BASIC STUDIES OF THE IMMUNOBIOLOGY OF SNAIL-DIGENEAN INTERACTIONS


Rediae of E. paraensei

 

The long-term goals of this NIH funded study are to gain a deeper appreciation of the underlying mechanisms governing the ability of digenetic trematodes to establish successful infection in the molluscan host, and to reveal the tactics used by snails in resisting infection. This will help us gain a comprehensive understanding of basic trematode biology and provide insights into novel methods of future control of medically relevant species like the schistosomes.

The basic model system used is the freshwater snail Biomphalaria glabrata and two compatible digeneans, Schistosoma mansoni and Echinostoma paraensei. Use of the echinostome has been invaluable in revealing immune capabilities of snails that might have gone undetected otherwise.



 


The following aims are being pursued:


1. Diversity and Expression of B. glabrata Fibrinogen-Related Proteins (FREPs):

 
 
 
Click image for enlargement and a short explanation
  
 
 

We found B. glabrata snails to possess a family of blood proteins capable of precipitating trematode antigens and binding to sporocysts. These proteins, termed fibrinogen-related proteins or FREPs, are upregulated following infection with E. paraensei. Using strains of B. glabrata known to be susceptible or resistant to infection with S. mansoni, the patterns of expression of the known members of the FREP family will be analyzed at different times post-exposure to both S. mansoni and E. paraensei. We will also examine the sequences of FREPs produced by a schistosome-resistant strain to see if they are different from those produced by the susceptible M line strain. These studies will enable us to determine if susceptible and resistant strains produce different FREPs and if the diversity and timing of expression of their FREPs differ following exposure to infection with the two parasites


2. Genomic Organization of FREPs:

Using recently made genomic libraries, the structure of FREP-encoding genes, including intron-exon boundaries and adjacent non-coding sequences will be determined and compared. These studies will provide additional insight into FREP regulation that can be correlated with the above-mentioned expression studies and inform us as to how diversity in antigen-recognizing molecules is generated and maintained in an invertebrate host.


3. Investigation of Parasite Secretory-Excretory Products (SEP) and Their Interaction with Hemocytes:

E. paraensei parasites mediate interference towards the immune capabilities of their snail hosts. The following movies demonstrate how this parasite affects hemocytes in vitro:

Click on an icon to view each Windows Media Movie (~ 3.5 mb each)

We are now in the process of obtaining sequence information for a 51 kDa polypeptide produced by E. paraensei sporocysts that is a candidate molecule for causing observed adverse effects on B. glabrata hemocytes. We intend to examine its effect on hemocytes in several assays. This aim will provide important clues as to how parasite molecules affect hemocytes.

 

Hertel L.A., Stricker S.A. and Loker E.S. 2000. Calcium dynamics of hemocytes of the gastropod Biomphalaria glabrata: effects of digenetic trematodes and selected bioactive compounds. Invertebrate Biology, v. 119(#1): 27-37. Abstract

This is a montage of photomicrographic images obtained using confocal microscopy of two hemocytes from B. glabrata. Fluorescent calcium indicators, Calcium Green and Fura Red (the pair of images at upper left, respectively), were used to monitor calcium dynamics in the snail's cells following exposure to sporocysts of digenetic trematodes, E. parensei. The remaining images represent ratios of the two calcium indicators, recorded at 10-sec intervals, the brighter ones documenting the occurrence of brief calcium transients in the cells. Click HERE for a Windows Media Time Lapse Movie of the exposure dynamics during a two hour period.. The first part of the movie shows control hemocytes in medium only followed by two repititions of the response of hemocytes to secretory/excretory products from E. paraensei.

  

4. Defining the Essential Components of Infectivity or Resistance - Use of a Neglected Model System:

B. glabrata exposed to irradiated echinostome parasites show markedly enhanced resistance when later challenged with normal parasites (Lie et al., 1975a, 1987; and recently confirmed by us). The ability of normal and irradiated parasites to interfere with hemocyte functions will be compared. Also, hemocyte and plasma composition of snails exposed to normal or irradiated parasites will be monitored and compared to those of unexposed control snails. This approach will enable us to address specific hypotheses regarding the underlying mechanism of acquired resistance. We hope to determine if successful infection is correlated with the ability to produce certain kinds of secretory products that affect hemocyte behavior, and if a state of acquired resistance is associated with production of increased numbers of hemocytes or plasma proteins, including FREPs.