Clearance of schistosome parasites by resistant genotypes at a single genomic region in Biomphalaria glabrata snails involves cellular components of the hemolymph

Abstract

Schistosomiasis is one of the most detrimental neglected tropical diseases. Controlling the spread of this parasitic illness requires effective sanitation, access to chemotherapeutic drugs, and control over populations of the freshwater snails, such as Biomphalaria glabrata, that are essential intermediate hosts for schistosomes. Effectively controlling this disease, while minimising ecological implications of such control, will require an extensive understanding of the immunological interactions between schistosomes and their molluscan intermediate hosts. Here we histologically characterise the clearance of schistosome larvae by snails that exhibit allelic variation at a single genomic region, the Guadeloupe resistance complex. We show that snails with a resistant Guadeloupe resistance complex genotype clear schistosomes within the first 24-48 h, and that this resistance can be transferred to susceptible snails via whole hemolymph but not cell-free plasma. These findings imply that Guadeloupe resistance complex-coded proteins help to coordinate hemocyte-mediated immune responses to schistosome infections in Guadeloupean snails.

Keywords: Biomphalaria glabrata; Hemocyte; Hemolymph transfer; Histology; Plasma transfer; Resistance; Schistosomiasis.

Fig 1

Resistance phenotypes are conserved among inbred Guadeloupean Biomphalaria glabrata (BgGUA) lines. The susceptibility of nine independent homozygous BgGUA lines (3× S1S1, S2S2and RR) after exposure to 20 Guadeloupean Schistosoma mansoni (SmGUA) miracidia (n= 32 S1¹,32 S1²,28 S1³, 35 S2¹, 33 S2², 27 S2³, 55 R¹, 31 R², 42 R³). Data are presented as the proportion of infected snails (+/− the S.E. of proportions). Significant differences (P <0.05, Z score of proportion) are denoted by asterisks (*).

Fig 2

Resistant genotypes do not have modified integument anatomy or thickness, nor do they prevent Schistosoma mansoni miracidial penetration in Guadeloupean Biomphalaria glabrata (BgGUA). (A) The head-foot integument architecture of S1S1, S2S2, and RR BgGUA (scale bar = 100 μm). (B) The thickness of the integument of the head-foot of S1S1, S2S2, and RR BgGUA (n= 4). (C) The number of miracidia remaining in a well 2 h post-BgGUA exposure to 20 miracidia (n=12). Data presented as means (+/− S.D.). No significant differences (P >0.05, ANOVA) were observed.

Fig 3

Resistant Guadeloupean Biomphalaria glabrata (BgGUA) have fewer free sporocysts, and destroy schistosomes during the initial stages of infection. (A) Free and encapsulated sporocysts in BgGUA 24 h post-exposure (scale bar = 200 μm). (B) Total number and (C) number of free (unencapsulated/undegenerated) sporocysts found in individual SS and RR BgGUA 24 h and 48 h post-exposure. Data presented as means (+/− S.D.). Significant differences (P <0.05, student’s t-test) are denoted by asterisks (*).

Fig 4

Transfer of resistant (RR) hemolymph, but not cell-free plasma or sensitive (SS) hemolymph, confers resistance to Guadeloupean Schistosoma mansoni (SmGUA). The susceptibility of homozygous S1S1 snails after exposure to 20 miracidia and transfer treatments. Groups of snails were treated with sham (SSS), S2S2 hemolymph (HL), RR plasma, or RR hemolymph (HL) 24 h pre-exposure (n=69 S1S1 + sham, n = 67 S1S1 + S2S2 HL, n = 31 S1S1 + RR plasma, n= 74 S1S1 + RR HL). Data are presented as the proportion of infected snails (+/− the S.E. of proportions), and significant differences (P <0.05, Z score of proportion) are denoted by an asterisk (*).