In insects it has been observed that the primary immune cells in the hemocoel, called hemocytes, exist both in circulation as well as attached to tissues (sessile).  While much work has been done to characterize the biology of circulating hemocytes, the distribution and activity of sessile hemocytes within the insect body cavity has not been quantified, and virtually nothing is known about the biology of these cells in mosquitoes.  In a paper published today in BMC Biology we:

1. 1.Create a physical map describing the distribution of sessile and circulating hemocytes throughout the mosquito body cavity.
   

2. 2.Show that aging results in a decrease in hemocyte numbers.
   

3. 3.Show that infection results in an increase in hemocyte numbers.
   

4. 4.Show that the infection-induced increase in hemocyte numbers is due to mitosis by circulating hemocytes.
   

Article Citation:

King, J.G., and J.F. Hillyer. 2013. Spatial and temporal in vivo analysis of circulating and sessile immune cells in mosquitoes: hemocyte mitosis following infection. BMC Biology. 11:55.  (Pubmed) (Download Free from BMC Biology)

Article Abstract:

- Background

Mosquitoes respond to infection by mounting immune responses. The primary regulators of these immune responses are cells called hemocytes, which kill pathogens via phagocytosis and via the production of soluble antimicrobial factors. Mosquito hemocytes are circulated throughout the hemocoel (body cavity) by the swift flow of hemolymph (blood), and data show that some hemocytes also exist as sessile cells that are attached to tissues. The purpose of this study was to create a quantitative physical map of hemocyte distribution in the mosquito, Anopheles gambiae, and to describe the cellular immune response in an organismal context.

- Results

Using correlative imaging methods we found that the number of hemocytes in a mosquito decreases with age, but that regardless of age, approximately 75% of the hemocytes occur in circulation and 25% occur as sessile cells. Infection induces an increase in the number of hemocytes, and tubulin and nuclear staining showed that this increase is primarily due to mitosis, and more specifically autonomous cell division, by circulating granulocytes. The majority of sessile hemocytes are present on the abdominal wall, although significant numbers of hemocytes are also present in the thorax, head, and several of the appendages. Within the abdominal wall, the areas of highest hemocyte density are the periostial regions (regions surrounding the valves of the heart, or ostia), which are ideal locations for pathogen capture as these are areas of high hemolymph flow.

- Conclusions

These data describe the spatial and temporal distribution of mosquito hemocytes, and map the cellular response to infection throughout the hemocoel.