When mosquitoes take a blood meal, they spill saliva into their host's skin, causing an immune response even without pathogens. The saliva alone causes an increase in cytokine levels in the blood due to the contained proteins and at the same time alters the size of the immune cell subpopulations. Such changes have been observed in studies with mouse models, but a new study goes a bit further. It describes the effects of mosquito bites on human immune cells in mice transplanted with human hematopoietic stem cells.
The new study by scientists at Baylor College of Medicine shows that the number of immune cell types affected is much greater than previously described, and that some immune responses to mosquito bites can be detected up to seven days after biting.
Although these findings are of uncertain biological significance, they could complement the findings of previous studies that mosquito saliva increases the pathogenicity of dengue, West, Nile, and other arbovirus infections. Also, one of the new findings that immune effects triggered by mosquito bites persist for so long may be relevant to the study of allergic reactions.
Details of the new study appeared on May 1
"We have demonstrated both human Th1 and Th2 immune responses as well as delayed effects on cytokine levels in the blood, and immune cell compositions in the skin and bone marrow, up to 7 days after the sting," wrote the authors of the article. "These are the first measurements of this kind, with human immune responses in whole animals bitten by live mosquitoes, compared to previous studies with incomplete mouse models and salivary gland extracts or needle-injected saliva."
At various times during the immune response there were also increases in natural killer T cell levels, natural killer cells, CD8 + T cells, mononocytes, and macrophages. Overall, detection of the immune response took up to 7 days after bite and was observed in various tissue types including blood, skin, and bone marrow.
"These results have major implications for the study of hematophagous insect saliva, its effects on the human immune system, with or without transmission of pathogens, and the ability to determine which of these proteins is eligible for vaccination to block the transmission of many tropical diseases, "conclude the authors of the study. "The long-lasting effects in humanized bone marrow and skin cells may explain how some of the mosquito-borne viruses may still be viable in these tissues or how they might serve as replication reservoirs."