Effect of PDVs on host haemocytes

Three main immune responses implicating haemo-cytes have been described in Lepidoptera. Phagocytosis corresponds to the engulfment of bacteria or yeast by individual cells, whereas nodulation and encapsulation involve haemocytes that will respectively, surround masses of bacteria or nec-rotic tissues, or form capsules around larger targets such as parasitoid eggs. Four circulating haemo-cyte types have been described in Lepidoptera and have been shown to be implicated in these cellular immune responses (Ribeiro and Brehelin, 2006). Plasmatocytes and granular cells are numerically the two main classes of haemocytes, accounting for 85-95% of all the haemocytes in a caterpillar (Loret and Strand, 1998). In Pseudoplusia includens it has been shown that capsule formation requires that both granular cells and plasmatocytes are modified from being non-adhesive to highly adhesive cells that can adhere to the foreign target (Pech and Strand, 1996). In this process release of granulocyte inclusions allows the recruitment of plasmatocytes to form a multilayer of cells before a final layer of granulocytes completes the capsule (Pech and Strand, 1996; Gillespie et al, 1997). Granulocytes of this species also have strong phagocytic abilities, which enable clearance of microbial pathogens and dying cells from the haemolymph (Akai and Sato, 1973). Oenocytoids are large fragile cells that synthesize prophenoloxidase (proPO), a precursor of phenoloxidase (PO) (Essawy et al, 1985). The proenzyme is locally released in the plasma when these cells lyse (Ashida et al., 1988) and directly participates in the darkening of melanin and hardening of capsules and nodules (Marmaras et al., 1996). Finally, all lepidopteran species studied so far possess spherule cells, which are filled with crystal-like inclusions (Ribeiro et al., 1996). It has been suggested that these cells transport cuticular components (Locke et al., 1994) but their exact functions remain unknown.

PDVs can severely alter the function of host haemocytes (Figure 9.3). Depending on the virus and the hosts studied, the adhesion properties of plasmatocytes and granulocytes can be inhibited and their spreading at the surface of non-self material strongly reduced (Guzo and Stoltz, 1987; Strand and Noda, 1991; Luckhart and Webb, 1996; Beck and Strand, 2003). These effects were notably observed in the following host-parasi-toid-PDV associations: Malacosoma disstria/H. fugitivus/HflV (Stoltz and Guzo, 1986), Pseudaletia separata/Microplitis mediator/M. mediator bracovirus (MmBV) (Tanaka, 1987), H. virescens/C. sonorensis/ CslV (Davies and Vinson, 1988), P. includens/M. demolitor/MdBV (Strand et al, 1997; Strand and Clark, 1999), and Spodoptera littoralis/C. inanitus/ CiBV (Stettler et al, 1998). Some PDVs can also trigger the apoptosis of granulocytes (Strand and Pech, 1995a), or target potential haematopoietic tissues (Strand and Pech, 1995b), thus quantitatively limiting the ability of the host's immune cells to encapsulate the parasitoid eggs (Figure 9.4). In H. virescens, CslV induces a reduction in the number of circulating plasmatocytes, and the remaining haemocytes are altered in morphology and in spreading ability (Davies and Vinson, 1988; Webb and Luckhart, 1996). The phagocytic abilities of granular cells can also be altered upon injection of PDVs as in the case of the pierid Pieris rapae infected by Cotesia rubecula bracovirus (CrBV)

Figure 9.3 Comparison of haemocytes from Manduca sexta larvae parasitized by the wasp Cotesia congregata (24 h post-oviposition) and from unparasitized larvae. Cells were stained with neutral red for staining of the cytoplasm and Hoechst 33258 for nuclei staining. (a, b) Haemocytes from unparasitized larvae. (c, d) Haemocytes from a parasitized larvae showing typical nucleus fragmentation and condensation evocative of parasitism-induced apoptosis. This phenotype was also reported by Amaya et al. (2005).

Figure 9.3 Comparison of haemocytes from Manduca sexta larvae parasitized by the wasp Cotesia congregata (24 h post-oviposition) and from unparasitized larvae. Cells were stained with neutral red for staining of the cytoplasm and Hoechst 33258 for nuclei staining. (a, b) Haemocytes from unparasitized larvae. (c, d) Haemocytes from a parasitized larvae showing typical nucleus fragmentation and condensation evocative of parasitism-induced apoptosis. This phenotype was also reported by Amaya et al. (2005).

(Asgari et al., 1997). Recently, it has been suggested (Schmidt et al., 2005) that these various virus-induced alterations in host haemocyte function could have in common actin-cytoskeleton breakdown and rearrangements (Tanaka, 1987; Li and Webb, 1994; Strand, 1994; Asgari et al., 1997). Thus certain PDVs, probably in combination with other maternal factors, converge in their respective functions to disrupt one subcellular component of the haemocytes, the cytoskeleton.

The pathologies induced by MdBV on the haemocytes of two lepidopteran hosts, P. inclu-dens and H. virescens, were compared to those observed upon injection of bracovirus particles from M. mediator (MmBV) and Microplitis cro-ceipes (McBV) (Kadash et al., 2003). Interestingly the effects induced by the three PDVs differed. For instance, while MdBV induced in both hosts the apoptosis of granulocytes and prevented plas-matocyte adherence to foreign surfaces, MmBV caused a loss of adhesion in less than 50% of plasmatocytes and McBV had little impact on the haemocytes. However McBV was able to cross protect approximately 50% of M. demolitor or M. mediator eggs from encapsulation in H. virescens. This observation indicates that protection of parasitoid eggs from encapsulation does not necessary rely on the induction of detectable haemocyte pathologies, as has been observed in other models involving parasitoids devoid of PDVs (Asgari et al., 2002; Moreau et al., 2003). However, the protected eggs were not able to develop fully, indicating that host suitability for the development of a parasitoid not only results from the ability of PDVs to manipulate the host immune system but also depends on other aspects of host physiology.

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