleatum reduced the viral yield whereas co-infection with HSV-1 and A. actinomycetemcomitans significantly enhanced the viral yield in oral epithelial cells. The enhancing effect of A. actinomycetemcomitans was not affected by bacterial heat-inactivation. Co-infection with HSV-1/A. actinomycetemcomitans increased intracellular levels of both viral and bacterial DNA. Secretion of IL-6 and IL-8 stimulated by A. actinomycetemcomitans infection was partly reduced by co-infection with HSV-1 in oral epithelial cells. In contrast to S. sanguinis and F. nucleatum, A. actinomycetemcomitans enhanced the yield of HSV-1. Either HSV-1 or A. actinomycetemcomitans may be benefited from co-infection, in aspects of increases in production of viral and bacterial DNA as well as reductions in cytokine secretion. These findings echoed with previous clinical studies showing co-infection of HSV and A. actinomycetemcomitans in patients with aggressive periodontitis.